RELEASE MODELS Antony Thanos Ph.D. Chem. Eng. [e-mail]

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RELEASE MODELS Antony ThanosPh.D. Chem.
Eng.antony.thanos_at_gmail.com
This project is funded by the European
Union Projekat finansira Evropska Unija

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• Consequence analysis framework

Release scenarios
Accident type
Hazard Identification
Event trees
Release models
Dispersion models
Consequence results
Release quantification
Fire, Explosion Models
Domino effects
Limits of consequence analysis
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• Release rates models
• Essential step as providing one of the main
  parameters required in Consequence Analysis
• General categories of releases based on sources
• Releases from vessels/tanks
• Releases from piping
• Releases from pools (pool evaporation rates)
• Releases from fire events (flue gas dispersion
  case)

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• 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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• Release rates models
• Essential step as providing one of the main
  parameters required in Consequence Analysis
• General categories of releases based on duration
  
• Continuous (constant/variable flow rate)
• 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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• Release rates categories based on physical state
  of released flow
• Liquid
• Gas
• Two-phase (gas-liquid mixture)

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• 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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• 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

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• Liquid phase release from piping
• Cd 0.61-1
• Cd0.61 for hole with rough edges (as for random
  seizures of tank wall)
• Cd1 hole with smooth edges, Full Bore Rupture
  (FBR)

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• Liquid phase release from piping (cont.)
• If piping is fed by tank, same approach as for
  release from tank.
• 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
• Especially important for releases from liquid
  pipelines with remote pump station

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• Liquid phase release from piping (cont.)
• In case of Full Bore Rupture downstream pump
• Release rate considered equal to pump flow rate
• 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
• Release from broken pipe downstream hole is
  usually ignored

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• Liquid phase release and refrigerated gases
• Typically, releases of refrigerated gas (storage
  at normal boiling point) are treated as simple
  liquid releases
• No severe shear forces are expected at release
  point
• No significant aerosol formation is expected
• 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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• 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

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• Gas phase release (cont.)
• When release point is in piping, pressure drop
  from feeding tank/vessel must be taken into
  account
• Especially important for releases in long
  pipelines
• Conservative approach release from point close
  to tank/vessel, equivalent to hole in tank/vessel

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• Some release points in LPGs

Release from PSV outlet
Release from gas phase piping
PSV
2 in, gas phase
Release from small hole in gas phase
to other tanks, compressor
GAS
LIQUID
to other tanks
Supply pipeline from refinery
6 in, liquid phase
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• Gas phase release (cont.)
• Gas flow expected
• Failures in gas phase piping of liquefied under
  pressure substance
• Pressure safety valves of liquefied under
  pressure substance tanks (e.g. LPGs)
• Small hole in gas phase of LPG tanks
• In case of rather big holes in gas phase ???

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• 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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• Some release types in LPGs

Gas release from PSV outlet
Gas release from gas phase piping
2-phase release from big hole in gas phase
PSV
2 in, gas phase
Gas release from small hole in gas phase
to other tanks, compressor
GAS
LIQUID
to other tanks
Supply pipeline from refinery
6 in, liquid phase
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• 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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• 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
• Quasi single phase
• Homogeneous Equilibrium Models (expanding
  liquid/gas phase have same velocity)
• Non-Homogenous Models (expanding liquid/gas phase
  have different velocities, phase slip)
• 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.)
• Gas expands and cools (density increase)
• Liquid vaporizes and cools (density increase)
• Air is entrained and provides heat for
  evaporation of liquid, air cools with
  condensation of humidity (density increase)
• After a time evaporation is completed
• Entrainment of air is diminished, gradually, due
  to less turbulence
• Heat from surrounding heats up cloud

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• 2-phase release (cont.)
• 2-phase jet is parted from a mix of
• expanding gas
• 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
• droplet dimensions,
• ambient and storage conditions
• substance properties
• release size/location/direction etc.
• Rule of thumb 2 x times the flashing liquid
  will be airborne (mix of liquid/gas)
• Propane T 29 C, rainout estimated to 14
• 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
• Released liquid forms pool
• Heat provided from/to pool via
• ground
• solar radiation
• ambient air
• 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
• Theoretically maximum pool diameter is set by
  balance of release feeding the pool and
  evaporation rate from pool

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• Evaporating pools (cont.)
• Unconfined pool (cont.)
• Real life pool dimensions are restricted by
  ground characteristics
• AreaVolume/Depth
• Typical values for assumed depth
• 0.5-2 cm (depending on ground type)

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• Evaporating pools (cont.)
• Example results for Dp10 m, depth 2 cm, T 25
  C, atmospheric conditions D5 (confined
  evaporating pool, Aloha)
• Methanol Propane

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• Evaporating pools (cont.)
• Example results for Methanol tank, Dtank20 m, H
  tank20 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, EFN SPON, 1994
• RIVM, Reference Manual Bevi Risk Assessments,
  2009
• ALOHA, Users Manual, US EPA, 2007
• ALOHA Two Day Training Course Instructor's Manual