TOP EVENTS CONSEQUENCE ANALYSIS MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

1. This project is funded by the European Union Projekat finansira Evropska Unija TOP EVENTSCONSEQUENCE ANALYSIS 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. Pool fire • Ignition of flammable liquid phase Main consequence Thermal radiation Liquid fuel tank fire

4. Pool fire characteristics • Pool dimensions (diameter, depth) • Confined pool (liquid fuels tank/bund fire) : • Tank fire pool : diameter equal to tank diameter dimension • bunds : pool diameter estimated by equivalent diameter of bund

5. Pool fire characteristics (cont.) • Pool dimensions (diameter, depth) (cont.) • Unconfined pool (LPG pool from LPG tank failure –no dike present): • Theoretically maximum pool diameter is set by balance of release feeding the pool and combustion rate from pool Release to pool Combustion rate

6. Pool fire characteristics (cont.) • Pool dimensions (diameter, depth) (cont.) • Unconfined pool : Min : release rate (kg/sec) Mcomb : combustion rate (kg/sec) mcomb : specific combustion rate (kg/m2.sec) • In real life, pool is restricted by ground characteristics. Typical values for assumed depth: 0.5-2 cm (depending on ground type, higher values reported for sandy soils)

7. Pool fire characteristics (cont.) • Flame height, inclination (angle of flame from vertical due to wind) • Long duration (hours to days) • Combustion rate

8. Pool fire models • Combustion rate per pool surface on empirical equations (Burges, Mudan etc.) • Example :

9. Pool fire models (cont.) • Combustion rate for liquids not exceeding 0.1 kg/m2.sec. Upper range for low boiling point hydrocarbons • Flame dimension from empirical equations (Thomas, Pritchard etc.) • Example, Thomas correlation : • Big pools : Hf/Dp in the range of 1-2

10. Pool fire models (cont.) • Point source model (cont.) • No flame shape taken into account • A fraction of combustion energy is considered to be transmitted by ideal point in pool center Thermal radiation transmitted semi spherically

11. Pool fire models (cont.) • Point source model (cont.) • Increased inaccuracies near pool end (important for Domino effects)

12. Pool diameter Flame height Pool depth • Pool fire models (cont.) • Solid flame radiation model, radiation emitted via flame surface

13. Pool fire models (cont.) • Solid flame radiation model (cont.) • Calculation based on : • flame shape (usually considered cylinder -tilted or not-), • distance from flame (View Factor), • emissive power (thermal radiation flux at flame surface)

14. Pool fire models (cont.) • Solid flame radiation model (cont.) • Calculation equation :

15. Pool fire models (cont.) • Solid flame radiation model (cont.) • View Factor : function of distance of receptor from flame and flame dimensions. Different equation for different flame shapes • Transmissivity coefficient : Absorbance of thermal radiation by atmosphere components - e.g. humidity, CO2 – • Correlation with relative humidity (R.H.) level and distance to “receptor”) • High R.H, low transmissivity coefficient • More important for far-field effects (due to increased distance)

16. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : • Depending on pool size, substance • For big pools, soot formation (20 kW/m2), masking of flame, significant reduction of average flame emissive power

17. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : (cont.)

18. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : (cont.) • Experimental Gasoline pool examples : Dp=1 m, E=120 kW/m2 Dp=50 m, E= 20 kW/m2 • Medium to low emissive power for big pools (thermal radiation flux, up to 60 kW/m2 for liquid fuels) • LPGs, LNG, provide higher emissive power (up to 150-270 kW/m2 for LPG, 250 kW/m2 for LNG)

19. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : (cont.) • One example of correlations available for max emissive power :

20. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : (cont.) • Final emissive power must take into account smoke production. Example correlations : s, smoke coverage of surface Dp, pool diameter (m) Esmoke, emissive power of smoke (kW/m2)

21. Pool fire models (cont.) • Solid flame radiation model (cont.) • Emissive power : (cont.) • Please be careful !!!! • Make sure radiation fraction used is in-line with experimental data if available • Evaluate calculation results for emissive power with experimental results, if available

22. Pool fire models (cont.) • UK HSE suggestions for LPGs : • Emissive power : 200 kW/m2 over half flame height • Some conservative assumptions • For unconfined LPG cases, for theoretical pool calculation : • butane fire instead of similar propane release (lower boiling rate, higher pool diameter • low ambient temperature examined (as above) • Low relative humidity examined (high transmissivity coefficient)

23. Pool fire models (cont.) • Example results for propane pool fire Dp=10 m, wind speed 5 m/sec T=25 °C (confined fire, Aloha), • flame height Hf : 21 m • combustion rate M : 400 kg/min

24. Pool fire models (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 (burning unconfined pool, Aloha) • pool diameter Dp = 27 m • flame length : 11 m

25. Fireball, BLEVE (Boiling Liquid Expanding Vapour Explosion) • Rapid release and ignition of a flammable under pressure at temperature higher than its normal boiling point Main consequence Thermal radiation Secondary consequences: • Fragments (missiles) • Overpressure LPG BLEVE (Crescent City)

26. Fireball/BLEVE characteristics • Very rapid phenomenon (expanding velocity 10 m/sec) • Limited duration (up to appr. 30 sec, even for very large tanks) • Significant extent of fireball radius (in the order of 300 m for very big tanks, ≈ 4000 m3) • Very high emissive power (in the order or 200-350 kW/m2) • No precise capability for prediction of when it will happen (usual initial step for tanks exposed to heat -pool fire, jet flame-, opening of PSVs)

27. Fireball/BLEVE characteristics and models (cont.) • Radius and duration from correlations with tank content, example(AIChE CCPS) : • t, duration (sec) • m, mass (tn) • No significant deviations for various correlations available, example results for full propane tank BLEVE (100 m3)

28. Fireball/BLEVE characteristics and models (cont.) • Radius and duration from correlations with tank content, example(AIChE CCPS) : • t, duration (sec) • m, mass (kg)

29. Fireball/BLEVE characteristics and models (cont.) • Mass in fireball calculations : • Typically whole tank content (worst case approach.) • Netherlands (BEVI method) : gas phase + 3 x flash fraction of liquid phase at failure pressure. • For typical failure pressure in LPGs with hot BLEVEs, results to whole tank content • For propane at usual atmospheric conditions, results to whole tank content

30. Fireball/BLEVE characteristics and models (cont.) • Solid flame model • radiation emitted via fireball surface, • Usually fireball considered as sphere touching ground (conservative approach, adopted by UK HSE) Evolution of fireball/BLEVE

31. Fireball/BLEVE characteristics and models (cont.) • Solid flame radiation model (cont.) • Calculation based on : • sphere shape at contact with ground, • distance from fireball (sphere View Factor), • fireball emissive power (thermal radiation flux at fireball surface)

32. Fireball/BLEVE characteristics and models(cont.) • Solid flame radiation model (cont.) • View Factor : function of distance of receptor from flame and fireball radius • Transmissivity coefficient : as in pool fire case

33. Fireball/BLEVE characteristics and models (cont.) • Emissive power in fireball calculations : • Correlations are available for emissive power calculation based on : • vapour pressure at failure conditions (AIChE CCPS) Pv, vapour pressure at failure (MPa) • and/or mass involved, duration, size of fireball • Experimental data provide values up to 350 kW/m2 • UK, HSE suggestion >270 kW/m2

34. Fireball/BLEVE models (cont.) • Example results (full 100 m3 propane tank BLEVE, Aloha) • But, duration is only 13 sec. For limit values set in TDU (not in kW/m2), the relevant thermal radiation flux limit must be calculated

35. Fireball/BLEVE models (cont.) • Example results (full 100 m3 propane tank BLEVE, Aloha) (cont.) • For t=13 sec, • 1500 TDU corr. to 35 kW/m2 • 450 TDU corr. to 14 kW/m2 • 170 TDU corr. to 6.9 kW/m2

36. Jet flame • Ignition of gas or two-phase release from pressure vessel Main consequence Thermal radiation Propane jet flame test

37. Jet flame characteristics • Results as outcome of gas or two phase releases of flammable substances • Cone shape • Long duration (minutes to hours, depends on source isolation) • Very high emissive power (in the order or 200 kW/m2) • Soot expected, but not affecting radiation levels

38. Jet flame models • Combustion rate determined by release rate • Dimensions from empirical equations. Example of simplified Mudan-Cross equation L= jet flame length d= release point diameter Ct= fuel content permole in stoichiometric mix of fuel/air ΜWa= air molecular weight MWf= fuel molecular weight

39. Jet flame models (cont.) • Dimensions from empirical equations. Example of simplified Considine-Grint equation for LPGs L= jet flame length M= release rate (kg/sec) W= jet radius at flame tip (m)

40. Jet flame models (cont.) • Point source models • Single point : all energy is released from flame “center”. Similar to relevant point source model for pool fires • Multipoint source : several point along jet trajectory taken into account

41. Jet flame models (cont.) • Solid flame radiation model • Radiation emitted via flame surface • Calculation based on : • shape (cylinder, tilted or not) • distance (View Factor) • emissive power

42. Jet flame models (cont.) (cont.) • Solid flame radiation model (cont.) • Calculation equation :

43. Jet flame models (cont.) • Solid flame radiation model (cont.) • View Factor : function of distance of receptor from flame and flame dimensions for shape assumed • Transmissivity coefficient : as in pool fires, fireball/BLEVE • Emissive power : Estimated by flame dimension (surface) and energy released E= Emissive power (kW/m2) M= release rate (kg/s) ΔΗc= combustion energy (kJ/s) A= jet surface area, m2 Fs= fraction of combustion energy radiated uj= expanding jet velocity (m/sec)

44. Jet flame models conservative approaches • Examination of horizontal jet • Produce more extended thermal radiation zones • Have direct effect via impingement in near by equipment • Wind speed (for models taking into account flame distortion due to wind) : • Vertical jets : High wind speed (UK HSE suggestion 15 m/sec) • Horizontal jets : Low wind speed (UK HSE suggestion 2 m/sec)

45. Jet flame models example results (cont.) • Example results, 2 in hole in top of propane tank/gas phase, vertical jet (Aloha)

46. END OF PART A

47. Vapour cloud dispersion (cont.) • Extent of cloud : dimensions, downwind/crosswind till specific endpoints (concentration)

48. Vapour cloud dispersion (cont.) • Endpoints : • Toxics : several toxicity endpoints (e.g. IDLH, LC50) • Flammables : LFL, ½ LFL • Deaths expected within cloud limits where ignition is possible (Flash fire) due to thermal radiation and clothes ignition • Reporting of LFL, ½ LFL is for theoretical extend of cloud, as no ignition is assumed on cloud path • Very extended clouds expected for LPGs, especially in catastrophic failure cases (in the order of 500-1500 m)

49. Vapour cloud dispersion (cont.) • Endpoints : (cont.) • Flammables : (cont.) • Usually ignition sources outside establishment premises limit actual cloud • Protection zones not justified to take into account flammable dispersion till LFL, ½ LFL

50. Vapour cloud dispersion (cont.) • Example results for LPG dispersion (SLAB) at ground level centerline