release models antony thanos ph.d. chem. eng. antony.thanos@gmail

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]



• 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



• 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)



• 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



• 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



• Release rates categories based on physical state of released flow Liquid Gas Two-phase (gas-liquid mixture)



• 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)



• 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



• 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)



• 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



• 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…



• 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



• 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



• 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



• 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



• 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



• 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 ???



• Evaporation mechanism in liquefied under pressure tanks



• 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



• 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



• 2-phase release Expected in failures of liquid phase

piping and tanks of liquefied under pressure substances

Overview of expansion of substance in pipe



• 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)



• 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)



• 2-phase release (cont.) Release is expanding also within

ambient air (2-phase jet)



• 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



• 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



• 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 %



• Example results for release rates LPG tank, T= 25 C°, 2 in hole at bottom of

tank (Aloha)

Propane Butane



• Evaporating pools Simple volatile liquid release (e.g.

methanol) and pool formation



• 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



• Evaporating pools (cont.) Liquid pool from liquefied under

pressure substance release (along with heavy gas formation)

Similar behaviour of pool



• 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



• 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



• 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)



• Evaporating pools (cont.) Example results for Dp=10 m, depth= 2

cm, T= 25 C°, atmospheric conditions D5 (confined evaporating pool, Aloha)

MethanolPropane



• 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)



• 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



• 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

release models antony thanos ph.d. chem. eng. antony.thanos@gmail

Documents