Gas Detectors & Detectability

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Gas Detectors & Detectability

  1. 1. Copyright 2010 ISA. All Rights Reserved. Gas Detectors and Detectability Simon J. O’Connor Detector Electronics (UK) LtdStandardsCertificationEducation & TrainingPublishingConferences & Exhibits Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  2. 2. Copyright 2010 ISA. All Rights Reserved.Presenter: Simon J. O’Connor• ~30 years as a scientist working for Shell Research. Projects included 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. 2 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  3. 3. Copyright 2010 ISA. All Rights Reserved.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. 3 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  4. 4. Copyright 2010 ISA. All Rights Reserved. How good are existing systems? UK Offshore hydrocarbon release database 2001-2008* Method of detection Human Smoke2 Heat2 Flame2 Gas Det. Process3 Hand- Total observation held4 Major1 9 0 0 0 23 4 0 36 Significant1 34 1 1 1 18 2 1 58 1. See reference (below) for definitions 2. Original database includes smoke, heat and flame detectors for ignited events 3. “Process” – release noticed from changes in process monitors. 4. “Hand-held” – releases noticed during hand-held sniffing of components* “Offshore hydrocarbon releases 2001 to 2008”; Alison McGillivray, John Hare, UK Health and Safety LaboratoryResearch Report RR672; UK Health and Safety Executive, 2008. 4 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  5. 5. Copyright 2010 ISA. All Rights Reserved.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? 5 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  6. 6. Copyright 2010 ISA. All Rights Reserved. The nature of hazardous gas releases • Liquid spills (including flashing liquids or liquefied gases): Generally low level, heavy Gas clouds and wind dispersed plumes • Pressurised gas releases: Release direction dominates wind. Rapid air entrainment improves dispersion• Combinations of the above in two phase releases. 6 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  7. 7. Copyright 2010 ISA. All Rights Reserved.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. 7 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  8. 8. Copyright 2010 ISA. All Rights Reserved.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? 8 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  9. 9. Copyright 2010 ISA. All Rights Reserved.Consider a release from a site component Site fence line Hazardous zone Point gas detector 9 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  10. 10. Copyright 2010 ISA. All Rights Reserved.Consider a release from a site component Release in the “westerly” direction interacts with site structures. The jet slows, this aided by additional turbulence, forms an accumulation cloud that is detected. 10 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  11. 11. Copyright 2010 ISA. All Rights Reserved.Consider a release from a site component Release in the “easterly” exits the site undetected. 11 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  12. 12. Copyright 2010 ISA. All Rights Reserved.Consider a release from a site component Larger releases do not necessarily become easier to detect; they form accumulations further from the source. LOS ? Acoustic ? 12 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  13. 13. Copyright 2010 ISA. All Rights Reserved.Pragmatic view of Gas Detection andMitigation Below acceptable detection frequency Liquid releases Gas releases Ineffective mitigation Potential consequence Effective range of application of Fixed detectors Event size 13 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  14. 14. Copyright 2010 ISA. All Rights Reserved.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. 14 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  15. 15. Copyright 2010 ISA. All Rights Reserved.Gas Detection Mapping Uses a 3D model of a facility. Releases “idealised” clouds from all points. Assess detector density. Idealised sphere constructed to allow for point or line detector Produces a coverage “map” and % coverage of zone to the idealised target. 15 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  16. 16. Copyright 2010 ISA. All Rights Reserved. Assessing detectability: NIST 5 FDS* Module 25x11x10m Releases of 1.3kg/s methane*Fire Dynamics Simulator, Version 5.0, Building and Research Laboratory, National Institute of Standardsand Technology http://www.fire.nist.gov/fds 16 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  17. 17. Copyright 2010 ISA. All Rights Reserved.Flammable gas release in an enclosed,partially enclosed and open module T = 1s T = 7s T = 18s 17 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  18. 18. Copyright 2010 ISA. All Rights Reserved.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. Percentage of releases detected by: LOS Points Enclosed 100 100 Partially Enclosed 83 50 Open 83 38 18 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  19. 19. Copyright 2010 ISA. All Rights Reserved.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: Enclosed Open Area 100% 100% 80% 80%% D e te c te d 60% % Detected 60% 40% 40% 20% 20% 0% 0% 0 25 50 75 100 0 25 50 75 100 Mapped Coverage Mapped Coverage 19 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  20. 20. Copyright 2010 ISA. All Rights Reserved. Toxic gas release on a process unit.Release point Zone 60x40x10m Releases of 10% H2S in methane X axis Y axis 20 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  21. 21. Copyright 2010 ISA. All Rights Reserved. Toxic gas release on a process unit.Five release rates, four wind directions; four release directions (80 releases in total) 21 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  22. 22. Copyright 2010 ISA. All Rights Reserved.Toxic gas release on a process unit. 5m Grid 10m Grid 15 20 15 10 10 5 5 0 0 -20 -15 -10 -5 0 5 10 15 20 -30 -20 -10 0 10 20 30 -5 -5 -10 -10 -15 -20 -15 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” 22 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  23. 23. Copyright 2010 ISA. All Rights Reserved.Toxic gas release on a process unit. 100 90 80 5m grid Detectability (%) 70 10m grid 60 20m grid 50 40 30 20 10 0 0.002 0.004 0.006 0.7 1.3 Release rate (kg/s)• Detectors initially set to alarm at 10ppm H2S. Relationship between detectability, release rate and detector density clearly evident. 23 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  24. 24. Copyright 2010 ISA. All Rights Reserved.Toxic gas release on a process unit. 39.0 38.0 Time to the first alarm (s) 37.0 36.0 5m grid Response time 10m grid 35.0 20m grid 34.0 33.0 32.0 0 20 40 60 80 100 120 Detector alarm setting (ppm)• 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. 24 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  25. 25. Copyright 2010 ISA. All Rights Reserved.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. 25 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org
  26. 26. Copyright 2010 ISA. All Rights Reserved.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! 26 Distributed with permission of author(s) by ISA 2010 Presented at ISA Automation Week 2010; http://www.isa.org

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