This project is funded by the European Union
Projekat finansira Evropska Unija
RELEASE MODELS
Antony Thanos
Ph.D. Chem. Eng.
antony.thanos@gmail.com
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Consequence analysis framework
Hazard
Identification
Release
scenarios
Event
trees
Dispersion models
Consequence
results
Accident
type
Release models
Release
quantification
Fire, Explosion Models
Domino effects
Limits of
consequence analysis
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)
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
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
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)
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)
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
M  A  Cd  l  2  g  l  H l  H h   2  ( P0  Pa )
M = release rate
Cd = dimensionless release coeff.
ρl = liquid density
Pa = atmospheric pressure
Hh = height of hole
This Project is funded
by the European Union
A = hole area
g = gravity acceleration constant
P0 = pressure in tank vapour space
Hl = liquid level in tank
Project implemented by Human
Dynamics Consortium
• 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)
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
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 rate
o 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…
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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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
 2 

M  Cd A  P0  0 
  1
 1
 1
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
 Cd values as for liquid phase releases
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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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
Supply pipeline
from refinery
This Project is funded
by the European Union
to other tanks
6 in, liquid phase
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 ???
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Evaporation mechanism in liquefied under
pressure tanks
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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




This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Some release types in LPGs
2-phase release
from big hole in
gas phase
Gas release from
PSV outlet
Gas release from
gas phase piping
PSV
2 in, gas phase
Gas release
from small
hole in gas
phase
to other tanks,
compressor
GAS
LIQUID
Supply pipeline
from refinery
This Project is funded
by the European Union
to other tanks
6 in, liquid phase
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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 2-phase release (cont.)
 Complex models used
o Quasi single phase
o 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)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 2-phase release (cont.)
 Release is expanding also within ambient air (2phase jet)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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 completed
o Entrainment of air is diminished, gradually,
due to less turbulence
o Heat from surrounding heats up cloud
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 2-phase release (cont.)
 2-phase jet is parted from a mix of :
o expanding gas
o droplets of liquid vaporising
 Aerosol characteristics
 Typical example of heavy-gas cloud formation
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 2-phase release and pool formation
 Formation of pool due to droplets
agglomeration (rain-out) depend on :
o droplet dimensions,
o ambient and storage conditions
o 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 %
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Example results for release rates
 LPG tank, T= 25 C°, 2 in hole at bottom of tank (Aloha)
Propane
This Project is funded
by the European Union
Butane
Project implemented by Human
Dynamics Consortium
• Evaporating pools
 Simple volatile liquid release (e.g. methanol)
and pool formation
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Evaporating pools (cont.)
 Simple volatile liquid pool mechanism
o Released liquid forms pool
o Heat provided from/to pool via :
 ground
 solar radiation
 ambient air
o Evaporation of pool due to diffusion and
convection (wind speed, turbulence)
mechanism above pool surface
 Similar mechanism for pool of refrigerated
gases
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Evaporating pools (cont.)
 Liquid pool from liquefied under pressure
substance release (along with heavy gas
formation)
 Similar behaviour of pool
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Evaporating pools (cont.)
 Unconfined pool :
o Theoretically maximum pool diameter is set
by balance of release feeding the pool and
evaporation rate from pool
Release
to pool
This Project is funded
by the European Union
Evaporation
from pool
Project implemented by Human
Dynamics Consortium
• Evaporating pools (cont.)
 Unconfined pool : (cont.)
o Real life : pool dimensions are restricted by
ground characteristics
o Area=Volume/Depth
o Typical values for assumed depth :
o 0.5-2 cm (depending on ground type)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• Evaporating pools (cont.)
 Example results for Dp=10 m, depth= 2 cm, T= 25 C°,
atmospheric conditions D5 (confined evaporating pool,
Aloha)
Methanol
This Project is funded
by the European Union
Propane
Project implemented by Human
Dynamics Consortium
• 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)
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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, CCPSAICHE, 2000
 Guidelines for Consequence Analysis of Chemical Releases, CCPSAICHE, 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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium
• 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
This Project is funded
by the European Union
Project implemented by Human
Dynamics Consortium