ACCIDENT SCENARIOS AND TOP EVENTS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

This Project is funded by the European Union

Project implemented by Human Dynamics Consortium

This project is funded by the European Union

Projekat finansira Evropska Unija


Projekat realizuje Human Dynamics Konzorcijum

ACCIDENT SCENARIOS AND TOP EVENTS

Antony ThanosPh.D. Chem. [email protected]



• Risk Analysis Framework

HazardIdentification

HazardIdentification

Consequence Analysis

Consequence Analysis

Accepted Risk

NO

Risk reductionmeasures

END

AccidentScenariosAccidentScenarios

Accident ProbabilityAccident

Probability

RiskAssessment

RiskAssessment

YES



• Scenarios selection No unique approach within EU, as

for rest of Risk Assessment Methodology

Nevertheless, worst case scenarios are almost always required for :

oEmergency Response

oLand Use Planning reason



• Scenarios selection, UK case :

• “representative” set of major accident scenario required

• minimum scenarios list examples referred in Assessment Guides (SRAG) for certain types of establishments (although probabilistic approach)



• Scenarios selection, UK case : (cont.) Example of scenarios list in HSE

SRAG for LPGs



• Scenarios selection, Netherlands case Detailed guidance in Reference

Manual Bevi Risk Assessments, as part of QRA

Step 1 : Sub-selection method (based on TNO selection method)

Screening method (relative ranking)

Basic principle: Identification of “Containment Systems” which contribute most to external risk

Not scenario selection, but Containment systems selection



• Scenarios selection, Netherlands case (cont.) Main criteria :

oEffects (1% lethality) extent out of fence (calculation of consequence required for worst case scenario for Containment) and/or



• Scenarios selection, Netherlands case (cont.) Main criteria : (cont.)

oEstimation of effects based on selection method :

Indication number A (intrinsic hazard)

Q, quantity

O1, O1, O1 factors for

process conditions

G, limit value (10000 kg for flammables)

G

OOOQA 321



• Scenarios selection, Netherlands case (cont.)

oSelection method : (cont.)Selection number S: (hazard level

at location -fence)

n : 2 for toxics,

3 for flammables and explosives

L : distance from fence (at least 8 points

examined)

S>1, candidate Containment Systems for inclusion in QRA

Comparison of S values for various

Containment Systems provides final selection

AL

Sn

100



• Scenarios selection, Netherlands case (cont.)

Sub-selection method

overview



• Scenarios selection, Netherlands case (cont.) Step 2 : Definition of Releases

oReleases to be included for selected Containment System based on tables per equipment type referring also frequency

oExample for gas containers



• Scenarios selection, Netherlands case (cont.) Step 3 : Top events (scenarios)

defined on event trees for releases

oEvent trees included for main cases in Manual

Releases general cut-off limit :

oprobability > 10-9 per year

o1% lethality distance extending outside fence



• Scenarios selection, Cyprus case Deterministic approach Generic minimum list of scenarios

oCatastrophic failure of vessels, tanks, pipes

oRupture of vessel/tank (hole with diameter equal to max pipe connected to tank/vessel), hole 20% of pipe diameter

oSmall leak in vessel tank, pipe (hole diameter 25mm or 50 mm)

Selection method for critical equipment (TNO Purple Book)



• Scenarios selection, a few other EU Member States cases Italy (Hybrid approach)

oDecree for LPGs : Certain scenarios are excluded based on available measures

France (Hybrid approach)

oHigh consequence scenarios must be included in consequence analysis, even for low probability



• Typical release scenarios per equipment type failure : Pipes

oCatastrophic failure (Full Bore Rupture –FBR- or guillotine break)

oPartial failure (hole diameter equivalent to a fraction of pipe diameter, e.g. 20%)



• Typical release scenarios per equipment type failure (cont.) : Pressure vessel (process vessel,

tank, tanker)

oCatastrophic failure: “instantaneous” rupture (complete release of content within short time e.g. 3-5 min)

oMechanical failure : equivalent hole set to e.g. 50 mm

oSmall leakage (e.g. corrosion), smaller hole with equivalent diameter of e.g. 20 mm



• Typical release scenarios per equipment type failure (cont.) : Pressure vessel connected

equipment

oRelease from PSV

oFailure of connecting pipes (as for pipes above)

Pumps/compressors

oRelease from PSV

oLeakage from seal (equivalent small hole diameter set, e.g. 20 mm)



• Typical release scenarios per equipment type failure (cont.) : Atmospheric liquid fuel tanks

o Ignition in floating roof tank (tank fire)

o Ignition of constant roof tank (tank fire)

oFailure of tank with release to dike (bund) of tank and subsequent fire in dike (dike fire)



• Worst case scenarios Although low probability expected,

indispensible for Land Use Planning and Emergency Planning

Worst case releases/scenarios to be provided for the different sections of Plant (type of activities) :

oEach Production Unit

oTank-farm

oMovement facilities (road/rail tanker stations, ports)



• Worst case scenarios (cont.) Worst case releases/scenarios

within sections :

oCatastrophic failure of vessel (process vessel, tank, tanker) with maximum inventory size

oCatastrophic failure of pipe : Full Bore Rupture (FBR)/Guillotine Break) for pipes, especially for movement facilities (import/export pipelines, hoses/loading arms)




within sections :

oFor liquid fuels tanks, fire in :Largest diameter tankDike with largest equivalent

diameter




must take into account :

oDifferent operating conditions (P/T/phase) e.g. :

For liquefied gases piping, worst case is usually expected from liquid phase pipe failure

For LPGs, worst case is usually expected from pure propane compared to butane (due to higher pressure)



• Worst case scenarios (cont.) Worst case scenarios selection

criteria (cont.) :

oDifferent operating conditions (P/T/phase) e.g. (cont.) :

Smaller tank of pressurized ammonia can produce more extended consequences than larger refrigerated ammonia tank




must take into account :

oDifferent substances, e.g. smaller tank of a very toxic substance can produce more extended consequence than a larger tank of a toxic substance

oProximity to site boundaries, especially if vulnerable objects are close



• Worst case scenarios (cont.) Worst case scenarios usual

convention : Only one failure can happen at a certain time

oNo simultaneous accidents expression, e.g. only single tank BLEVE in LPG tank farm at a time

oNo double containment failure, e.g. in refrigerated tanks with secondary containment only primary containment failure is taken into account, if no special reasons are present



• Hazard identification usually specify release expected and not final accident (top event)

• Example : Initial event (release) : failure of

LPG pipeline due to corrosion Top events:

o jet flame

ovapour cloud explosion



• Event Tree and Top Events Logic evolution of potential

outcomes (top event) of an initial event (release) identified

Usually used in categorisation of final accidents (top events) per initial release identified

Scenario evolution parameters (e.g. ignition) produce differences in top events



• Event tree and Top Events (cont.) Example: Gas phase release from

LPG tankPHASE

IGNITION CONFINEMENT TOP EVENT

DIRECT JET FLAME

GAS DELAYED NO CONFINEMENT FLASH FIRE

CONFINEMENT UVCE

NO IGNITION

SAFE DISPERSION



• Why Event Trees? Consequence analysis requires top

events to be identified

Technique in the borderline of hazard identification and consequence analysis



• Consequence analysis framework

Releasescenarios Release

scenarios Accident

typeAccident

typeEvent

trees

Releasequantification

Releasequantification

Hazard

Identification

Release models

Consequenceresults

Consequenceresults

Domino effectsDomino effectsLimits of

consequence analysis

Dispersion models

Fire, Explosion Models



• Main top event categories

Toxicdispersion

Toxicdispersion

ExplosionExplosion

FireFire

Hazardoussubstance

release

Initial event Top event

Toxic effectsToxic effects

OverpressureOverpressure

ThermalRadiationThermal

Radiation

Consequences

FireFire ThermalRadiationThermal

Radiation

Toxicdispersion

Toxicdispersion Toxic effectsToxic effects

FireFire ThermalRadiationThermal

Radiation



• Top events related with thermal radiation “Fire” categories:

Pool fire

FLEVE (fire ball)

Flash fire



• Pool fire Ignition of flammable liquid phase

Liquid fuel tank fire

Main consequenceThermal radiation




• Jet flame Ignition of gas or two-phase

release from pressure vessel

Propane jet flame test





• Fireball, BLEVE (Boiling Liquid Expanding Vapour Explosion)

Rapid release and ignition of a flammable under pressure at temperature higher than its normal boiling point

LPG BLEVE (Crescent City)



Secondary consequences: oFragments (missiles)oOverpressure



• Fireball, BLEVE Mechanism (exposure of tank to fire)

Shell at gas phase collapses due to weakening and in combination to pressure increase. Massive release of tank content. Rapid evaporation and ignition of the whole tank content

GAS PHASELOW HEAT TRANSFER,LOW HEAT CAPACITY,

RAPID INCREASE OF SHELL TEMPERATURE,POSSIBLE FAILURE

LIQUID PHASEHIGH HEAT TRANSFER RATE,

HIGH HEAT CAPACITYRATHER LOW SHELL TEMPERATURE



• Vapour cloud (gas) dispersion Passive (neutral) dispersion

(Gauss) :

oRelease of gas with density equal or higher than air



• Vapour cloud (gas) dispersion (cont.) “Positive” buoyant dispersion :

oRelease of gas at elevated temperature (e.g. flue gas at stack)

oTreated as special case of Gauss models (plume rise)



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion, e.g. liquefied

under pressure gas releases Common characteristic of

substances : Normal Boiling Point (BP) less

than ambient temperature and Pressure higher than ambient



• Vapour cloud (gas) dispersion (cont.) Typical example of heavy gas

dispersion, LPGs :

oPropane BP = - 42 °C

oButane BP = - 0.5 °C

and

ostorage at ambient temperature (high pressure), propane case : T = 17 °C, P = 6,7 barg



• Vapour cloud (gas) dispersion (cont.) Other examples of heavy gas cases

:

oAmmonia BP= -33°C and

Storage at ambient temperature, usually in bullets (high pressure : T = 15°C, P = 6,3 barg) or

Semi-refrigerated storage (T = 0 °C,

P = 3,2 barg), usually in spheres

oPropylene (BP = -47.7 °C) at semi refrigerated storage (T = 6°C, P = 6 barg)



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion case -

Released gas has lower density than air

Why heavy gas dispersion is different ?

oAt release point, pressure reduction occurs (from vessel pressure to atmospheric)



• Vapour cloud (gas) dispersion (cont.) General thermodynamics of heavy

gas dispersion



• Vapour cloud (gas) dispersion (cont.) Why heavy gas dispersion is

different ? (cont).

oGas phase release: Lower pressure incurs lower

temperature of gas (adiabatic expansion, Joule/Thomson effect). Colder gas has density higher than surrounding air (fells to ground).

Additional effect by entrainment of air in expanding gas and condensation of humidity



• Vapour cloud (gas) dispersion (cont.) Why heavy gas dispersion is

different ? (cont).

oLiquid phase release: Reduction of pressure causes

evaporation of liquid to gasEvaporation causes lower

temperature in both gas and liquid (equal to normal boiling point temperature, freezing effect)

Expanding liquid/gas entrains air who is getting cold by boiling, condensing also humidity



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion : Vapour

cloud remains for long distance at ground level

Propane cloudHeavy gas behaviour



• Vapour cloud (gas) dispersion (cont.) Refrigerated gases not considered

in general as producing heavy gas dispersion :

oStorage at cryogenic conditions (close to normal boiling point, atmospheric pressure)

oExamples:Liquefied Natural Gas (LNG)Cryogenic ammonia



• Vapour cloud (gas) dispersion (cont.) Refrigerated gases (cont.)



• Vapour cloud (gas) dispersion (cont.) Effects:

Toxic substances (e.g. HF) : toxic effect via inhalation

Flammables (Flash fire) : Ignition of cloud in area with no confinement (obstacles)

odeaths expected within cloud limits where ignition is possible (LFL-HFL), due to thermal radiation and clothes ignition

olow flame front propagation velocity (as per wind speed)

oinsignificant overpressure



• Vapour Cloud Explosion (VCE) Delayed ignition of flammable

vapour cloud under partial confinement (obstacles within cloud) producing overpressure during flame front propagation

Main consequenceOverpressure

Main consequenceOverpressure

VCE results (Flixborough)Secondary consequences: oFragments (e.g. broken glasses)



• Explosion Explosion : General term for vapour

cloud ignition event, in which turbulence (necessary for combustion surface increase) leads to significant flame front propagation velocity and overpressure

oTurbulence sources:Leaks (release with jet

characteristics)Obstacles :

Equipment, e.g. air-coolersWalls, tanks, or other

confinement in space



• Explosion (cont.) Explosion feedback mechanism

High combustionrate

High combustionproducts volume rate

High heat release rate

Overpressure

Expandingflue gasesCombustion

Turbulence



• Deflagration Rather fast combustion rate Molecular diffusion limitation Small ignition energy requirements

(10-4 J for hydrocarbons) Flame front propagation velocity :

5-30 m/sec Flash fire : deflagration with no

flame front acceleration



• Detonation High ignition energy (106 J) Compression in flame front

exceeds autoignition temperature Supersonic flame front propagation

velocity. High overpressure produced Highly homogenous cloud required

– not feasible in real life

Explosives case



• Literature for Accident Scenarios and Top Events

Lees’ Loss Prevention in the Process Industries, Elsevier Butterworth Heinemann, 3nd Edition, 2005

Methods for the Determination of Possible Damage to People and Objects Resulting from Releases of Hazardous Materials , Green Book, CPR 16E, TNO, 1992

Guidelines for Chemical Process Quantitative Risk Analysis, CCPS-AICHE, 2000

Guidelines for Consequence Analysis of Chemical Releases, CCPS-AICHE, 1999

Guidelines for Evaluating the Characteristics of Vapour Cloud Explosions, Flash Fires and BLEVEs, CCPS-AICHE, 1994

Guidelines for quantitative risk assessment, Purple Book, CPR 18E, RVIM, 2005

RIVM, Reference Manual Bevi Risk Assessments, 2009



• Literature for Accident Scenarios and Top Events (cont.)

HSE, Safety Report Assessment Guide : LPG

Benchmark Exercise in Major Accident Hazard Analysis, JRC Ispra, 1991

Methodology for Evaluation of Safety Reports, Cyprus Ministry of Labour and Social Security, 2007 (in Greek)

Assael M., Kakosimos K., Fires, Explosions, and Toxic Gas Dispersions, CRC Press, 2010

Crowl D., Louvar J., Chemical Process Safety Fundamentals with Applications, Prentice Hall, 2nd Edition, 2002

Taylor J., Risk Analysis for Process Plant, Pipelines and Transport, E&FN SPON, 1994

Documents

ACCIDENT SCENARIOS AND TOP EVENTS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail