Ip_15_research Report-dispersion Modelling and Calculations in Support of Ei Model Code of Safe Practice Part 15-Area Classification Code for Installations Handling Flammable Fluids,2ed_(March

energy I N S T D T U T E O J

Research report: Dispersion modelling and calculations in support of El Model code of safe practice Part 15: Area classification code for installations handling flammable fluids

2nd edition

RESEARCH REPORT: DISPERSION MODELLING AND CALCULATIONS IN SUPPORT OF El MODEL CODE OF SAFE PRACTICE PART 15: AREA CLASSIFICATION CODE FOR

INSTALLATIONS HANDLING FLAMMABLE FLUIDS

Second edition

March 2008

Published by ENERGY INSTITUTE, LONDON

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RESEARCH REPORT: DISPERSION MODELLING AND CALCULATIONS IN SUPPORT OF El MODEL CODE OF SAFE PRACTICE PART 1 5

CONTENTS

Page

Acknowledgements vi

Foreword vii

1 Introduction 1

2 Objective and scope of work 2

3 Approach 3 3.1 Investigation of research topics 3

3.1.1 Sensitivity effects of input parameters on dispersion characteristics 3 3.1.2 Area classification for liquid pools 5 3.1.3 Application of area classification methodology to LNG 6

3.2 Software 7

4 Validation of previous work 9 4.1 Release flow rates 9 4.2 Dispersion results 9

5 Investigation of sensitivity of results to variation of parameters 16 5.1 Variation with weather category (windspeed and stability) 16

5.1.1 R1 distances to LFL (releases at 5 m height above ground) 16 5.1.2 R2 distances to LFL (releases at 1 m height above ground) 18 5.1.3 Distance to 0.5 LFL 23

5.2 Variation with ambient temperature 26 5.3 Variation with relative humidity 28 5.4 Variation with surface roughness 29

6 Investigation of area classification for liquid pools 30 6.1 Spill volumes 30 6.2 Hazard distances: base case 30 6.3 Hazard distances: sensitivities 33

6.3.1 Variation with ambient temperature 33 6.3.2 Variation with surface roughness 33

7 Investigation of application of EI15 methodology to LNG 36 7.1 Liquid rain-out 36 7.2 Base case and variation with weather category 36 7.3 Variation with ambient temperature 42 7.4 Variation with relative humidity 42 7.5 Variation with surface roughness 42 7.6 Variation with surface type 44


Contents Cont... Page

8 Conclusions and recommendations 45 8.1 Sensitivity to parameter variations 45

8.1.1 Fluid category A 45 8.1.2 Other fluid categories 45

8.2 Liquid pools 46 8.2.1 Base case 46 8.2.2 Sensitivity to parameter variations 46

8.3 LNG 47

Annex A Sensitivity analysis on release angle 48

Annex B Glossary of terms and abbreviations 53 B.1 Introduction 53 B.2 Glossary of terms 53 B.3 Glossary of abbreviations 53

Annex C References 54

Addendum 1: El Calculations in support of IP 15: The area classification code for petroleum installations (first edition) 56

Tables: Table 3.1: Parameter values used for dispersion values 3 Table 3.2: Pool dimensions modelled 5 Table 3.3: Example LNG compositions 6 Table 4.1: Release flow rates from El 15 and DNV Phast 9 Table 4.2: Comparison of dispersion results (distances to LFL, m) from EI15 and DNV Phast:

Hazard distance R ; release height 5 m 12 Table 4.3: Comparison of dispersion results (distances to LFL, m) from EI15 and DNV Phast:

Hazard distance R2; release height 1m 13 Table 4.4: Summary of sensitivity of hazard distance to release angle 14 Table 4.5: Comparison of dispersion results (distances to LFL) from EI15 and DNV Phast:

Hazard distance R2; release height 1m 15 Table 5.1: Ratios of R, hazard distance to 0.5 LFL7LFL (5 m elevation release) 25 Table 6.1: Spill volumes modelled (m3) 30 Table 6.2: Distances from edge of pool to LFL and 0.5 LFL for base case, concrete 31 Table 6.3: Distances from edge of pool to LFL and 0.5 LFL for base case, dry soil 32 Table 7.1: Percentage rain-out of LNG (maximum from all weather categories) 36 Table 7.2: Summary of base case hazard radii R, to LFL and 0.5 LFL for LNG releases at

5 m height 37 Table 7.3: Comparison of distances to LFL (m) for fluid category G(i) and LNG 37 Table 7.4: Summary of base case hazard radii R, to LFL and 0.5 LFL for LNG releases at

1 m and 0.1 m height 38 Table 7.5: Distances to LFL for releases at 5 m height at different ambient temperatures . . . 42 Table 7.6: Variation in distances to LFL for releases at 5 m height for different relative

humidities 43 Table 7.7: Distances to LFL for selected releases at 5 m height onto concrete and water . . . . 44

iv


Contents Cont... Page

Table 7.8: Distances to LFL for releases at 0.1 m height onto concrete and water 44 Table A.1: Hole diameter 1 mm 49 Table A.2: Hole diameter 2 mm 50 Table A.3: Hole diameter 5 mm 51 Table A.4: Hole diameter 10 mm 52

Figures: Figure 3.1: Definitions of hazard radii R, and R2 for releases at height H (reproduced

from El 15) 4 Figure 5.1: Example comparisons of R, hazard distances to LFL for different weather

categories 17 Figure 5.2: Example comparisons of R2 hazard distances to LFL for different weather

categories 20 Figure 5.3: Example side views of plumes for releases at 1 m height, showing LFL contour

for each weather category 21 Figure 5.4: Example comparisons of hazard distances R, to 0.5 LFL for different weather

categories 24 Figure 5.5: Example comparisons of R, hazard distances to LFL between different ambient

temperatures 27 Figure 5.6: Example comparisons of R, hazard distances between different relative

humidities 28 Figure 5.7: Example comparisons of R, hazard distances between different surface

roughnesses 29 Figure 6.1: Example comparisons of R, hazard distances between different ambient

temperatures 34 Figure 6.2: Example comparisons of R, hazard distances to LFL between different surface

roughnesses 35 Figure 7.1: Example side views of plume dispersion 39 Figure 7.2: Variation of distances to LFL with weather category (release height 5 m) 41 Figure 7.3: Variation of distances to LFL with surface roughness 43

v


The technical development project that forms the basis of the second edition of this report was undertaken by Philip Nalpanis (DNV Energy)1, and assisted by colleagues Dr Phil Crossthwaite, Stavros Yiannoukas and Jeff Daycock. It was directed by the Energy Institute's Area Classification Working Group, which comprised during the project:

The Institute wishes to record its appreciation to the technical direction provided by them.

The Institute also wishes to record its appreciation to the assistance provided by Peter Roberts (Shell Global Solutions) regarding technical issues in the first edition of this report.

Project management and technical editing were carried out by Dr Mark Scanlon (Energy Institute).

Affiliations refer to the time of participation.

ACKNOWLEDGEMENTS

Howard Crowther Jon Ellis Tony Ennis Geoff Fulcher Kieran Glynn Martin Hassett Peter Murdoch Sonia Quintanilla Dr Mark Scanlon Dr Sam Summerfield Edmund Terry Mick Wansborough Dirk Wong

Consultant (Chairperson) ConocoPhillips Haztech Consultants F.E.S. (Ex) BP International M W Kellogg SIRA Certification Energy Institute (Secretary) Energy Institute Health and Safety Executive Sauf Consulting Consultant M W Kellogg

1 Philip Nalpanis, Principal Specialist, Det Norske Veritas (DNV) Ltd., Palace House, 3 Cathedral Street, LONDON, SE1 9DE.

vi


FOREWORD

The third edition of El Area classification code for installations handling flammable fluids ('EI15') was published in July 2005 and incorporated both technical clarifications and editorial amendments. EI15 is widely used in both the upstream and downstream sectors of the petroleum industry, as well as in other industry sectors that handle flammable fluids. In addition, it is regarded as a key methodology for addressing the area classification requirements of the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR).

In developing the second edition of EI15, some research studies were commissioned to strengthen its evidence base; this included the research published as the first edition of El Calculations in support of I PI 5: The area classification code for petroleum installations.

Following publication of the third edition of EI15, this research was commissioned to further extend the dispersion analysis and calculation basis of EI15: the findings are described here in the second edition of El Research report: Dispersion analysis and calculations in support of Ell5: The area classification code for installations handling flammable fluids.

The research described here focused on the following technical issues:

— Sensitivity analysis of ambient temperature, relative humidity, weather and surface roughness on dispersion characteristics from point sources for various fluid categories.

— Sensitivity analysis of the nature and ambient temperature of the underlying surface (e.g. concrete, dry soil), weather, and surface roughness on dispersion characteristics from pools of various diameters and depths, for various fluid categories.

— Determination of whether any existing IP15 fluid categories apply to liquefied natural gas (LNG), or if it requires specific modelling and sensitivity analysis of parameters (hole diameter, release pressure, temperature, relative humidity, weather, surface roughness, surface type) to determine dispersion characteristics.

Publication of the new research augments the following technical issues in the first edition:

— Work item 2: Shape factors and hazard radii for pressurised releases - see Section 3 of El Calculations in support of IP 15: The area classification code for petroleum installations (see Addendum 1).

— Work item 4: Liquid pools due to spillage - see Section 5 of El Calculations in support of IP 15: The area classification code for petroleum installations (see Addendum 1).

In practice, information on these issues in the earlier report is not invalidated, but rather is augmented in the second edition. Consequently, and because no further technical work has been carried out on other work items in the earlier report, the first edition is entirely replicated as Addendum 1.

vii


The research reported here should be considered independent of the Energy Institute's Area Classification Working Group, although commissioned and reviewed by them; going forward, they intend to consider how the findings of the new research affects the continuing technical integrity of pertinent aspects of the third edition of EI15.

The information in this publication should assist process safety engineers, safety advisors, designers, or others with responsibility for hazardous area classification to better determine the extent of hazardous areas in a consistent manner for specific fluid and process, weather and environment dependent values. The information is internationally applicable provided it is read, interpreted and applied in conjunction with relevant national and local requirements.

For further information on the suite of El publications on hazardous area classification see http://www.energyinst.org.uk/ei15.

The information contained in this publication is provided as guidance only and while every reasonable care has been taken to ensure the accuracy of its contents, the Energy Institute and the technical representatives listed in the Acknowledgements, cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The Energy Institute shall not be liable to any person for any loss or damage which may arise from the use of any of the information contained in any of its publications.

This publication may be further reviewed from time to time. It would be of considerable assistance in any future revision if users would send comments or suggestions for improvement to:

The Technical Department Energy Institute 61 New Cavendish Street LONDON, W1G 7AR e: [email protected]

viii

http://www.energyinst.org.uk/ei15



1 INTRODUCTION

The third edition of El Area classification code for installations handling flammable fluids ('EI15') was published in July 2005 and incorporated both technical clarifications and editorial amendments. EI15 is widely used in both the upstream and downstream sectors of the petroleum industry, as well as in other industry sectors that handle flammable fluids. In addition, it is regarded as a key methodology for addressing the area classification requirements of The Dangerous Substances and Explosive Atmospheres Regulations (DSEAR).

In developing the second edition of EI15, some research studies were commissioned to strengthen its evidence base; this included the research published as the first edition of El Calculations in supportoflP15: The area classification code for petroleum installations (see Addendum 1).

Following publication of the third edition of EI15, this research was commissioned to further extend the dispersion analysis and calculation basis of EI15: the findings are described here in the second edition of El Research report: Dispersion analysis and calculations in support of Ell5: The area classification code for installations handling flammable fluids. This research report sets out project findings: the results obtained from DNV's analysis and the conclusions that can be drawn in respect of El 15 in the areas researched.

Page 1


2 OBJECTIVE AND SCOPE OF WORK

The project has aimed to strengthen the evidence base of EI15 by researching the following technical issues, as requested by the El Area Classification Working Group:

1. Sensitivity effects of parameters on dispersion characteristics. EI15 Annex C sets out atmospheric dispersion results for a range of fluid compositions, modelled for a single set of atmospheric conditions and other release parameters. The dispersion modelling included in EI15 Annex C will be repeated as a calibration of the models to be used. The new research will extend the parameter ranges of atmospherictemperature, relative humidity, wind speed, reference height, stability, reservoir temperature and surface roughness. In addition, modelling would be carried out for both the existing El 15 hazard radius lower flammable limit (LFL) boundary and a 0.5 LFL boundary.

2. Area classification arising from pools formed as a result of instantaneous releases. Ell 5 currently only addresses continuous releases (leaks) for particular mass flow rates/hole sizes, continuing over a period of time. Additional research is required to investigate the factors that affect vapour dispersion from gasoline spills resulting from an instantaneous release that would be subject to area classification. These factors potentially include pool size, pool depth, confinement by kerbs or bunds, the nature and temperature of the underlying surface (e.g. concrete, water), wind speed, and topography.

3. Application of the EI15 methodology to liquefied natural gas (LNG). El 15 does not specifically address LNG, a rapidly growing market and a material that recent work has shown presents particular challenges in modelling dispersion. The research for EI15 will determine whether it aligns with any of the existing and well established fluid categories or warrants separate treatment. In the latter case hazard radii would be modelled equivalent to those for materials already addressed. The intent of each of these technical issues was further discussed with the El Area Classification Working Group, which led to the agreed approach described in Section 3. This research report delivering the project findings aims, so far as is practicable, to align with Ell 5, to enable the findings to be integrated into any future edition.

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3 APPROACH

3.1 INVESTIGATION OF RESEARCH TOPICS

3.1.1 Sensitivity effects of input parameters on dispersion characteristics

The previous dispersion analysis, as published in EI15 Annex C, was carried out essentially for a single set of parameters as set out in Table 3.1.

With one exception, the parameters were initially varied singly in turn to identify any small dependencies and determine whether combinations of variations of more than one parameter simultaneously could potentially give rise to significantly enhanced influence. All four stability-windspeed combinations were modelled for the other parameter variations. The investigations were carried out for fluid categories A, B, C, G(i) and G(ii).

Table 3.1: Parameter values used for dispersion values

Parameter Value modelled for EI15 Range of values modelled here

Ambient temperature 30°C -20°C, 0°C, 20°C, 40°C

Storage/process temperature

20°C Same as ambient and fluid category A (liquefied petroleum gas (LPG)-like)

at -40°C

Relative humidity 70% 50%, 90%

Wind speed 2 m/s 1.5 m/s 2 m/s 5 m/s 9 m/s

Stability class D F D D D

Surface roughness length 0.03 m 0.1 m, 0.3 m, 1.0 m

Release direction Horizontal Horizontal

Release height For R,: 5 m For R2: 1 m

For R,: 5 m For R2: 1 m

Release angle For R,: horizontal For R2: unknown

For R,: horizontal For R2: -30°, -45°, -60°

Sample time 18.75 s No variation (Note 1)

Reference height 10 m No variation (Note 2)

Hazard distances To LFL To LFL and 0.5 LFL Notes 1 This is a standard value derived from CCPS Guidelines for use of vapor cloud dispersion models

and TNO Yellow book. 2 This is the standard height for wind speed measurements, e.g. HMSO Meteorological glossary.

The meaning of R, and R2, referred to in Table 3.1, is illustrated by Figure 3.1: R, represents the hazard distance for a release that does not interact with the ground; whereas, R2

represents the hazard distance for a release that does interact with the ground.

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Figure 3.1: Definitions of hazard radii R, and R2 for releases at height H (reproduced from Ell 5)

(a) Releases where H > Ri + 1 m

Ground

(b) Releases where 1 m < H < Ri + 1 m

(c) Releases where H < 1 m

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3.1.2 Area classification for liquid pools

Notwithstanding the original specification for this task set out in section 2, the following parameter variations were agreed:

— Fluid categories: A; B; and C. — Pool sizes and depths: as set out in Table 3.2. — Surface types: concrete; and dry soil. — Ambient temperatures: -20°C; 0°C; 20°C (base case); and 40°C. — Surface roughness length: 0.03 m (base case); 0.1 m; 0.3 m; and 1.0 m. — Range of weather categories: 2D; 1,5F; 5D; and 9D.

Table 3.2: Pool dimensions modelled

Pool diameter (m)

Pool depth Pool diameter

(m) Minimum (Note 1)

0.1 m 1 m

1 /

3 / /

10 / / /

30 / /

100 /

Spill temperatures

Ambient (Note 2)

and 100°C

Ambient Ambient

Notes 1 The minimum pool depth depends on the surface type:

- Concrete: 5 mm - Dry soil: 20 mm

2 See 3.1.2.

Since the modelling input requires spill mass ratherthan volume, the pool dimensions given in Table 3.2 were combined with the fluid densities at the release temperature to obtain the masses.

At ambient temperature, fluid category A can only be maintained as a liquid under pressure. A release of this fluid under these conditions will have a large vapour fraction (especially at temperatures of 0°C and above) and any release, unless at low level, is not likely to result in significant rain-out and liquid pool formation. Hence a refrigerated spill of fluid category A, stored at -40°C has been modelled instead, as representing better the types of scenario being addressed by this part of the study.

Fluid category B, with its wide range of hydrocarbon constituents from methane up to n-decane, was modelled as follows.

1. Discharge was modelled using DNV Process hazard analysis software tool (PHAST) 6.6 (see 3.2).

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2. The composition of the liquid part of this was used to model the spill using DNV PHAST 6.51.

A release of fluid at atmospheric pressure and a temperature of 100°C is only relevant to fluid category C. Fluid category A will be entirely vapour at this temperature, while fluid category B will be roughly 50% vapour (hence a release will be two-phase and not a true 'liquid spill'). Hence a temperature of 100°C is only modelled for fluid category C.

3.1.3 Application of area classification methodology to LNG

LNG is largely methane but also typically includes small quantities of ethane, propane and C02 and sometimes other components. Two typical compositions are given in Table 3.3. Although DNV PHAST can model mixtures, based on DNV's experience in several LNG studies it has been modelled as pure methane for the present study.

Typical rundown, storage and loading temperatures for LNG are in the range -170° to -160°C; therefore releases from a storage temperature of -165°C have been modelled.

Typical pressures in LNG systems range from 1.5 bar(a)to 10 bar(a); therefore these two release pressures have been modelled, and also an intermediate release pressure of 5 bar (a).

Table 3.3: Example LNG compositions

Component LNG example 1 LNG example 2

Methane 93.2375% 90.6410%

Ethane 6.5487% 8.3232%

Propane 0.0427% 0.0022%

Carbon dioxide 0.0058% 0.0049%

Hydrogen sulfide 0.0003% 0

Nitrogen 0.1650% 1.0281%

Helium 0 0.0006%

The sensitivity analysis for the EI15 fluids shows limited variation of hazard distances with ambient temperature (see section 5.2). On this basis only two ambient temperatures have been modelled, approximately bracketing the range of values given in Table 3.1: -20°C and 30°C.

The same relative humidities, weather categories (i.e. combinations of windspeed and stability) and surface roughness lengths as in the sensitivity analysis for the EI15 fluids, i.e. those in Table 3.1 have been adopted. In addition, the effects of releases over water rather than over land have also been investigated. DNV PHAST can model different types of water body; for this study 'shallow open water' was chosen as likely to be most representative of, for example, the situation at a marine terminal where LNG is loaded or unloaded, and where spills may occur. The appropriate corresponding surface roughness length is not universally agreed; a value of 0.001 m was adopted for the purposes of this study. As for the El 15 sensitivity analysis, two release heights above ground have been modelled: 5 m and 1 m. As for the sensitivity analysis, the lower height is intended to

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represent releases where ground effects (i.e. the plume contacting the ground) influence the dispersion. The analysis (see section 7.2) shows that this is the case for all 10 mm leaks and some 5 mm leaks, but not for smaller leaks (1 mm, 2 mm). Another issue is rain-out of liquid droplets on the ground to form a pool that then vaporizes. The dispersion modelling (see section 7.1) showed that liquid rain-out does not occur for releases at 5 m, and only occurs at 1 m for 1.5 bar(a) releases from the larger hole sizes (5 mm, 10 mm). Hence 1.5 bar(a) releases have also been modelled at 0.1 m height above ground in order to investigate the influence of both ground effects and liquid rain-out.

All releases were modelled as initially horizontal.

3.2 SOFTWARE

The proprietary, commercial software DNV PHAST has been used for this study. Its key features are:

— It is a general purpose consequence modelling package covering: Discharge of fluids from pressurised, atmospheric or refrigerated containment. Atmospheric dispersion including liquid rain-out and re-evaporation.

Jet, pool and flash fires; explosions; boiling liquid expanding vapour explosions (BLEVEs)/fireballs (radiation and blast). Toxic impact.

(The latter two are not relevant to this study.)

— It is licensed to over 100 organisations world-wide (operating companies, contractors, regulatory authorities, academic institutions, consultants).

— It is widely used in DNV, being their main tool for consequence analysis.

— Its dispersion modelling has been validated in the SMEDIS project (Daish et al. 1999, CERC Model evaluation report on UDM version 6.0), where it performed well.

— Concentration in air is calculated as mol%. The previous work reported in El Calculations in support of IP!5 (see Addendum 1) indicated mass concentration rather than vol% should be used to determine flammable material concentrations in plumes containing liquid droplets. In using mol%, the formulation in DNV PHAST correctly accounts for liquid content in calculating flammable material concentrations and so is equivalent to using mass concentration. For gaseous plumes, mol% is equivalent to vol%.

In the main, the latest released version (6.51) has been used. This models mixtures as 'pseudo pure materials' by averaging the thermophysical properties according to the components' contribution to the mixture. In general this gives acceptable results, provided the calculated saturation temperature at atmospheric pressure matches the atmospheric boiling point. However, for fluid category B (a hot straight run gasoline type material in a process environment between a fractionating column and a depropaniser) this was found not to be the case and so a pre-release version 6.53, which models mixtures using multicomponent thermodynamics, was used to model the discharge parameters and composition of each phase. For fluid categories A and C, versions 6.51 and 6.53 give

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consistent values for the saturation temperature at atmospheric pressure and the atmospheric boiling point, so it was considered acceptable to use version 6.51 for these fluid categories.

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4 VALIDATION OF PREVIOUS WORK

4.1 RELEASE FLOW RATES

Table 4.1 compares the release flow rates obtained from DNV PHAST with those presented in Table C9(a) of EI15. Allowing for rounding, correspondence is good for all fluids, release pressure and hole sizes.

Table 4.1: Release f low rates from EI15 and DNV PHAST

Fluid category

Release pressure (bar(a))

Ell 5 flow rate (kg/s) DNV PHAST flow rate (kg/s) Fluid category

Release pressure (bar(a)) Release hole diameter Release hole diameter

Fluid category


1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm

A

6.8 (Note 1) 0.01 0.04 0.3 1 0.01 0.05 0.30 1.21

A 10 0.03 0.06 0.4 1.5 0.02 0.06 0.38 1.50

A 50 0.03 0.14 0.9 3.5 0.04 0.14 0.88 3.50

A

100 0.05 0.2 1.2 5 0.05 0.20 1.24 4.97

B

5 0.01 0.04 0.3 1 0.012 0.05 0.30 1.18

B 10 0.02 0.07 0.4 1.7 0.02 0.07 0.44 1.76

B 50 0.04 0.15 1 4 0.04 0.17 1.03 4.10

B

100 0.06 0.2 1.4 5.5 0.06 0.24 1.50 5.82

C

5 0.01 0.06 0.3 1.1 0.01 0.05 0.31 1.23

C 10 0.02 0.1 0.4 1.7 0.02 0.07 0.46 1.84

C 50 0.04 0.2 1 4 0.04 0.17 1.07 4.30

C

100 0.06 0.25 1.4 6 0.06 0.24 1.52 6.10

G(i)

5 0.001 0.002 0.02 0.06 0.001 0.003 0.016 0.06

G(i) 10 0.001 0.005 0.03 0.1 0.001 0.005 0.032 0.13

G(i) 50 0.007 0.03 0.2 0.7 0.007 0.027 0.170 0.68

G(i)

100 0.015 0.06 0.4 1.5 0 .015 0.059 0.370 1.48

G(ii)

5 0 .0004 0.001 0.01 0.04 0.0003 0.0012 0.0075 0.03

G(ii) 10 0.001 0.003 0.02 0.07 0.001 0.003 0.02 0.06

G(ii) 50 0.004 0.02 0.1 0.4 0.003 0.012 0.08 0.30

G(ii)

100 0.007 0.03 0.2 0.7 0.006 0.024 0.15 0.60

Notes 1 At the fluid storage temperature of 20°C, the nominal release pressure of 5 bar(a) is below

the saturated vapour pressure of fluid category A. The saturated vapour pressure (6.8 bar(a)) was used to calculate the discharge rate and dispersion.

4.2 DISPERSION RESULTS

Table 4.2 compares the R, hazard distance results (horizontal release at 5 m elevation) obtained from DNV PHAST with those in EI15 Table C9(a).

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For fluid categories A and B, the distances predicted by DNV PHAST vary from 32% less than the corresponding EI15 value to 45% more. For the most part, the largest variances are for the smaller hole sizes.

For fluid category C, DNVPHASFs hazard distances are all larger than those in EI15, by up to 25% . No pattern is discernible in these variances.

For fluid categories G(i) and G(ii) (i.e. the two gases) DNV PHAST's hazard distances are all larger than those in EI15 by between 20% and 80% . Again, no pattern in these variances is discernible although the largest variances are for the smallest hole sizes.

Overall, the results obtained from DNV PHAST were sufficiently consistent with those in EI15 to continue using DNV PHAST without modifying the standard approach to modelling these releases.

The R2 hazard distances are intended to indicate the hazard range for releases close to ground level, where ground effects may increase the distances compared with dispersion of a plume that does not touch the ground. Table 4.3 compares results obtained from DNV PHAST for horizontal releases at 1 m elevation with the R2 results presented in EI15 Table C9(b). It is colour coded to indicate how these compare with the respective distances presented in Table 4.2 for releases at 5 m elevation obtained from DNV PHAST and presented in EI15 Table C9(a). Overall, Table 4.3 shows that:

— As presented in EI15, R2 > R, in most cases; for 5 mm and 10 mm holes, by around a factor of two.

— By contrast, only for 10 mm holes, fluid categories A, B and C, are the distances obtained by DNV PHAST greater for 1 m release height; only for fluid categories B and C at 5 bar(a) and 10 bar(a) do they approach a factor of two larger.

This is as would be expected, namely that the plume from small horizontal releases even as low as 1 m above the ground will not spread vertically sufficiently to interact with the ground within the distance along the plume centreline where the concentration falls to LFL. However, the supporting research report to EI15 Annex C (see Addendum 1) indicates that releases at angles below horizontal were modelled. With definitive information not being available as to how the results from this model were translated into the R2 distances presented in EI15, a sensitivity study has been carried out on how the distance to LFL (and also 0.5 LFL) varies with the following release angles below horizontal: -30°, -45°, -60°.

Table 4.4 summarises the results from this; Annex A provides tables showing the detailed results for each weather category and release angle modelled. The following general conclusions are drawn from Table 4.4:

— Releases that interact with the ground (whilst still close to flammable) give longer hazard distances than the equivalent horizontal releases that do not interact with the ground. This is particularly marked for the larger release sizes modelled.

— For 1 mm holes, a horizontal release gives the longest hazard distance in almost all cases. This is because there is no significant interaction of these small releases with the ground.

— For 5 mm and 10 mm holes, in most cases the longest hazard distance results from a release at 60° below the horizontal (-60°).

Page 10


For many of these cases, this occurs for weather category F1.5; for higher release pressures (50 bar(a), 100 bar(a)) it may occur with weather categories D5 or D9 (i.e. the higher windspeeds have a significant influence in carrying the plume horizontally). Where it occurs for weather category F1.5, it is because the plume interacts with the ground for this windspeed but not for the higher windspeeds; where it occurs with weather categories D5 or D9, the plume grounds for all windspeeds and the higher windspeeds carry it further.

— For 2 mm holes, the findings are mixed, with a release angle of 60° below horizontal giving the longest hazard distances for fluid categories A and B at all release pressures, and the same angle giving the longest hazard distances for fluid categories C and G(ii) only at release pressures of 50 bar(a) and 100 bar(a); whereas, for the remaining cases, the longest hazard distance is for horizontal releases.

As a comparison with the R2 values presented in EI15 Table C9(b), the maximum distances to LFL with weather category D2 are given in Table 4.5. This shows the following:

— For 1 mm holes, the distances from DNVPHAS7~are generally shorter than the EI15 values, in particular for fluid category A, where they are 21 % to 36% shorter. This is consistent with no ground effect being seen in Table 4.4 (see discussion above).

— For 2 mm holes there is a wide scatter in the comparisons, with the DNV PHAST distances being from 30% shorter to more than double those in EI15.

— For 5 mm holes and 10 mm holes there is a trend towards DNV PHAST predicting longer hazard distances than those in El 15.

From the discussions above on Table 4.4 and Table 4.5, it appears that releases from 2 mm holes are particularly sensitive to release angle, release conditions, weather category and proximity to the ground; for smaller and larger holes, clear patterns emerge.

Page 11

Table 4.2: Comparison of dispersion results (distances to LFL, m) from El l5 and DNV PHAST: Hazard distance R,; release height 5 m

Fluid category


Ell5 Table C9(a) DNV PHAST Change from EI15 to DNV PHAST

(Note 1) Fluid category

Release pressure (bar(a)) Release hole diameter Release hole diameter Release hole diameter

Fluid category


1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm

A

6.8 2 4 8 14 1.45 2.9 7 13.6 -28% -28% -13% -3%

A 10 2.5 4 9 16 1.7 3.2 8 15.3 -32% -20% -11 % -4% A 50 2.5 5 11 20 2.2 4.6 11.1 23 -12% -8% +1% + 15% A

100 2.5 5 11 22 2.6 5.1 11.9 23.4 +4% +2% +8% +6%

B

5 2 4 8 14 1.3 2.6 6.1 11.2 -35% -35% -24% -20%

B 10 2 4 9 16 2 3.9 8.7 14.5 0% -3% -3% -9% B

50 2 4 10 19 2.7 5.6 13 25 +35% +40% +30% +32% B

100 2 4 10 20 2.9 5.8 13.7 25.8 +45% +45% +37% +29%

C

5 2 4 8 14 2.5 4.5 9.1 15 +25% + 13% + 14% +7%

C 10 2.5 4.5 9 17 2.6 5 10.8 18 +4% + 11% +20% +6% C 50 2.5 5 11 21 2.8 5.6 13.1 24 + 12% + 12% + 19% +14% C

100 2.5 5 12 22 2.8 5.7 13.8 26 + 12% + 14% +15% +18%

G(i)

5 < 1 < 1 < 1 1.5 0.1 0.2 0.85 1.9 +27%

G(i) 10 < 1 < 1 1 2 0.15 0.55 1.3 2.8 +30% +40%

G(i) 50 < 1 1 2.5 5 0.5 1.3 3.4 7 +30% +36% +40% G(i)

100 < 1 1.5 4 7 0.95 2 5 9.5 +33% +25% +36%

G(ii)

5 < 1 < 1 1.5 3 0.4 0.8 2.1 4.1 +40% +37%

G(ii) 10 < 1 1 2 4 0.6 1.2 2.7 5.5 +20% +35% +38%

G(ii) 50 < 1 2 4 8 1.3 2.5 6 11.2 +25% +50% +40% G(ii)

100 1 2 6 11 1.8 3.4 8.5 16.2 +80% +70% +42% +47% Notes 1 Only calculated where an exact value is presented in El 15.

Table 4.3: Comparison of dispersion results (distances to LFL, m) from EM 5 and DNV PHAST: hazard distance R2; release height 1

Fluid category


EI15 Table C9(b) DNV PHAST Fluid category


Release hole diameter Release hole diameter Fluid

category Release pressure (bar(a)) 1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm

A

6.8 2 4 16 40 1.4 2.8 6.5 13.7

A 10 2.5 4.5 20 50 1.6 3.0 7.1 15.7 A 50 3 5.5 20 50 2.2 4.5 10.0 23.7

A

100 3 6 20 50 2.4 4.9 11.2 26.6

B

5 2 4 14 40 2.1 3.6 8.6 24.9

B 10 2.5 4 16 40 2.4 4.2 10.8 30.2 B 50 2.5 5 17 40 2.6 5.2 12.6 30.8

B

100 3 5 17 40 2.7 5.4 13.2 31.6

C

5 2.5 4 20 50 2.2 3.8 8.9 31.6

C 10 2.5 4.5 21 50 2.4 4.2 11.1 30.7 C 50 3 5.5 21 50 2.7 5.3 13.1 31.3

C

100 3 6 21 50 2,8 5.5 13.5 32.2

G(i)

5 < 1 < 1 1 2 0.1 0.3 1.0 2.0

G(i) 10 < 1 < 1 1.5 3 0.2 0.5 1.3 2.6

G(i) 50 < 1 1.5 3.5 7 0.6 1,3 3.3 6.2 G(i)

100 1 2 5 11 0.9 2.0 4.8 8.9

G(ii)

5 < 1 < 1 2 4 0.3 0.7 1.9 3.7

G(ii) 10 < 1 1 2.5 5 0.5 1.1 2.5 5.1

G(ii) 50 1 2 6 11 1.2 2.4 5.5 10.3 G(ii)

100 2 3 8 14 1.7 3.1 7.4 15.9 Key

Distance < corresponding distance for 5 m release height (i.e. R2 < R,) Distance > corresponding distance for 5 m release height (i.e. R2 > R,)

Table 4.4: Summary of sensitivity of hazard distance to release angle

1 mm hole 2 mm hole 5 mm hole 10 mm hole

Fluid Release Max. LFL Weather Max. LFL Angle Weather Max. LFL Weather Max. LFL Angle Weather Max. LFL Weather Max. LFL Angle Weather Max. LFL Weather Max. LFL Angle Weather category pressure dist. for category dist. for O category dist. for category dist. for category dist. for category dist. for n category dist. for category dist. for n category

(bar(a)) horiz. any horiz. any horiz. any horiz. any release release release release release release release release

(m) (m) (m) (m) (m) (m) (m) (m)

A

0 (refr.) 1.5 D2 1.5 Horiz 2.4 D5 4.8 -60 F1.5 4.2 D9 16 1 - 4 5 F1.5 35.4 F1.5 35.4 Horiz

A

5 (refr.) 2.2 D2 2.2 Horiz 3.7 02 5.7 - 3 0 F1.5 13.4 F1.5 28 A, -60 F1.5 33.8 F1.5 58.9 MR F1.5

A 6.8 1.4 F1.5 1.4 Horiz 2.8 F1.5 3.7 -60 F1 5 6.7 F1.5 14 / -60 F1.5 14.6 F1.5 35.3 D5 A

10 1.6 F1.5 1.6 Horiz 3.0 F1.5 4.4 -60 F1.5 7.4 F1.5 16./ -60""* D5 16.5 F1.5 39.4 m D5 A

50 2.3 F1.5 2.3 Honr 4.5 F1.5 7.1 60 F1.5 10.5 F1.5 26 C 60 0 5 24.6 F1.5 60.0 60 D5

A

100 2.4 F1.5 2.7 60 F1.5 4.9 F1.5 7.8 60 F1.5 11.7 F1.5 29 4 60 D9 27.4 F1.5 63.0 60 D9

B

5 2.1 F1.5 2.1 Horiz 3.7 F1.5 7.5 - 6 0 F1 5 10.5 F1.5 25 9 F1.5 27.7 F1.5 55.8 sir- F1.5

B 10 2.4 F1.5 2.4 Horiz 4.3 F1.5 9.5 - 6 0 F1.5 12.9 F1.5 30 2 - s o ' F1.5 32.8 F1.5 65.9 sir-

F1.5 B

50 2.6 F1.5 3.2 " S B 5.3 F1.5 10 0 - 6 0 F1.5 13.7 F1.5 32 0 -60 D5 32.0 F1.5 65 6 fin D5 B

100 2.7 F1.5 3.4 -60 F1.5 5.5 F1.5 10.1 - 6 0 F1.5 13.9 F1.5 34 (, -60 D9 32.4 F1.5 66 1 D9

C

5 2.2 F1.5 2.2 Mi in/ 3.8 F1.5 3.8 1 Inn.' 10.4 F1.5 28 5 60 F1.5 32.7 F1.5 58..: H . 5

C 10 2.4 F1.5 2.4 1 ion. 4.3 F1.5 4.3 1 Ion.- 13.3 F1.5 34.8 - 60 F1.5 32.7 F1.5 71 .- F1.5 C

50 2.7 F1.5 3.4 60 F1.5 5.4 F1.5 9.6 60 D2 13.9 F1.5 3.3 / m D5 32.9 F1.5 103.4 uj F1.5 C

100 2.8 F1.5 3.0 60 F1.5 5.6 F1.5 6.6 60 D2 14.2 F1.5 37 1 60 D5 33.1 F1.5 109.1 Horiz

F1.5

G(i)

5 0.1 D2 0.1 Hon/ 0.3 02 0.3 1loi 1/ 1.0 F1.5 1.0 Hi.i . 2.0 F1.5 2.0 Horiz

G(i) 10 0.2 02 0.2 Horiz 0.5 02 0.5 Horiz 1.3 F1.5 1.3 I ll !I / 2.6 F1.5 3 6 -60 F1.5

G(i) 50 0.6 02 0.6 Horiz 1.3 F1.5 1 3 Horiz 3.3 F1.5 4.2 -60 F1.5 6.3 F1.5 11.2 - t o F1.5 G(i)

100 0.9 F1.5 0.9 Horiz 2 0 F1.5 2.0 Horiz 4.8 F1.5 7 6 -60 F1.5 9.1 F1.5 20.0 60 F1 5

G(li)

5 0.3 D2 0.3 Horiz 0.7 D2 0.7 Horiz 2.0 F1.5 2 0 Hin z 3 8 F1.5 5.9 60 1 1.5

G(li) 10 0.5 02 0.5 Horiz 1.1 F1.5 1.1 Horiz 2.6 F1.5 3.3 60 F1.5 5.2 F1.5 9 0 60 1 1.5

G(li) 50 1.2 F1.5 1.2 Horiz 2.4 F1.5 2.6 60 F1.5 5.6 F1.5 10.7 60 D5 11.1 F1.5 26.3 60 D9 G(li)

100 1.7 F1.5 1.7 Horiz 3.2 F1 5 4.9 -60 F1.5 7.7 F1.5 17.1 D5 16.9 F1.5 38.1 -60 D5

Key: cells shaded: ndicate that the maximum distance for LFL is for a release anqle be low horizontal

Table 4.5: Comparison of dispersion results (distances to LFL) from EI15 and DNV PHAST: hazard distance R2; release height 1

Fluid Release EI15 Table C9(b) DNV PHAST Change from EI15 to DNV PHAST (Note 1)

category pressure (bar(a)) Release hole diameter Release hole diameter Release hole diameter pressure (bar(a))

1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm 6.8 2 4 16 40 1.4 2.8 13 31 -32% -30% -16% -22%

A 10 2.5 4.5 20 50 1.6 3.9 15 34 -36% -14% -25% -31% A 50 3 5.5 20 50 2.2 6.3 23 50 -25% 14% 16% -1% 100 3 6 20 50 2.4 6.9 25 56 -21% 14% 25% 11% 5 2 4 14 40 2.1 5.5 23 46 3% 39% 66% 15% 10 2.5 4 16 40 2.4 8.4 27 52 -6% 110% 68% 31% 50 2.5 5 17 40 2.6 9.0 28 52 3% 80% 67% 31% 100 3 5 17 40 2.7 10.0 27 54 -10% 100% 60% 36% 5 2.5 4 20 50 2.2 3.8 26 54 -11 % -5% 29% 8%

C 10 2.5 4.5 21 50 2.4 4.2 30 62 -5% -6% 42% 24% C 50 3 5.5 21 50 2.7 9.4 30 74 -11 % 70% 42% 49% 100 3 6 21 50 2.8 6.4 32 84 -8% 6% 50% 67% 5 < 1 < 1 1 2 0.1 0.3 1.0 2 -4% 0%

G(i) 10 < 1 < 1 1.5 3 0.2 0.5 1.3 3 -12% 3% G(i) 50 < 1 1.5 3.5 7 0.6 1.3 4.1 11 -16% 17% 51% 100 1 2 5 11 0.9 2.0 7.1 16 -6% -1% 41% 49% 5 < 1 < 1 2 4 0.3 0.7 1.9 5 -3% 26%

G(ii) 10 < 1 1 2.5 5 0.5 1.1 2.5 9 9% 2% 81% G(ii) 50 1 2 6 11 1.2 2.4 11 21 18% 18% 78% 91% 100 2 3 8 14 1.7 4.0 17 25 -15% 35% 113% 77%

Notes 1 Only calculated where an exact value is presented in EN 5.


5 INVESTIGATION OF SENSITIVITY OF RESULTS TO VARIATION OF PARAMETERS

5.1 VARIATION WITH WEATHER CATEGORY (WINDSPEED AND STABILITY)

5.1.1 R, distances to LFL (releases at 5 m height above ground)

Figure 5.1 compares the R, hazard distances for different weather categories, for releases modelled at the base case conditions, i.e. as used in EI15. Note that the weather category 2D was used for the EI15 work.

For lower release pressure (illustrated in Figure 5.1 (a),(c) by 10 bar(a)), the weather category has no discernible impact on the hazard distances for the two gases G(i) and G(ii). This is because the releases are momentum driven jets, with the velocity difference between the released fluid and its surroundings driving the entrainment of air and resulting lowering of concentration. For higher release pressure (illustrated in Figure 5.1(b),(d) by 100 bar(a)), higher windspeeds do lead to more rapid dilution and hence shorter hazard distances for the two gases, compared with lower windspeeds. The higher windspeeds also lead, at all release pressures, to shorter hazard distances for the three liquid categories A, B and C released at ambient temperature and for fluid category A (LPG-like), refrigerated and released under 5 bar(a) head (e.g. pump pressure). Hence, the original choice of 2D for the modelling in EI15 was in general conservative as it gave longer hazard distances than more usual choices (e.g. 5D and 1,5F) might have done. However, for a windspeed of 2 m/s, D stability rarely occurs in the United Kingdom (UK). According to Pasquill and Smith (1983), this windspeed would tend to give unstable conditions (A to C stabilities) in the daytime (more unstable with stronger insolation, i.e. less cloud) and stable conditions (E or F stabilities) at night (more stable with less cloud).

However, for fluid category A (LPG-like), refrigerated and stored at atmospheric pressure (modelled as a discharge under 10 m tank head; Figure 5.1(e)), a case not considered in EI15, higher wind speeds give longer dispersion distances. The refrigerated fluid forms an evaporating pool on the ground, with evaporative mass transfer therefore increasing with wind speed.

Page 16

Figure 5.4: Example comparisons of hazard distances R, to 0.5 LFL for different weather categories

(a) 10 bar(a), 2 mm hole

(c) 10 bar(a), 10 mm hole

(e) Refrigerated LPG under 10 m liquid head I „ 1 1

E~ Q -S O LL • A

• A O c ro 1 4

• B • •

• • • •

0 2D 1.5F 5D 9D • 1 mm hole • 2 mm hole

5 mm hole • 10 mm hole

(b) 100 bar(a), 2 mm hole

• fi •

rE — 4

(rt 2 5

2D • Fluid A

Fluid G(i)

1.5F • Fluid B • Fluid G(ii)

5D 9D I Fluid C

(d) 100 bar(a), 10 mm hole

(f) Refrigerated LPG under 5 bar(a) pressure

Notes 1 Larger sized points in all graphs depict the EI1 5 base case; here, the 2D combination.

Page 17


It may also be noted that, contrary to what is often believed regarding atmospheric dispersion, stability F does not always give the longest hazard distances (except for fluid category B released from a 10 mm hole, and then the distance is only marginally longer than for weather category 2D: see Figure 5.1(c),(d)). Stability F does occur sufficiently frequently (at night: see section 5.1.1) that it cannot be discounted. The received wisdom only applies when passive (sometimes called 'Gaussian') dispersion is the dominant mechanism of dispersion. In the case of flammable substances discharged under pressure, the dominant dispersion mechanisms in the early stages are turbulence induced by the momentum of the release and then dense gas dispersion (i.e. dispersion of a cloud substantially denser than air). Usually the LFL is reached before the cloud is moving with a velocity close to the ambient windspeed and the cloud's density has approached neutral with respect to air, the conditions for passive dispersion to dominate. Hence, atmospheric stability tends to have less influence on dispersion to LFL. However, as shown in 5.1.3, it does influence dispersion to lower concentrations.

5.1.2 R2 distances to LFL (releases at 1 m height above ground)

Based on the analysis in section 4.2 of release angle, releases at 1 m height above ground have been modelled at an angle of 60° below horizontal in order to examine the influence of ground effects on the hazard distances. Figure 5.2 compares the R2 hazard distances for different weather categories, for releases modelled at the base case conditions, i.e. as used in Ell5.

At 10 bar(a) and for all hole sizes (see Figure 5.2(a),(c)), fluid category C shows much longer hazard distances for weather categories 2D and 1,5F, i.e. low windspeeds, than for weather categories 5D and 9D, i.e. moderate to high windspeeds. For fluid categories A and B the same behaviour is exhibited at 10 bar(a) for the 2 mm hole size (Figure 5.2(a)). The behaviour can be understood by looking at side views of the plumes: examples are shown in Figure 5.3. Here, the plot (top left) for fluid category A shows that, for the low windspeeds, the plume centreline hits the ground whilst the concentration is still well above LFL. This results in the plume losing much of its momentum and being directed horizontally (so long as it is not buoyant), with the concentration decreasing to the LFL. At the higher windspeeds, the centreline concentration is close to or below LFL when the centreline hits the ground. By contrast, for fluid category C (plot at top right of Figure 5.3) similar behaviour is shown for all weather categories, with the plume centreline having hit the ground with the concentration well above LFL.

For fluid category G(i), Figure 5.2 shows distances to LFL that vary reasonably consistently with windspeed. By contrast, for releases of fluid category G(ii) at 100 bar(a), for a 2 mm hole (Figure 5.2(a),(b)) there is sharp decrease in hazard distance from low windspeeds to a windspeed of 5 m/s, and for a 10 mm hole (Figure 5.2(c),(d)) there is a sharp increase in hazard distance from low windspeeds to a windspeed of 5 m/s. Figure 5.3(c),(d) help to understand this. For the smaller hole size (2 mm), the behaviour is similar to that noted above for fluid category C. For the larger hole size (10 mm), at the lower windspeeds the plume becomes buoyant and lifts off, whereas at the higher windspeeds it is 'knocked down' to remain at ground level.

For refrigerated fluid category A (LPG-like), both under tank head (10 m)and under pressure (5 bar(a)), the hazard distances are much shorter at higher windspeeds as compared with low windspeeds, with the longest hazard distances being for weather category 1,5F (Figure 5.2(e),(f)). The side view plot (not presented here) is similar to that for fluid category A at ambient temperature and under 10 bar(a) pressure, as shown in

Page 18


Figure 5.3(a). Figure 5.2(e) indicates hazard distances in most cases much smaller for leaks up to 5 mm that might be expected from scaling down of the hazard distances for 10 mm leaks. Closer examination of the dispersion modelling, and in particular the pool evaporation, indicates that the pool area decreases much more quickly with hole size than the release rate and that in many cases there is no significant pool evaporation. The fraction of the initial release that rains out (and forms a pool) decreases slightly with hole size (74% to 81 % for 10 mm leaks vs. 65% to 72% for 2 mm leaks, depending on wind speed).

Overall, for all fluids except fluid category G(ii), weather category F1.5 gives the longest hazard distances for releases where ground effects influence dispersion. For fluid category G(ii), the low molecular weight renders the fluid buoyant even when the concentration approaches LFL, so at high windspeeds the plume resulting from larger releases is knocked down and higher windspeeds give the longest hazard distances.

Page 19

Figure 5.4: Example comparisons of hazard distances R, to 0.5 LFL for different wea ther categories

(a) 10 bar(a), 2 mm hole (b) 100 bar(a), 2 mm hole

• Fluid A • Fluid B • Fluid c Fluid G(i) • Fluid G(ii)

(c) 10 bar(a), 10 mm hole 80

60

40

• Fluid A Fluid G(i)

_ 9D > Fluid B • Fluid C Fluid G(ii)

25

20

15

10

* v 1 1 •

J

•

2D • Fluid A

Fluid G(i)

1 5F • Fluid B • Fluid G(ii)

5D 9D l Fluid C

(d) 100 bar(a), 10 mm hole

(e) Refrigerated LPG under 10 m liquid head (f) Refrigerated LPG under 5 bar(a) pressure

Notes 1 Larger sized points in all graphs depict the EI15 base case; here, the 2D combination.

Page 20


Figure 5.3: Example side views of plumes for releases at 1 m height, showing LFL contour for each weather category

(a) Fluid category A, 10 bar(a), 2 mm hole

• Category 2/D @ 0.6096 s •Category 1 5/F@0 841 s Category 5/D @ 0 09596 s

• Category 9/D @ 0 05311 s

Side View 1 1 1

0 9 0 8

E 0 7 x= cr. n fi r> 1 0 !) z» O 04

0 3 02 0 1

0 r

S

v \ m o m m <N M) r> u> ^ id w

Distance Downwind (m)

(b) Fluid category C, 100 bar(a), 10 mm hole

— Category 2/D @ 1888 s — Category 1 5/F @ 2028 s

Category 5/D @ 1156s — Category 9/D @ 1119 s

1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

Side View



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

/

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

i \

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3 / \

3



1 4 1 3 1 2 1 1

1

I 0 9 E 0 3

I

3



X U '

3



o 0 5 04 0 3 02 0 1

0 c

1

3



o 0 5 04 0 3 02 0 1

0 c 3



o 0 5 04 0 3 02 0 1

0 c 3



o 0 5 04 0 3 02 0 1

0 c 3



o 0 5 04 0 3 02 0 1

0 c

\

3



o 0 5 04 0 3 02 0 1

0 c \ \

3



o 0 5 04 0 3 02 0 1

0 c 3 O C D C - C D C * O

Dist

D O C

ance Dc

c -> (i wnv

I» o o o o c ) K CD D O ' vind (m)

3

Page 21


Figure 5.3: Example side v iews of plumes for releases at 1 m height, showing LFL contour for each weather category (continued)

(c) Fluid category G(ii), 100 bar(a), 2 mm hole

(d) Fluid category G(ii), 100 bar(a), 10 mm hole

1 4

— Category 2/D @ 1888 s 13 — Category 1 5/F @ 2028 s 12

Category 5/D @ 1156 s 11 — Category 9/D @ 1119 s ^

1 0 9

2 0 3 o> £ 0 7 | 0 6 § 0 5

04 0 3 0 2 0 1

0 o o o o o o o o o o o o o


Side View

L jf K \ / \ \ f \ \

\ v \ \ J V \ \

\ I

s

I \ \ \ \ \ \ \ \ \ \

I \

Page 22


5.1.3 Distance to 0.5 LFL

Figure 5.4 shows distances to 0.5 LFL for the different fluids and for a selection of release pressures and hole sizes. Apart from those for a 5 mm hole, they may be compared with the graphs in Figure 5.1.

With a few notable exceptions (see below), they show the same variation with weather category, but a steeper reduction in distance with increasing windspeed.

The exceptions are for stability F, windspeed 1.5 m/s; 10 mm hole size (Figure 5.4(e),(f)); fluid categories A (refrigerated), B and C, where the distance to 0.5 LFL is rather larger than any of those for D stability and about twice as long for refrigerated fluid category A. This shows that, for larger releases, the dispersion does reach a point where passive dispersion dominates before 0.5 LFL is reached. Consequently, if a criterion of 0.5 LFL were to be adopted for area classification, which in itself would be much stricter as the distances are so much longer (see below), the necessity to take into account the influence of stability F would further extend the classified areas.

Table 5.1 gives the ratios (distance to 0.5 LFL)/(distance to LFL) for the different fluid categories, weather categories, hole sizes and release pressures. They typically range from 1.5 to 2.0 for the liquids (fluid categories A, B, and C) and from 1.5 to 2.5 for the gases (fluid categories G(i) and G(ii)). Other general characteristics are that they are larger for:

— Smaller hole sizes (more markedly so for the gases).

— Lower release pressures.

— Lower windspeeds.

The behaviour described above for F stability, 1.5 m/s windspeed is apparent for fluid categories A, B and C released from a 10 mm hole, in particular for release pressures of 5 bar(a) and 10 bar(a).

The dispersion modelling results to 0.5 LFL provide information on how much larger hazard radii would be, compared to applying an LFL boundary. With EI15, the hazard radius boundary has been set at LFL (see EI15 section 1.6.8) reflecting the smaller releases subject to hazardous area classification than those major hazards-type releases for which a possible worst-case boundary of 0.5 LFL is sometimes applied. For major hazards-type releases, actual vapour dispersion is inevitably inhomogeneous and therefore there is a risk of pockets of flammable vapour forming beyond the modelled LFL boundary; however, this is less likely for the smaller and largely momentum driven releases subject to hazardous area classification. In addition, outside the hazardous radii defined by LFL contours, any significant inhomogeneity in cloud concentration at a particular location is likely to be relatively shortlived and continuous strong ignition sources are also unlikely to be present at all locations.

Page 23

Figure 5.4: Example comparisons of hazard distances R, to 0.5 LFL for different wea ther categories

(a) 10 bar(a), 2 mm hole (b) 100 bar (a), 2 mm hole

B 6

o 4

5 2

H I • HI 1 Hi • — m mm H

A A a 2D 1.5F

• Fluid A Fluid G(i)

• Fluid B • Fluid G(ii)

5D 9D • Fluid C

12

o ® o c 2 4 <A Q

2D • Fluid A

Fluid G(i)

1.5F i Fluid B • Fluid C Fluid G(ii)

(c ) 10 bar(a), 5 mm hole (d) 100 bar(a), 5 mm hole 16 16

1 1 2

1 • • • *

• •

1 1 2 • • •

in o •

0) o re to Q 4

re to Q 4

2D 1.5F 5D 9D • Fluid A • Fluid B • Fluid C

Fluid G(i) • Fluid G(ii)

25

_ 20 E

15 o o 0) " 10

0 1

• f

• •

• • : • -

•

- •

2D • Fluid A Fluid G(i)

1.5F 5D 9D (Fluid B

Fluid G(ii) I Fluid C

(e) 10 bar(a), 10 m m hole (f) 100 bar(a), 10 m m hole

Notes 1 Larger sized points in all graphs depict the El 15 base case; here, the 2D combination.

Page 24

RESEARCH REPORT: DISPERSION MODELL ING AND CALCULATIONS IN SUPPORT OF El MODEL CODE OF SAFE PRACTICE PART 1 5

F igure 5.4: Examp le compar i sons of hazard d is tances R, t o 0.5 LFL for d i f f e ren t w e a t h e r ca tegor ies (continued)

(g) Refrigerated LPG under 10 m liquid head (h) Refrigerated LPG under 5 bar(a) pressure

Notes 1 Larger sized points in all graphs depict the El 15 base case; here, the 2D combination.

Tab le 5.1: Rat ios of R, hazard d is tance to 0.5 LFL/LFL (5 m e l e va t i on re lease)

Fluid category


Hole size: 1 mm Hole size: 2 mm Hole size: 5 mm Hole size: 10 mm Fluid

category Release pressure (bar(a)) 2D 1.5F 5D 9D 2D 1.5F 5D 9D 2D 1.5F 5D 9D 2D 1.5F 5D 9D

A

0 (refr.) 1.3 1.3 1.4 1.4 1.4 1.3 1.4 1.4 1.4 1.3 1.4 1.4 1.2 2.5 1.4 1.4

A

5 (refr.) 1.5 1.6 1.5 1.5 1.4 1.5 1.5 1.5 1.3 1.3 1.4 1.4 1.5 2.6 1.3 1.3

A 6.8 1.9 1.9 1.8 1.8 1.8 1.8 1.7 1.5 1.7 1.7 1.7 1.5 1.6 1.6 1.5 1.4 A 10 1.9 1.8 1.8 1.8 1.8 1.8 1.7 1.5 1.7 1.6 1.6 1.5 1.6 1.6 1.6 1.4

A

50 2.0 1.9 1.8 1.6 1.8 1.7 1.6 1.5 1.6 1.6 1.6 1.4 1.5 1.6 1.4 1.5

A

100 1.9 1.9 1.8 1.7 1.8 1.8 1.7 1.5 1.8 1.7 1.6 1.5 1.6 1.7 1.5 1.4

B

5 1.5 1.6 1.5 1.5 1.4 1.5 1.6 1.5 1.4 1.4 1.4 1.4 1.4 2.1 1.4 1.4

B 10 1.7 1.7 1.5 1.5 1.5 1.5 1.5 1.5 1.4 1.4 1.5 1.5 1.6 2.3 1.3 1.4

B 50 1.9 1.9 1.8 1.8 1.8 1.7 1.7 1.5 1.6 1.6 1.6 1.5 1.6 1.9 1,4 1.4

B

100 1.8 1.8 1.8 1.8 1.8 1.8 1.7 1.7 1.7 1.7 1.6 1.6 1.7 1.9 1.5 1.4

C

5 1.5 1.4 1.4 1.4 1.4 1.3 1.4 1.4 1.3 1.3 1.4 1.4 1.4 2.4 1.3 1.4

C 10 1.7 1.5 1.5 1.4 1.5 1.5 1.4 1.4 1.4 1.4 1.4 1.4 1.6 2.4 1.3 1.4

C 50 1.8 1.8 1.7 1.5 1.6 1.6 1.5 1.4 1.5 1.5 1.4 1.4 1.7 1.9 1.4 1.3

C

100 1.8 1.8 1.7 1.7 1.8 1.6 1.6 1.5 1.6 1.6 1.5 1.4 1.7 1.8 1.4 1.4

G(i)

5 2.6 2.6 2.6 2.5 2.4 2.3 2.3 2.2 2.1 2.1 2.0 2.0 2.0 2.0 1.9 1.8

G(i) 10 2.5 2.4 2.4 2.4 2.2 2.1 2.1 2.1 2.1 2.0 1.9 1.9 2.0 2.0 1.9 1.9

G(i) 50 2.1 2.1 2.1 2.0 2.0 2.0 1.9 1.8 2.0 2.0 1.8 1.7 1.9 1.9 1.9 1.8

G(i)

100 2.1 2.1 2.0 2.0 2.0 2.0 1.9 1.8 2.0 2.0 1.7 1.6 2.0 1.9 1.7 1.6

G(ii)

5 2.4 2.3 2.2 2.1 2.1 2.0 1.9 1.9 2.0 1.9 1.7 1.6 1.9 1.8 1.6 1.7

G(ii) 10 2.1 2.1 2.0 2.0 2.0 2.0 1.9 1.8 1.9 1.9 1.7 1.7 1.9 1.8 1.6 1.5

G(ii) 50 2.0 2.0 1.9 1.8 1.9 1.9 1.7 1.6 1.9 1.8 1.6 1.5 1.8 1.8 1.5 1.6

G(ii)

100 2.0 1.9 1.7 1.8 2.0 1.8 1.8 1.6 1.8 1.7 1.7 1.5 1.6 1.6 1.5 1.5

P a g e 2 5


5.2 VARIATION WITH AMBIENT TEMPERATURE

Figure 5.5 compares the R, hazard distances for releases at different ambient temperatures. Note that the base case value (as used in EI15) is 30°C.

For the gases G(i) and G(ii), variations with ambient temperature are mostly small and not systematic. The exception is for G(i) at -20°C, which has a hazard distance 10% to 20% higher than under the EI15 base case conditions.

For fluid categories B and C, there is a small systematic increase in hazard distance with ambient temperature but this is less than 10%. Compared with the base case results, the variation is only a few percent. Hence the variation with ambient temperature for these fluids is not significant.

For fluid category A under pressure there is an apparent decrease in hazard distance with increasing ambient temperature, especially for the smaller leak sizes. This results from the thermodynamics: as the ambient temperature increases, the liquid fraction in the discharge and the liquid droplet diameter decrease, and the velocity increases. The velocity increase enhances entrainment of air and hence promotes more rapid dispersion, reducing the hazard distance. The larger liquid fraction and droplet size at lower ambient temperatures maintain the centreline concentration well above LFL to a greater distance, also increasing the hazard distance at lower temperatures. Hence in colder climates it may be necessary to increase the classified area dimensions compared with the values given in EI15 around equipment containing a fluid similar to category A (i.e. LPG-like) under pressure, by up to 70%.

For fluid category A (LPG-like), refrigerated at -40°C, the hazard distances increase slightly with ambient temperature both for atmospheric storage with 10 m tank head and for a release pressure of 5 bar(a) (Figure 5.4(e),(f)). For the case of a release under pressure, the increase is less than 10% over the ambient temperature range modelled except for 10 mm holes, for which the increase is 11%. However, compared with releases under EI15 conditions (i.e. fluid category A under pressure of 6.8 bar(a) and at 20°C), these releases give hazard distances up to 80% higher. For the case of a release under tank head, a 1 mm hole gives hazard distances up to 50% longer than a release under EI15 conditions, a 2 mm hole hazard distances only slightly longer, and a 10 mm hole hazard distances up to 40% shorter. Based on these results, hazard distances for refrigerated LPG significantly exceed those given in EI15 for fluid category A stored under pressure.

Page 26

F i g u r e 5.5: E x a m p l e c o m p a r i s o n s o f R , h a z a r d d i s t a n c e s t o LFL b e t w e e n d i f f e r e n t a m b i e n t t e m p e r a t u r e s

(a) 10 bar(a), 2 m m hole 6 6

• • • £ r 4 UL

£ r 4 UL SsSlpip^iE

•

o 0) o

H g g g g • • < > o 0) o I 2 (/> Q I 2 (/> Q

-20 0 20 Temperature (°C)

40

• Fluid A • Fluid B • Fluid C Fluid G(i) • Fluid G(ii)

(b) 100 bar(a), 2 m m hole

M 2 5

o - 2 0 0 20 40

Temperature (°C)

• Fluid A • Fluid B Fluid G(i) • Fluid G(ii)

l Fluid C

(c) 10 bar(a), 10 m m hole (d) 100 bar(a), 10 m m hole



(e) Refr igerated LPG under 10 m liquid head (f) Refr igerated LPG under 5 bar(a) pressure

o 20 Temperature (°C)

• 1 mm hole 5 mm hole

• 2 mm hole • 10 mm hole

0 20 Temperature (°C)



P a g e 2 7


5.3 VARIATION WITH RELATIVE HUMIDITY

Figure 5.6 compares the R, hazard distances for releases at different relative humidities. Note that the base case value (as used in EI15) is 70%.

Figure 5.6 shows that there are no significant variations in hazard distance with relative humidity for any of the fluids, hole sizes or release pressures, including fluid category A (LPG-like) refrigerated.

Figure 5.6: Example comparisons of R, hazard distances between different relative humidities

( a ) 10 ba r ( a ) , 2 m m h o l e (b ) 1 0 0 ba r ( a ) , 10 m m h o l e

50% 70% 90% Relative Humidity


(c ) R e f r i g e r a t e d L P G u n d e r 10 m l iqu id h e a d

10

c 4

2 •-





• Fluid A • Fluid B I Fluid C Fluid G(i) * Fluid G(ii)

(d ) R e f r i g e r a t e d L P G u n d e r 5 b a r ( a ) p r e s s u r e



l 2 mm hole »10 mm hole

Page 28


5.4 VARIATION WITH SURFACE ROUGHNESS

Figure 5.7 compares the R, hazard distances for releases with different surface roughnesses. Note that the base case value (as used in EI15) was 0.03 m.

For all fluid categories except G(i) the hazard distance falls off with increasing surface roughness. This is as expected, since increasing surface roughness promotes turbulence, enhancing dilution of the vapour cloud and hence reducing the hazard distances. For fluid category G(i) the hazard distance is more or less independent of surface roughness. This is because the dispersion is driven by the initial momentum of the release and atmospheric turbulence does not influence the dispersion before the cloud centreline concentration has reached LFL.

The effect of surface roughness on R2 is not presented. The same trends as identified above for R, are seen and are more marked for R2: in some cases its value reduces by as much as 60% as the surface roughness increases from 0.03 m to 1 m.

Figure 5.7: Example comparisons of R, hazard distances between different surface roughnesses

( a ) 10 ba r ( a ) , 2 m m h o l e (b ) 1 0 0 ba r ( a ) , 2 m m h o l e

6

5

? _ J 4 Li. _J O z 3 0 c 2 2 01 '

5

m • » • • •

• • •

• I A 4 A j

0.01 0.1 Sur face Roughness (m)


(c ) R e f r i g e r a t e d L P G u n d e r 10 m l iqu id h e a d

0.01 0.1 1 Surface Roughness (m)

• 1 mm hole • 2 mm hole 5 mm hole • 1 0 mm hole

30

25

i 20

; 15

! 10

5

0 0.01 0.1 1

Sur face R o u g h n e s s (m)

• Fluid A • Fluid B Fluid G(i) • Fluid G(ii)

I Fluid C

(d ) R e f r i g e r a t e d L P G u n d e r 5 b a r ( a ) p r e s s u r e

0.01 0.1 1 Surface Roughness (m)


12 mm hole > 10 mm hole

Page 29


6 INVESTIGATION OF AREA CLASSIFICATION FOR LIQUID POOLS

6.1 SPILL VOLUMES

The spill volumes modelled for the pool sizes set out in Table 3.2 are given in Table 6.1.

Table 6.1: Spill volumes modelled (m3)

Pool diameter

(m)

Pool depth Pool

diameter (m)

Minimum: concrete (0.005 m)

Minimum: dry soil (0.02 m)

0.1 m 1 m

1 0.0039 0.015 (not modelled) (not modelled)

3 0.035 0.14 0.71 (not modelled)

10 0.39 1.6 7.9 79

30 (not modelled) (not modelled) 70 707

100 (not modelled) (not modelled) (not modelled) 7 854

The spill volume has been modelled as being released instantaneously onto the ground and allowed to spread until it reaches the pool diameter shown in Table 6.1. Evaporation and dispersion will take place as soon as the pool starts spreading outwards.

6.2 HAZARD DISTANCES: BASE CASE

There is no distinction between R, and R2 for the results presented in this section, since all releases are at ground level.

Distances from the edge of the pool to LFL and 0.5 LFL for all four weather categories modelled are set out in Table 6.2 (concrete) and Table 6.3 (dry soil) for releases into an ambient (air) and ground temperature of 20°C. This is for refrigerated fluid category A (at -40°C) and for fluid categories B and C at ambient temperature, also small spills of fluid category C released at 100°C.

Several general observations can be made on these results:

— Pools of fluid category A generally give larger hazard distances than those for other fluids studied for various pool diameters/depths, surface types and weather categories.

— Going from the smallest spills to the largest, there is a trend in the maximum distance with weather category from 9D to 1.5F. This means that, for very small spills (such as might result from the breaking of a coupling), the hazard distances are longest for high winds; for larger spills, the hazard distances are longest for low windspeeds and stable conditions.

Page 30


For spills of 0.1 m and 1 m depth, the hazard distance is almost independent of surface type, with marginally longer distances for dry soil.

For fluid category C released at 100°C, the hazard distances are longer than for the corresponding releases at 20°C at low windspeeds.

Table 6.2: Distances from edge of pool to LFL and 0.5 LFL for base case, concrete

Fluid category

Case: diameter (m) x depth (m)

Distance m) to LFL Distance (m) to 0.5 LFL Fluid category

Case: diameter (m) x depth (m) 1.5F 2D 5D 9D 1.5F 2D 5D 9D

A

1 X 0.005 5 8 12 12 6 11 18 17

A

3 x 0.005 18 17 12 15 28 27 19 17

A

10 x 0.005 53 58 28 35 66 87 49 51

A 3 x 0.1 15 15 11 14 24 23 13 18

A 10 x 0.1 33 76 49 42 47 115 78 52 A 30 x 0.1 212 207 147 115 279 257 208 161

A

10 x 1 46 46 30 36 70 66 41 43

A

30 x 1 163 130 111 117 234 182 188 161

A

100 x 1 814 729 389 316 1 086 995 569 422

B

1 x 0.005 1 3 6 11 2 5 10 17

B

3 x 0.005 15 14 12 17 21 22 17 23

B

10 x 0.005 19 18 25 26 30 29 37 31

B 3 x 0.1 15 15 13 18 22 23 17 26

B 10 x 0.1 32 28 16 20 47 45 23 27 B 30 x 0.1 174 196 91 75 218 246 132 114

B

10 X 1 58 54 27 29 77 75 43 38

B

30 x 1 186 149 68 56 251 208 106 86

B

100 x 1 826 734 304 185 1 088 961 454 303

C (ambient)

1 x 0.005 2 3 5 4 2 3 5 6

C (ambient)

3 x 0.005 0 1 1 2 0 2 3 5

C (ambient)

10 x 0.005 11 12 11 14 18 18 16 18

C (ambient) 3 x 0.1 0 0 1 2 0 2 3 4

C (ambient) 10 x 0.1 15 16 15 16 21 21 17 17 C (ambient) 30 x 0.1 54 49 34 32 75 65 47 50

C (ambient)

10 x 1 17 19 17 15 25 29 22 17

C (ambient)

30 x 1 58 54 34 35 79 72 47 40

C (ambient)

100 x 1 233 197 121 100 321 279 150 134

C (100°C) 1 x 0.005 1 2 4 5 1 3 5 6

C (100°C) 3 x 0.005 6 11 1 2 8 17 3 4 C (100°C) 10 x 0.005 12 12 9 14 20 19 16 18

Page 31


Tab l e 6.3: Distances f r o m e d g e of poo l t o LFL a n d 0.5 LFL fo r base case, d ry soil

Fluid category

Case: diameter (m) x depth (m)

Distance m) to LFL Distance (m) to 0.5 LFL Fluid category

Case: diameter (m) x depth (m) 1.5F 2D 5D 9D 1.5F 2D 5D 9D

A

1 X 0.005 2 4 7 9 3 6 12 11

A

3 x 0.005 15 15 10 14 23 23 16 19

A

10 x 0.005 17 18 28 28 28 28 45 33

A 3 x 0.1 15 15 13 18 22 23 17 26

A 10 x 0.1 32 28 17 20 47 45 23 27 A 30 x 0.1 174 196 91 75 218 246 132 114

A

10 X 1 58 54 27 29 77 75 43 38

A

30 x 1 186 149 68 56 251 208 106 86

A

100 x 1 827 735 304 185 1 087 961 454 303

B

1 x 0.005 2 4 7 9 3 6 12 11

B

3 x 0.005 15 15 10 14 23 23 16 19

B

10 x 0.005 17 18 28 28 28 28 45 33

B 3 x 0.1 15 15 13 18 22 23 17 26

B 10m x 0.1 32 28 17 20 47 45 23 27 B 30m x 0.1 174 196 91 75 218 246 132 114

B

10m x 1 58 54 27 29 77 75 43 38

B

30m x 1 186 149 68 56 251 208 106 86

B

100m x 1 827 735 304 185 1 087 961 454 303

C (ambient)

1 x 0.005 1 2 3 4 1 3 5 5

C (ambient)

3 x 0.005 0 0 1 2 0 1 3 4

C (ambient)

10 x 0.005 11 13 11 11 17 17 16 14

C (ambient) 3 x 0.1 0 0 1 2 0 2 3 4

C (ambient) 10 x 0.1 15 16 15 16 21 22 17 17 C (ambient) 30 x 0.1 54 49 34 32 75 71 47 50

C (ambient)

10 X 1 17 19 17 15 25 29 22 17

C (ambient)

30 x 1 58 54 34 35 79 72 47 40

C (ambient)

100 x 1 233 197 121 100 321 279 150 134

C (100°C) 1 x 0.005 1 2 3 4 1 2 4 5

C (100°C) 3 x 0.005 15 13 10 10 22 19 13 10 C (100°C) 10 x 0.005 13 56 10 12 22 83 16 15

Page 32


6.3 HAZARD DISTANCES: SENSITIVITIES

As the results presented in section 5.3 showed no significant variation of hazard distances with relative humidity, this sensitivity has not been examined. The sensitivities with temperature and surface roughness are presented in 6.3.1 and 6.3.2 respectively.

6.3.1 Variation with ambient temperature

Figure 6.1 shows distances to LFL for a range of ambient temperatures (releases of fluid category A are refrigerated; releases of fluid categories B and C are at ambient temperature), for weather categories 1.5F and 9D (i.e. the lowest and highest windspeeds).

For spills of fluid category A onto concrete (Figure 6.1(a)) there is a general increase with ambient temperature in the distance to LFL for low windspeed; for high windspeed the smallest spill gives a much longer hazard distance for higher ambient temperatures whereas the ambient temperature has little influence on the hazard distance for larger spills. For spills of fluid category A onto dry soil (Figure 6.1(b)), the hazard distances are generally shorter than for spills onto concrete but the trends with ambient temperature are stronger.

Spills of fluid category B onto concrete (Figure 6.1(c)) generally show a similar influence of ambient temperature to fluid category A. For low windspeed, at ambient temperatures of -20°C and 0°C there is minimal pool evaporation and hence the hazard distance under these conditions is zero or negligible. Fluid category B shows little dependence on surface type (compare with Figure 6.1(c),(d)).

Spills of fluid category C (Figure 6.1(e),(f)) show very little variation with ambient temperature other than for high windspeed and an ambient temperature of 40°C, which results in much longer distances to LFL for a 3 m diameter, 0.1 m deep pool. Fluid category C shows little dependence on surface type.

6.3.2 Variation with surface roughness

Figure 6.2 shows distances to LFL for a range of surface roughnesses, for weather categories 1,5F and 9D (i.e. the lowest and highest windspeeds).

In general these show the expected decrease in hazard distance with increasing surface roughness, by a factor of at least two over the range modelled for both low and high windspeeds.

For spills of fluid category A onto concrete (Figure 6.2(a)) with weather category 9D, the smallest surface roughness (0.03 m) appears to give shorter hazard distances than the higher surface roughnesses. It is likely that this is an artefact of the modelling, which is sensitive to the degree of accuracy specified: a lower accuracy was reguired for the higher surface roughness in order to obtain results, and hence there is greater confidence in the results for the base case surface roughness of 0.03 m.

For spills of fluid category C onto concrete (Figure 6.2(e)), the smallest size spill modelled (diameter 1 m, minimum depth of 0.005 m) with weather category 9D gives the opposite trend to that generally found. As with fluid category A, this is likely to be an artefact of the accuracy required to obtain results from the modelling.

In both cases these anomalous results should be disregarded and it should be assumed that the trend indicated by the other results would be followed in reality.

Page 33

Figure 6.1: Example comparisons of R, hazard distances be tween different ambient temperatures

(a) Fluid category A (refrigerated) spill on concrete (b) Fluid category A (refrigerated) spill on dry soil

Ambient Temperature (C) • 1m x 0 005m, 1 5F • 3m x 0.005m, 1.5F 3mx0.1m,1.5F 01m x 0 005m, 9D • 3m x 0.005m, 9D 3mx0.1m,9D

-20 -10 0 10 20 30 401

Ambient Temperature (C) I • 1m x 0 02m, 1 5F • 3m x 0 02m. 1.5F 3m x 0 1m. 1 5F

01m x 0.02m, 9D D 3m x 0.02m. 9D 3mx0.1m,9D

(c) Fluid category B spill on concrete (d) Fluid category B spill on dry soil

0 10 20 30 4 Ambient Temperature (CJ

3m xO 1m, 1 5F • 1m x 0 005m . 1 5F • 3m x 0.005m. 1.5F 01m x 0 005m. 9D • 3m x 0.005m, 9D 3m x 0 1m. 9D

(e) Fluid category C spill on concrete

0 10 20 30

Ambient Temperature (C) • 1m x 0 005m. 1 5F • 3m x 0 005m, 1.5F 3m x 0.1m. 1 5F 01m x 0 005m, 9D O 3m x 0.005m, 9D 3mx0.1m.9D

-10 0 10 20 30

Ambient Temperature (C) • 1m x 0 02m, 1 5F 01m x 0.02m, 9D

• 3m x 0 02m. 1 5F n 3m x 0.02m, 90

3m x0.1m, 1.5F 3m xO 1m, 9D

(f) Fluid category C spill on dry soil

0 10 20 30

Ambient Temperature (C) • 1m x 0.02m. 1 5F 01m x 0 02m. 9D

• 3m x 0 02m. 1 5F • 3m x 0 02m, 9D

3m x 0.1m, 1 5F 3m x 0.1m, 9D

Page 34

Figure 6.2: Example comparisons of R, hazard distances to LFL be tween different surface roughnesses

(a) Fluid category A (refrigerated) spill on concrete (b) Fluid category A (refrigerated) spill on dry soil

H

! 0.1 1

Surface Roughness (m)

1m x 0 005m, F1.5 • 3m x 0.005m, F1.5 3mx0.1m,F1.5 01m x 0 005m, D9 • 3m x 0.005m, D9 3m x 1m, D9


• 1m x 0.02m. F1.5 • 3m x 0.02m. F1.5 3m x 0.1m, F1.5 I o 1m x 0.02m, D9 • 3m x 0.02m, D9 3mx1m,D9

(c) Fluid category B spill on concrete

0.01 0.1 Surface Roughness (m)

• 1m x 0 005m, F1.5 • 3m x 0.005m, F15 3m x 0 1m, F1.5 01m x 0.005m, D9 O 3m x 0.005m, D9 3mx1m, D9

(d) Fluid category B spill on dry soil

0 -o


1m x 0.02m, F1 5 • 3m x 0.02m, F1.5 3m x 0.1m, F1.5 1m x 0 02m, D9 • 3m x 0.02m, D9 3m x 1m, D9

(e) Fluid category C spill on concrete (f) Fluid category C spill on dry soil

6

a 2 S- 4

o o 0

o o

• • A • l •

/I M r. 0.1

Surface Roughness (m) • 1m xO 005m, F1.5 • 3m x 0.005m, F1.5 3mx0.1m. F1.5 O 1m x 0.005m, D9 • 3m x 0.005m, D9 3m x 1m, D9

0.01 0 1 1 Surface Roughness (m)

I • I m x 0.02m. F1.5 • 3m x 0.02m. F1.5 3m x 0.1m, F1.5 01m x 0.02m, D9 • 3m x 0 02m, D9 3mx1m, D9

Page 35


7 INVESTIGATION OF APPLICATION OF EI15 METHODOLOGY TO LNG

7.1 LIQUID RAIN-OUT

When a liquid is released, the droplets will evaporate as they travel through the air but will also fall towards the ground under the influence of gravity. Whether or not they reach the ground depends on the release height, rate and velocity as well as the material properties. If they do reach the ground, they form a spreading and evaporating liquid pool.

Table 7.1 shows the amount of rain-out for releases initially horizontal at three heights: 5 m, 1 m, and 0.1 m. Rain-out only exceeds 50% for releases at 0.1 m height and 1.5 bar(a). For higher release pressures there is no rain-out at all; for releases at 0.1 m, the larger hole sizes (5 mm, 10 mm) do give some rain-out for a release pressure of 1.5 bar(a). In order to see the effect of significant rain-out and evaporation, releases at 0.1 m height and 1.5 bar(a) have been modelled in addition to releases at 1 m and 5 m height and all three release pressures.

Table 7.1: Percentage rain-out of LNG (maximum from all weather categories)

Hole diameter Release height (m) % of release raining out for release pressure (mm) 1.5 bar(a) 5 bar(a) 10 bar(a)

5 0 0 0 1 1 0 0 0

0.1 6 6 % 0 0 5 0 0 0

2 1 0 0 0 0.1 7 2 % 0 0 5 0 0 0

5 1 17% 0 0 0.1 7 4 % 0 0 5 0 0 0

10 1 3 1 % 0 0 0.1 7 8 % 0 0

Note that the droplet formation and rain-out modelling in DNV PHAST has not specifically been validated for LNG. It is considered possible by DNV's modelling experts that the droplet sizes and hence amount of rain-out are over-estimated although this cannot be confirmed, and work is in progress to improve this modelling. The hazard distances where rain-out is involved may therefore be conservative (i.e. over- rather than under-predicted).

7.2 BASE CASE AND VARIATION WITH WEATHER CATEGORY

Table 7.2 summarises the base case (30°C) results for releases at 5 m height above ground. Broadly, the distances to LFL (equivalent to R, in Section 5) increase with windspeed for the lower release pressures (1.5 bar(a) and 5 bar(a)) but decrease with windspeed for the highest release pressure (10 bar(a)).

Page 36


Table 7.2: Summary of base case hazard radii R, to LFL and 0.5 LFL for LNG releases at 5 m height

Hole diameter

(mm)


Release rate

(kg/s)

Distance (m) to LFL Distance (m) to 0.5 LFL

Ratio of distances (to 0.5 LFL)/(to LFL)

Hole diameter

(mm)


Release rate

(kg/s) 2D 1.5F 5D 9D 2D 1.5F 5D 9D 2D 1.5F 5D 9D

1 1.5 0.003 2.2 2.0 2.3 2.4 3.1 2.6 3.3 3.2 1.4 1.3 1.4 1.3

1 5 0.009 2.6 2.6 2.5 2.5 4.5 4.3 4.4 4.3 1.7 1.6 1.8 1.7 1 10 0.013 2.9 2.7 2.8 2.7 5.0 4.6 4.7 4.5 1.7 1.7 1.7 1.6

2 1.5 0.012 3.4 3.2 3.6 3.8 5.1 4.3 5.5 5.2 1.5 1.3 1.5 1.4

2 5 0.035 5.0 4.6 4.9 4.8 7.3 6.9 7.1 6.8 1.5 1.5 1.5 1.4 2 10 0.052 5.5 5.3 5.2 5.1 9.0 8.1 7.8 7.4 1.6 1.5 1.5 1.5

5 1.5 0.077 6.2 5.9 6.9 7.1 8.7 7.9 10.3 9.7 1.4 1.3 1.5 1.4

5 5 0.22 10.3 9.8 9.9 9.4 13.8 13.3 13.4 13.1 1.3 1.3 1.4 1.4 5 10 0.33 12.1 12.1 11.3 10.4 16.9 16.4 15.9 14.5 1.4 1.4 1.4 1.4

10 1.5 0.31 9.8 9.1 10.5 10.7 12.9 22.7 15.0 15.2 1.3 2.5 1.4 1.4

10 5 0.87 16.6 17.1 16.2 15.5 27.0 46.5 23.1 21.1 1.6 2.7 1.4 1.4 10 10 1.3 20.4 20.8 18.7 17.6 33.2 50.8 25.6 24.0 1.6 2.4 1.4 1.4

The distances to 0.5 LFL range from 30% to 70% longer than the distances to LFL, except for 10 mm holes, weather 1,5F, where they are around two and half times longer. This is because in this case, the plume touches down (as a result of the plume density and low windspeed), with ground effect maintaining the concentration above 0.5 LFL for longer than if the plume remained above the ground.

Of the materials for which hazard distances are presented in El 15, fluid category G(i) is the closest in composition to LNG. However, releases of this fluid category were originally modelled for a storage temperature of 20°C and have been modelled in this study for temperatures down to -20°C. It is interesting to compare the results for LNG with these other results. From Figure 5.5 (section 5.2) it can be seen that the dispersion behaviour of fluid category G(i) shows little variation with temperature, hence a comparison is made in Table 7.3 between the results in El 15 Table C9(a) and the base case results from section 4.2 for 5 bar(a) and 10 bar(a) release pressures.

Table 7.3: Comparison of distances to LFL (m) for fluid category G(i) and LNG

Fluid category


El 15 Table C9(a) DNV PHAST Fluid

category


Hole diameter (mm) Hole diameter (mm) Fluid category

Release pressure (bar(a)) 1 2 5 10 1 2 5 10

G(i) 5 < 1 < 1 1 2 0.15 0.55 1.3 2.8

G(i) 10 < 1 1 2.5 5 0.5 1.3 3.4 7

LNG 5

No results 2.6 5.0 10.3 16.6

LNG 10

No results 2.9 5.5 12.1 20.4

Page 37


Clearly LNG gives much longer hazard distances than fluid category G(i). This is not surprising as the release rates are much higher, LNG being liquid and fluid category G(i) gas; also, the modelled release velocity is about 43 m/s for LNG but sonic for fluid category G(i), which means that fluid category G(i) will entrain air much more rapidly through induced turbulence at the edge of the plume. Hence, results for fluid category G(i) should not be used as a surrogate for LNG.

The corresponding distances to LFLand 0.5 LFLfor releases at 1 m and 0.1 m height above ground are given in Table 7.4. The same trends with windspeed are seen as for releases at 5 m height (Table 7.2). Considering first the releases at 1 m height, the distances to LFL are shorter than for releases at 5 m height for the lower release pressures and smaller hole sizes, increasing to exceed them for the higher release pressures and larger hole sizes. These variations can be explained by a combination of plume interaction with the ground and rain-out. Figure 7.1 shows some example side views of the plume dispersion, for weather categories 1,5F and 9D, release pressures 1.5 bar(a) and 10 bar(a), release heights 5 m and 1 m, and all four hole sizes. The plots show that only the largest releases at 5 m interact with the ground, and then only at the lowest windspeed, but the larger releases at 1 m do interact with the ground, even for the highest release pressure and highest windspeed.

For releases at 0.1 m height, Table 7.4 shows that the distances to LFL are smaller than from releases at 1 m height for the smaller hole sizes (1 mm, 2 mm) but larger for the larger hole sizes (5 mm, 10 mm).

Table 7.4: Summary of base case hazard radii R, to LFL and 0.5 LFL for LNG releases at 1 m and 0.1 m height

Hole diameter

(mm)


Release height

(m)

Distance (m) to LFL Distance (m) to 0.5 LFL

Ratio of distances (to 0.5 LFL)/(to LFL)

Hole diameter

(mm)


Release height

(m) 2D 1.5F 5D 9D 2D 1.5F 5D 9D 2D 1.5F 5D 9D

1

1.5 0.1 1.2 1.1 0.9 0.8 1.6 2.1 1.5 1.1 1.4 1.9 1.7 1.4

1 1.5 1.0 1.8 1.5 1.5 1.4 2.4 1.9 2.1 2.0 1.4 1.3 1.4 1.4

1 5 1.0 2.2 2.2 2.0 1.8 3.2 3.2 2.7 2.5 1.4 1.4 1.3 1.4

1

10 1.0 2.5 2.5 2.2 2.0 4.1 3.7 3.2 2.7 1.7 1.5 1.4 1.3

2

1.5 0.1 7.3 7.8 3.8 2.0 11.3 11.6 5.9 3.3 1.5 1.5 1.5 1.7

2 1.5 1.0 2.8 2.5 2.6 2.2 3.7 3.4 3.4 3.1 1.3 1.4 1.3 1.4

2 5 1.0 3.9 4.0 3.4 3.0 5.5 6.9 4.8 4.1 1.4 1.7 1.4 1.4

2

10 1.0 4.5 4.6 3.8 3.4 6.5 8.0 5.4 4.8 1.4 1.7 1.4 1.4

5

1.5 0.1 19.8 19.6 13.3 8.7 27.0 31.3 17.8 12.1 1.4 1.6 1.3 1.4

5 1.5 1.0 6.6 14.8 4.7 4.4 18.4 24.6 6.2 6.0 2.8 1.7 1.3 1.4

5 5 1.0 11.3 15.1 6.9 6.5 30.6 39.2 12.9 8.2 2.7 2.6 1.9 1.3

5

10 1.0 12.6 15.3 8.1 7.3 31.3 40.6 17.4 10.3 2.5 2.6 2.1 1.4

10

1.5 0.1 38.3 41.1 23.0 19.0 51.7 79.4 32.3 25.8 1.3 1.9 1.4 1.4

10 1.5 1.0 30.2 34.7 12.9 6.8 45.2 53.2 27.3 12.9 1.5 1.5 2.1 1.9

10 5 1.0 40.1 42.5 26.7 15.2 77.7 86.4 50.2 33.2 1.9 2.0 1.9 2.2

10

10 1.0 37.5 40.2 30.3 20.3 87.6 99.5 60.5 44.5 2.3 2.5 2.0 2.2

Page 38


Figure 7.1: Example side v iews of plume dispersion

(a) Release at 1.5 bar(a), 5 m height, weather category 1,5F

— Base Case @ 120 s — 2mm @ 120 s

5mm @ 120 s — 10mm @ 120 s

1 OJ o X TJ Z3 &

o


Side view

1 H \ i \ \ \ \ \ \ \ \ \ \ \

\ \

(b) Release at 10 bar(a), 5 m height, weather category 9D

Page 39


Figure 7.1: Example side v iews of plume dispersion (continued)

(c) Release at 1.5 bar(a), 1 m height, weather category 1,5F

— Base Case® 120 s — 2mm @ 120 s

5mm @ 120 s — 10mm @ 120 s

Side View


(d) Release at 10 bar(a), 1 m height, weather category 9D

Page 40


Figure 7.2 shows the variation with weather categories graphically. The effect referred to above weather 1.5F on the distance to 0.5 LFL for 10 mm holes is clearly seen. More generally, the distances are more sensitive to weather categories for the lower release pressures; alternatively, the windspeed is less important for the higher release pressures as dispersion is driven more by the turbulence induced by the resulting higher release velocities. The distance to 0.5 LFL is more sensitive to the weather category: as the plume becomes more dispersed, it is also losing the effect of the initial momentum and hence the windspeed becomes more significant.

Figure 7.2: Variation of distances to LFL wi th weather category (release height 5 m)

(a) LNG at 1.5 bar(a), 30°C

25

20

15

10

_

_ •••

. . . n a. A

' -1 . -V ; j::

1 i l l

_2EL 1.5F _5D_ JJQ_ • 1 mm hole LFL

5 mm hole LFL O 1 mm hole 0.5 L F L

5 mm hole 0.5 L F L

• 2 mm hole LFL • 10 mm hole L F L • 2 mm hole 0.5 L F L

10 mm hole 0.5 L F L

(b) LNG at 5 bar(a), 30°C 60 - I 60 -

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10

u

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10 A A

A A A

50 -

? -J 40 -u_ _ J o Z 30 o c TO « 20 -Q

10

1 A A

A A

0 -2D 1.5F 5D 9D

• 1 mm hole LFL 5 mm hole LFL

O 1 mm hole 0.5 LFL 5 mm hole 0.5 LFL

• 2 mm hole LFL • 10 mm hole LFL • 2 mm hole 0.5 LFL

10 mm hole 0.5 LFL

(c) LNG at 10 bar(a), 30°C

60 60

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

50

? _ i 40 U_ _ l O S 30 0

1 20 Q

10

: c

1

ffi . ' mmm • t i

2D 1 5F 5D 9D • 1 mm hole LFL

5 mm hole LFL O 1 mm hole 0.5 LFL

5 mm hole 0.5 LFL

• 2 mm hole LFL • 10 mm hole LFL • 2 mm hole 0.5 LFL

10 mm hole 0.5 LFL

Page 41


7.3 VARIATION WITH AMBIENT TEMPERATURE

Table 7.5 shows the distances to LFL for releases at 5 m height above ground at ambient temperatures of -20°C and 30°C. At the colder temperature the distances are up to 20% shorter. A similar pattern emerges for releases at 1 m height.

Table 7.5: Distances to LFL for releases at 5 m height at different ambient temperatures

Hole diameter

(mm)


Weather category 2D

Weather category 9D Hole

diameter (mm)

Release pressure (bar(a)) Ambient temperature (°C)

Hole diameter

(mm)


-20 30 -20 30

1 1.5 1.8 2.2 2.2 2.4

1 5 2.4 2.6 2.5 2.5 1 10 2.6 2.9 2.6 2.7

2 1.5 3.0 3.4 3.5 3.8

2 5 4.1 5.0 4.1 4.8 2 10 4.7 5.5 4.4 5.1

5 1.5 5.3 6.2 6.6 7.1

5 5 8.6 10.3 8.4 9.4 5 10 10.1 12.1 9.6 10.4

10 1.5 8.1 9.8 10.5 10.7

10 5 14.0 16.6 14.2 15.5 10 10 17.1 20.4 16.4 17.6

7.4 VARIATION WITH RELATIVE HUMIDITY

The effect of varying the relative humidity is shown in Table 7.6 for releases at 5 m height above ground. The results for releases at 1 m height are similar. The variation is no more than about 10%, and no clear trend is apparent.

This contrasts with the results in major hazard releases of LNG, where the effect of relative humidity is marked: as relative humidity increases, hazard distances decrease. These results are seen in modelling and have been observed in experiments (e.g. Maplin Sands). However, the size of the releases in both trials and major hazard modelling is much greater than in the present study and so the absence of any identifiable trend with relative humidity for such small releases does not conflict with the results for large releases.

7.5 VARIATION WITH SURFACE ROUGHNESS

The variation with surface roughness is shown in Figure 7.3 for selected cases. For releases at 5 m height above ground (Figure 7.3(a)) there is a consistent decrease in hazard distances from the base case surface roughness of 0.03 m to 1 m, the largest roughness modelled, of around 25%: this trend is as expected. For releases at 1 m height (Figure 7.3(b)), most cases exhibit the same trend but the largest releases (10 mm) under weather category 2D do not show such a clear trend. This is because the plume centre grounds and hence dispersion behaviour is modified by the interaction with the ground.

Page 42


Table 7.6: Variation in distances to LFL for releases at 5 m height for different relative humidities

Hole diameter

(mm)


Variation from base case (70% relative humidity) Hole

diameter (mm)


Weather category 2D Weather category 9D Hole

diameter (mm)

Release pressure (bar(a)) Relative humidity

Hole diameter

(mm)


50% 90% 50% 90%

1 1.5 -2% 2% 0% -4%

1 5 3% 3% 1% -1% 1 10 0% 0% 1% -1%

2 1.5 -1% 1% 11% 0%

2 5 1% 1% 2% -1% 2 10 0% 7% 1% -4%

5 1.5 -1% 1% 0% 0%

5 5 5% -2% -4% -1% 5 10 4% -2% -3% -5%

10 1.5 -1% 0% 0% 0%

10 5 -1% -1% 2% -1% 10 10 1% -2% 0% 0%

Figure 7.3: Variation of distances to LFL wi th surface roughness

(a) Release at 5 m height (b) Release at 1 m height

0.01 0.1 1 Su r f ace roughness length (m)

• 2 mm, 1.5 bara. 2D • 2 mm, 10 bara. 2D 10 mm. 1.5 bara, 2D • 10 mm, 10 bara. 2D

0 2 mm. 1.5 bara. 9D • 2 mm, 10 bara. 9D 10 mm. 1.5 bara, 9D 10 mm, 10 bara. 9D

0.01 0.1 1 Surface roughness length (m)

• 2 mm, 15 bara, 2D • 2 mm, 10 bara, 2D 10 mm, 1.5 bara, 2D • 10 mm. 10 bara 2D

0 2 mm, 1.5 bara, 9D • 2 mm, 10 bara. 9D 10 mm, 1.5 bara. 9D 10 mm. 10 bara, 9D

Page 43


7.6 VARIATION WITH SURFACE TYPE

All releases over water will result in longer hazard distances than the equivalent releases over land because the surface roughness is lower, following the trend demonstrated in section 7.5. This is shown in Table 7.7 for releases at 5 m height above ground. Modest increases in distance to LFL (up to about 7 % for weather category 2D, 25% for weather category 9D) are apparent.

An additional effect can be expected when the release results in a spill onto the water surface due to the different heat transfer properties of land and water. As shown in Table 7.1, only the releases at 0.1 m height and 1.5 bar(a) result in significant rain-out Table 7.8 shows the distances to LFL for these releases. There are larger increases in hazard distance (up to 38% longer) for weather category 2D, showing the enhanced vapour generation on water compared with land. For weather category 9D the increase is generally less marked than for releases at 5 m.

Table 7.7: Distances to LFL for selected releases at 5 m height onto concrete and water

Hole diameter

(mm)


Weather category 2D

Weather category 9D

Hole diameter

(mm)

Release pressure (bar(a)) Concrete Water Concrete Water

1 1.5 2.2 2.3 2.4 3.0

1 10 2.9 3.1 2.7 3.6

10 1.5 9.8 10.4 10.7 13.5

10 10 20.4 21.8 17.6 21.9

Table 7.8: Distances to LFL for releases at 0.1 m height onto concrete and water

Hole diameter

(mm)


Weather category 2D

Weather category 9D

Hole diameter

(mm)

Release pressure (bar(a)) Concrete Water Concrete Water

1

1.5

1.2 2.9 0.8 1.2 2

1.5 7.3 9.8 2.0 2.7

5 1.5

19.8 27.7 8.7 10.6 10

1.5

38.3 52.7 19.0 20.5

Note that, for spills onto water, the phenomenon of 'rapid phase transition (RPT)' has not been considered. (RPT is defined in IChemE Nomenclature for hazard and risk assessment in the process industries as 'the rapid change of state of a substance which may produce a blast wave and missiles'; i.e., in this context it is an explosive boiling of LNG on contact with water.)

Page 44


8 CONCLUSIONS AND RECOMMENDATIONS

The conclusions and recommendations presented here focus on the hazard distances as defined in EI15, i.e. to LFL and not to 0.5 LFL.

8.1 SENSITIVITY TO PARAMETER VARIATIONS

8.1.1 Fluid category A

For fluid category A under pressure:

— The variation in hazard distance with weather category compared with the base case (2D) is mostly less than ±20%, whereas for higher windspeeds and larger hole sizes the reduction in hazard distance compared with base case reaches 27%. Given the uncertainties in dispersion modelling, this is not considered significant.

— The variation of hazard distance with relative humidity is negligible.

— The reduction in hazard distance with increasing surface roughness (up to 1 m) is less than 20%. Given the uncertainties in dispersion modelling, this is not considered significant.

For fluid category A, refrigerated at -40°C and stored at atmospheric pressure:

— The hazard distances are up to 80% higher than for the same fluid stored under pressure at ambient temperature: Figure 5.5 (e),(f) indicates suitable hazard distances for refrigerated LPG.

— Higher wind speeds give longer dispersion distances than the same fluid stored under pressure at ambient temperature. This should be taken account of for installations storing refrigerated LPG.

— The variation of hazard distance with relative humidity is negligible.

— The reduction in hazard distance with increasing surface roughness (up to 1 m) is up to 31 % ; however, in most cases the surface roughness will be significantly less than this hence the distances shown in Figure 5.5 (e),(f) should be used.

8.1.2 Other fluid categories

For fluid categories B, C, G(i) and G(ii):

— For lower storage/process pressures, the weather category has no discernible impact on the hazard distances for the gases G(i) and G(ii). For fluid category G(ii), high windspeeds give the longest hazard distances. Higher windspeeds also lead, at all release pressures, to shorter hazard distances for fluid categories B and C (as well as fluid category A).

Page 45


The variation of hazard distance with ambient temperature is less than 20% over the range modelled. Given the uncertainties in dispersion modelling, this is not considered significant.

The variation of hazard distance with relative humidity is negligible.

The reduction in hazard distance with increasing surface roughness (up to 1 m) is less than 20%. Given the uncertainties in dispersion modelling, this is not considered significant.

8.2 LIQUID POOLS

8.2.1 Base case

A comprehensive approach to area classification for liquid pools due to spills is set out in Table 6.2 and Table 6.3 for various fluid categories, pool diameters/depths and weather categories for two surface types. The following specific points should be noted:

— For very small spills, the hazard distances are longest for high windspeeds; for larger spills, the hazard distances are longest for low windspeeds and stable conditions.

— In the majority of cases, the surface type (concrete or dry soil) makes no significant or systematic difference to hazard distances.

8.2.2 Sensitivity to parameter variations

— For fluid category A there is significant variation with increasing ambient temperature compared with the base case results: Figure 6.1 (a),(b) indicates the range of variation.

— For fluid category B the variation for a higher ambient temperature than base case (20°C) is less than 10%. Given the uncertainties in dispersion modelling, this is not considered significant and no variation from the hazard distances given in Table 6.2 and Table 6.3 is recommended. For lower temperatures, it is recommended that the hazard distances in Figure 6.1 (c),(d) are used as they are significantly shorter than the base case.

— For fluid category C the variations of hazard distance with ambient temperature are mostly small (see Table 6.2 and Table 6.3).

— For all fluid categories A, B and C the variation of hazard distance with relative humidity is negligible (see Table 6.2 and Table 6.3).

— For all fluid categories A, B and C the variation of hazard distance with surface roughness is significant as illustrated in Figure 6.2.

Page 46


8.3 LNG

— Figure 7.2 provides a suitable basis for guidance on hazard distances for EI15.

— The variation in hazard distance between ambient temperatures of -20°C and (+)30°C is less than 20%. Given the uncertainties in dispersion modelling, this is not considered significant and no variation from the hazard distances shown in Figure 7.2 is recommended.

— Except for very small releases (i.e. 1 mm hole size), the hazard distances for releases initially horizontal at 0.1 m height are mostly longer than for the corresponding releases at 1 m height due to rain-out, liquid pool formation and re-evaporation occurring.

— The variation in hazard distance with relative humidity is less than 10%. Although not negligible, this is insufficient to justify any variation from the hazard distances shown in Figure 7.2.

— The reduction in hazard distance with increasing surface roughness (up to 1 m) is less than 25%. However, in most cases the surface roughness will be significantly less than this and it would be prudent to retain the hazard distances shown in Figure 7.2.

— For spills onto water, the phenomenon of RPT has not been considered. The modelling results show that, for releases resulting in significant rain-out (those at 0.1 m height and 1.5 bar(a) release pressure), the hazard distances are significantly increased at low windspeed compared with releases onto land.

Page 47


ANNEX A SENSITIVITY ANALYSIS ON RELEASE ANGLE

Tables A. 1 to A.4 show the variation in distances to LFL and 0.5 LFL for every weather category, hole size, fluid and release pressure at various release angles.

Page 48

Table A.4: Hole diameter 10 mm

•t. ID

Fluid category


D2 F1.5 D5 D9 Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release

Angle Weather Fluid category


Distance (m) to LFL Distance (m) to 0.5 LFL Distance (m) to LFL Distance (m) to

0.5 LFL Distance (m) to LFL Distance (m) to 0.5 LFL Distance (m) to LFL Distance (m) to

0.5 LFL

Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release



Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60

Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release

Angle Weather

A

0 (refr.) 1.5 0.9 0.6 0.3 1.8 1.1 0.7 0.4 1.2 0.6 0.5 0.4 1 5 0.9 0.6 0.4 1.4 0.8 0.6 0.5 2.0 1.2 0.9 0.7 1.4 0.7 0.6 0.6 1.9 1.2 1.0 0.9 1.5 D 2 1 5 Hz.

A

5 (reft.) 2.2 1.1 0.8 0.6 3.0 1.7 1.3 1.0 2.2 1.3 1.0 0.7 2.9 3 6 5.1 5.2 1.9 1 0 0.7 0.6 2.6 1.5 1 2 1.0 1.7 0.8 0.7 0.5 2.3 1.4 1.2 1 0 2.2 D2 2 2 Hz.

A 6 8 1.4 1.0 0.7 0.5 2.5 1.4 1.1 0.8 1.4 1.0 0.8 0.5 2.5 1.8 2.4 3.0 1.2 0.8 0.5 0.4 2.3 1.1 0.9 0.7 1.2 0 6 0.5 0.3 2.1 1.1 0.8 0.6 1.4 F1.5 1.4 Hz.

A 10 1.6 1.0 0.8 0.5 2.6 1.5 1.3 1.8 1.6 1.1 0.9 0.6 2 7 2.4 3.2 3.9 1.4 0.9 0.6 0.4 2.4 1 2 1.0 0.7 1.3 0.7 0.5 0.4 2.2 1.2 0.8 0.7 1 6 F1 5 1.6 Hz.

A

50 2.2 1.4 1.1 1.2 4.1 3.8 5.0 5.9 2.3 1.6 1.8 2.1 3.8 5.1 5.8 7 0 2.1 1.2 0.9 0.6 3.5 1 7 1.6 2 2 1.9 1.0 0.7 0.5 2.8 1.5 1.2 0.9 2 3 F1.5 2.3 Hz.

A

100 2.4 1.6 1.6 2.0 4.4 5.0 5.8 7.3 2.4 2.0 » m 4 5 5.9 6.7 8.0 2.1 1.3 0.9 0.7 3.7 2.4 3.9 5.1 2.1 1.1 0.8 0.6 3.3 1.7 1.3 1 1 2.4 F1.5 -60 F1 5

B

5 2.1 1.1 0.9 0.6 2.9 1.7 1.2 1.0 2.1 1.3 1.0 1.0 2.9 3 1 4.7 5.8 1.8 0.9 0.7 0.5 2 6 1.4 1.2 0.9 1.5 0.8 0.6 0.5 2.2 1.3 1.0 0.9 2.1 F1.5 2.1 Hz.

B 10 2.4 1.3 1.0 0.7 3.5 2.0 2.9 4.3 2.4 1.5 1.8 2.3 3.5 5.5 6.9 8.0 2.1 1.1 0.8 0.6 2.9 1.6 1.3 1.0 1.9 0.9 0 7 0.6 2.6 1.5 1.2 1.0 2.4 F1.5 2.4 Hz.

B 50 2.6 1.6 1.7 2.2 4.3 5 5 7.1 8.5 2.6 2 3 2.7

2.3

4.3 7.0 3.0 9.2 2.4 1.3 0.9 0.7 4.0 1.9 2.5 3.9 2.2 1.2 0.8 0.6 3.3 1.7 1.3 1.0 2.6 F , 5 • -60 F1.5 B

100 2.7 1.8 2 3 2.7 4.8 6.3 7.4 8.6 2.7 2.5 2.9

0.4

4.9 7.3 8.2 9.6 2.5 1.4 1.0 0.8 4.3 3.2 5.4 7.0 2.4 1.2 0.9 0.6 3.5 1.8 1.5 1.4 2.7 F1.5 • -60 F1.5

C

5 2.2 1.0 0.5 0.4 3.1 1.1 0.7 0.5 2.2 1.0 0.7 0.4 2.9 1.2 0.9 0.6 2.0 0.9 0.6 0.5 2.6 1.1 0.7 0.7 1.7 0.8 0.5 0.6 2.4 1.2 0.6 0.8 2.2 F1.5 2.2 Hz.

C 10 2.4 1.3 1.0 0.6 3.5 2.1 2.5 1.7 2.4 1.4 1.0 0.6 3.5 4.7 4.3 4.1 2.1 1.1 0.8 0.6 3 0 1.6 1.3 1.0 1,8 0.9 0.7 0.6 2.6 1.5 1.2 1.0 2.4 F1.5 2.4 Hz.

C 50 2.7 1.6 1.9 2.4 4.6 5.7 7.2 8.4 2.7 2.4 2.9 4.4 7.1 8.1 9 6 2.4 1.3 1.0 0.7 4.0 2.0 2.9 4.2 2.3 1.2 0.8 0.6 3 3 1.7 1.4 1.0 2.7 F , 5 m -60 F1.5

C

100 2.8 1.8 2.1 2.3 4.9 6 2 7.0 7 5 2.8 2.5 2.5

0.1

5.0 7.1 7.2 8.7 2.5 1.4 1.0 0.7 4.2 3.4 5.0 6.7 2.3 1.2 0.9 0.6 3.5 1.8 1.5 1.6 2 8 F , 5 m -60 F1.5

G(i)

5 0.1 0.1 0.1 0.0 0.3 0.2 0 2 0.1 0.1 0.1 0.1 0.1 0.3 0.3 0.2 0.1 0.1 0.1 0.1 0.0 0.3 0 2 0.1 0 1 0.1 0.1 0.0 0.0 0.3 0.1 0.1 0.1 0.1 D2 0 1 Hz.

G(i) 10 0.2 0.2 0.1 0.1 0.5 0.3 0.3 0.2 0.2 0 2 0.1 0.1 0.5 0.4 0.3 0.2 0 2 0.1 0.1 0 1 0.5 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0.4 0.2 0.2 0.1 0.2 D2 0 2 Hz.

G(i) 50 0.6 0.5 0.4 0.2 1.2 0.9 0.6 0.4 0.6 0.5 0.4 0.3 1.2 1.0 0.7 0.5 0.6 0.4 0.3 0.2 1.2 0.7 0.5 0.4 0.6 0.3 0.3 0.2 1.1 0.6 0.5 0.3 0.6 D2 0.6 Hz.

G(i)

100 0.9 0 7 0.5 0.4 1.9 1.2 0.9 0 7 0.9 0.8 0.6 0.4 2.0 1.4 1.3 1 5 0.9 0.6 0.5 0.3 1.8 1.1 0.8 0.5 0,9 0.5 0.4 0.3 1.6 0.9 0.6 0.5 0.9 F1 5 0.9 Hz

G(n)

5 0.3 0.2 0.2 0.1 0.7 0.4 0.3 0 2 0.3 0.2 0.2 0.1 0.7 0.5 0.4 0.3 0.3 0.2 0.1 0.1 0.6 0.3 0.2 0.2 0.3 0.1 0.1 0.1 0.6 0.3 0.2 0.2 0 3 D2 0.3 Hz.

G(n) 10 0.5 0.3 0.2 0 2 1.1 0.6 0.5 0.4 0.5 0.4 0 3 0.2 1.1 0.8 0.5 0.4 0.5 0.3 0.2 0.1 1.0 0.5 0.4 0.3 0.4 0.2 0.2 0.1 0.8 0.4 0.3 0.3 0.5 D2 0.5 Hz.

G(n) 50 1.2 0 8 0.6 0.4 2.3 1 3 0.9 0.7 1.2 0.9 0.7 0 5 2.3 1.5 1.7 2.0 1.1 0.6 0.5 0.3 2.0 1.1 0.8 0.6 1.0 0.5 0.4 0.3 1.8 1.0 0.7 0 6 1.2 F1.5 1.2 Hz.

G(n)

100 1.7 1.1 0.8 0 5 3.1 1 8 2.0 2.9 1.7 1 2 0.9 0.8 3 2 3.2 4.0 4.8 1.5 0.9 0.6 0.4 2 5 1.4 1 0 0.8 1.3 0.8 0.5 0.4 2.3 1.2 0.9 0.7 1 7 F1.5 1.7 Hz.

The cell shading indicates the following: Horizontal release gives longest distance to LFL Release at this angle below horizontal gives longest distance to LFL

Notes 1 The 'maximum LFL distance for a horizontal release' and the 'maximum LFL distance for any release angle1 give the maxima based on several decimal places; whereas, data in the table only refer

to one decimal place.


Fluid category



for hz. release

Weather

Max. LFL dist.

for any release



Distance (m) to LFL Distance (m) to 0.5 LFL Distance (m) to LFL Distance (m) to 0.5 LFL Distance (m) to LFL Distance (m) to 0.5 LFL Distance (m) to LFL Distance (m) to 0.5 LFL

Max. LFL dist.

for hz. release

Weather

Max. LFL dist.

for any release



Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60 Hz. -30 -45 -60

Max. LFL dist.

for hz. release

Weather

Max. LFL dist.

for any release

Angle Weather

A

0 (refr.) 2.3 0.9 0.6 0.4 2.5 1.3 0.9 0.7 1.9 4.0 4.2 • M 2 5 8.9 8.3 8.4 2.4 1.2 0.8 0.6 2.9 1.5 0.9 0.7 2.2 1.3 1.1 0.8 3.0 1.8 1.5 1.0 2.4 05 • • -60 F1.5

A

5 (refr.) 3.7 3.3 3.2 3.4 5.0 11.8 11.0 10.4 3.7 4.7 4.8 6.6 14.1 12.3 11.8 3.2 1.7 1.3 0.9 4.4 2.6 3.1 3.8 2.9 1 5 1.3 0.9 4.0 2.4 2.0 1.6 3.7 D2 -30 F1.5

A 6.8 2.8 1.8 2.3 2.7 4.9 6 6 7.8 8.8 2 8 2.6 3.1 3.7 5.0 7.9 8.8 10.4 2.5 1.4 1.0 0.7 4,3 2 7 4.9 6.6 2.4 1.2 0.9 0 6 3 5 1.8 1.4 1.1 2.8 F1.5 M i i M p M l -60 F1.5

A 10 3.0 2.4 3.1 1<t 5.2 8.0 9.2 11.1 3.0 3.2 3.8 4,4 5.3 9.1 10.2 1 1.8 2.8 1.5 1.1 1.0 4.6 4.6 7.4 9.2 2.6 1.3 1.0 0 7 3.8 2.0 1.8 2.5 3.0 F1.5 -60 F1.5

A

50 4.5 5.0 1 t. 7.0 12.9 14.0 15.4 4.5 5.5 6.3 7.3 13.9 15.2 17.6 4.0 3.6 4.9 » 5.9 12.8 15.6 17.5 3.6 1.9 2.3 3.4 5.3 9.5 12.8 14.0 4.5 F1.5 -60 F1.5

A

100 4.9 5.8 i -0 7.9 14.2 15.4 16 5 4.9 6.3 7 0 8.1 15.5 16 8 18.9 4.5 4.9 6.3 3 » « * 6.6 14.9 17.3 19.9 4.1 3.0 4.9 m

5 8 13.3 17.1 18.6 4 9 F1.5

1 -60 F1.5

B

5 3.6 3.1 4 ; 5.0 11.5 12.9 13.4 3.7 5.4 5 6 14.1 15.0 15.3 3.1 1.7 1.2 0.9 4 3 2 6 4.2 5.4 2,8 1.5 1.1 0.9 3.8 2.2 1.8 1.5 3.7 F1.5 1 -60 F1.5

B 10 4.2 5.5 7.0 f • 6.0 15.5 16.7 17 5 4.3 7.1 8.1 ' j 1

7.1 17.6 18.7 19.1 3.6 1.9 1.8 2.6 5.1 7.1 9.7 10.2 3 2 1.7 1.3 1.0 4.4 2.5 2.2 2.5 4.3 F , 5 1 -60 F1.5 B

50 5.2 7.0 8.2 9 Z 7.8 17,7 19.2 21.2 5.3 7.8 8.6 • ' 8.8 18.4 20.0 23.4 4.6 5.5 7,4 6.5 17.2 19.3 20.4 4.1 2.4 4.4 6.0 5.8 12.0 14,9 16 1 5.3 F1.5 '"WrT -60 F1.5 B

100 5 4 7,2 8.1

0.5

8 5 17.7 18 6 2 1 9 5.5 7.8 8 7 m i 9.2 18.6 20.4 23.5 4.9 6.4 8.1 " a 7.3 18.9 21.3 23.2 4.5 4.4 7.0 6.4 16.7 19 7 20.7 5 5 F1.5 -60 F1.5

C

5 3.8 1.3 0.8 0.5 5.1 1.9 1.8 2.1 3.8 1.4 1.0 0.8 6.0 3.9 3.9 4.1 3.2 1.4 0.8 0.5 4.4 1.5 1.2 0.8 3.0 1 3 0.9 0.6 4.0 1.5 1.0 0.7 3.8 F1.5 3.8 Hz

C 10 4.2 3,3 2.6 2.3 5.9 11.1 9 0 8.4 4.3 4.1 3.3 3.0 7.7 10.8 8.9 8.6 3.7 1.9 1.2 0.8 5.2 7.3 6.6 6 1 3.2 1.7 1.3 1.0 4.5 2.5 2.3 1.9 4.3 F1.5 4,3 Hz

C 50 5.3 7.3 8.2 7.9 17.2 18.5 21.4 5.4 8.0 8.8 8 / 9.1 18.2 20.1 20.1 4.7 5.8 8.0 6.5 17.2 19.0 20.0 4.2 2.6 4.9 w 5.8 12.3 15.1 15.6 5.4 F1.5 3 5 - -60 D2

C

100 5.5 5.0 5.8 8.7 12.4 14.0 15.3 5.6 5.3 6.0 - 9.5 13.0 14 3 15.6 5.0 4.8 6 6 S j j j mP 7.2 14.3 17.5 17 4 4.5 3 6 6.4 h' 6.3 14.6 18.3 16.0 5.6 F1.5 • 5 5 - -60 D2

GO)

5 0.3 0.2 0.2 0.1 0.7 0,5 0.4 0.3 0.3 0.3 0.2 0.1 0.7 0.5 0.4 0.3 0.3 0.2 0.2 0.1 0.7 0.4 0.3 0.2 0.3 0.2 0.1 0.1 0.6 0.3 0.3 0.2 0.3 D2 0.3 Hz

GO) 10 0.5 0.4 0.3 0.2 1.1 0.8 0.5 0.4 0.5 0.4 0.3 0.2 1.1 0.9 0.6 0.4 0.5 0.3 0.2 0.2 1.0 0 6 0.5 0.3 0,5 0.3 0.2 0.2 1.0 0.5 0.4 0.3 0.5 D2 0.5 Hz

GO) 50 1.3 1.0 0.7 0.5 2.5 1.7 1.9 2.5 1.3 1.0 0.8 0.5 2.5 2.2 2.5 2 9 1.2 0.8 0.6 0.4 2.3 1.3 1.0 0.7 1.2 0.7 0.5 0.4 2 0 1.2 0.8 0 6 1.3 F1.5 1.3 Hz

GO)

100 2.0 1.3 1.1 1.3 3.9 3 8 4.4 5.3 2.0 1.5 1.5 1.7 4.0 4.4 4.9 5.8 1.9 1.2 0.9 0.6 3.5 2.3 3.4 4.0 1.8 1.1 0.8 0.5 3.0 1.6 1.4 1.6 2.0 F1.5 2.0 Hz

GOO

5 0.7 0.5 0.4 0.2 1.4 0.9 0.6 0.4 0.7 0.5 0.4 0.3 1.4 1.0 0.7 0.5 0.7 0.4 0.3 0.2 1 2 0.6 0.5 0.4 0.6 0.3 0.2 0.2 1.2 0.6 0.5 0.4 0.7 D2 0.7 Hz

GOO 10 1.1 0.7 0.5 0.4 2.1 1.2 0.8 0.6 1.1 0.8 0.6 0.4 2 1 1.4 1.1 1.4 1.0 0.6 0.4 0.3 1.9 1.0 0.7 0.5 1.0 0.5 0.4 0.3 1.6 0.8 0.6 0.5 1.1 F1.5 1.1 Hz

GOO 50 2.4 1.5 1.2 1 5 4.9 6.5 7 7 2.4 1 8 2 2 ) I 4.5 6.3 7.4 8.3 2.2 1.2 0.9 0.6 3.5 1.9 1.9 2.7 2.0 1.0 0.8 0.6 3.0 1.6 1.3 1.0 2 4 F1.5 -60 F1.5

GOO

100 3.1 2.7 3.5 « S 5.6 9.0 11.2 12.5 3.2 3.6 4.1 •><1 5.7 9 7 11.5 12.0 2.8 1.6 1 3 1.2 4.8 6.2 9.1 10.4 2.6 1.4 1.0 0.8 4.0 2.2 2.8 4.2 3.2 F1.5 -60 F1.5

The cell shading indicates the following: _ _ _ _ _ _ _ _ _ Horizontal release gives longest distance to LFL j R e l e a s e at this angle below horizontal gives longest distance to LFL




Fluid category



for hz release

Weather

Max. LFL dist.

for any release

Angle Weather Fluid

category



Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release

Angle Weather Fluid

category


Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60

Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release

Angle Weather

A

0 (refr.) 3.7 2.1 2.2 2.8 6.4 9.3 9.5 9.3 4.0 16.0 15.4 12.6 22.6 22.4 21.2 4.1 1.5 1.0 0.7 4.5 2.1 1.6 1.3 4.2 1.6 1.0 0.8 5.1 2.0 1.6 1.2 4.2 D9 -45 F1.5

A

5 (refr.) 9.9 23.4 25.0 2=9 28.5 38.5 38.8 38.2 13.4 26.5 28.2 s i 35.2 41.1 40,4 38.9 6.5 13.0 17.5 17.3 11.6 22.7 27.2 27.3 5.9 7.6 ,", 7.8 7.8 17.3 17.1 16.4 13.4 F1.5 -60 F1.5

A 6.8 6.5 10.5 11.2 11.6 23.8 25.0 30 6 6.7 11.3 12.8 14 7 13.3 25.6 28.9 34.0 5.7 10.0 12.2 14.6; 8.5 25.9 27.9 29.2 5.3 8.5 11.9 13,3 7.6 22.8 25.0 25.7 6.7 F1.5 -60 F1.5

A 10 7.1 11.9 12.5 13.4 26.5 27.7 32 8 7.4 12.9 13.5 16 2 15.1 28.9 30.5 37.1 6.3 12.0 13.3 16 7 9.6 29.6 30.9 32.9 5.7 11.2 14.1 8.0 26.7 28.9 29.4 7.4 F1.5 -60 05

A

50 10.0 17.0 19.3 21.6 36.7 40.9 47.6 10.5 18.9 21.9 24.6 23.1 40.9 47.0 54.3 8.7 18.8 22.4 26,6. 17.4 43.3 46.6 48.8 7.6 18.6 22.3 13.1 41.0 43.2 45.7 10.5 F1.5 -60 D5

A

100 11.2 18.3 21.3 25 0 24.7 39.7 44.9 51.4 11.7 21.5 24.2 25.8 46.3 51.5 60.8 9.6 19.7 23.4 26.$ 21.3 46.7 51.9 56.0 8.6 21.6 22.4 29.4 16.8 48.4 48.5 53.3 11.7 F1.5 ; . -60 D9

B

5 8.6 18.8 20.1 2:3 2 21.1 37.0 37.5 38.5 10.5 19.9 21.5 > :3 26.6 41.0 41.3 41.2 6.4 16.3 17.4 176 10.6 27.0 27.9 27.3 5.8 10.2 12.4 12 4 7.7 20.5 22.1 22.5 10.5 F1.5 25.9 -60 F1.5

B 10 10.8 22.4 23.5 27.0 27.0 44.8 46.0 46.7 12.9 23.4 25.2 30 2 32.1 50.4 51.0 51.5 7.6 21.4 22.5 22,3 15.5 34.2 34.3 34.0 6.6 16.6 18.6 • IS.? ' 9.5 28.3 29.7 29.8 12.9 F1.5 30 2 -60 F1.5

B 50 12.6 23.0 24.1 28.3 27.7 47.5 49.5 57.6 13.7 25.2 26.5 29 0 30.8 52.7 57.7 68.3 10.0 25.9 27.8 32.0 22.6 50.3 50.8 52.0 8.5 27.1 28.7 30.4 16.4 46.2 47.7 47.8 13.7 F1.5 32 0 -60 D5

B

100 13.2 22.5 24.3 27 2 29.0 47.6 48.2 55.9 13.9 25.6 27.0 29,7 30.5 53.0 56.2 63.5 11.0 26.1 27.0 31,5 25.4 55.2 56.8 59.1 9.4 27.8 30.5 20.2 53.5 54.5 56.8 13.9 F1.5 34 6 -60 D9

C

5 8.9 23.6 25.0 2 p ; 24.8 38.0 38.4 37.5 10.4 25.8 28.1 28.5 26.1 39.9 40.3 38.5 6.6 16.7 17.6 17 7 12.4 26.8 27.6 27.0 6.0 10.1 12.2 12 3 7.9 20.2 21.9 21.8 10.4 F1.5 28 5 -60 F1.5

C 10 11.1 23.9 26.4 29 8 28.5 45.7 46.4 46.9 13.3 25.9 31.0 34 3 31.4 50.4 50.8 49.9 7.7 21.6 22,8 22.7 15.9 34.1 34.6 34 1 6.7 16.9 18.1 9.7 28.2 29.2 29.0 13.3 F1.5 i l s -60 F1.5

C 50 13.1 23.1 26.2 297 28.4 46.9 52.8 61.0 13.9 25.8 28.7 32 8 30.8 54.1 67.1 74.4 10.3 26.2 29.9 • • ; 23.0 50.3 52.3 52.0 8.7 27.2 29.5 31,2 16.8 45.5 47.3 47.3 13.9 F1.5 -60 D5

C

100 13.5 25.9 27.1 31.5 29.1 53.4 55.8 66.8 14.2 27.8 29.3 35.2 30.9 58.5 62.1 83.5 11.3 28.3 29.7 371 25.7 57.4 58.5 60.3 9.5 30.7 34.0 20.5 53.9 55.8 56.5 14.2 F1.5 -60 D5

G(i)

5 1.0 0.7 0.5 0.4 2.0 1.2 0.9 0.7 1.0 0.8 0.6 0.4 2.0 1.4 1.4 1.5 0.9 0.6 0.5 0.3 1.8 1.1 0.8 0.6 0.9 0.5 0.4 0.3 1.7 0.9 0.7 0.5 1.0 F1.5 1.0 Hz

G(i) 10 1.3 1.0 0.8 0.5 2.6 1.9 2.4 2.9 1.3 1.1 0.8 0.6 2.6 2.5 2.9 3.4 1.3 0.9 0.6 0.4 2.5 1.4 1.1 0.9 1,3 0.7 0.5 0.4 2.3 1.3 0.9 0.7 1.3 F1.5 1.3 Hz

G(i) 50 3.3 3.1 3.4 4. t 6.0 8.8 9.3 11.5 3.3 3.4 3.7 4.2 6.2 9.5 10.1 11.7 3.0 2.3 2.9 3.6 5.2 8.2 9.5 12.0 2.8 1.6 1.8 2.4 4.9 7.0 9.3 11.0 3.3 F1.5 -60 F1.5

G(i)

100 4.8 5.6 6.3 7.1 8.5 14.1 15.2 18.2 4.8 6.1 6.7 7.6 8.8 15.6 17.0 19.3 4.5 5.3 6.2 7 7 7.5 14.8 16.3 19.6 4,3 4.5 5.9 7.3 6.8 15.1 17.4 22.2 4.8 F1.5 -60 F1.5

G(ii)

5 1.9 1.2 0.9 0.6 3.6 2.3 3.4 4.4 2.0 1.3 1.2 1.4 3.7 4.1 5.0 6.0 1.7 1.0 0.7 0.5 2.8 1.6 1.1 0.9 1.5 0.8 0.6 0.5 2.5 1.4 1.0 0.8 2.0 F1.5 2.0 Hz

G(ii) 10 2.5 1.6 1.8 2.2 4.7 6.3 7.7 9.0 2.6 2.3 2.9 3.3 4.8 7.4 8.8 9.6 2.4 1.3 1.0 0.7 3.8 2.4 4.1 5.2 2.2 1.2 0.8 0.6 3.4 1.8 1.4 1.1 2.6 F1.5 3 3 -60 F1.5

G(ii) 50 5.5 7.9 8.9 107 9.3 17.5 22.4 24.7 5.6 8.5 9.1 9,5 9.9 16.6 15.8 14.8 4.8 6.8 8.7 }Q;7: 7.4 18.3 23.8 25.4 4.0 3.9 6.5 8.0 6.4 17.1 20.3 21.7 5.6 F1.5 10.7 -60 D5

G(ii)

100 7.4 12.4 13.6 17,0 13.6 22.9 31.8 34.4 7.7 12.7 13.1 13 2 14.9 21.1 20.0 19.0 6.3 12.2 14.9 10.2 24.8 34.5 35.6 5.7 11.1 14.4 15:2 8.2 27.9 33.1 34.4 7.7 F1.5 17.1 -60 D5

The cell shading indicates the following: Horizontal release gives longest distance to LFL

I I I I I I I I I I I I I j Release at this angle below horizontal gives longest distance to LFL




Fluid category



for hz release

Weather

Max. LFL dist.

for any release




Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release



Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60 Hz -30 -45 -60

Max. LFL dist.

for hz release

Weather

Max. LFL dist.

for any release

Angle Weather

A

0 (refr.) 30.3 32.1 32.0 31.3 45.0 44.6 44.3 43.5 35.4 35.0 33.8 32.1 48.8 46.1 44.5 43.0 14.4 21.1 20.1 19,6 27.3 31.7 31.1 30.5 6.7 12.4 13.9 14.4 13.6 23.8 25.1 25.5 35.4 F1.5 35.4 Hz

A

5 (refr.) 32.8 51.8 53.0 n t 68.5 76.0 75.2 73.0 33.8 58 9 58.6 57.5 80.8 79.6 77.4 75.2 22.9 37.1 37.4 362 44.5 54.4 54.5 52.9 12.7 31.7 31.8 310 28.9 47.1 47.8 46.7 33.8 F1.5 58 9 • -30 F1.5

A 6.8 13.7 24.7 26.2 30.4 50.8 52.8 64.7 14.6 27.3 28.9 • 32 6 57.2 62.5 78.5 11.0 26.8 27.9 35.3 25.8 53.4 54.9 58.8 9.1 28.8 32.5 316 19.6 50.9 53.1 52.2 14.6 F1.5 35 3 -60 D5

A to 15.7 27.6 29.2 §11 34.6 56.4 60.1 69.7 16.5 30.9 32.1 38 S 37.3 65.4 68.8 90.9 12.9 31.7 33.6 : 4 30.7 62.3 63.6 64.9 10.3 31.4 34.1 37.7 24.3 55.7 58.7 60.7 16.5 F1.5 ; a -60 D5 A

50 23.7 40.6 42.7 49,7 52.4 82.3 83.0 101.3 24.6 45.2 47.6 562 54.5 92.2 98.6 135.6 21.7 45.5 48.4 60.0 51.7 92.2 94.1 97.6 18.6 50.1 52.4 57,7 48.4 86.5 86.5 90.1 24.6 F1.5 &0.0 -60 D5

A

100 26.6 44.8 46.9 55 6 58.7 90.1 92.0 114.1 27.4 50.9 53.3 62.5 60.6 100.2 106.4 142.3 25.1 49.4 52.3 Ml 59.4 106.1 107.6 111.0 22.7 55.7 58.4 63 0 58.2 99.4 101.1 101.0 27.4 F1.5 -60 D9

B

5 24.9 39.8 41.3 46.0 60.8 73.4 75.3 76.5 27.7 44.5 50.6 '55.8'' 67.5 84.7 85.1 85.0 18.1 38.3 38.0 38.3 41.6 54.8 54.8 54.0 11.7 32.9 32.7 31.9 26.9 48.4 48.0 47.8 27.7 F1.5 'S -60 F1.5

B 10 30.2 45.1 46.7 52 4 71.0 88.0 89.3 92.1 32.8 50.4 54.1 659 77.0 104.2 104.7 105.1 24.4 46.6 46.8 46.9 53.1 66.7 66.9 65.2 16.7 40.6 40,8 40.4 38.6 58.4 59.0 58.0 32.8 F1.5 65,9 -60 F1.5

B 50 30.8 46.6 48.3 68.9 91.9 94.2 104.1 32.0 52.0 54.7 61.9 70.4 111.2 125.4 151.9 28.5 55.3 57.7 68.5 100.0 100.4 100.8 24.7 56.8 57.9 :<»5, 60.8 87.7 89.0 89.1 32.0 F1.5 65 6 -60 D5

B

100 31.6 47.6 49.6 544 63.3 92.1 96.6 103.5 32.4 54.1 57.2 60.7 71.5 106.4 112.1 124.7 30.1 54.1 56.4 532 70.3 110.8 112.6 116.4 27.6 61.0 62.6 66.5 69.4 102.7 102.7 103.3 32.4 F1.5 -60 D9

C

5 31.6 52.5 53.9 67.9 76.1 75.9 73.3 32.7 58.3 58.4 57.3 79.2 79.5 78.6 77.3 23.5 36.8 36.4 35.1 45.1 52.1 51.5 50.3 13.5 30.9 31.0 30.1 30.1 46.3 46.0 45.0 32.7 F1.5 -45 F1.5

C 10 30.7 54.9 60.3 62.2 69.9 89.6 89.8 87.5 32.7 67.5 71.3 71,7 75.2 100.3 98.7 96.2 24.9 46.4 4S7 43.9 54.2 65.2 63.9 61.6 17.3 39.7 39.0 39.3 57.8 57.6 56.4 32.7 F1.5 71 7 -60 F1.5

C 50 31.3 52.2 62.6 i f ? ! 1 67.5 104.7 125.4 132.1 32.9 58.2 73.8 1.03,4 70.0 141.6 151.9 152.6 29.0 62.3 67.2 mx. 69.8 99.5 99.2 96.2 25.4 56.8 ' • 59.6 60.7 86.9 88.2 85.6 32.9 F1.5 -60 F1.5

C

100 32.2 56.6 66.9 83.S 68.7 110.9 136.3 158.2 33.1 62.4 73.6 i p s 71.1 139.5 176.7 182.5 30.7 68.5 76 6 81,4 69.9 116.1 117.0 114.7 28.1 67.5 69.7 70 9 68.9 102.3 102.5 101.7 33.1 F1.5 -60 F1.5

G(i)

5 2.0 1.4 1.2 1.3 4.0 3.9 4.5 5.4 2.0 1.5 1.6 1.7 4.0 4.5 5.0 6.1 1.9 1.2 0.9 0.6 3.6 2.5 3.6 4.2 1.7 1.1 0.8 0.5 3 1.6 1.4 1.7 2.0 F1.5 Hz

G(i) 10 2.6 2.3 2.7 31 5.3 7.0 7.8 9.2 2.6 2.7 3.1 3.6 5.4 7.6 8.4 10.4 2.5 1.7 2.0 2.5 4.8 6.3 7.5 9.1 2.4 1.4 1.1 1.0 4.2 4.6 6.8 8.4 2.6 F1.5 -60 F1.5

G(i) 50 6.2 8.7 9.0 105 11.8 20 1 21.3 25.9 6.3 9.0 9.8 11.2 12.3 22.5 25.0 28.5 5.7 8.3 8.9 fltiS 10.0 21.3 23.2 27.9 5.1 8.0 9.0 10-3 8.6 23.0 25.5 30.3 6.3 F1.5 11 2 -60 F1.5

G(i)

100 8.9 13.5 14.1 16.4 19.4 31.3 32.5 38.2 9.1 14.9 16.5 20.0 20.2 36.7 40.2 49.3 8.2 13.9 14.8 ' 17.4 34.7 36.8 47.0 7.6 14.1 15.8 17.5 14.9 38.2 42.0 50.2 9.1 F1.5 -60 F1.5

G(ii)

5 3.7 3.6 4.3 5.G 6.3 10.5 13.1 14.5 3.8 4.4 5.0 5.9 6.5 11.0 13.0 11.1 3.1 1.8 2.0 2.8 5.2 8.8 12.0 13.1 2.8 1.5 1.1 0.8 4.3 3.1 5.9 7.3 3.8 F1.5 -60 F1.5

G(ii) 10 5 1 6.8 7.8

i f . 18.5

8.4 15.4 20.2 21.9 5.2 7.5 7.9 -8,6 8.9 15.2 14.5 13.2 4.4 5.2 7.1 8.3 6.9 16.2 20.6 22.0 3.8 2.4 4.1 6.0 13.5 17.3 18.3 5.2 F1.5 9.0 -60 F1.5 G(ii)

50 10.3 18.0 21.0

24.7

i f . 18.5 21.3 30.4 43.8 27.0 11.1 182 18.2 17.7 23.0 27.0 25.9 24.7 8.5 18.7 23.2 262 17.9 32.8 48.0 50.0 7.5 19.2 24.1 2S.3 13.8 36.0 48.5 49.6 11.1 F1.5 26.3 -60 D9

G(ii)

100 15.9 24.7

21.0

24.7 24.5 30.9 38.9 37.5 34.2 16.9 24.5 24.0 22.9 31.7 35.0 33.8 31.9 13.1 25.9 34.2 38 1 28.5 41.7 67.3 67.0 10.5 27.5 35.8 37 6 25.2 47.0 68.1 69.7 16.9 F1.5 38.1 -60 D5

The cell shading indicates the following: Horizontal release gives longest distance to LFL

2 Release at this angle below horizontal gives longest distance to LFL




ANNEX B GLOSSARY OF TERMS AND ABBREVIATIONS

B.1 INTRODUCTION

For the purpose of this research report, the interpretations for terms in B.2 and abbreviations in B.3 apply, irrespective of the meaning they may have in other connections.

B.2 GLOSSARY OF TERMS

rapid phase transition (RPT): the rapid change of state of a substance which may produce a blast wave and missiles

B.3 GLOSSARY OF ABBREVIATIONS

BLEVE: boiling liquid expanding vapour explosion DNV: Det Norske Veritas DSEAR: The Dangerous Substances and Explosive Atmospheres Regulations El 15: El Area classification code for installations handling flammable fluids LFL: lower flammable limit LNG: liquefied natural gas LPG: liquefied petroleum gas PHAST: Process hazard analysis software tool RPT: rapid phase transition UK: United Kingdom

Page 53


ANNEX C REFERENCES

The information provided in this annex comprises publications that are referred to in this research report. All were current at the time of its writing. Users should consult the pertinent organisations for details of the latest versions of publications. To assist, web site addresses are provided.

CERC (Cambridge Environmental Research Consultants) http://www.cerc.co.uk Model evaluation report on UDM version 6.0, SMEDIS/00/9/E, version 1.0 (2002).

CCPS (Center for Chemical Process Safety) http://www.aiche.org/ccps Guidelines for use of vapor cloud dispersion models, 2nd ed. (1996), New York.

DNV (Det Norske Veritas) http://www.dnv.com Process hazard analysis software tool (PHAST), versions 6.51, 6.53 and 6.6.

IChemE (Institution of Chemical Engineers) http://www.icheme.org Nomenclature for hazard and risk assessment in the process industries, Jones D. (ed.), 2nd ed. (1992), Rugby.

El (Energy Institute) http://www.energyinst.org Area classification code for installations handling flammable fluids ('EI15'), Model Code of safe practice Part 15, 3rd ed. (2005), ISBN 978 0 85293 418 0, London.

Calculations in support of IP15: The area classification code for petroleum installations, 1st ed. (2001), London. (See Addendum 1.)

HMSO (Her Majesty's Stationery Office) http://www.opsi.gov.uk

Meteorological glossary, 5th ed. (1972), London: Meteorological Office.

Monographs

Pasquill F. and Smith F.B., Atmospheric diffusion, 3rd ed. (1983), Chichester: Ellis Horwood.

Research papers Daish et al. 1999: Daish N.C., Britter R.E, Linden P.F., Jagger S.F. and Carissimo, B. SMEDIS: Scientific model evaluation techniques applied to dense gas dispersion models in complex situations, presented to International conference and workshop for modelling the consequences of accidental releases of hazardous material$CCPS, San Francisco, California, September 1999, New York: CCPS.

Page 54

http://www.cerc.co.uk

http://www.aiche.org/ccps

http://www.dnv.com

http://www.icheme.org

http://www.energyinst.org

http://www.opsi.gov.uk


TNO http://www.tno.nl Yellow book, 2nd ed. (1992).

Office of Public Sector Information (OPSI) http://www.opsi.gov.uk The Dangerous Substances and Explosive Atmospheres Regulations 2002 (2002 SI 2776).

Page 55

http://www.tno.nl

http://www.opsi.gov.uk


ADDENDUM 1: El CALCULATIONS IN SUPPORT OF IP 15: THE AREA CLASSIFICATION CODE FOR PETROLEUM INSTALLATIONS (FIRST EDITION)

Explanatory note

Publication of the new research set out in the second edition of El Research report: Dispersion analysis and calculations in support of El 15: The area classification code for installations handling flammable fluids augments the following technical issues in the first edition of El Calculations in support of IP 15: The area classification code for petroleum installations:

— Work item 2: Shape factors and hazard radii for pressurised releases - see Addendum 1 Section 3.

— Work item 4: Liquid pools due to spillage - see Addendum 1 Section 5.

In practice, information on these issues in the earlier report is not invalidated, but rather is augmented in the second edition. Consequently, and because no further technical work has been carried out on other work items in the earlier report, the first edition is entirely replicated here as Addendum 1.

Some editorial errors in the first edition has been corrected: — Section A2, paragraph 5 the second citation of R, has been amended to R2. — Executive summary, paragraph 2 fluid category G has been amended to Gi.

Page 56

Calculations in Support of IP15: The Area Classification Code

for Petroleum Installations

November 2001

By P. T. Roberts

OGCH/2 HSE Business Group

Shell Global Solutions (UK), Cheshire Innovation Park, P.O. Box 1, Chester CH1 3SH, England

Report No. OP.00.47110

Published by The Institute of Petroleum, London

A charitable company limited by guarantee

Copyright © 2001 by The Institute of Petroleum, London: A charitable company limited by guarantee. Registered No. 135273, England

All rights reserved

No part of this book may be reproduced by any means, or transmitted or translated into a machine language without the written permission of the publisher.

ISBN 0 85293 339 8

Published by The Institute of Petroleum

Further copies can be obtained from Portland Press Ltd. Commerce Way, Whitehall Industrial Estate, Colchester C02 8HP, UK. Tel: +44 (0) 1206 796 351 email: [email protected]


CONTENTS

Page

Foreword vii

Acknowledgements viii

Executive summary 1

1. Introduction 2

2. Flammability limits for two phase releases 3

3. Shape factors and hazard radii for pressurised releases 6

4. Hazard radii from vents 10 4.1 Vents from the storage of petroleum products 11 4.2 Process vents 20

5. Evaporation from pools and sumps 22 5.1 Vapour pressure comparisons of some commonly used

Category C fluids 27

6. Releases into confined areas 29

7. Discussion and conclusions 34

8. References 35

Appendix A: Methodology 37

Appendix B: Preliminary investigation of hazard radii and shape factors for the Revision of IP15: the Area classification code for petroleum installations 40

V

vi

FOREWORD

The Institute of Petroleum commissioned this report to address concern over the effect of a release containing droplets or a mist on the dispersion distances determined by the methodology used in IP publication A Risk-Based Approach to hazardous area classification, 1998. The report also reviews flammability limits, evaporation from pools and releases into confined areas.

The aim of this publication is to provide a record of the calculations, methodology and assumptions used to calculate dispersion distances. It provides a traceable scientific basis that will be applied to the 2nd edition of IP publication Model Code of Safe Practice Part 15: Area Classification code for petroleum installations 1st edition, 1990.

Although it is believed that the adoption of the recommendations of this report will assist the user, the Institute of Petroleum cannot accept any responsibility, of whatsoever kind, for damage or loss, or alleged damage or loss, arising or otherwise occurring as a result of the application of this report.

vii

ACKNOWLEDGEMENTS

This report was prepared by Dr Peter Roberts and Dr Les Shirvill and was reviewed by members of the Institute of Petroleum's Area Classification Working Group:

Phil Cleaver Advantica Technology Howard Crowther Consultant (formerly BP) Kieran Glynn BP Alan Tyldesley Health and Safety Executive Mick Wansborough Shell

The Institute wishes to record its appreciation of the work carried out by the members of the group.

The Institute also wishes to record its thanks to the Health and Safety Executive for co-sponsoring this research.

viii

Calculations in Support of IP 15: The Area Classification Code for Petroleum Installations

Executive Summary

The Area Classification Code for Petroleum Installations published by the Institute of Petroleum (IP 15) offers guidance on the immediate area of hazard associated with the normal processing and handling of petroleum products and is in very wide use.

A major revision of IP 15 is being prepared which aims not only to update the guidance based upon best current practice but also to provide a traceable and scientific basis for the guidance given. This latter is not a trivial task and necessarily depends to a great extent on the methodology developed for assessing the consequences of accidental releases on a large scale - much larger than would arise from normal processing and handling. The quantification of hazard necessarily starts with specifying the type of material and the size of release which is very much unknown in the case of small spills and leaks. Material types have been simulated using 5 example fluid compositions coded (A, B, C, Gi and Gii) following earlier work to update IP 15. Release rate values used here represent the lower end of the "hazardous release" scale. These should be larger than arise in normal handling and certainly should not be taken as indicative of the magnitude of "acceptable" spills. In all circumstances the potential for spills to occur should be rigorously assessed and a full hazard assessment carried out where necessary.

This note contributes a methodology and the physical basis for the deriving several guidance parameters relating to:

• the characteristics of two-phase releases compared to single phase releases. • the definition of shape factors for pressurised releases of both heavier than air and

lighter than air fluids (fluid categories A, B, C, Gi, Gii). • the flammability limits for the fluids used as examples of categories A, B, C, Gi, Gii. • hazards arising from the evaporation of category C fluids. • releases into confined areas.

The major findings arising from this work are summarised below. It has generally been possible to defend the key recommendations of IP 15 as conservative. Where revisions are recommended these are strongly dependent on scenario and fluid type.

• The hazard radii for pressurised releases of category B and C fluids should be derived assuming a mechanically generated flammable mist; previously gaseous releases were assumed. The Hazard radii for category B and C fluids are increased relative to previous guidance.

• Numeric flammability limits published in Annex D of "A Risk-Based Approach to Hazardous Area Classification" for the category B and C fluids have been updated to take account of the composition of the flammable mist; previously low vapour components were assumed to rain-out and not contribute to the lower flammability limit evaluation.

• Shape factors for pressurised releases are revised to take better account of the role of initial jet momentum on the jet trajectory. In particular the lighter than air gases (category Gi and Gii fluids) are found to have qualitatively more similar shape factors to

1

the two-phase category A, B and C releases; previously buoyancy was assumed to dominate their dispersion.

• Hazard radii for discharges from vents are evaluated. The hazard radius varies from slightly smaller to slightly larger than that in the existing guidance depending on the properties of the vented vapour.

• The composition of vapour from vents on storage facilities maintained at atmospheric pressure may be variable (in composition, density and flammability) and the user of the new guidance should be aware of the effect of this variability because of the consequence for hazard radii.

• The example range of venting rate and vent sizes used in the guidance are not wholly consistent with the assumption that the discharge takes place at atmospheric pressure. The relationship between venting rate and pressure of discharge is investigated and a value of 300 mb suggested as a threshold above which the consequences of pressure should be assessed. This is of significance for multicomponent fluids where condensation may occur.

• The existing guidance for liquid spillages is conservative, judged by the volatility of the model category C fluid. The guidance is applicable to materials with approximately twice the vapour generation rate of category C fluid under the specimen conditions. Relative vapour pressures for some common hydrocarbon compounds are listed.

• The existing guidance for sumps is conservative, judged by the volatility of the model category C.

• Vapour generation at the source of spillage of category C fluids is a potential hazard dictated by the spill rate and conditions and not the rate of evaporation of the liquid pool. The new guidance should emphasise the role of release conditions in determining the initial vapour generation from spills of category C fluids.

• For releases into confined areas the relative size of spillage and building are of key importance. The classification "Adequate ventilation" has been assessed with respect to these parameters.

1. Introduction

Shell Global Solutions, on behalf of the Institute of Petroleum (IP), has worked to establish a methodology by which certain guidance parameters in the IP 15 document can be calculated from specific scenarios. The benefit is two-fold. Firstly, the existence of a methodology enables the guidance to be independently verified, secondly, it allows an end user to derive specific fluid and process dependent values in a consistent way when required. The methodology closely follows that used for assessing the consequence of hazardous events. Several of the scenarios adopted to illustrate the effect, say on hazard radius, of changing release scenarios were found to produce events that in practice would require a formal assessment of risk; i.e. they fall outwith the definition of normal processing and handling of petroleum products. None of the discharge rates used in this report should be taken as representing normal or acceptable routine practice.

This work took place in two stages. A preliminary investigation was carried out for Shell (UK) in order to verify the hazard radii reported in "A Risk-Based Approach to Hazardous Area Classification", Institute of Petroleum, November 1998 and to see if the shape factors reported in IP 15 were adequate.

• the values of the hazard radius for category B and category C fluids in the Risk-Based Approach Document were too small, and the release scenarios unrealistic.

• the shape factors for lighter than air gases (category Gi and Gii fluids), and to a lesser extent, the low vapour pressure category B and C fluids needed to be revised.

2

An abridged version of the report of this preliminary investigation is included as Appendix B to this note. The major findings are restated in the body of this note.

The implied changes to key values in IP 15 were significant. The Institute of Petroleum requested that Shell Global Solutions pursue 6 work items to verify and quantify the necessary changes for the revision to IP 15. These were (in short)

Work Item 1: To examine results of the AlChE Release modelling program and other recent data and derive, if possible, an improved estimate of the flammability limit for high flash-point releases (e.g. taking account droplet size, rain-out) that would allow the category C hazard radii to be better assessed.

Work Item 2: To describe the method used to calculate the hazard radii; define the shape factors for the fluid releases; state the hazard radii.

Work Item 3: Cross check the hazard radii for the process vents.

Work Item 4: Liquid Pools due to Spillage (section 5.11) To determine the hazard radii/shape factor for shallow liquid pools.

Work item 5: Open Sumps and Interceptors (section 5.12) To determine the hazard radii/shape factor for deep liquid pools by taking the steady state evaporation rate and a steady dispersion calculation.

Work item 6: Propose a simple 'low momentum' calculation method; implement this method and use it to assess the external hazard for releases of category A and category B fluid inside a building and evaluate the hazard radii.

Progress on these work items was reviewed. Specific scenarios were discussed and amended in discussion with the Area Classification Committee of the IP to give the results below. These results are in a form suitable for inclusion in the IP 15 revision.

A consequence of changes to some scenarios is that numeric values obtained in the original investigation on behalf of Shell (UK)(Appendix B) are changed. Only values from the main body of this note should be transferred to the new guidance.

Where possible, publicly available and publicly evaluated hazard assessment models have been used in this work.

2. Flammability Limits for Two Phase Releases.

A major advance in hazard assessment has been the development of models capable of describing the dispersion of two-phase liquids. The AEROPLUME model is one example, being part of the HGSYSTEM v3.0 (1995) suite of models developed by Shell for industry consortia as publicly available tools subject to peer review and acting as a standard benchmark in model evaluation exercises.

Two-phase releases can arise in two ways:

• by the atomisation accompanying the expansion and phase change of material that is liquid under storage conditions of high pressure, and gaseous at atmospheric temperature and pressure.

3

• by the mechanical break-up of a (volatile) liquid into small droplets and their subsequent evaporation.

Mists of fine droplets and dust clouds of combustible material can be very highly flammable. Unlike a gaseous mixture that is combustible only within narrow flammability limits each droplet can act as a fuel source surrounded by a plentiful air supply. Mists are optically thick and, once ignition has occurred, heat transfer by radiation very effectively preheats droplets/particles distant from the ignition front. In extreme cases and especially for dust clouds, this preheating is sufficient for auto-ignition to take place causing the cloud to burn throughout its volume. This can be a much more vigorous process than a gas cloud fire where a flame-front passes through the mixture.

The flammability hazard arising from a two-phase release depends in a complicated way upon the ease of ignition of the fluid, the droplet size distribution and the concentration of droplets and vapour in air. Unfortunately very little is known about precise flammability criteria for mists arising from "real" releases. The summary guidance based upon a review of available literature (Appendix B, Lees, 1998) is that the potential of a mist to burn should be assessed: take all the droplets present in a volume, evaporate them and see if the resulting vapour and air mixture lies within the known vapour phase flammability limits. The easiest way to evaluate this is to use a mass based flammability limit (kg fuel/m3 air) in place of the standard and familiar volume based limit (m3 fuel/m3 air) used for vapours.

This definition of a flammability limit:

• has NO effect on hazard distances calculated for gaseous mixtures or two-phase mixtures of very volatile components'.

• has a profound effect on the hazard distances calculated for the category B and category C example fluids resulting in a substantial INCREASE in hazard radius over previous advice based upon volumetric flammability limit values.

Not all of the liquid released from a pressurised source might remain airborne. Thus, loss of fluid to the ground through rain-out may mitigate the hazard associated with two-phase releases with a low volatility component. The question of rain-out has been investigated at length by the Center for Chemical Process Safety of the American Institute of Chemical Engineers by means of a series of experiments and an extended modelling exercise. Unfortunately the problem has not been satisfactorily solved. The combined results of this study are reported by Johnson and Woodward (1999) and expressed in software form as a model, called Release.

The Release model does not account for all of the physical processes involved in two-phase releases of low volatility materials and is strongly tuned to account for the results of the experiments that were carried out. These experiments aimed to measure the total liquid deposition from pressurised releases of several materials under a limited and non-ideal set of environmental conditions. An obvious concern is that the validity of the model outside of the range of the calibration data is unknown. The model performance is also poor in several respects.

1 This applies to the category A (two-phase), Gi and Gii (gaseous) example fluids used in the IP 15 revision at the reference atmospheric conditions and release conditions therein.

4

Two versions of the model are supplied on CDROM by AlChE; an original model and a corrected model. These have different functionality. The corrected model is intended for end-use and is referred to hereafter in this report.

The Release model was reviewed by AEA Technology for the UK Health and Safety Executive (Ramsdale and Tickle (2000)). We conclude from the AEAT report, together with our less detailed investigation of the model, that:

• the Release model would over-predict rain-out by a substantial margin for category C fluids.

We also believe that the relationship between over-pressure and rain-out is not robustly developed and may not be extrapolated to conditions outside of the tests. With these reservations in mind, but for completeness, we used the Release model to calculate rain-out for cyclo-hexane using the conditions of release temperature, pressure and hole size used in this study for the IP. Cyclo-hexane is the closest to a category C fluid of those tested.

We found that: • mass flow rates calculated by the Release model as a function of hole size and pressure

were realistic. • rain-out as a fraction of mass-flow rate was independent of hole size and a function of

pressure only. The calculated rain-out values were:

Pressure (bar) Rain-out (fraction of mass released) 5 0.98 10 0.44 50 0 100 0

Table 1 Results of the "Release" model for Cyclo-Hexane.

• the rain-out fraction did not depend upon the axial location in the jet.(input parameter ZJ)

We do not believe that the values in Table 1 are necessarily correct although they confirm our intuition that pressurised releases should become atomised. Further the consensus of the model reviewers is that Release overestimates the amount of rain-out. For the purposes of this work we therefore assume that:

• rain-out of liquid from pressurised releases can be neglected in calculating hazard radii.

This is in accord with the concluding remarks (5.2) of the AEAT review which suggest that low volatility materials should conservatively be treated as a mist. There must be a lower limit to the drive pressure for which this is true and further work is needed to establish the proper limits.

It does seem credible, from intuition and from Table 1, that the 5 bar pressure releases would rain-out. However, if they do not form a flammable mist they would instead form an initially coherent liquid jet. The throw of a liquid jet can be quite substantial and could credibly extend the same distance as we calculate here for dispersed phase jets. A liquid jet of gasoline, say, would present a contact hazard to electrical equipment on exposed surfaces and also form a liquid pool on the ground that will flow away from the source. The result on ignition would be a pool fire rather than a jet-fire or cloud deflagration. For these reasons we retain the hazard radii for Category C fluid down to the 5 bar condition.

5

It is worth noting that, for flammable as opposed to toxic hazards, the Release model prediction of total rain-out does require qualification as to at what distance from the source the rain-out occurs. Clearly to mitigate the hazard this must be smaller than the calculated hazard radius. For toxic hazards (evaluated at a long distance from the release point) only the total released needs to be known.

The revision of the IP guidelines is based on model fluids for the five fluid categories. These first appeared in the Addendum to IP 15: A Risk Based Approach to Hazardous Area Classification. The fluid properties are quoted in Table 2. We note:

• The vapour phase flammability limits have been re-calculated and differ from those in the risk-based addendum to IP 15. The derivation of those flammability limits is not known. However, we infer that this is because of a changed assumption that all of the released material might participate in a fire. Certainly the previously published values are consistent with the assumption that only the light ends of the mixture would burn and match those calculated for a pseudo-mixture of hydrocarbons smaller than C8.

Stream component (mol %)

Fluid Cat. A

Fluid Cat. B

Fluid Cat. C

Fluid Cat. G

(i)

Fluid Cat. G

(ii)

Comp. LFL

(vol %)

MW Boiling point °C

N2 Nitrogen 0.00 0.00 0.00 2.00 2.00 - 28.01 -196 Ci Methane 0.00 4.00 0.00 88.45 10.00 5.00 16.04 -161 C2 Ethane 0.00 0.00 0.00 4.50 3.00 3.00 30.07 -87 C3 Propane 70.00 6.00 1.00 3.00 3.00 2.10 44.09 -42 C4 Butane 30.00 7.00 1.00 1.00 1.00 1.80 58.12 -1 C5 Pentane 0.00 9.00 2.00 1.00 0.00 1.40 72.15 36 C6 Hexane 0.00 11.00 3.00 0.00 0.00 1.20 86.17 69 C7 Heptane 0.00 16.00 3.00 0.00 0.00 1.05 100.20 98 C8 Octane 0.00 22.00 27.00 0.00 0.00 0.95 114.23 126 C9 Nonane 0.00 0.00 25.00 0.00 0.00 0.85 128.26 151 C-io Decane 0.00 25.00 38.00 0.00 0.00 0.75 142.28 173 H20 Water 0.00 0.00 0.00 0.05 0.00 - 18.02 100 Carbon Dioxide

0.00 0.00 0.00 0.00 1.00 - 44.01 -78 (sub)

Hydrogen 0.00 0.00 0.00 0.00 80.00 4.00 2.02 -253 Average MW

48.30 100.06 125.03 18.74 7.03

LFL (vol %) 2.00 1.05 0.86 4.6 4.00 LFL (kg/m3) 0.039 0.042 0.043 0.034 0.011

Table 2 Composition of the example category A,B,C, G(i) and G(ii) Fluids and their lower flammability limits (LFL)

3. Shape Factors and Hazard Radii for Pressurised Releases

The major findings of this study, compared with earlier guidance are that:

• Pressurised releases give rise to an approximately spherical hazard zone" for all categories of fluid, except where the release comes into ground contact where the hazard distance is extended.

11 In the context of flammable hazards.

6

• Hazard radii for category B and category C fluids are substantially greater than those quoted in the Addendum to IP 15: A Risk-Based Approach to Hazardous Area Classification.

• The increase in hazard radius is due to the redefinition of flammability limit and not to substantial differences in modelling changes.

The new shape factors are shown in Figure 1. The shape factor depends upon the height of the release and the hazard radius.

7

(a) Releases where H > R̂ + 1

b) Releases where 1 < H < Ri + 1

c) Releases where H < 1

Figure 1 Shape Factors for Pressurised Releases

The key features are:

• Releases below a height (H) of 1 m are declared to be influenced by the ground and to have a hazard radius R2.

j • Releases above 1 m, but at heights below the hazard radius Ri + 1 m are declared to be influenced by the ground if the release is directed downward and passes below 1 m.

• Releases at a height above the hazard radius R-i +1 m are declared independent of the ! ground.

The Hazard radii are given in Table 3 for the primary radius R-i and in Table 4 for the ground radius R2. For small releases, giving a dimension R-i not substantially larger than 1 m, the radii are similar.

i i

The numerical values given in Table 3 and Table 4 are specific to the example fluids. The release rate for these fluids is only weakly dependent upon small variations in the assumed storage temperature about 20 °C, which is chosen to reflect a daily average UK summer temperature. Other fluids may be more sensitive to temperature changes.

I

Table 3 Primary Hazard radius, Ri for example releases. (Rounded figures)

*At the fluid storage temperature of 20 °C the nominal discharge pressure of 5 bara is below the saturated vapour pressure of the category A fluid. The saturated vapour pressure (6.8 bara) was used to calculate the discharge rate and dispersion.

Fluid Release Category pressure

Release flow rate Hazard radius Ri (kg/s) (metres)

Release hole diameter Release hole diameter

(bara) 1mm 2mm 5mm 10mm 1mm 2mm 5mm 10mm A 5* 0.01 0.04 0.3 1.0 2 4 8 14

10 0.01 0.06 0.4 1.5 2.5 4 9 16 50 0.03 0.14 0.9 3.5 2.5 5 11 20 100 0.05 0.2 1.20 5.0 2.5 5 11 22

B 5 0.01 0.04 0.30 1.0 2 4 8 14 10 0.02 0.07 0.40 1.7 2 4 9 16 50 0.04 0.15 1.0 4.0 2 4 10 19 100 0.06 0.2 1.4 5.5 2 4 10. 20

C 5 0.01 0.06 0.3 1.1 2 4 8 14 10 0.02 0.1 0.4 1.7 2.5 4.5 9 17 50 0.04 0.2 1.0 4.0 2.5 5 11 21 100 0.06 0.25 1.4 6 2.5 5 12 22

G(i) 5 0.001 0.002 0.02 0.06 < 1 < 1 <1.0 1.5 10 0.001 0.005 0.03 0.10 < 1 < 1 1.0 2 50 0.007 0.03 0.2 0.7 < 1 1.0 2.5 5 100 0.015 0.06 0.4 1.5 < 1 1.5 4.0 7

G(ii) 5 0.0004 0.001 0.01 0.04 < 1 <1 1.5 3 10 0.001 0.003 0.02 0.07 < 1 1 2 4 50 0.004 0.02 0.1 0.4 < 1 2 4 8 100 0.007 0.03 0.2 0.7 1 2 6 11

9

Release flow rate Hazard radius R 2

(kg/s) (metres) Fluid Release Release hole diameter Release hole diameter Category pressure

(bara) 1mm 2mm 5mm 10mm 1mm 2mm 5mm 10mm A 5* 0.01 0.04 0.3 1.0 2 4 16 40

10 0.01 0.06 0.4 1.5 2.5 4.5 20 50 50 0.03 0.14 0.9 3.5 3 5.5 20 50 100 0.05 0.2 1.2 5.0 3 6 20 50

B 5 0.01 0.04 0.3 1.0 2 4 14 40 10 0.02 0.07 0.4 1.7 2.5 4 16 40 50 0.04 0.15 1.0 4.0 2.5 5 17 40 100 0.06 0.2 1.4 5.5 3 5 17 40

C 5 0.01 0.06 0.3 1.1 2.5 4 20 50 10 0.02 0.1 0.4 1.7 2.5 4.5 21 50 50 0.04 0.2 1.0 4.0 3 5.5 21 50 100 0.06 0.25 1.4 6 3 6 21 50

G(i) 5 0.001 0.002 0.02 0.06 < 1 < 1 1.0 2 10 0.001 0.005 0.03 0.10 < 1 < 1 1.5 3 50 0.007 0.03 0.2 0.7 < 1 1.5 3.5 7 100 0.015 0.06 0.4 1.5 1.0 2.0 5 11

G( i i) 5 0.0004 0.001 0.01 0.04 < 1 < 1 2 4 10 0.001 0.003 0.02 0.07 < 1 1 2.5 5 50 0.004 0.02 0.1 0.4 1 2 6 11 100 0.007 0.03 0.2 0.7 2.0 3 8 14

Table 4 Hazard Radius at Ground level, R2, for the example releases

For larger releases R2 can be approximated from R1 using Table 5. The ratio decreases as the release pressure increases because mixing improves.

Fluid Category R2/R1 Low pressure High Pressure

A 3.0 2.2 B 2.6 2.0 C 3.5 2.0

G(i) 1.4 1.5 G(ii) 1.3 1.3

Table 5 Quick estimator for the hazard radius at ground level, R2.

4. Hazard Radii from vents

In this chapter we address hazard radii for discharges from vents using a standard matrix of conditions and ambient conditions of neutral atmospheric stability and a temperature of 30 °C. A vent is defined as a means of release of vapour at or near to atmospheric pressure. This is distinct from the pressurised releases considered in section 3.

The results of the calculations for the storage of petroleum products are given in section 4.1 and for process vents in section 4.2. The main findings, which result from the use of the prescribed matrix of conditions, are that:

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• low flow rates through the larger orifices implies insufficient exit momentum to disperse the vented material as a jet. The dispersion process then becomes dependent on the detailed flow interaction at the vent tip. Downwash, contacting of material below the vent height and dense vapours in the vicinity and below the vent exit are to be expected in these cases.

• some combinations of venting rates and vent sizes are incompatible with the assumption of discharge at atmospheric pressure.

• a guideline value of 300 mb for the pressure drop across a vent is suggested as an upper bound to "atmospheric pressure" releases. Above this pressure account needs to be taken of density changes due to pressure differences in calculating discharge rates for ideal gases. The effect of pressure on vapour composition should be screened because components of high flashpoint fluids may condense.

• Emissions from the storage of category B and category C fluids necessarily imply that the fluid vapour are in some admixture with air or an inert gas. Hazard radii are derived for a range of possible vapour compositions treated as ideal gases with molecular weights between 48 and 100. The impact of vapour composition on flammability limits is also taken approximately into account.

• The shape factor from the existing guidance is retained. The actual shape of the plume from a vent varies from an upright jet, to a bent-over plume, to a plume subsiding below the vent height depending upon the released gas composition, flow rate, vent size and wind speed. With this complexity it is appropriate and conservative to retain a spherical hazard radius around the exit plane.

• The hazard radii vary from just smaller to just larger than those in the existing guidance depending upon the vapour composition and flammability limit.

• The existing guidance for process vents is in agreement with these results.

4.1. Vents from the Storage of Petroleum products

Section 3.2.5 of Revision 6 of IP 15 provides hazard radii for differing emission rates and vent sizes for Class I, ll(2) and lll(2) materials. Model fluids are used to derive the hazard radii. The composition of these fluids was given Table 2. It is assumed that the vents are remote from any structure and, if attached, are elevated sufficiently for the dispersion to the lower flammability limit to be unaffected by building induced flow. The comments below are specifically addressed for the early stages of dispersion for flammability assessments. Other problems, such as the assessment of odours, assessment of health impacts or environmental impacts, require a more detailed treatment.

Work Item 3 is to verify/update the hazard radii given in the original report according to the matrix given in Table 6 for materials in Categories B and C.

Vapour Emission Rate (filling rate) m3/h

Vent diameter (mm)

50 80 100 250 250 x X X X 500 X X X X 1000 X X X X 2500 X X X X

Table 6. Matrix of vent flow rates and diameters used in this study

Flow from a vent is of vapour only and it is implicit that there is only a small pressure drop across the vent. This contrasts with releases from pressurised containment which may exhibit two-phase behaviour as well as density changes near to the discharge point.

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Venting usually takes place in a vertical direction. Material vented upwards will rise above the vent stack as a consequence of vertical momentum. As it mixes with the ambient air it acquires horizontal momentum and bends over toward the horizontal. If the vented gas is less dense than air it will then slowly rise, if it is heavier than air it will slowly subside. The extent to which the plume path is affected by density differences depends upon the gas properties, the venting rate and the prevailing atmospheric conditions.

The degree of plume rise in the immediate vicinity of the vent depends mainly upon the exit momentum and hence on the fluid density as well as the exit velocity. However, most of the modelling that has been carried out is for stack gas effluent and uses velocity based criteria for design purposes. The most commonly applied rule is to guard against plume downwash. Experiment shows that, when the exit velocity is less than 1.5 times the wind speed at the vent height, the aerodynamic interaction between the wind and the vent stack causes the pollutant to be drawn down into the near wake of the vent tip. This is called downwash and results in a reduction in the height of release, extra mixing of the pollutant and contacting of the pollutant with the external stack.

For releases where the flow rate is sufficient to fully avoid downwash then dilution to below flammable concentrations will occur in the vicinity of the vent and through the mechanism of jet mixing rather than ambient turbulence.

In order to be conservative we have neglect the effect of downwash on plume dilution. An ambient wind speed of 2 m/s at the vent top is assumed for these calculations.

We observe that: for wind speeds greater than 2 m/s the plume trajectory will be flattened toward the horizontal. The shape factor is thus conservative in the vertical for wind speeds of 2 m/s and higher. For wind speeds less than 2 m/s the hazard will be above the vent point so that the shape factor is conservative in the downwind direction. The hazard radii given in this report take account of the contribution of the 2 m/s wind speed to the downstream extent of the plume. This should render the estimation of hazard radius conservative for lower wind speeds. In critical cases the user should check by calculation.

The shape factor given in the existing guidance is retained because it is conservative.

For very slow venting rates and under low-wind conditions denser than air gases might flow down the outside of the vent pipe. The situation should not occur for simple vents, designed with an adequate exit velocity. It is more likely to occur for large area vents on a structure. There is no simple model available to treat this problem which needs experimental or computational investigation on a case by case basis. The flow phenomenon is complicated and may involve substantial mixing within the exit of the vent pipe. We believe that it is necessary, but conservative, to retain the zone 2 classification outside of and beneath the zone 1 hazard radius to account for this possibility.

Nominal exit velocities for the vent matrix are given in Table 7. These cover a large range. We observe that:

• For the range of flow rates shown the 250 mm diameter vent in particular shows nominal exit velocities below and close to 10 m/s. This implies downwash under common wind conditions and so these vent combinations may not be realistic in practice for continuous emissions.

• Exit velocities above 100 m/s are indicated for the 50 and 80 mm diameter vents at the higher flow rates. The implications of this are assessed below for the different fluids.

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Exit Velocity m/s

Vent diameter (mm) 50 80 100 250

250 35 14 9 1 500 71 28 18 3 1000 141 55 35 6 2500 283 110 71 11

Table 7. Nominal exit velocity based on vent area (m/s). The shaded cells represent combinations of vapour rate and vent diameter that need special consideration. Exit velocities below 10 m/s in the top right of the table imply possible downwash effects. Exit velocities above 100 m/s in the bottom left imply a significant pressure forcing.

The exit velocity of the vented flow is illustrative but, because it is the exit momentum of the flow that determines the rate of dispersion, the density of the effluent needs to be taken into account as well.

We assume that the effluent is an ideal gas with properties evaluated at atmospheric pressure. In reality any rate of discharge requires some driving force and a positive pressure differential is the most common. We need to evaluate what a "negligible" pressure is and how it affects the realistic range of vent sizes and venting rates for the different category fluids. We also need to assess what are the likely properties of the material that is vented. For liquid storage at atmospheric pressure the vapour space will necessarily be occupied by some mixture of fluid vapour and air or fluid vapour and an inert gas. The mixture composition should not be flammable but will be variable according to the filling level and the history of changes in level, ambient conditions etc. Thus for vents :

• The vented mixture may vary in composition. • The flammability limit and physical properties of the vapour may vary. • For some conditions (high venting rates and small vents) it may be necessary to take

account of the effects of pressure on the release. • If the release does not take place close to atmospheric pressure then the ideal gas

assumption may not be appropriate for mixtures with low vapour pressure components.

Because the effective molecular weight, and hence density, of the vented vapour is not closely defined it is appropriate to make some example calculations using a range of values. We assume'" that the average molecular weight of the gas vented from the storage of category B and C fluids will lie in the range 48 - 100.

Figure 2 shows how the mixture molecular weight varies with dilution. For no-added air the molecular weight is that of the pure vapour (Table 2) and as more and more air is added then the mixture molecular weight tends to that of air (29) or if mixed with nitrogen (28).

Note: because the model fluids are illustrative we have made no attempt to reconcile the example ideal gas properties with the actual equilibrium air-vapour-liquid compositions of the model fluids at atmospheric pressure and the reference temperature of 30 °C used throughout this guidance

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Mean Molecular Weight for Mixtures of Category A, B, C fluids with Air, g/mol

MW 48 - A MW 100 - B MW 125 - C

0.2 0.4 0.6 0.8

Mol fraction of Air in Mixture

F igure 2 The effect on mixture molecu lar we ight of mixing category C (top), ca tegory B (middle) and ca tegory A vapour with air a s s u m i n g volat i l isat ion of all componen t s .

The upper flammability limits of the Category B and C fluids lie far to the right of Figure 2 as shown in Table 8.

Fluid Upper Flammability Limit of vapour %

Mol fraction Air in mixture

B 6.84 0.93 C 6.08 0.94

Table 8. Upper F lammabi l i ty l imits of Category B and C vapour . The mol fract ion of air in the vapour in a s torage tank mus t be substant ia l ly less than these v a l ues to avo id f lammable/explos ive mixtures.

The mixture molecular weight of 48 corresponds to the following mixtures, Table 9:

Fluid Type Mol Fraction Air Mol Fraction Fluid Vapour A 0 1 B 0.72 0.28 C 0.81 0.19

Table 9: Equ iva len t mixtures for a vapour molecu la r wt of 48.

The Category B and C fluids in Table 2 have a lower flammability limit falling into a narrow range of 0.039-0.043 kg/m3, 0.86-2.0 %v. We also included Category A vapour as an additional example in this sensitivity analysis although, of course, for practical reasons it is not stored at atmospheric pressure and temperature.

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As the pure vapour becomes diluted then the lower flammability limit of the mixture increases. The effect of dilution on the flammability limit of the mixture is shown qualitatively in Figure 3 where we have used Le Chatelier's law and treated air as inert fuel component. This is not strictly accurate for high dilutions but it does illustrate that lower flammability limit increases as air is added. Only mixtures initially richer than the upper flammability limit are considered which is why the mol fraction of air is terminated at 0.9.

Lower Flammability Limit for Mixtures of Category A, B, C fluids with air, %

100 F MW 48 - A MW 100 - B MW 125 - C

E > 10

re E E re

0) 5 o

1 r...

0.1 -

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Mol fraction of Air in Mixture

Figure 3. Lower Flammability l imits of the three model f luids used in the Area Classif ication guidance as a funct ion of di lut ion with air. Only mixtures originally above the upper f lammabil i ty l imit are considered.

We now verify and quantify the range of venting rates and vent diameters that qualify as "negligible pressure". Figure 4 shows the variation in mass-flow rate from a short pipe as the applied pressure, expressed in millibar gauge, increases.

Specific calculations (points) were calculated for an ideal gas of molecular weight 48 and for a pipe diameter of 50 mm. At low drive pressure the mass flow rate is proportional to the square root of the applied pressure as would be expected from the Bernoulli equation for an incompressible fluid i.e. keeping the density unchanged from its base value. A correlation lineiv is drawn to indicate this relationship. At high pressure the mass flow rate (points) increases more quickly with increasing pressure than indicated by the correlation line. This is because the increase in fluid density in the vent is significant. To identify a suitable cut-off we compared the goodness of fit of the Bernoulli equation to the flow calculations. We found that the fit became progressively worse as we included results for pressures above 300 mb. Therefore we adopt 300 mb as marking the upper limit of "negligible" pressure.

IV A correlation line was used for convenience. Equally the slope could be derived from the discharge model for an ideal gas through a round hole using a discharge coefficient of 0.8. This was the basis for the model calculations to which the regression line was fitted.

15

To see the consequence of this for the flow conditions in Table 6 we take the maximum flow rate of 1.34 kg/s corresponding to the volume flow-rate of 2500 m3/h (evaluated at 1 atm. pressure and 30 °C) and plot this as a horizontal line on Figure 4. We then construct the (incompressible) flow rates for the additional hole sizes of 80, 100 and 250 mm as parallel lines to the 50 mm pipe calculations. Figure 4 shows that for pipes smaller than 80 mm diameter the 2500 m3/h flow rate implies a drive pressure greater than 300 mb.

Mass flow rate as a function of applied pressure

0.001

100 mm pipe 250 mm pipe

2500 m3/hr . I i —I

100 1000 10000

over-pressure, mb

Figure 4. Mass Flow rate th rough pipes of di f ferent diameter for an ideal gas of molecular weight 48. Points denote calculat ions wi th a d ischarge model. Lines denote a correlat ion based on dr ive pressures less than 300 mb. The hor izontal line denotes the mass f low rate consis tent wi th a vo lume f low rate of 2500 m3 /h at atmospher ic pressure and 30 °C.

Figure 5 shows the equivalent graph for a vapour of molecular weight 100. For the larger molecular weight greater pressures are needed to achieve a given volumetric flow. We find that pipe diameters smaller than 100 mm require a drive pressure greater than 300 mb to achieve a flow rate of 2500 m3/h.

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Mass flow rate as a function of applied pressure

over-pressure, mb

Figure 5. Mass Flow rate through pipes of different diameter for an ideal gas of molecular weight 100. Points denote calculations with a discharge model. Lines denote a correlation based on drive pressures less than 300 mb. The horizontal line denotes the mass f low rate consistent with a volume f low rate of 2500 m3/h at atmospheric pressure and 30 °C

These calculations suggest that for ideal gases a threshold of 300 mb is a suitable choice to distinguish between "vent" flows where difference in the physical properties of the vapour within the vent and at atmospheric pressure can be neglected. If the vent design implies higher over-pressures then the implications for the possible change in properties of mixtures with low vapour pressure components needs to be assessed.

Hazard radii were obtained for vertical discharges of an ideal gas with molecular weight in the range 48 - 100 fluid. The effect of changing flammability limit was tested using values appropriate to mixtures of category B and category C vapour with air and including category A vapour as a worse case. Results are given in Table 10 and have been rounded. Values marked with a double asterisks(**) are conservative, at such low exit velocities mixing is dependent on the detail flow in the vent exit.

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Hazard radius, m


250 2 2.0 2.5 3** 500 2.5 2.5 2.5 4 1000 3 3.5 3.5 6. 2500 4 5 5 7

a) Assuming category B vapour mixed with air. The shaded cells require a drive pressure greater than 300 mb to achieve


Hazard radius, m


250 2 2.0 2 3** 500 2.5 2.5 2.5 4.0 1000 3.0 3.5 3.5 5.5 2500 4.0 5.0 5 6

b) Assuming category C vapour mixed with air. The shaded cells require a drive pressure greater than 300 mb to achieve


Hazard radius, m


250 2.5 4.0 6.0 6** 500 3.5 3.5 4.5 6.5 1000 4.5 4.5 5.0 9 2500 6.5 7 7 13

c) Assuming pure category A vapour. The shaded cells require a drive pressure greater than 300 mb to achieve

Table 10. Hazard radii for a fluid of molecular weight 48 g/mol treated as an ideal gas and three example flammability limits

The results show a trend for the hazard radius to increase as the lower flammability limit of the mixture decreases as would be expected. The hazard radius values for the category B and category C vapour/air mixtures are quite similar but substantially smaller than those for the "pure vapour" category A simulation. This suggests that the dilution with air that characterises an open venting system is key to reducing hazard distances and should be taken account of in any specific site assessment.

Table 11 below gives results for a molecular weight 100 fluid, evaluated as a mixture of category C fluid and air and as pure category B fluid. This is almost certainly an extreme example but, when compared to Table 10, illustrates that as the molecular weight of the

18

mixture increases the hazard radius also increases for a fixed volume flow rate. We also notice in the detailed calculations and showing in Table 11 that, for a fixed venting rate and increasing vent diameter, the trajectory of the vent gases alters as the exit velocity decreases. At low exit velocities the vent gases subside quite quickly after leaving the vent and the downward path of the gases passes relatively close to the vent. The hazard radius, measured from the exit plane to the point of dilution to LFL starts to decrease because of the trajectory whereas, for lighter (but still dense) gases the vented material passes away from the stack and the hazard radius increases as the exit velocity and hence the jet mixing component of entrainment decreases.

When the molecular weight is as large as 100 g/mol then discharge from a vent sized 250 mm or larger has insufficient bulk velocity to disperse the gases according to the assumptions made here.

In practice, and especially for substantially larger vents, such as hatches, complicated flow interactions can take place in the actual opening with a dense gas flow preferentially around the edges and a degree of inflow and mixing taking place inside the centre of the vent opening. A different methodology, such as computational fluid dynamics or experimental measurement must be used if the mixing within the geometric features of the holding vessel is to be understood and quantified.


Hazard radius, m

7 Vent diameter (mm) 50 80 100 250

250 4 6.5 6 n/a 500 5 5.5 8 n/a 1000 6.5 7 7 n/a 2500 9 10 10 11

a) Assuming category C vapour mixed with air


Hazard radius, m

7 Vent diameter (mm) 50 80 100 250

250 3.5 6.5 6.0 n/a 500 5 6 8.5 n/a 1000 6.5 7 7.5 n/a 2500 9.5 10.5 10.5 11.5

b) Assuming pure category B vapour

Table 11. Hazard radii for a fluid of molecular weight 100 g/mol, treated as ideal gas. For venting rates less than 2500 m3/h the vent of 250 mm diameter gives too small an exit velocity to assure dilution of the gas and a down flow in the vicinity of the vent stack is to be expected. Affected combinations of vent rate and vent size are marked as n/a.

In Table 10 and Table 11 we note that, for the highest flow rates and the smaller vent sizes, the hazard radius is shaded. This denotes that more than 300 mb pressure drop is needed

19

to achieve this flowrate and a check on the vapour composition should be carried out to see if condensation of any components is implied.

Comparison with section 3.2.5 of revision 6 of IP 15 shows that hazard radii range from just below (molecular weight 48) to just above (molecular weight 100) the existing guidance using flammability limits for category B and category C vapour diluted with air.

4.2. Process vents

Section 5.7 of Revision 6 describes releases from process vents to atmosphere. It is again assumed that venting is restricted to vapour phase releases. Release rates in the guidance and required for work item 3 follow the matrix shown in Table 12 within which the implied exit velocities are included. We note that the range of values is extremely small compared with those used above for vents from storage. This seems counter intuitive as one might expect venting from a process to involve quite large flow rates.

Vapour Emission Rate m3/h

Exit Velocity m/s

Vent diameter (mm) 50 100 250

10 1.5 0.5 0.1 100 14 3.5 1 250 35 9 1.5

Table 12. Matrix of conditions for assessing hazard radii from process vents and their associated exit velocity. Shading denotes combinations prone to downwash.

The exit velocity from a vent should exceed the wind speed by a factor 1.5 if downwash is to be avoided and exit velocities less than about 10 m/s may be assumed to give rise to downwash under common meteorological conditions. Downwash is to be avoided as it can lead to soiling and corrosion of the vent pipe and to low level exposure of structures and personnel to the emitted gases.

Table 12 shows exit velocities very substantially less than 10 m/s for a majority of cases. Where the exit velocity is less than 1 m/s some very complicated interactions between the external flow and the pipe flow can occur. It is known that for lighter than air gases the external flow can enter the vent pipe from above and that mixing can take place within the vent pipe itself to a good degree. Experimental and/or computational fluid calculations are needed to make accurate calculations for these cases.

Hazard radii are calculated in the same manner as for the vents from storage. The vented fluid is assumed to be an ideal gas of molecular weight, 7, 19 and 48 corresponding to the category G(ii), G(i), and A fluids used in the guidance.

The hazard radii are given in Tables 13-15 below. In view of our comments above with respect to the overall low flow rates in the guidance we have included larger values in the tables.

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Hazard radius, m


10 < 2** < 2.5** < 5** 100 < 2** < 2.5** < 5** 250 2 3 < 5** 500 2 3 < 5** 1000 3 3.5 < 5** 2500 4.5 4.5 5.5

Table 13. Hazard radii for Fluid category G(ii) as ideal gas of Molecular weight 7 g/mol.


Hazard radius, m


10 < 1** < 2** < 4** 100 1 < 2** < 4** 250 1.5 2 < 4** 500 2 2 < 4** 1000 3 3 4 2500 4 4 5

Table 14. Hazard radii for Fluid category G(i) as ideal gas of Molecular weight 19 g/mol.


Hazard radius, m


10 2.5 < 4.5** < 6** 100 2 5 < 6** 250 2.5 6 < 6** 500 3.5 4.5 7 1000 4.5 5 9 2500 6.5 7 13

Table 15. Hazard radii for Fluid category A as ideal gas of Molecular weight 48 g/mol.

No solutions were obtainable for the smallest releases. The dilution is entirely dominated by the flow interactions at the vent tip. We suggest that, to be conservative, the largest calculated hazard radius for a given vent diameter is used. These values are indicated by a double asterisk(**) in the tables.

The results are in agreement with those in the existing guidelines.

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5. Evaporation from pools and sumps.

IP 15 provides good qualitative guidance on the evaporation and dispersion of vapour from category B and C fluids discharged to pools and from sumps. This work sets out to derive the relevant hazard radii as requested in work items 4 and 5.

The transient nature of a spill problem and the multicomponent nature of category C and B fluids leads to a semantic problem when providing a reference document for the IP guidance.

The hazard radius defined for a pool of specific size should reflect the hazard from the pool itself and be independent of the manner of release of the fluid if it is to be of generic use. Investigation of several scenarios leads to the conclusion that the evaporation rate of the heavier components of category C fluid is very low under the reference conditions. The evaporation rate of the lighter components on the other hand is rapid such that the maximum vapour generation rate is actually dictated by amount and rate of release of the liquid. Consequently we find that:

• the existing guidance for liquid spillages and for sumps is conservative in so far as it applies to the hazard posed by the residual liquid in a spill of category C fluid. The guidance is appropriate to a spill with an average double the evaporation rate of category C fluid.

• the hazard arising early in a spill arises from the more volatile components and is dictated by the conditions of the release rather than the size of the pool itself.

• the existing guidance may still be conservative because the transient nature of the vapour release results in a rapidly dispersing "puff' of vapour rather than a large cloud. Some scenario investigation suggests that releases would have to be large ones to present substantial hazards and thus be more typical of incidents as opposed to more commonly occurring events.

• evaporation from sumps will be less than from pools and therefore the existing guidance is conservative for sumps as well. This is verified for releases of temperature 50°C.

In this section the basic phenomenology of liquid spills is described and the hazards quantified.

We define:

• a pool to contain a shallow layer of liquid e.g. arising from the spillage of a category C fluid onto a surface such as concrete.

• a sump to contain a much deeper layer of liquid; possibly floating upon a sub-layer of water.

This distinction is necessary because the physical process of evaporation requires the supply of energy equal to the latent heat of evaporation of the fluid. For a shallow layer this energy can be obtained from the substrate by conduction. Clearly the availability of energy depends on the actual depth of the pool, the temperature and physical properties of the substrate and the external conditions but, for very shallow pools, evaporation will take place at a rate determined by the ground temperature. For a deep layer this energy is obtained from the internal energy of the spilled fluid. The temperature of the pool will drop until a heat balance is reached between the evaporation rate and the total energy transfer to the liquid by conduction and insolation. As the liquid cools then the evaporation rate decreases.

22

Clearly these are extremes and spills can behave as "shallow" or "deep" or have mixed behaviour depending upon the fluid properties and spill conditions and whether or not the spill is physically contained. For a given fluid we would generally expect a greater evaporation rate from a shallow spill onto a warm substrate than from a deep pool. Implicit in this expectation is some consideration as to how the fluid enters the pool or sump, how long it remains there and what the time history of the event is.

A pool will originate with a breach in a tank or pipe containing liquid. As the spill commences, liquid spreads out over the ground until it meets a restriction or until the pool thickness has reached a limiting value determined by surface tension. While the spill is spreading over warm ground the lighter components evaporate preferentially. There may also be some vapour released at the early in the spill which adds to the overall vapour generation rate. The rate of total evaporation increases until the pool stops spreading. If the pool is unconfined then the vapour generation rate then falls quickly because the pool dries out. If the spread of liquid is confined by a bund or similar then a finite depth of liquid remains. The vapour generation rate will then remain constant or decline gradually in time depending on the overall heat balance on the pool. When the pool reaches the minimum depth then the evaporation rate will fall rapidly as the pool dries.

A sump may behave as a deep pool or may contain a residual amount of material that is refreshed with new releases. If the fill-up rate exceeds the potential evaporation rate of the fluid components then the sump will behave as a deep pool. If the fill up rate is smaller than the evaporation rate of the lighter components then the sump will behave more like a shallow pool.

For work items 4 and 5, hazard radii are sought for pools of 5, 10, 15 and 20 m diameterv

formed by the retention of spilled materials by planned bunds, naturally occurring blockages such as kerbs, or emergency measures.

There are a number of ways in which such pools can occur:

• A fixed volume of fluid could be spilled. If this is captured in bunds of the different sizes above then the smallest pool will be deeper than the largest pool.

• A variable volume of fluid could be spilled. This could result in the different sized pools having more similar depth.

Each of these scenarios will result in a different evaporation history. Quantifying all of the possible scenarios is a large task. Before investigating some actual releases we consider the simpler problem of steady state evaporation of a category C fluid.

Using the steady state modelling assumptions for the model category C fluid and for pools of size 5 - 15 m it can be shown that the evaporation flux must exceed certain limits if the existing guidance is correct. Some example hazard distances are given in Table 16. The pool size is the length of the pool in the wind direction and the hazard distance is the distance to LFL measured from the downward edge of the pool. Two values are given for the hazard distance. The minimum value uses a lower flammability limit appropriate to the lightest vapour components of a category C fluid. The maximum uses the lower flammability limit appropriate to the heaviest components of a category C fluid, c.f. Table 2,

v For practical reasons the hazard radii are calculated for square sources of equivalent area as described in Appendix A: Methodology.

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Evaporation Flux (kg/m2/s)

Evaporation Rate (kg/s)

Pool Size L (m)

Min. Hazard Distance D

(m)

Max. Hazard Distance D

(m)

0.002 0.05 5 2.9 5.2 0.45 15 13.0 21

0.0015 0.04 5 0 3.2 0.36 15 7.5 14.5

0.001 0.03 5 0 0 0.23 15 0 0

Table 16. Hazard distance ranges for category C vapour from area sources

The existing guidance gives hazard distances for ranges of pool sizes, Table 17:

Pool Size L (m) Hazard distance D (m) Less than 5 3

Between 5 and 10 7.5 Greater than 10 15

Table 17. Original Hazard distances from the Guidance.

Comparing Table 16 and Table 17 and taking the maximum hazard distance we see that the existing guideline is equivalent to a source with an evaporation flux of between 0.0015 and 0.002 kg/m2/s depending upon the flammability limit that is used. Indeed, considering that the more volatile components evaporate first, the existing guidance is equivalent of a source decreasing in strength from a flux of 0.002 to 0.0015 kg/m2/s over the life of the spill.

These points are illustrated with some examples below:

Figure 6 shows the time dependent evaporation flux for 4.5 m3 of category C fluid rapidly dumped into a 15 m pool. This volume is chosen to adequately fill the pool and leave fluid still present after a time of 30 minutes (1800 s). Details are given in the methodology section, Appendix A.

The horizontal lines show the evaporation flux range 0.0015 - 0.002 kg/m2/s relevant to the existing guidance. For the release the evaporation flux decreases from a value of 0.0012 kg/m2/s to a value of 0.0005 kg/m2/s after a time of 300 s. This marks the end of the period in which the lighter components are evolved. Subsequently the evaporation rate is nearly constant. In fact it shows a slight increase because the simulations assume noon-time insolation and the pool is in fact warming slightly over the later period of the modelled spill.

Thus a rapid spill of category C fluid gives evaporation fluxes a fraction 0.6-0.4 smaller than accounted for by the existing guidance.

24

Spill rate 0.45 m3/s

time from spill start, s

Figure 6. Evaporation rate of a rapid dump of category C f luid

Figure 7 shows the effect of spilling the same volume over the longer period of 300 s. Again there is no special significance to the choice of a volume of 4.5 m3 other than that this is sufficient to adequately fill a pool of 15 m diameter.

Here three stages of the spill can be seen. There is an initial high rate as the liquid spreads to fill the pool and is passing over a hot concrete substrate. The liquid quickly reaches the bund walls and the pool depth starts to increase. The vapour generation rate falls because the substrate and pool are cooled by the evaporation. The evaporation of the light components of the newly spilled fluid is visible as a plateau lasting just longer than the 300 s of the actual spill. Thereafter the evaporation rate falls again to give results similar to those for the rapid spill.

25

Spill rate 0.015 m3/s


Figure 7. Time history of a five minute spil l of 4.5 m3 of category C fluid.

For sumps there is a concern that hot liquid may be introduced. Figure 8 shows that preheating the fluid to 50 °C from 20 °C enhances the vapour rate in the early spill stages by a factor of about two. Even so it only reaches the 0.002 kg/m2/s value for the few seconds in which the liquid spreads to cover the pool. For a sump, where liquid is being added to existing material, this initial transient would be absent. It is notable that over a half-hour period the effect of the initial temperature difference on evaporation rate has disappeared.

26

Spill rate 0.015 m3/s Storage Temperature 50 C


Figure 8. Time history of evaporation for l iquid initially at 50 C. The liquid in the pool rapidly cools so that the residual evaporation rate is similar to the other cases.

These calculations show that in so far as the evaporation of the model category C fluid is concerned the existing guidance is conservative with respect to the evaporation of the bulk of the fluid. They also show that the manner of discharge itself (rate, amount) coupled to the initial volatilisation of light components determines the early peak in vapour rate. This cannot be characterised simply in terms of a pool dimension. For the example we have used here - the release of 4.5 m3 of fluid into a 15 m bund over time scales of 10 to 300 s -which perhaps is not an unreasonable scenario, the guidance is conservative even for this initial spill period. Larger spill possibilities would likely occur as incidents and thus require explicit modelling.

5.1. Vapour pressure comparisons of some commonly used Category C fluids.

Table 18 shows the vapour pressures of some hydrocarbons (C7+) relative to n-octane. Liquid Spill Hazard for all of these compounds exceeds that of n-octane and specific account of volatility must be taken into account when the ratio exceeds 2.

Name Formula Vapour Pressure relative to n-octane

at 30 °C n-octane C8H18 1 cis 1,2-dimethylcyclohexane C8H16 1.02 3-vinylcyclohexene C8H12 1.09 2,2,4-trimethylhexane C9H20 1.13 isopropyl cyclopentane C8H16 1.14 di-sec-butyl ether C8H180 1.15 2,2,5-trimethylhexane C9H20 1.17

27

trans-2-octene C8H16 1.19 octene-1 C8H16 1.23 tra 1,3-dimethylcyclohexane C8H16 1.24 cis 1,4-dimethylcyclohexane C8H16 1.26 2-methyl bicyclo 221 heptane C8H14 1.29 tra 1,2-dimethylcyclohexane C8H16 1.35 3-methyl heptane C8H18 1.38 2-methyl-1-heptene C8H16 1.39 2,4-heptadiene C7H12 1.41 3-ethyl hexane C8H18 1.42 1,7-octadiyne C8H10 1.43 2-methyl heptane C8H18 1.45 1,7-octadiene C8H14 1.49 cis 1,3-dimethylcyclohexane C8H16 1.5 cycloheptane C7H14 1.52 3,4-dimethylhexane C8H18 1.52 1,1 -dimethylcyclohexane C8H16 1.58 3-methyl 3-ethyl pentane C8H18 1.6 2,3-dimethylhexane C8H18 1.64 2-methyl 3-ethyl pentane C8H18 1.67 di-tert-butyl peroxide C8H1802 1.78 2,3,3-trimethylpentane C8H18 1.87 2,3,4-trimethylpentane C8H18 1.88 1,1,2-trimethyl cyclopentane C8H16 1.93 toluene C7H8 1.98 3,3-dimethylhexane C8H18 1.99 2,4-dimethylhexane C8H18 2.11 2,5-dimethylhexane C8H18 2.12 2,2,3-trimethylpentane C8H18 2.22 1,3-heptadiene C7H12 2.24 2,2-dimethylhexane C8H18 2.36 1,6-heptadiyne C7H8 2.38 2,4,4-trimethyl pentene-2 C8H16 2.5 heptyne-1 C7H12 2.73 2,2,3,3-tetramethylbutane C8H18 2.73 ethyl cyclopentane C7H14 2.75 2,4,4-trime-1 -pentene C8H16 3.06 n-heptane C7H16 3.15 cis-2-heptene C7H14 3.15 methyl cyclohexane C7H14 3.17 trans-2-heptene C7H14 3.21 cis 1,2-dimethylcyclopentane C7H14 3.24 2,2,4-trimethylpentane C8H18 3.37 trans-3-heptene C7H14 3.5 cis-3-heptene C7H14 3.52 1,4-heptadiene C7H12 3.67 1,5-heptadiene C7H12 3.75 heptene-1 C7H14 3.86 3-ethyl pentane C7H16 3.97 3-methyl hexane C7H16 4.21 2-methyl-1-hexene C7H14 4.22 1,6-heptadiene C7H12 4.31

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I

tra 1,2-dimethylcyclopentane C7H14 4.35 tra 1,3-dimethylcyclopentane C7H14 4.39 2-methyl hexane C7H16 4.49 cis 1,3-dimethylcyclopentane C7H14 4.56 2,3-dimethylpentane C7H16 4.68 1,1-dimethylcyclopentane C7H14 5.12 3,3-dimethylpentane C7H16 5.58 2,4-dimethylpentane C7H16 6.63 2,2,3-trimethylbutane C7H16 6.85 2,2-dimethylpentane C7H16 7.06

Table 18 vapour pressures of some hydrocarbons (C7+) relative to n-octane

6. Releases into confined areas.

A release of flammable material into a confined space, such as a building, is potentially an extremely hazardous event. Ignition may lead to the development of over-pressure causing structural damage to the enclosure and neighbouring buildings. Consequently, any events leading to a sustained release of flammable material should not be considered as normal operation but be subject to a detailed consequence analysis.

For the purposes of area classification there is a need to quantify the difference between an event definitely leading to a hazardous condition and an event that is potentially hazardous but which might be managed by precautionary action such as the active control of ignition sources near to the point of handling of flammable material.

A key concern is how to assess the conditions under which a release might escape a building at a flammable concentration and require external ignition prevention precautions.

General guidance for the safe ventilation of building enclosures given in IP 15 is, quite soundly given by dividing activities that may lead to releases into categories requiring different grades of ventilation. Grades of ventilation are then parameterised by the air change rate, a, expressed as the number of times per hour that the air in the building is changed. Four categories are identified:

Adequate To quickly reduce possibly flammable concentrations to safe Ventilation: concentrations in the event of a leak or spillage, and following

action to stop the fluid source, 12 air changes per hour is recommended.

Dilution Ventilation: Forced ventilation at sufficient rate to limit the formation of a gas volume at a concentration of 20% of the Lower Flammability limit is recommended. Typically dilution ventilation will be vigorous (30 - 90 air changes per hour) and the output diverted to vents.

Local Artificial Ventilation:

The use of either small scale dilution ventilation (use of extractors etc.) or an enhancement of flow in obstructed areas to attain adequate ventilation is recommended.

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Overpressure Prevention of the ingress of flammable material to a confined Ventilation: area by maintaining an over-pressure within it is

recommended for cases where buildings are close to potential sources but do not contain sources themselves.

Of these categories we need only to try to quantify the first, "Adequate Ventilation" as applying generally to small spills. The Dilution Ventilation rate needs to be specially designed for each application. Local Artificial Ventilation is essentially the same as Adequate Ventilation for small enclosures within a larger confined workplace, unless the effluent is ducted to a vent in which case it is an example of Dilution Ventilation. Overpressure ventilation precludes the need for assessment.

What does "adequate ventilation" mean in terms of a release? Let V be the ventilated volume. Consider the volume to be well mixed then the concentration within the enclosure

•

of a gaseous flammable material released at time 0 at a constant rate m kg/s is given by a simple mass balance:

d C • p V — = m - a p V C

where a s"1, is the air change rate, p kg/m3, is the density of the flammable gas and C m3/m3

is the volumetric (molar) concentration. This equation has the solution:

C = — ( l - e - ) a p V v '

for initial conditions C = 0 when t = 0.

The maximum concentration (volume fraction) that can occur throughout the well mixed volume is given by:

m

a p V

To estimate some values for the leak rate than would give rise to flammable concentrations in an enclosure we can take some typical values:

For a dense gas:

V = 1000 m3

p = 2 kg/m3

a = 12/3600 = 0.0033 s"1

[0.02,0.10]

corresponding to a building with typical dimension 10 m. density of the flammable gas e.g Cat A. an adequate ventilation rate of 12 changes per hour: a typical flammable limit range, [LFL.UFL] by volume fraction

30

from which we find that m e [ o . l 3 , 0 . 6 6 ] kg/s. A factor 2 variation on these end range values would be a reasonable uncertainty to apply. The time taken to reach 90% of maximum concentration is -700 s.

Thus, to round figures, if the gas is well mixed, a release of s 0.1 kg/s could give rise to potentially flammable conditions through an adequately ventilated building, of volume 1,000 m3, on a time scale of -700 s. This would be an extremely hazardous condition. Larger releases might lead to potential flammable regions outside of the building because the material leaving would be above the lower flammability limit for the gas. Certainly a release as large as = I kg/s, capable of filling the building to the upper flammability limit should not be tolerated.

The basic premise of the ventilation calculation is that the volume should be well mixed. Mixedness will not occur through the ventilation flow itself as the implied ventilation wind speed is very small, of order 33 mm/s assuming a typical building length-scale of 10 m. Some other "stirring" mechanism is needed.

If we consider the pressurised releases treated in section 3, then a release rate of =0.1 kg/s for a category A or B fluid could arise for a hole size as small as 2 mm for pressures below 50 bar. The hazard radius for an unconfined release of this magnitude is = 5 m which is half the characteristic length scale of the example building and the order of magnitude of the free path of a centrally placed release. The hazard radius is also the distance over which the unconfined jet entrains enough air to dilute the mass flow to the LFL. This entrainment is comparable to the circulation in the building and it is thus reasonable to propose that the volume of air passing through the building can be well mixed by a pressurised release.

For a lighter than air gas:

We can rework the above example for the category G(i) and G(ii) gases. We have:

V = 1000 m3 corresponding to a building with typical dimension 10 m.

p = 0.78, 0.29 kg/m3 corresponding to a category G(i) and G(ii) gases respectively.

a = 12/3600 = 0.0033 s"1 an adequate ventilation rate of 12 changes per hour:

C L F L = 0.046, 0.04 m3/m3 LFL for category G(i) and G(ii) gases respectively.

giving m = 0.12 , 0.04 kg/s for category G(i) and G(ii) fluids respectively.

From these simple arguments we can deduce that quite small pressurised releases of the type used to underpin the hazard radius recommendations for unconfined releases and of size ^ 0 . 1 kg/s could result in the establishment of a flammable mixture within a 1000 m3

building having a ventilation rate of 12 air changes per hour for Category A and G(i) fluids, and a somewhat lower value of = 0.04 kg/s for category G(ii) fluid.

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External Hazards

The implication of this is that there may be an efflux of flammable material from the buildings V V

for releases of order O l y ^ ^ kgls f o r category A and G(i); k 9 / s f o r category G(ii)

gases where V is the volume adequately ventilated. The fate of this material needs to be assessed.

Flow and mixing around buildings

Flow and dispersion are strongly influenced by the building shape and orientation to the wind. Buildings are classified as bluff rather than streamlined bodies and this greatly complicates the description of the flow. Consequently here is a large body of work reporting the study of flow and dispersion in building wakes.(e.g. Castro & Robins, 1977; Hosker, 1979; Britter, Hunt & Puttock, 1976; Snyder and Lawson, 1994).

The main feature of a bluff rather than a streamlined body is that the wind flow cannot pass smoothly around the body but instead separates from the upwind edges of the body. This forms a region along the sides and behind the body in which pressure is reduced. The pressure difference across this region causes the separated flow from each edge and side of the body to curve towards each other and eventually intercept to form a recirculation region behind the body in which the mean flow is actually reversing as fluid is returned towards the low pressure regions. Depending on the shape of the body this region can extend back up the building sides to the building front. The flow within the recirculation region is highly time varying and unsteady.

It was originally thought that the recirculation region was a closed "bubble" bounded by a separation streamline and that material was transported in and out of the region by turbulent mixing across this boundary. This remains a useful simplification of the flow but it is now known that transfer of material in and out of the recirculation region is not limited to turbulent transport across the boundary. Material is also advected into the region along entering mean streamlines and advected out via vertical spiral vortices or through entrainment by horseshoe vortices. The entire recirculation region may even collapse intermittently causing all the contents to be flushed downstream.

There is no simple and accurate way of assessing the fate of material released into the wake of a building. Dispersion models in regulatory use (ADMS, ISC, AERMOD) recognise that significant mixing takes place in a building wake and this can reduce concentrations downwind of the building by up to an order of magnitude compared with an unconfined release. The models do not attempt to describe events in the near wake and treat this as a well-stirred region of constant concentration i.e. any material leaving the building is predicted to undergo a step change in concentration.

The models should not be used closer to the building than 3-5 times the extent of the recirculation zone. Typically the extent of the recirculation zone for squat shaped buildings scales as the building height and for tall thin buildings it scales as the building width. For a building of characteristic dimension 10 m this imposes an a region of modelling uncertainty of 30-50 m. This is of greater extent than the present guidelines for the external hazard radii.

An alternative to regulatory models is to use experimentation or Computational Fluid Dynamics to explore some release scenarios. The disadvantage, in addition to that of cost, is that generalisation of the results is difficult. This particular problem is also difficult to solve with computational methods because of the physical complexity of the problem and the requirement for validation.

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What can we say about the external hazard zone?

i) An external hazard zone will only exist if the release within the building is sufficiently large to raise concentrations within the building above the LFL. For a building of size ~ 1000 m3 this implies pressurised releases have to exceed ~ 0.1 kg/s and assumes that the releases take place within the body of the building.

ii) The efflux from the building will comprise a flow of volume aV ~ 3.33 m3/s for a building of size ~ 1000 m3 and with adequate ventilation and take place through the normal ventilation openings in the building fabric.

iii) The efflux is a relatively small flow and will be mixed into the recirculation zone behind the building in the presence of wind. This mixing process is a result of highly unsteady flow and is wind speed and wind direction dependent but is highly effective.

iv) Hazardous areas will be restricted to the immediate vicinity of vent openings and the building fabric.

If releases are significantly larger than 0.1 kg/s then a greater hazard will pertain.

For slightly higher flow rates then, if the efflux buoyancy (positive or negative) is not negligibly small then it will influence the dynamics of the ventilation flow. Specifically it will preferentially direct hazards to roof or floor and enhance ventilation and if the buoyancy driven outflow exceeds the ventilation flow a counter flow (additional ventilation) will be induced.

For much larger flow rates then specific cases need to be considered. Containment and building effects will keep external concentrations low but events that create an opening in the fabric of the building, such as opening a door, that make greater ventilation possible would lead to the outflow of flammable material presenting a localised hazard.

Recommendations:

Enclosure of facilities handling flammable materials should be subject to a hazard assessment to determine the level of adequate ventilation.

The adequate ventilation recommended in the existing guidance is appropriate to pressurised releases of Cat A, G(i) fluid of up to ~ 0.1 kg/s for a 1000 m3 building provided that the reaction time to respond to a leak is less than ~ 700s.

For an adequately ventilated building the hazard zone is confined to the immediate building fabric. Escaping vapour will be rapidly entrained and mixed into the recirculation zone behind the building. This is a region of unsteady but average reverse flow toward the building at ground level and can be considered to extend to a dimension given by the smaller of the building height or width. Conventional dispersion models are only accurate at about 3 -5 times this distance for neutrally buoyant releases.

If ventilation is not adequate then hazard zones will extend at ground level or roof level depending on the buoyancy of the release. Buoyancy driven flow around vents/openings will probably enhance the ventilation flow.

33

If ventilation is far from adequate then changing the open area of the building may result in a large volume of potentially flammable material.

The present guidance is conservative as far as shape factors are concerned but should relate the size of release and the volume of the building in some more explicit way.

Notes:

The case where a release takes place in a doorway arose in discussion and, by implication, exists in the present guidelines. The ventilation rate for structures with open doorways will probably be higher than the 12 changes per hour considered as adequate ventilation.

7. Discussion and Conclusions

In this work we have used tools, developed to investigate the consequence of major releases, to quantify appropriate guidance for the Area Classification of Petroleum Installations. The quantification of hazard necessarily starts with specifying the type of material and the size of release which is very much unknown in the case of small spills and leaks. Material types have been simulated using 5 example fluid compositions coded (A, B, C, G(i) and G(ii) following earlier work to update IP 15 and published as an addendum a "Risk Based Approach to Area Classification". That work utilised some very large flow rates that certainly exceeded any definition of normal spillage. The release rate values used here necessarily represent the lower end of the "hazardous release" scale. These should be larger than arise in normal handling and certainly should not be taken as indicative of the magnitude of "acceptable" spills. In all circumstances the potential for spills to occur should be rigorously assessed and a full hazard assessment carried out where necessary.

It has generally been possible to defend the key recommendations of IP 15 as conservative. Where revisions are recommended these are strongly dependent on scenario and fluid type.

The major findings of the study are that:

• The hazard radii for pressurised releases of category B and C fluids should be derived assuming a mechanically generated flammable mist; previously gaseous releases were assumed. The Hazard radii for category B and C fluids are increased relative to previous guidance.

• Numeric flammability limits published in Annex D of "A Risk-Based Approach to Hazardous Area Classification" for the category B and C fluids have been updated to take account of the composition of the flammable mist; previously low vapour components were assumed to rain-out and not contribute to the lower flammability limit evaluation.

• Shape factors for pressurised releases are revised to take better account of the role of initial jet momentum on the jet trajectory. In particular the lighter than air gases (category G(i) and G(ii) fluids) are found to have qualitatively more similar shape factors to the two-phase category A, B and C releases; previously buoyancy was assumed to dominate their dispersion.

• Hazard radii for discharges from vents are evaluated. The hazard radius varies from slightly smaller to slightly larger than that in the existing guidance depending on the properties of the vented vapour.

• The composition of vapour from vents on storage facilities maintained at atmospheric pressure may be variable (in composition, density and flammability) and the user of the new guidance should be aware of the effect of this variability because of the consequence for hazard radii.

34

• The example range of venting rate and vent sizes used in the guidance are not wholly consistent with the assumption that the discharge takes place at atmospheric pressure. The relationship between venting rate and pressure of discharge is investigated and a value of 300 mb suggested as a threshold above which the consequences of pressure should be assessed. This is of significance for multicomponent fluids where condensation may occur.

• The existing guidance for liquid spillages is conservative, judged by the volatility of the model category C fluid. The guidance is applicable to materials with approximately twice the vapour generation rate of category C fluid under the specimen conditions. Relative vapour pressures for some common hydrocarbon compounds are listed.

• The existing guidance for sumps is conservative, judged by the volatility of the model category C fluid.

• Vapour generation at the source of spillage of Category C fluids is a potential hazard dictated by the spill rate and conditions and not the rate of evaporation of the liquid pool. The new guidance should emphasise the role of release conditions in determining the initial vapour generation from spills of category C fluids.

• For releases into confined areas the relative size of spillage and building are of key importance. The classification "Adequate Ventilation" has been assessed with respect to these parameters.

The major uncertainties remaining are:

• The role of liquid rain-out in influencing flammability hazards for releases from low (~ 5 bar) processes.

• How properly to advise the effect of different fluid compositions in hazards. • How to properly balance guidance for area classification and necessary procedures for

hazard assessment.

8. References

A Risk-Based Approach to Hazardous Area Classification, Institute of Petroleum, November 1998.

Shell FRED 3.1 User and Technical Manual, Shell Global Solutions, (2000).

HGSYSTEM v3.0; Technical Reference. Shell Global Solutions (1996) available in electronic form (pdf) at http://www.hgsystem.com/.

Johnson D.W. and Woodward J.L.; Release: a model with data to predict Aerosol Rainout in Accidental Releases. AlChE. (1999).

Ramsdale S., Tickle G. Review of Release rain-out model and the Center for Chemical Process Safety (CCPS) data, Contract Research report 277/2000, Publ. HSE Books.

Lees F. Loss prevention in the Process Industries, ed 2. Publ. Butterworth/Heinemann, 1996.

Bearman P.W., 1972, "Some Recent Measurements of the Flow Around Bluff Bodies in Smooth and Turbulent Streams", Paper presented at a synopsium on external flows, University of Bristol.

35

http://www.hgsystem.com/

Britter, R E. Hunt, J.C.R. and Puttock, J.S., 1976, "Predicting Pollution Concentrations Near Buildings and Hills." Presented at the Institution of Measurements and Control Conference on Systems and Models in Air and Water Pollution, London, Sep 22-24, pp 7-1 to 7-15.

Castro, I.P., & Robins, A.G., 1977, "The Flow Around a Surface-Mounted Cube in Uniform and Turbulent Streams", J. Fluid Mech., vol. 79, part 2, pp. 307-335.

Hosker, R.P., 1979, "Empirical Estimation of Wake Cavity Size Behind Block Type Structures." In the reprints of Fourth Symposium on Turbulence, Diffusion and Air Pollution, Reno, NV, Jan. 15-18, pp 603-609.

Snyder, W.H. and Lawson, R.E., Jr., 1994, "Wind tunnel measurements of flow fields in the

vicinity of buildings", Reprint vol.: 8th AMS Conf. on Appl. Air Poll. Meteor., with AWMA, Jan.

23-28, Nashville, TN.

36

Appendix A: Methodology

A1. Calculating Flammability limits.

The flammability limit of a mixture, L (%v), should therefore be computed by first using Le Chatelier's law to establish the volumetric flammability limit of a vapour only mixture:

Where y denotes the mol fraction of the species i and U the species vapour phase flammability limit. This flammability limit can then be converted to a mass-based criteria -most simply by the assumption of an ideal gas whereby the mixture density is given by:

p

Pmix k a / m 3

where Mi are the species molecular weights, P the pressure, R the ideal gas constant and T the temperature.

The mass based limit is.

L . L m = P m i x ^ kg/m

The calculation procedure for flammability limits is embodied in the Shell FRED 3.0 (and higher) software tool available from Shell Global Solutions.

A2. Calculating hazard radii for pressurised releases.

Hazard radii need to be calculated using a multicomponent and multi-phase dispersion model. Shell Global Solutions has developed such a capability as part of HGSYSTEM which is a set of dispersion models, developed at the specification of industry consortia, and made publicly available via the internet. HGSYSTEM has been used as a reference model in model evaluation exercises and the model components set a standard against which commercial models are tested. Model vendors should be able to relate the performance and physical basis of their models to HGSYSTEM.

The calculations used the fluid properties given in Table 2 of the main report and the following conditions:

Storage Temperature - 20 °C Ambient Temperature - 30 °C Wind Speed - 2 m/s Stability Class - D Roughness - 0.03 m Horizontal Release @ 5 m for R^ Horizontal Release @ 1 m for R2

37

Of these the surface roughness length 0.03 m has been increased from the value of 0.003 m used in the original guidance. The 2-phase jet calculations used for the hazard radii are not sensitive to this parameter so the change is semantic rather than critical. A value of 0.003 m is appropriate to extremely flat terrain with even and low lying vegetation. This is not characteristic of process sites where values as large as 0.3 m could in some circumstances be justified. The choice of 0.03 m is at the lower end of realistic values for process sites and is conservative. The role of roughness length in dispersion models is to characterise the ambient turbulence. This is the dominant mixing mechanism for low-momentum releases.

For convenience, and following the methodology in the previous update to IP 15, the discharge rates were calculated using the Shell Global Solutions Generalised Release Model (GENREL) incorporated in FRED 3.1 and setting pipe friction to be negligible. The HGSYSTEM model AEROPLUME has a simpler release model and, for two phase releases, generates a range of possible mass flow rates for a set of input conditions rather than a single value. The GENREL and AEROPLUME suggested rates are, for most mixtures, comparable.

The hazard radii were calculated using the two-phase jet dispersion model AEROPLUME. AEROPLUME calculates cross-sectional average concentrations. The hazard radius Ri should be taken as the distance at which the average concentration has fallen to a value of 0.7 LFL. The hazard radius R2 should be taken as the distance at which the average concentration has fallen to a value of 0.5 LFL. This arises because the hazard radius is evaluated with respect to the maximum concentration in the jet and the relationship between the maximum and average jet concentrations depends upon the shape of the concentration profile.

Within FRED 3.1 the AEROPLUME results are automatically processed to take account of the concentration profiles and give hazard radii directly and in a manner transparent to the user. We note that a key result of this study, that a mass-based flammability limit should be used for two-phase flow calculations; is not implemented in the post-processor tools supplied with HGSYSTEM 3.0. The post-processor PROFILE from v 3.0 should not be used for high flash-point fluids.

A3. Calculating Hazard Radii for vents.

Ambient conditions for calculating the hazard radii for vents were the same as for the pressurised releases. The key feature of the vent flows is that they are assumed to be ideal gases, i.e. no account is taken of the potential for two phase flow.

The dispersion calculations were carried out using AEROPLUME assuming a vertical release. The hazard radius was defined as:

R = V ( ( Z - H ) 2 + X 2 )

where H was the assumed vent height of 10 m and Z, X the vertical and downstream co-ordinates of the point of maximum concentration in the jet equal to the flammability limit. This is the centroid position of the jet calculated by AEROPLUME with an average concentration equal to 0.7 LFL.

38

A4. Calculating pool evaporation and dispersion

The pool evaporation calculations were carried out using the liquid spills model LPOOL which is part of HGSYSTEM. LPOOL is an implementation of the ExxonMobil Liquid Spills Model LSM90. It takes account of the multicomponent evaporation of material either spilled into a bund or onto an unconstrained surface.

The basis of pool evaporation models is set out in Lees (1996). The evaporation rate is based on a forced convection analogy and increases with wind speed and with pool area. For non-boiling pools the evaporation rate increases with the vapour pressure of the spilled liquid. A pool model adds a liquid spread, a heat balance and a component balance to the base evaporation correlation.

For consistency with the other hazard radii a wind speed of 2 m/s wind speed and an ambient temperature of 30 °C was used. To enhance the evaporation rate further, maximum insolation (spill at mid-day, cloudless sky) was assumed. The insolation factors in LPOOL are conservative (high) for mid-latitudes.

As noted in the text, pool evaporation is a transient process that gives rise to a time-varying gas cloud. In particular we found that the highest evaporation rates came during the first moments of the spill when the lightest components were evaporating.

Calculations made using the HGSYSTEM unsteady dense gas dispersion model HEGADAS-T showed that the vapour cloud dispersed extremely quickly and, with the target conditions, it was not possible to attain the hazard radii in the previous guidance.

We then carried out a sensitivity test with the HGSYSTEM steady state dense gas dispersion model HEGADAS-S to determine what continuous release rates of dense gas would be consistent with the existing guidance. We found, as given in the main text, that these rates are higher than the vapour rates for category C spills.

39

Appendix B: Preliminary Investigation of Hazard Radii and Shape Factors

for the revision of IP15: the Area Classification Code for Petroleum Installations.

Summary

As part of the revision of the Institute of Petroleum publication "Area Classification Code for Petroleum Installations", more commonly known as IP 15, dispersion calculations were carried out to cross check tables D3.2 and D3.3 of the publication "A risk-based Approach to Hazardous Area Classification".

The objectives of revisiting the calculations were twofold:

• To provide a subset of results more in line with petroleum installation needs. • To verify shape factors for releases - especially those representing a refinery hydrogen

stream.

In repeating the calculations it was noted that the usual practice of specifying flammability limits in terms of volumetric concentrations for gases leads to possibly incorrect and too low estimates of hazard distances for class B and class C liquids. These are materials that have high flash points at ambient conditions but may be placed under high pressure when pumped from one location to another. The leak of high pressure liquid may give rise to a flammable mist or, in any event, lead to the jetting of potentially flammable liquid over distances much greater than those given in the present IP 15 guidance.

This note describes the revised calculations and discusses the problem of how to assess the hazard range for high flash-point fluids. The calculations in this report were carried out with the HGSYSTEM models DATAPROP, AEROPLUME and PROFILE to facilitate the presentation of the many results. These models are equivalent to the components of the FRED model version 2.3.

As a result of this work the definition of flammable limit used in deriving contours with the software package FRED has been changed. FRED versions 3.0 and higher are compatible with these results.

B1. Introduction

Hazardous area classification requires that an assessment be made of the extent of the zone enclosing an operational area where flammable materials are handled and where small leaks might occur unnoticed for a short period of time. With this knowledge precautionary steps can be taken to reduce the potential for ignition, for example by placing constraints on electrical installations.

The Institute of Petroleum (IP) publication: "A risk-based Approach to Hazardous Area Classification" adopted (in Annex D) tables of hazardous distances from an internal Shell source, which contained calculations made with the FRED dispersion model, version 2.1, that were appropriate to an exploration and production environment where high pressures and hence high flow rates are the norm. These tables included events that would only arise as a result of mechanical damage on a petroleum refinery and which would in any case demand an emergency response. Including these events as part of advice on Area

40

Classification gives a misleading impression of the hazard distance to be associated with normal operation.

This work was intended to rework the content of current IP publication advice and verify that the shape factors (Figure 6.2 and Figure 6.3 of IP 15) for dense and buoyant gas releases were realistic. It was expected that the advice would be essentially unchanged. However, It was noticed that the published data gave very small (more than ten times smaller) hazard distances for the Class Cvi fluid, compared with the Class A fluid; for example:

Release Conditions Hazard radius (m) Pressure (bar) Diameter (mm) category A category C

100 10 39 3

Table B.1 Extract from Annex D of "A Risk Based Approach to Hazardous Area Classification"

Physical intuition suggests that, to take the example above, a the hazard radius of 3 m is far too small compared with the likely "throw" of a non-volatile but flammable liquid jet driven by a 100 bar pressure through a 10 mm hole. There have been discussions and project proposals to investigate the "throw" of water jets for area clarification and electrical safety purposes and unpublished work carried out for ICI has shown that small holes and low drive pressures can effectively transport material well beyond the present category C fluid guideline distances.

Pressurised releases can also atomise material with a high flash-point to give rise to mists that are flammable. The flammability limits for fine aerosols are akin to those for vapours although there is a paucity of informational data in the literature. It is believed that, in general, the flammability limit of fine aerosols, when expressed as the mass of fuel per unit volume, is similar to the flammability of the vapour alone (Lees, F.J. Loss Prevention in the Process Industries, 2nd ed., vol. 2, section 16.4.3). This is significant because it is common practice is to use volumetric units to express flammability limits for gases and for mixtures of gases. For a given mass concentration the equivalent volume fraction is substantially reduced in the presence of a small liquid component because of the very large density difference between the two phases. The use of a volumetric criterion for evaluating the hazard distance may therefore be misleading when two-phase mixtures are involved and, worse, give too small an estimate of the hazard distance.

In this note we compare the use of mass and volume based flammability criteria for establishing hazard zones on the assumption that releases of high flashpoint material will form a flammable mist and hence that their hazard can be addressed in a comparable manner to gaseous and low flash-point 2-phase releases. The issue remains of whether more coherent jets exist and are able to deliver flammable liquid to an even more distant target but we do not attempt to quantify this here.

We should also comment upon the mitigating effect of liquid droplet rainout from jet releases. For high flashpoint liquids that are atomised and ejected as jets the process of droplet collision will lead to a growth in the droplet size with time and the subsequent deposition of liquid. An extensive body of work has been carried out for single component fluids by the American Institute of Chemical Engineers (AlChE) and a release model developed to predict the loss of liquid from such events (Johnson, D.W., Woodward, J.L., Release: A model with data to predict Aerosol Rainout in Accidental Releases, AlChE, 1999). For area classification purposes we are considering material transport over very short distances and with transit times of a few seconds and over trajectories that are

vi Model fluid compositions are given in Table B2.

41

determined almost entirely by the initial jet momentum and orientation. To give conservative estimates of the hazard zone it is reasonable to assume that liquid droplet rainout in the vicinity of the source is a second order effect. For the larger releases that appear in risk assessment scenarios where fluid transport times of several minutes may occur, the AlChE work should be reviewed to see if accounting for droplet rainout makes a substantial difference to model predictions.

B2. Calculation Procedure.

Of necessity only a limited number of variables can be considered in this investigation. The area classification process is NOT a full hazard assessment which takes account of the range of meteorological and process conditions, release events etc. Accordingly a single set of release conditions was chosen and these are the same as those used in the IP addendum "A risk-based Approach to Hazardous Area Classification" with the exception that the surface roughness length has been increased by a factor of 10 from an unreasonable overland value to a value that is still low (by a factor of 10) compared to that used for process sites. This has no significant effect on any of the calculations but makes for a more physically realistic input data set.

The fluid compositions used are given in Table B2 and the base calculation conditions in Table B3. The two parameters in Table B3 that were varied to investigate the shape of the hazardous zone were the release orientation and the release height.

A reduced set of operating conditions were assumed. Four drive pressures (100, 50, 10, 5) bara. and five hole sizes (1, 2, 5, 10 , 20) mm (as diameters). The shape factors for the releases were calculated using the largest hole size and the greatest drive pressure.

42

Stream Component (mol perc)

Cat. A Cat. B Cat. C Cat. G (I)

Cat. G (ii)

Comp. LFL

MW Boiling point 0

C N2 Nitrogen 0 0 0 2 2 - 28.01 -196 Ct Methane 0 4 0 88.45 10 5.3 16.04 -161 C2 Ethane 0 0 0 4.5 3 3 30.07 -87 C3 Propane 70 6 1 3 3 2.2 44.09 -42 C4 Butane 30 7 1 1 1 1.86 58.12 -1 C5 Ethane 0 9 2 1 0 1.50 72.15 36 C6 Hexane 0 11 3 0 0 1.2 86.17 69 C7 Heptane 0 16 3 0 0 1.2 100.20 98 C8 Octane 0 22 27 0 0 0.95 114.23 126 C9 Nonane 0 0 25 0 0 0.83 128.26 151 Cm Decane 0 25 38 0 0 0.77 142.28 173 H 2 0 Water 0 0 0 0.05 0 - 18.02 100 C0 2 0 0 0 0 1 - 44.01 -78(sub) Hydrogen 0 0 0 0 80 4.1 2.02 -253 Average MW

48.3 100.06 125.03 18.74 7.03

LFL (vol%) 2.09 1.70 1.52 4.82 4.03

Table B.2 - Definition of category Fluids and their properties. The vapour phase mixture flammability limits are taken from the IP addendum publication and were evaluated using Le Chatelier's Law.

Standard conditions Base Case values Range of values Ambient temperature 20 ° C Relative humidity 7 0 % Wind speed 2 m/s Reference Height 10.0 m Stability class D Surface roughness 0.03 m Sample time 18.75 s (~ instantaneous) Release height 1.0 m 0 to 15 m Reservoir Temperature 20 ° C Release angle (relative to horizontal) 0° -30 0 to 900

Table B.3 - Base parameters for the calculation

Release rates for the five test fluids were evaluated using the Generalised Release Model from FRED 2.3 and assuming negligible friction losses from any pipe runs. The release rates calculated in this way have proved more consistent than the default release rate used by the Aeroplume model which is a literature correlation for two phase flow. Aeroplume also has a built in two phase discharge mode, used for advice purposes under normal program execution, and this usually agrees to within a few percent of the Generalised Release Model predictions.

As expected it was found that the release mass flux per unit area was hole size independent for the release conditions and the release rates are plotted as a function of hole size and pressure for each of the fluids in Figure B2. The results are straightforward except for category A fluid at the lowest drive pressure. Here the standardised 5 bar drive pressure at ambient temperature is below the saturated vapour pressure of propane and above the

43

bubble point of propanevii. This means that in a vessel the category A vessel would exist as discrete two phases giving three discharge scenarios: a vapour only release of a propane rich vapour; a liquid only release of a butane rich butane/propane mix and a two phase release; all depending on the location of the release point. Any significant length of piping between the storage vessel and the release would favour a two phase release because dissolved propane would vaporise in the pipe as the pressure decreases. We have therefore carried out two sets of calculations for this low pressure scenario. A high mass rate discharge of a 45:55 propane/butane mixture corresponding to the liquid phase composition of the 70:30 propane/butane total mixture and a low mass rate discharge of the original 70:30 propane/butane mixture as a 2 phase release. Figure B1 shows mass flow rates for both circumstances.

Release Rate as a function of hole size and drive pressure Category A Fluid

10

-52 05 JSC <1)

+-I ro * 1 0) c/> ro o> o 0£

0.1

0 2 4 6 8 10 12 14 16 18 20

Hole size (dia.), mm

Figure B.1: Release rate as a funct ion of hole size and drive pressure for each of the five condit ions, category A fluid.

—•• » 100 bar —••

50 bar 10 bar

5 bar - l iqu id C U'.- o

50 bar 10 bar


—

50 bar 10 bar


—

* * " *

pi i aoc * 9 *

, * *

—4 *

+—

* J

» 0 • t

h li It

v" In the Addendum to IP 15 it was recognised but not explicitly stated that the 5 bar scenario was ambiguous. The approach there was to decrease the ambient temperature for this run and hence the saturation vapour pressure of propane.

44

Release Rate as a function of hole size and drive pressure Category B Fluid

0 2 4 6 8 10 12 14 16 18 20


Release Rate as a function of hole size and drive pressure Category C Fluid

0 2 4 6 8 10 12 14 16 18 20

Hole size (dia.), mm Figure B.2: Release rate as a funct ion of hole size and drive pressure for each of the condit ions, categories B and C fluids.

45

Release Rate as a function of hole size and drive pressure Category G(i) Fluid

10

O) •SC <1) TO * 1 0 </> to 0 0 a:

0.1

100 bar mmm

50 bar — • 1 u 5

Udl bar

1 u 5

Udl bar

• • • •

4

/ f

/ f y 0 0 0

V • • * *

f ~ t

/ §

0 2 4 6 8 10 12 14 16 18 20


Release Rate as a function of hole size and drive pressure Category G(ii) Fluid

10

O) Jtf 0 ro a: 1 0 </> CD

0 tr

0.1

100 bar 50 bar 1 u 5

uar bar

1 u 5

uar bar —

4 / *

— 4 fi— , *

0 • • / 0

0 2 4 6 8 10 12 14 16 18 20

Hole size (dia.), mm Figure B.3 - Release rate as a funct ion of hole size and drive pressure for each of the condit ions, categories G(i) and G(ii) f luids.

46

The AEROPLUME model was run using automatically generated input files for the different operating conditions. AEROPLUME outputs a cross-sectional average jet concentration. To derive concentration contours the output of the AEROPLUME model can be post-processed using the PROFILE program. This superposes a Gaussian profile upon the AEROPLUME results and constructs the required contour. This is the normal operating procedure in the FRED model where the results are automatically displayed as a graphic and the underlying calculations are not visible to the user.

It is important to note that PROFILE assumes a ground-reflected Gaussian distribution which means that the relationship between the maximum plume concentration and the average plume concentration depends upon the height of the plume above ground. Where we are just interested in the furthest extent of the maximum LFL concentration from the source we can estimate this directly from AEROPLUME by calculating the distance to a slightly smaller concentration. The ratios are shown in Table B.4.

Peak Concentration Averaged Concentration Elevated Plume LFL 0.7 LFL Grounded Plume LFL 0.5 LFL

Table B.4 : Equivalence between peak and averaged concentrations in the HGSYSTEM and FRED models.

B3. Effect of using mass and concentration based LFL criteria:

The mass equivalent LFL was derived from the vapour properties in Table B.2 by multiplying the vapour phase volume fraction by the vapour phase material density assuming an ideal gas, one atmosphere pressure and the ambient temperature.

The downwind LFL position on a mass and on a volume basis was determined by linear interpolation of distance between bounding entries in the AEROPLUME output file and using the ratios given in Table B.4. This is adequate for the purposes of this study.

Figure B.4 shows a composite of all results for a 1 m horizontal release. The distance to LFL calculated using the mass concentration and the distance to LFL using the volume fraction criteria are plotted against each other.

• For the two gas mixtures (category G(i) and G(ii) ) the flammability limits equivalent and an identity results.

In the presence of a liquid phase, volumetric concentrations underestimate the amount of flammable material present because the contribution of liquid to volume is negligible. We see:

• For the low flashpoint material (fluid A) sufficient liquid has evaporated for the jet to be wholly gaseous at LFL so the two flammability definitions and calculated LFL distances are equivalent

• For fluid category B the volume based definition noticeably understates the hazard distance by ~ 30%.

• For fluid category C there is almost an order of magnitude difference in the hazard radius.

47

Influence of fluid type on LFL definition

0.1 1 10

Distance to LFL using mass concentration

100

Figure B.4 : Effect of using mass concentrat ion and volume concentrat ion definit ions on distance to LFL for the five f luids.

This is a significant finding. It suggests that using volume based flammability criteria to derive distances to LFL may lead to underestimating the hazard of two-phase releases.

To illustrate this more clearly on a linear scale. Figure B.5 shows some example shape factor calculations for the category B fluid. An elevated release (15 m height) is assumed and calculations carried out for different release directions. The resulting overlapping cigar-shaped contours are results that would be obtained by following normal practice and using volumetric limits (e.g. with the FRED 2.3 model). The points show where the maximum distance to LFL occurs using the mass based criterion.

48

IsoConcentration contours for CATB fluid, 100 bara, 20 mm hole Releases at different inclinations

50

40

30 E

f 20 x

10

0

0 10 20 30 40 50

Downwind distance, m

Figure B.5 - Visual Example of the difference in using volumetric (contours) and mass concentrat ion definit ions (points) for the LFL of a category B fluid.

B.4 Shape factors

Shape factors for the hazardous area are based on calculations of jet dispersion in different directions and the existing classification advice implicitly assumes that, because of the effects of body forces, the shape factor for buoyant releases is different to that for dense gas releases.

This work shows that the hazard radius for the different fluids is determined by jet momentum rather than body forces, but that there is an effect of the ground on entrainment which is different for dense and for buoyant releases.

Figure B.6 shows the shape factor for a category A fluid released from a 1 m height (maximum flow rates are used in all of these calculations). It is assumed that the wind direction is from the left of the picture. For a category A fluid there is no effect of liquid load on the calculation procedure and we have included both the full iso-concentration contours as would be obtained using the FRED model and the points derived from the AEROPLUME model using the mass concentration definition of LFL.

For releases that do not touch the ground the hazard range is essentially a flattened circular locus. An increasing wind speed would flatten the shape further by decreasing the vertical penetration of the jet because the contribution to entrainment from cross-wind mixing is increased.

49

For releases that do touch the ground the dispersion distance is increased on two counts:

• Within the assumptions of the jet model, the overall entrainment is reduced. Initially the perimeter area of the jet is reduced by the physical presence of the ground and then, as it spreads out over the ground and slows down, the entrainment is inhibited by the density difference between the plume and the air.

• The reflected Gaussian shape of the concentration distribution means that the ratio of the peak concentration to the average concentration within the jet changes when the jet hits the ground. For an elevated plume the peak concentration is -1.42 times greater than the average concentration across the plume and occurs on the plume centroid. For a grounded plume the peak concentration is ~ 2 times the averaged concentration and occurs at the ground. The locus of points from AEROPLUME shown in Figure B.6 reflect this assumption.

IsoConcentration contours for CATA fluid, 100 bara, 20 mm hole Releases at different inclinations

0 20 40 60 80 100


Figure B.6 : LFL isopleths for a category A f luid released at angles f rom -20 to 90 degrees to the horizontal and centroid posit ion of LFL from Aeroplume (points). The ground level LFL concentrat ion posit ion f rom Aeroplume for the three lowest trajectory releases are shown as vertical bars.

These two factors substantially increase the hazard distance for horizontal releases near to the ground. This methodology is conservative because extra mixing caused by the friction between the jet and the ground is neglected. Also it results in a step change increase in hazard radius for releases that only just touch the ground as shown in Figure B.7 below.

50

IsoConcentration contours for CATA fluid, 100 bara, 20 mm hole Releases at different inclinations

50

40

30 E

x

10

0

0 20 40 60 80 100


Figure B.7 : Shape factor for an elevated release of category A f luid

Figure B.7 shows the effect of increasing the release height to 15 m. Calculations to the required concentrations can only be made for release angles greater than -20 degrees with the present version of the AEROPLUME model which does not model steep impacts of high momentum jets. We do not expect these steeper impacts to lead to greater dispersion distances as the impact will lead to the generation of additional turbulence which will increase the mixing rate.

Figure B.8 shows the shape factor for the release of category G(i) fluid from a height of 1 m We find that the distance to LFL along the plume centroid is nearly independent of release angle.

The distance to LFL at the ground for those releases that hit the ground is again greater than that of the elevated jet so that the overall shape factor is similar to that of a dense gas release. The effect of the ground is less dramatic because the spreading fluid is less dense than the air and is convectively unstable. This means that entrainment into the grounded lighter than air gas is greater than it would be for a dense gas. Consequently the hazard distance at the ground is not as great.

i i i Values from Aeroplume O

Peak Values from Aeroplume

51

IsoConcentrat ion contours for CATG(i) f luid, 100 bara, 20 mm Releases at 90, 70, 50, 30, 10. 0.-10 degrees from the horizontal

20

15

E 10 x: D> 0) X

5

0

0 5 10 15 20


Figure B.8 : Shape factor for the category G(i) Fluid. Points are centroid distance to LFL and the vertical bars show the ground level distance to LFL for the two lowest trajectory releases.

The category G(ii) fluid is markedly more buoyant than the category G(i) fluid and to investigate whether the extra buoyancy significantly modifies the shape factor these have been derived for release heights of 1m, 5 m, 10 m and 15 m.

Figure B.9 shows that, for a 1 m release height the results are qualitatively very similar to those for a category G(i) fluid. The grounded jets entrain air more efficiently because the flow is more unstable and there is less enhancement of the distance to LFL at the ground..

Increasing the release height to 5 m barely alters the shape factor. As the release height is increased to 10 m the basic shape is still maintained with a downward directed jet at -30 degrees hitting the ground before diluting to LFL. The ground footprint is very similar to that for the 1 m release. Increasing the release height to 15 m again results in a fan shaped hazard range that is almost symmetric about the release height showing that, in the absence of ground contact, the hazard radius is determined by the jet orientation. This contrasts with Figure 6.3 of the IP 15 publication that suggests that there is a smaller hazard zone below the release point than above it.

52

L F L Concentration contours for CATG(i i ) fluid 100 bara, 20 mm hole

Centroid LFL posit ion from Aeroplume ^ Groundlevel LFL from Aeroplume

Centroid LFL posit ion from Aeroplume ^ Groundlevel LFL from Aeroplume

0 5 10 15 20 25 30 35


LFL Concentrat ion contours for CATG(ii) f luid 100 bara, 20 mm hole

0 5 10 15 20 25 30 35


Release height: 1 m

Release height: 5 m

53

35

30

25

20

E 15

f 10 I

5

0

LFL Concentration contours for CATG(i i ) fluid 100 bara, 20 mm hole

Centroid LFL posit ion f rom Aerop lume^ Groundlevel LFL f rom Aeroplume

Release height: 10 m

5 10 15 20 25


30 35

LFL Concentration contours for CATG(ii) f luid 100 bara, 20 mm hole

Release height: 15 m

5 10 15 20 25


30 35

Figure B.9 : Shape Factors for a release of Cat. G(ii) f luid released from a height of 1 m, 5m , 10 m and 15 m above the ground. Points indicate the plume centroid posit ion at LFL and the vertical bars the ground level distance to LFL for the three lowest trajectories all derived from the Aeroplume model. The isopleths are calculated with the Profile model and are equivalent to the output f rom the FRED 2.3 model.

54

B.5 Hazard Radii

The calculations carried out for each of the releases suggest that for elevated jets the shape factor should comprise a semi-spherical hazard zone evaluated as the distance to LFL on the plume centroid for a horizontal jet in the downwind direction. The calculations given in Tables B.5 & B.6 show that the effect of the ground is significant for low level releases of all materials and therefore the shape factor should be extended at the ground.

Distance to LFL at the plume centroid height for a 1 m high horizontal release, m

Fluid Category Pressure 20 mm 10 mm 5mm 2 mm 1mm (bara)

A 100 59.6 27.8 11.1 4.0 2.0 50 59.8 28.0 10.9 3.9 2.0 10 60.2 28.4 10.8 3.4 1.8 5 63.5 29.2 10.9 3.1 1.7

B 100 39.2 17.4 7.0 2.9 1.5 50 39.0 17.2 6.7 2.8 1.4 10 30.0 12.5 5.0 2.1 1.1 5 23.4 8.6 3.9 1.7 0.9

c 100 36.6 16.0 6.6 2.7 1.4 50 36.7 16.0 6.4 2.7 1.4 10 37.0 16.4 5.8 2.5 1.3 5 37.2 16.6 5.3 2.4 1.3

G(i) 100 13.2 6.4 3.3 1.3 0.7 50 8.4 4.2 2.1 0.9 0.4 10 3.5 1.8 0.9 0.4 0.2 5 2.6 1.3 0.7 0.3 0.2

6(H) 100 15.6 9.3 4.9 2.0 1.0 50 12.3 6.8 3.5 1.4 0.7 10 6.3 3.3 1.6 0.7 0.3 5 4.7 2.4 1.2 0.5 0.3

Table B.5 : Summary of Distances to LFL at the plume centroid height for the horizontal release f rom a height of 1 m.

5 5

Distance to the LFL at Ground Level for a 1 m high horizontal release, m.

Fluid Category Pressure 20 mm 10 mm 5mm 2 mm 1mm (bara)

A 100 83.0 39.8 17.1 5.1 2.7 50 83.1 40.2 17.1 4.9 2.6 10 85.5 41.0 17.2 4.1 2.3 5 93.5 42.7 17.3 3.7 2.1

B 100 54.6 25.2 9.9 3.8 1.9 50 54.2 25.0 9.6 3.6 1.9 10 42.0 18.8 6.3 2.7 1.4 5 33.1 13.8 4.8 2.1 1.1

C 100 50.7 23.2 9.1 3.5 1.8 50 50.9 23.4 8.8 3.4 1.8 10 51.0 23.8 8.4 3.1 1.6 5 52.3 24.0 8.3 2.9 1.6

G(i) 100 18.7 9.2 4.4 1.8 0.9 50 12.0 5.7 2.9 1.2 0.6 10 4.9 2.5 1.2 0.5 0.3 5 3.6 1.8 0.9 0.4 0.2

G(ii) 100 21.3 12.8 6.6 2.8 1.4 50 16.7 9.3 4.7 2.0 1.0 10 8.6 4.4 2.3 0.9 0.5 5 6.3 3.2 1.7 0.7 0.4

Table B.6 : Distance to LFL at ground level for the horizontal release from a height of 1 m.

B6. Conclusions

The purpose of this study was to verify shape factors and to cross check tables D3.2 and D3.3. of the publication " A risk-based Approach to Hazardous Area Classification" with a set of conditions more in line with petroleum installation needs.

Dispersion calculations were carried out for 5 types of fluids (Categories A, B, C, G(i) and G(ii)) under a range of pressure and release rate conditions.

We found that:

The usual practice of specifying flammability limits in terms of volumetric concentrations for gases leads to possibly incorrect and too low estimates of hazard distances for class B and class C liquids. These are materials that have high flash points at ambient conditions but may be placed under high pressure when pumped from one location to another. The leak of high pressure liquid may give rise to a flammable mist or, in any event, lead to the jetting of potentially flammable liquid over distances much greater than those given in the present IP 15 guidance.

Shape factors for the hazardous area, based on calculations of jet dispersion in different directions, show that the hazard radius for the different fluids is determined by jet momentum rather than body forces. Consequently, the shape factor for buoyant releases is qualitatively similar to that for dense gas releases.

56

For releases that do not touch the ground, the hazard radius is nearly independent of release angle and the hazard range is essentially a circular locus, slightly flattened at the top owing to the wind. For a buoyant elevated release, the hazard radius is almost symmetric about the release height. This contrasts with Figure 6.3 of the IP15 publication that suggests that there is a smaller hazard zone below the release point than above it.

We recommend that:

Mass based flammability criteria should be used for multiphase releases. The Shell Global Solutions FRED model (version 3.0 and higher) has been modified on this basis.

The shape factor should comprise a semi-spherical hazard zone evaluated as the distance to LFL on the plume centroid height for a horizontal jet in the downwind direction. There should be an extra margin of allowance for grounded jets.

57

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