Gas Detectors & Detectability

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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

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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.



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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.



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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



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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?



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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



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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.



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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?



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Consider a release from a site component

Site fence line

Hazardous zone

Point gas detector



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Release in the “westerly”direction interacts with sitestructures. The jet slows, this aided by additional turbulence, forms an accumulation cloud that isdetected.



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Release in the “easterly”exits the site undetected.



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LOS ?

Acoustic ?

Larger releases do notnecessarily become easier to detect; they form accumulations further from the source.



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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



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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.



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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.



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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



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Flammable gas release in an enclosed,partially enclosed and open module

T = 1s

T = 7s T = 18s



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• 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:



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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


• Introducing one detector at a time, assessing coverage in both CFD and volumetric Mapping software. Mapped coverage shows a correlation to the detectability:



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X axis

Y axis

Toxic gas release on a process unit.

Zone 60x40x10m

Releases of 10% H2S inmethane

Release point



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Five release rates, four wind directions; four release directions (80 releases in total)



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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


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”



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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.



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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.



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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.



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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!



Gas Detectors & Detectability

Technology

Transcript of Gas Detectors & Detectability