RELEASE MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail
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Transcript of RELEASE MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail
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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
RELEASE MODELS
Antony ThanosPh.D. Chem. [email protected]
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• 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
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Release rates models Essential step as providing one of the
main parameters required in Consequence Analysis
General categories of releases based on sources :
o Releases from vessels/tanks
o Releases from piping
o Releases from pools (pool evaporation rates)
o Releases from fire events (flue gas dispersion case)
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Release rates categories based on physical state of substance to be released
Release of substance stored/handled at liquid state and temperature below normal boiling point (e.g. leak from Diesel tank release)
Release of liquefied gas stored/handled at temperature above normal boiling point (liquefied gas under pressure), e.g. leak of LPG from LPG tank bottom)
Release of liquefied gas stored/handled at liquid state at normal boiling point (refrigerated gas), e.g leak of liquid ammonia from failure of refrigerated tank shell wall
Release of gases (adiabatic expansion at hole), e.g. leak from hydrogen piping
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Release rates models Essential step as providing one of the
main parameters required in Consequence Analysis
General categories of releases based on duration :
o Continuous (constant/variable flow rate)
o “Instantaneous” : Usually refers to catastrophic failures, i.e. release of the whole content of a vessel, tank within short time e.g. 3-5 min
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Release rates categories based on physical state of released flow Liquid Gas Two-phase (gas-liquid mixture)
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Liquid phase release from tank Release of substance stored/handled at
liquid state and temperature below normal boiling point (e.g. leak from Diesel tank release)
Released substance is expected to form pool in surroundings (no aerosol expected)
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Liquid phase release from tank (cont.) Release driven by pressure
difference between pressure in container and atmosphere
Rate is affected by hole size and shape
Model : Bernoulli equation )(22 0 ahllld PPHHgCAM
M = release rate A = hole area Cd = dimensionless release coeff. g = gravity acceleration constant ρl = liquid density P0 = pressure in tank vapour space Pa = atmospheric pressure Hl = liquid level in tank Hh = height of hole
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• Liquid phase release from piping Cd= 0.61-1
o Cd=0.61 for hole with rough edges (as for random seizures of tank wall)
o Cd≈1 hole with smooth edges, Full Bore Rupture (FBR)
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Liquid phase release from piping (cont.) If piping is fed by tank, same approach
as for release from tank.o Pressure at hole must take into account
pressure drop from tank to hole location due to release flow rate (Fanning equation etc.)
If piping is supplied by pump : pressure drop from pump till hole location (normal pressure at hole location) must be taken into account
o Especially important for releases from liquid pipelines with remote pump station
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Liquid phase release from piping (cont.) In case of Full Bore Rupture downstream
pump:o Release rate considered equal to pump
flow rateo Better estimation, if pump performance
curves are available (increase of pump flow rate above nominal due to decreased DH at pump discharge).
Initial estimation : flow rate appr. 120% of nominal flow rate
Conservative approach: assume release point very close to pump
o Release from broken pipe downstream hole is usually ignored…
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Liquid phase release and refrigerated gases Typically, releases of refrigerated
gas (storage at normal boiling point) are treated as simple liquid releases
o No severe shear forces are expected at release point
o No significant aerosol formation is expected
o Simple pool is formed
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• Gas phase release Release from contained gas phase Example : Release of hydrogen from
pressure vessel at discharge of hydrogen compressor
Expansion of gas at hole as pressure is reduced (typically consider as adiabatic), cooling of gas at expansion, as also in tank
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This Project is funded by the European Union
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• Gas phase release (cont.) For most gases and pressure
higher than 1.4 barg, choked flow is established with sonic of supersonic flow at hole
Cd values as for liquid phase releases
1
1
00 1
2
PACM d
M = release rate Cd = Dimensionless release coeff. A = hole area γ = heat capacities ratio (Cp/Cv) P0 = initial gas pressure at source (tank, etc.) ρ0 = initial gas density
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Gas phase release (cont.) When release point is in piping,
pressure drop from feeding tank/vessel must be taken into account
o Especially important for releases in long pipelines
o Conservative approach : release from point close to tank/vessel, equivalent to hole in tank/vessel
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Some release points in LPGs
2 in, gas phase to other tanks,compressor
to other tanksSupply pipelinefrom refinery 6 in, liquid phase
PSV
LIQUID
GAS
Release fromgas phase piping
Release fromPSV outlet
Release from small hole in gas phase
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Gas phase release (cont.) Gas flow expected :
o Failures in gas phase piping of liquefied under pressure substance
o Pressure safety valves of liquefied under pressure substance tanks (e.g. LPGs)
o “Small” hole in gas phase of LPG tanks
In case of rather “big” holes in gas phase ???
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Evaporation mechanism in liquefied under pressure tanks
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• Evaporation mechanism in liquefied under pressure tanks (cont.) Pressure drops In order to achieve equilibrium liquid is
evaporated. Evaporation via bubble formation Bubbles development produce swell
(expansion of liquid phase) Small release hole, small depressurisation,
minimal bubble formation, small swell, no effect on released phase
Big hole, rapid depressurisation, increased bubble formation, increased swell, liquid phase expansion may reach release point, 2-phase flow
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• Some release types in LPGs
2 in, gas phase to other tanks,compressor
to other tanksSupply pipelinefrom refinery 6 in, liquid phase
PSV
LIQUID
GAS
Gas release fromgas phase piping
Gas release fromPSV outlet2-phase release
from big hole in gas phase
Gas release from small hole in gas
phase
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This Project is funded by the European Union
Project implemented by Human Dynamics Consortium
• 2-phase release Expected in failures of liquid phase
piping and tanks of liquefied under pressure substances
Overview of expansion of substance in pipe
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This Project is funded by the European Union
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• 2-phase release (cont.) If failure is on tank shell, the expansion of
liquid happens totally outside tank For failures in piping, establishment of
liquid/gas equilibrium or not within pipe depends on distance of release point from tank (or other constant pressure point)
For less than 1 m distance of failure point from tank, no equilibrium is established
Consideration of vessel state during depressurisation (flashing/evaporation, liquid phase swell)
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• 2-phase release (cont.) Complex models used
o Quasi single phaseo Homogeneous Equilibrium Models
(expanding liquid/gas phase have same velocity)
o Non-Homogenous Models (expanding liquid/gas phase have different velocities, phase slip)
o Frozen models (expanding liquid/gas phase have same velocity and constant mass ration)
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• 2-phase release (cont.) Release is expanding also within
ambient air (2-phase jet)
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• 2-phase release (cont.) 2-phase jet evolution : (cont.)
o Gas expands and cools (density increase)
o Liquid vaporizes and cools (density increase)
o Air is entrained and provides heat for evaporation of liquid, air cools with condensation of humidity (density increase)
o After a time evaporation is completedo Entrainment of air is diminished,
gradually, due to less turbulenceo Heat from surrounding heats up cloud
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• 2-phase release (cont.) 2-phase jet is parted from a mix
of :o expanding gaso droplets of liquid vaporising
Aerosol characteristics
Typical example of heavy-gas cloud formation
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• 2-phase release and pool formation Formation of pool due to droplets
agglomeration (rain-out) depend on :o droplet dimensions,o ambient and storage conditionso substance properties o release size/location/direction etc.
Rule of thumb : 2 x times the flashing liquid will be airborne (mix of liquid/gas)
o Propane : T= 29 °C, rainout estimated to 14 %o Butane : T= 29 °C, rainout estimated to 66 %
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• Example results for release rates LPG tank, T= 25 C°, 2 in hole at bottom of
tank (Aloha)
Propane Butane
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• Evaporating pools Simple volatile liquid release (e.g.
methanol) and pool formation
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• Evaporating pools (cont.) Simple volatile liquid pool mechanism
o Released liquid forms poolo Heat provided from/to pool via :
groundsolar radiationambient air
o Evaporation of pool due to diffusion and convection (wind speed, turbulence) mechanism above pool surface
Similar mechanism for pool of refrigerated gases
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• Evaporating pools (cont.) Liquid pool from liquefied under
pressure substance release (along with heavy gas formation)
Similar behaviour of pool
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• Evaporating pools (cont.) Evaporation rates provided by rather
complex models (GASP, LPOOL, SUPERCHEMS) taking into account of all former parameters affecting
Simpler models for low boiling liquids
Significant parameter of pool : pool dimensions (mainly pool area)
Pool formation within bund : pool diameter is equal to bund equivalent diameter
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• Evaporating pools (cont.) Unconfined pool :
o Theoretically maximum pool diameter is set by balance of release feeding the pool and evaporation rate from poolEvaporation
from poolRelease to pool
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• Evaporating pools (cont.) Unconfined pool : (cont.)
o Real life : pool dimensions are restricted by ground characteristics
o Area=Volume/Deptho Typical values for assumed
depth :o 0.5-2 cm (depending on
ground type)
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• Evaporating pools (cont.) Example results for Dp=10 m, depth= 2
cm, T= 25 C°, atmospheric conditions D5 (confined evaporating pool, Aloha)
MethanolPropane
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• Evaporating pools (cont.) Example results for Methanol tank,
Dtank=20 m, H tank=20 m, T= 25 C°, atmospheric conditions D5, 2 in hole on tank shell at ground level (unconfined evaporating pool, Aloha)
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• Literature for Release Models Lees’ Loss Prevention in the Process Industries,
Elsevier Butterworth Heinemann, 3nd Edition, 2005
Methods for the Calculation of Physical Effects due to Releases of Hazardous Materials (Liquids and Gases), Yellow Book, CPR 14E, VROM, 2005
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
Safety Report Assessment Guides (SRAGs), Health and Safety Executive, UK
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• Literature for Release Models (cont.) Assael M., Kakosimos K., Fires, Explosions, and Toxic
Gas Dispersions, CRC Press, 2010 Benchmark Exercise in Major Accident Hazard Analysis, JRC Ispra, 1991
Taylor J., Risk Analysis for Process Plant, Pipelines and Transport, E&FN SPON, 1994
RIVM, Reference Manual Bevi Risk Assessments, 2009
ALOHA, Users Manual, US EPA, 2007
ALOHA Two Day Training Course Instructor's Manual