RELEASE MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

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

2. Consequence analysis framework Release scenarios Accident type Hazard Identification Event trees Dispersion models Release models Consequence results Release quantification Fire, Explosion Models Domino effects Limits of consequence analysis

3. 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)

4. 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 liquefiedgas 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 stateat 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

5. 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

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

7. 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)

8. Liquid phaserelease 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

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

10. 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

11. 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…

12. Liquid phaserelease 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

13. 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

14. 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

15. 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

16. 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

17. 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 ???

18. Evaporation mechanism in liquefied under pressure tanks

19. 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

20. 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

21. 2-phase release • Expected in failures of liquid phase piping and tanks of liquefied under pressure substances • Overview of expansion of substance in pipe

22. 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)

23. 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)

24. 2-phase release (cont.) • Release is expanding also within ambient air (2-phase jet)

25. 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

26. 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

27. 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 %

28. Example results for release rates • LPG tank, T= 25 C°, 2 in hole at bottom of tank (Aloha) • Propane Butane

29. Evaporating pools • Simple volatile liquid release (e.g. methanol) and pool formation

30. 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

31. Evaporating pools (cont.) • Liquid pool from liquefied under pressure substance release (along with heavy gas formation) • Similar behaviour of pool

32. 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

33. Evaporation from pool Release to pool • Evaporating pools (cont.) • Unconfined pool : • Theoretically maximum pool diameter is set by balance of release feeding the pool and evaporation rate from pool

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

35. Evaporating pools (cont.) • Example results for Dp=10 m, depth= 2 cm, T= 25 C°, atmospheric conditions D5 (confined evaporating pool, Aloha) Methanol Propane

36. 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)

37. 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

38. 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