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Minimal criteria for Rapid Phase Transition explosion of cryogenic gases Roberto Bubbico 1 , Ernesto Salzano 2

Minimal criteria for Rapid Phase Transition explosion of cryogenic gases Roberto Bubbico 1 , Ernesto Salzano 2. 1 Dipartimento di Ingegneria Chimica Università di Roma “La Sapienza” Roma, Italy 2 Istituto di Ricerche sulla Combustione Consiglio Nazionale delle Ricerche Napoli, Italy.

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Minimal criteria for Rapid Phase Transition explosion of cryogenic gases Roberto Bubbico 1 , Ernesto Salzano 2

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  1. Minimal criteria for Rapid Phase Transition explosion of cryogenic gasesRoberto Bubbico1, Ernesto Salzano2 1 Dipartimento di Ingegneria Chimica Università di Roma “La Sapienza” Roma, Italy2 Istituto di Ricerche sulla Combustione Consiglio Nazionale delle Ricerche Napoli, Italy

  2. Introduction • Liquefied natural gas (LNG) market is increasingly expanding • Storage, handling and transportation of large volumes is involved • Large-scale hazards ??

  3. General data • LNG is transported mostly by ship (4 to 6 tanks for a total of 125000-160000 m3) • Methane (85-95%), ethane, propane + heavier hydrocarbons • It is kept at atmospheric pressure and refrigerated at about 111 K

  4. LNG hazards Besides “minor” damages (direct contact with cryogenic fluid, asphyxiation, breathing cold vapours), major hazards are: • Structural damage to tank/ship due to low T • Vapour cloud explosions (deflagration/detonation) • Vapour cloud fires • Pool fire • Rapid Phase Transition - RPT

  5. Rapid Phase Transition RPT • It is a fast expansion of vapour due to phase transition (phase change) • When vapour generation is very fast, localized overpressure can result • It can occur when cold LNG comes in contact with water at much higher (ambient!) temperature • It can be considered a physical explosion (no combustion)

  6. LNG release on water • LNG density is half that of water • LNG vapour density at boiling T is about 1.5 times the density of air • LNG will float on water • Pool spreading • More or less fast evaporation • A low-lying visible (moisture condensation) cloud will form

  7. Release dynamics A. Luketa-Hanlin /Journal ofHazardous Materials A132 (2006) 119—140

  8. Experimental data From past experimentation on LNG release on sea-water, for an RPT to occur it seems that: • A minimum CH4 content (40-80 %, depending on release size) is required; • Water temperature should be higher than 12/17°C (depending on degree of mixing with LNG) • RPT strength depends on spill rate (5 orders of magnitude increase over 0.3 m3/s)

  9. LNG composition LNG composition will affect vaporization dynamics: • Different boiling temperatures (vapour pressures): methane 111 K, C2 185 K, C3 231 K. • Different latent heats of vaporization Methane will boil off first Varying composition of the pool

  10. Uncertainties Among others (modelling, etc.): • Drake et al. (‘75), Boe (’98), etc.: • Heavier hydrocarbons will increase evaporation rate • Conrado & Vesovic (2000): • Heavier hydrocarbons will decrease evaporation rate

  11. Pool boiling Due to the temperature difference between LNG and water (about 180 °C) film boiling will result:

  12. Pool boiling • At high methane concentrations (initial stages): High temperature difference Film boiling / lower heat transfer rates (Vapour film acts as an insulator) • At later stages: Lower temperature difference Nucleate boiling / higher heat transfer rates (Very fast evaporation and RPT)

  13. RPT modelling Prevalent theory for RTP explosion is based on the superheat temperature TSH: TSH ( 170 K for methane; 326 K for propane) < Twater Source: SuperChemsExpert v5.7, ioMosaic Corp.

  14. RPT modelling Phase envelope for an LNG mixture Source: SuperChemsExpert v5.7, ioMosaic Corp.

  15. RPT modelling The propagation of blast wave may be reproduced by the acoustic analysis from conservation equations of mass and momentum: and by the definition of potential φ as:

  16. RPT modelling Under acoustic assumption: and in spherical coordinates for radius r: POTENTIAL WAVE EQUATION where co is the ambient speed of sound.

  17. RPT modelling The potential wave equation has been solved to give the peak overpressure P as a function of the distance R from the acoustic far-field source point (considering a ground explosion in open atmosphere) as: where g is the ratio of specific heats, co is the ambient speed of sound, R is the distance from source and Φ is the volume source strength (m3/s).

  18. RPT modelling Recently, van den Berg et al. (2004), have applied the correlation for blast wave produced by BLEVE modelling. For a vessel of volume V, if the flash fraction is F and the expansion ratio of liquid to vapour is , it can be written: integration

  19. Example of application These equations have been applied to LNG phase transition after release on sea level. Conservative option (worst-worst case analysis): V = 10000 m3 (Moss sphere) Time to release = 1 s – 10 s (instantaneous release) Flashing ratio F = 1 LNG composition = methane 100% liquid density ρ = 423 kg/m3 (at ambient temperature) vapour density ρ = 1.819 kg/m3 at boiling point vapour density ρ = 0.68 kg/m3 (at ambient temperature) expansion ratio  620

  20. Results Calculated acoustic RPT overpressure as a function of distance Dashed line: 0.08 bar = structural threshold value for atmospheric equipment Discharge time: Red = 1 s; Green = 10 s

  21. Results Acoustic model: max release time for reaching threshold values for overpressure Dashed lines: 0.08 and 0.3 bar Discharge time: Red = 2.75 s; Green = 5 s

  22. Results Acoustic model: overpressure profiles at different release time Dashed lines: 0.08 and 0.3 bar Discharge time: Red = 2 s; Green = 1 s

  23. Alternative model By adopting Brode’s equation with P1=24.6 bar (corresponding to TSH for methane), and P0=1.01 bar:

  24. Simulation results Release dynamics from a 27 m diameter tank, almost full ( 10000 m3) Catastrophic release

  25. Simulation results Release dynamics from a 27 m diameter tank, almost full ( 10000 m3) • 100 cm dia. hole • Hole level 2 m • Pin = 1.5 bar

  26. Simulation results Release dynamics from a 27 m diameter tank, almost full ( 10000 m3) • 100 cm dia. hole • Hole level 2 m • Pin = 1.5 bar

  27. Simulation results Release dynamics from a 27 m diameter tank, almost full ( 10000 m3) • 100 cm dia. hole • Hole level 2 m • No padding

  28. Simulation results Release dynamics from a 27 m diameter tank, almost full ( 10000 m3) • 100 cm dia. hole • Hole level 2 m • No padding

  29. Conclusions • LNG presents various sources of hazards • RPT explosions do not generate large distance impact areas • Thus RPTs don’t seem to represent a main hazard to public safety • However, they still can generate further damages close to the spill location, due to: • Brittle fracture • Thermal effects • Overpressure

  30. References • W.E. Baker, P.A. Cox, P.S. Westine, J.J. Kulesz, R.A. Strehlow, Explosion hazards and evaluation, Elsevier, Amsterdam, 1983. • G.B. Whitham, On the propagation of weak shock waves, Journal of Fluid Mechanics 1 (1956) 290. • A.C. van den Berg , M.M. van der Voort, J. Weerheijm, N.H.A. Versloot Expansion-controlled evaporation: a safe approach to BLEVE blast, Journal of Loss Prevention in the Process Industries 17 (2004) 397–405 • Lighthill, J.(1978). Waves in fluids.Cambridge : Cambridge University Press. • Reid, R.C.(1976).Superheated liquids. American Scientist, 64, 146–156. • Reid, R.C.(1979). Possible mechanisms for pressurized-liquid tank explosions or BLEVE’s. Science, 203, 3. • Strehlow, R.A. (1981).Blast wave from deflagrative explosions: an acoustic approach. 13th AIChE loss prevention symposium, Philadelphia (PA). • A. Luketa-Hanlin, A review of large-scale LNG spills: Experiments and modeling, Journal of Hazardous Materials A132 (2006) 119–140 • C. Conrado, V. Vesovic, The influence of chemical composition on vaporization of LNG and LPG on unconfined water surfaces, Chem. Eng. Sci. 5 (2000) 4549–4562.

  31. Thank you for your attention

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