9s davis et al presentation
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GexCon US
Using CFD to Analyze Gas Detector Placement in Process Facilities
Presented by:
Scott G. Davis, Olav R. Hansen, Filippo Gavelli and Are Bratteteig
GexCon
GexCon US Outline
§ Background
§ CFD based dispersion study – Gas Detector System
§ Examples § Danger clouds § Optimize small leak rates
GexCon US 3 3
Barriers
Consequence Development!
Consequence 1!
Consequence 2!Consequence 3!
Consequence 4!
Threat 1!
Threat 2!Threat 3!
Threat 4!
Chain of Causes!
Top Event!
Barriers
Probability Reducing Measures
Consequence Reducing Measures
Consequence Development!
Consequence 1!
Consequence 2!Consequence 3!
Consequence 4!
Threat 1!
Threat 2!Threat 3!
Threat 4!
Chain of Causes!
Top Event!
Threat 5
GexCon US 4
Mitigation Measures Triggered
q Activation of q Emergency Shutdown (ESD) or Blowdown – isolating the inventory
and limiting the size of the release
q Ignition source control q Minimize the likelihood of ignition of flammable gases or liquids due to
loss of containment (large leak rates)
q Shutdown of ventilation in HVAC inlets
q Activation of fire water pumps and deluge
q Activation of PA/alarms to alert personnel
GexCon US 5
Detector types
q Infrared point q Superior to catalytic detection
q Cannot detect hydrogen
q Infrared line q Better coverage than point detectors
q Prone to misalignment or beam blockage
q Acoustic
q Large coverage
q False alarms (pneumatic systems)
q Cannot detect low velocity leaks (pools)
Gas Detection System
GexCon US 6
Design
q Number of detectors q Regulations vs. company practice
q Proportional to module volume
q Layout q Equally spaced vs. staggered (cloud size)
q Clustering around leak sources (not recommended)
q Distribution according to ventilation patterns
q Number of line vs. point detectors
Gas Detection System
GexCon US 7
Example of clustering detectors around leak source
Gas Detection System
Gas detectors
Gas leak, not detected
GexCon US 8
Design
q Set points and voting q 10%-25% low, 30%-60% high
q 1-2 LELm line
q 2ooN (2 out of N detectors)/3ooN
q Measures to be taken
Gas Detection System
GexCon US 9
q Facility specific gas dispersion q Potential inventories
q Types of release q High/low momentum jets, flashing, liquid spills, lighter or heavier than
air gases
q Air movement and ventilation patterns (open, confined, dead zones)
q Topography
q Design Basis q Prescriptive and qualitative assessment – conservative
q Performance based - CFD
q Cases that are virtually impossible to anticipate
Complexities involved in GDS
ANSI/ISA-RP12.13.02 Recommended Practice for the Installation, Operation, and Maintenance of Combustible Detection Instruments
GexCon US CFD - FLACS
§ Allows for 3D representation of geometry and detectors § Model the ventilation, dispersion and explosion potential
GexCon US Detection Criteria
§ Leak rate and cloud size important § Cloud is what is detected and affects risk (ignition)
GexCon US Detection Criteria § Recall need to detect the cloud not the leak § Site dependent
GexCon US Detection Criteria
§ Correlation to compare inhomogeneous cloud to “dangerous” gas cloud (if ignited unacceptable consequences) § Mapping to determine when dispersed clouds are “dangerous”
GexCon US Detection Criteria
§ Comparison of detectable cloud size vs. hazard potential
GexCon US GDS Design Parameters
§ Unrealistic and not impossible to detect all leaks § Small leaks can lead to very small clouds < 1m3
§ Dangerous leaks – typically a relaxed requirement § Cloud size is generally quite large
§ ALARP – optimize for small, higher frequency leaks § For example – 0.1 kg/s
§ False Alarms – raise detection level/voting? § Redundancy § Caution against optimizing detectors around chosen
scenarios
GexCon US GDS Design Parameters
§ Time to detection § Important parameter to evaluate effectiveness § Relative to the leak rate § Many reasonably designed systems will detect clouds that can
lead to escalation or unacceptable consequences
§ Minimizing time to detection can play crucial role in initiating mitigation measures (ignition source, ESD or deluge)
GexCon US Explosion Risk during leak
17
GexCon US Confirmed Gas Detection
■ Some explosion risk prior to gas detection
18
GexCon US Deluge activated
■ Additional risk accumulation until deluge activated (full coverage)
19
GexCon US CFD based dispersion study
§ Geometry model § Important to predict flow phenomena in complex geometry
GexCon US CFD based dispersion study
§ GDS will be compared to dispersion results § Vary wind direction and speed, leak characteristics § Typically 100-600 scenarios are performed per area § Gas density is very important – need to simulate both lighter and
heavier than air gases § Record data at over 1000 sensor locations (line as well)
GexCon US CFD based dispersion study
§ Detection of “Dangerous” clouds § Larger leak rates – relaxed requirement § Clouds quite large § Limited options with respect to mitigation measures
§ Optimization of GDS against small leaks § Increased benefit of initiating mitigation measures § High probability events (0.1-0.3 kg/s leak rates) § Typically never reach “dangerous” sizes
GexCon US CFD based dispersion study
GexCon US CFD based dispersion study
GexCon US “Dangerous” Clouds
Leak
Leak
Pmax = 1.0 barg Pmax = 1.3 barg* Pmax = 0.6 barg *Design Accidental Load = 1.2 barg Stoichiometric Cloud
GexCon US Layout Comparison
§ Ensure all “dangerous” clouds are detected
GexCon US False Alarms
§ Numerous shutdowns costing $4-8 million § Performance going from 1x60%LEL to 2x20%LEL
GexCon US Minimum Number of Detectors
§ Maximize probability of detection – medium/large leaks
GexCon US Optimize Small Leaks – 0.1 kg/s
§ Optimize probability of detection for layouts
GexCon US Optimize Small Leaks – 0.1 kg/s
§ Compare detected vs. max potential cloud size
GexCon US Summary
§ CFD allows one to quantity the detection of GDS to potential hazard § Time to detection and probability of detection are typically reported § Relative to leak rate
§ Couple the results to measures that can be initiated by detection § Improving a GDS by a few seconds may not be important for ESD (~ 30 seconds) § But could be very important for shutting down ignition sources
§ Limited scenarios are performed – do not get trapped into optimizing sensors around chosen leaks sources
§ Requires the input of multiple disciplines
§ CFD is not stand alone but provides firm and repeatable base upon which complex reasoning for GDS is based
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