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Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Gas Detectors and Detectability
Simon J. O’ConnorDetector Electronics (UK) Ltd
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
2
Presenter: Simon J. O’Connor
• ~30 years as a scientist working for Shell Research. Projectsincluded research into:– Spark and compression ignition of gases; Open path FTIR; Flare
Spectroscopy; Open path Air Quality monitoring; Differential Absorption LIDAR
• Last eight years as a Senior Consultant in the Fire and Gas Detection group of Shell Global Solutions– Over 50 projects on terminals; offshore facilities (manned and un-
manned); refineries; gas plants; chemical sites; FPSO and FLNG. Shell and non-Shell customers.
• Joined Det-Tronics April 2010.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
3
Introduction
• In this presentation we discuss the “detectabilty” of gas releases.
• A brief introduction to the concept of “volumetric coverage” and “Gas Detection Mapping” will be presented.
• Finally, results from simple CFD analysis of releases are presented and the correlation between these and Mapping results are shown.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
4
How good are existing systems?
UK Offshore hydrocarbon release database 2001-2008*
* “Offshore hydrocarbon releases 2001 to 2008”; Alison McGillivray, John Hare, UK Health and Safety Laboratory Research Report RR672; UK Health and Safety Executive, 2008.
Human observation
Smoke 2 Heat2 Flame 2 Gas Det. Process 3 Hand-held 4
Total
Major 1 9 0 0 0 23 4 0 36Significant 1 34 1 1 1 18 2 1 58
Method of detection
1. See reference (below) for definitions2. Original database includes smoke, heat and flame detectors for ignited events3. “Process” – release noticed from changes in process monitors.4. “Hand-held” – releases noticed during hand-held sniffing of components
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
5
How good are existing systems?
• Approximately 2/3 of “Major” and 1/3 of “Significant” releases were detected by the fixed gas detection system:
– Is this acceptable?– Given the significance of human observation (25% of Major and
60% of Significant), will general reductions in staffing levels lead to reduced detection?
– Can we identify best practice for fixed gas detection systems?
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
6
The nature of hazardous gas releases
• Liquid spills (including flashing liquids or liquefied gases):
• Pressurised gas releases:
• Combinations of the above in two phase releases.
Generally low level, heavyGas clouds and wind dispersed plumes
Release direction dominates wind. Rapid air entrainment improves dispersion
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
7
Tools of the trade: Gas detectors
• Point Gas Detectors: – Measure the concentration of gas at the location of the detector.
May be flammable or toxic gases (Catalytic / Pellister detectors, Electrochemical cells, Infra Red (IR) point detectors).
• Line of Sight (LOS) Detectors:– Measure the integrated concentration of gas along a (generally)
IR beam.
• Acoustic Detectors:– Listen for the characteristic noise associated with high pressure
gas releases.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
8
Tools of the trade: QRA? Exceedence?
• Release frequency (failure rates) combined with 3D dispersion and ignition models provides important information about the risk associated with hazardous gases.
• However:– Resolution limited– Link to “tolerance” unclear– Can assess incorrect
parameters (average detectability x average mitigation efficiency)
• Accept tolerable cloud?
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
9
Consider a release from a site component
Site fence line
Hazardous zone
Point gas detector
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
10
Consider a release from a site component
Release in the “westerly”direction interacts with sitestructures. The jet slows, this aided by additional turbulence, forms an accumulation cloud that isdetected.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
11
Consider a release from a site component
Release in the “easterly”exits the site undetected.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
12
Consider a release from a site component
LOS ?
Acoustic ?
Larger releases do notnecessarily become easier to detect; they form accumulations further from the source.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
13
Event size
Pot
entia
l con
sequ
ence
Liquid releases
Gas releases
Below
acceptable detection frequency
Effective range of applicationof Fixed detectors
Ineffective mitigation
Pragmatic view of Gas Detection and Mitigation
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
14
Volumetric coverage protection
• Following on from the Lord Cullen report after Piper Alpha. Several studies were initiated to assess the tolerable volumes of gas that could exist on offshore facilities.
• The concept of a 5m diameter LFL equivalent sphere was born.
• Detection target was to detect gas clouds before they could achieve potentially damaging volumes and escalate the situation.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
15
Gas Detection Mapping
Uses a 3D model of a facility.
Releases “idealised” clouds from all points.
Assess detector density.
Idealised sphere constructed toallow for point or line detector
Produces a coverage “map” and% coverage of zone to the idealisedtarget.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
16
Assessing detectability: NIST 5 FDS*
*Fire Dynamics Simulator, Version 5.0, Building and Research Laboratory, National Institute of Standardsand Technology http://www.fire.nist.gov/fds
Module 25x11x10m
Releases of 1.3kg/s methane
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
17
Flammable gas release in an enclosed,partially enclosed and open module
T = 1s
T = 7s T = 18s
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
18
Flammable gas release in an enclosed,partially enclosed and open module
• 6 point gas and 2 LOS (4 LOS in open module) simulated.
• 6 release directions for enclosed and partially enclosed modules with one wind direction. 6 release directions and 4 wind directions for open module. 36 releases in all.
LOS PointsEnclosed 100 100Partially Enclosed 83 50Open 83 38
Percentage of releases detected by:
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
19
Enclosed
0%
20%
40%
60%
80%
100%
0 25 50 75 100
Mapped Coverage
% D
etec
ted
Open Area
0%
20%
40%
60%
80%
100%
0 25 50 75 100
Mapped Coverage%
Det
ecte
d
Flammable gas release in an enclosed,partially enclosed and open module
• Introducing one detector at a time, assessing coverage in both CFD and volumetric Mapping software. Mapped coverage shows a correlation to the detectability:
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
20
X axis
Y axis
Toxic gas release on a process unit.
Zone 60x40x10m
Releases of 10% H2S inmethane
Release point
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
21
Toxic gas release on a process unit.
Five release rates, four wind directions; four release directions (80 releases in total)
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
22
5m Grid
-15
-10
-5
0
5
10
15
-20 -15 -10 -5 0 5 10 15 20
10m Grid
-20
-15
-10
-5
0
5
10
15
20
-30 -20 -10 0 10 20 30
Toxic gas release on a process unit.
5m grid 10m grid
• Three grids of detectors were simulated; these were a 5m grid, a 10m grid and a 20m grid (not shown)
• The grids are unrealistic; they are simulated to test the relationship between detector density and “detectability”
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
23
Toxic gas release on a process unit.
0
10
20
30
40
50
60
70
80
90
100
0.002 0.004 0.006 0.7 1.3
Release rate (kg/s)
Det
ecta
bilit
y (%
)
5m grid
10m grid
20m grid
• Detectors initially set to alarm at 10ppm H2S. Relationship between detectability, release rate and detector density clearly evident.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
24
Toxic gas release on a process unit.
32.0
33.0
34.0
35.0
36.0
37.0
38.0
39.0
0 20 40 60 80 100 120
Detector alarm setting (ppm)
Tim
e to
the
first
ala
rm (
s)5m grid
10m grid
20m gridR
espo
nse
time
• Varying the detector alarm set point; the 5m grid at 100ppm alarm level, on average, responds faster than a 10m grid or 20m grid with detectors set at 10ppm.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
25
Summary
• Guaranteeing high detection rates for hazardous gas releases on industrial sites is not trivial due to the large range of possible scenarios and release behaviour.
• A combination of detection technology generally provides the best solution.
• Simple volumetric Mapping can be used to assess conformity to target clouds in complex geometries.
• CFD modelling indicates that detectability is a strong function of detector density (number of detectors) and this correlates to volumetric Mapping.
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
26
Summary
• Detector sensitivity (alarm point) is secondary; if detectors do not intercept the plume, the situation cannot be corrected by reducing the detector setting.
• It is critically important to assess “upper limit” of mitigation actions and not to use these events to justify detection systems. Detection targets should align to “lower level” tolerable clouds.
Thank you for your attention!
Copyright 2010 ISA. All Rights Reserved.
Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org