Minimal criteria for Rapid Phase Transition explosion of cryogenic gases

Roberto Bubbico1, Ernesto Salzano2

1 Dipartimento di Ingegneria Chimica Università di Roma “La Sapienza” Roma, Italy

2 Istituto di Ricerche sulla Combustione Consiglio Nazionale delle Ricerche Napoli, Italy

IntroductionIntroduction• Liquefied natural gas (LNG) market is Liquefied natural gas (LNG) market is

increasingly expandingincreasingly expanding

• Storage, handling and transportation Storage, handling and transportation of large volumes is involvedof large volumes is involved

•Large-scale hazardsLarge-scale hazards ?? ??

General dataGeneral data

• LNG is transported mostly by ship LNG is transported mostly by ship (4 to 6 tanks for a total of 125000-(4 to 6 tanks for a total of 125000-160000 m160000 m33))

• Methane (85-95%), ethane, Methane (85-95%), ethane, propane + heavier hydrocarbonspropane + heavier hydrocarbons

• It is kept at atmospheric pressure It is kept at atmospheric pressure and refrigerated at about 111 Kand refrigerated at about 111 K

LNG hazardsLNG hazardsBesides Besides “minor”“minor” damages (direct contact damages (direct contact with cryogenic fluid, asphyxiation, with cryogenic fluid, asphyxiation, breathing cold vapours), major hazards breathing cold vapours), major hazards are:are:

•Structural damage to tank/ship due to Structural damage to tank/ship due to low Tlow T

•Vapour cloud explosions Vapour cloud explosions (deflagration/detonation)(deflagration/detonation)

•Vapour cloud firesVapour cloud fires

•Pool firePool fire

•Rapid Phase Transition - RPTRapid Phase Transition - RPT

Rapid Phase Transition RPTRapid Phase Transition RPT

• It is a fast expansion of vapour due to It is a fast expansion of vapour due to phase transition (phase change)phase transition (phase change)

• When vapour generation is very fast, When vapour generation is very fast, localized overpressure can resultlocalized overpressure can result

• It can occur when cold LNG comes in It can occur when cold LNG comes in contact with water at much higher contact with water at much higher ((ambient!ambient!) temperature) temperature

• It can be considered a physical It can be considered a physical explosion (no combustion)explosion (no combustion)

LNG release on waterLNG release on water• LNG density is half that of water LNG density is half that of water • LNG vapour density at boiling T is LNG vapour density at boiling T is

about 1.5 times the density of airabout 1.5 times the density of air

• LNG will float on waterLNG will float on water– Pool spreadingPool spreading– More or less fast evaporationMore or less fast evaporation

• A low-lying visible (moisture A low-lying visible (moisture condensation) cloud will formcondensation) cloud will form

Release dynamicsRelease dynamics

A. Luketa-Hanlin /Journal of Hazardous Materials A132 (2006) 119—140

Experimental dataExperimental data

From past experimentation on LNG release on From past experimentation on LNG release on sea-water, for an RPT to occur it seems sea-water, for an RPT to occur it seems that:that:

• A minimum CHA minimum CH44 content (40-80 %, content (40-80 %, depending on release size) is required;depending on release size) is required;

• Water temperature should be higher than Water temperature should be higher than 12/1712/17 °C (depending on degree of mixing °C (depending on degree of mixing with LNG)with LNG)

• RPT strength depends on spill rate (5 orders RPT strength depends on spill rate (5 orders of magnitude increase over 0.3 mof magnitude increase over 0.3 m33/s)/s)

LNG compositionLNG composition

LNG composition will affect vaporization LNG composition will affect vaporization dynamics:dynamics:

• Different boiling temperatures (vapour Different boiling temperatures (vapour pressures): methane 111 K, Cpressures): methane 111 K, C22 185 K, C 185 K, C33 231 K.231 K.

• Different latent heats of vaporizationDifferent latent heats of vaporization

Methane will boil off firstMethane will boil off first

Varying composition of the poolVarying composition of the pool

UncertaintiesUncertainties

Among others (modelling, etc.):Among others (modelling, etc.):

• Drake et al. (‘75), Boe (’98), etc.:Drake et al. (‘75), Boe (’98), etc.:– Heavier hydrocarbons will Heavier hydrocarbons will increaseincrease

evaporation rateevaporation rate

• Conrado & Vesovic (2000):Conrado & Vesovic (2000):– Heavier hydrocarbons will Heavier hydrocarbons will decreasedecrease

evaporation rateevaporation rate

Pool boilingPool boilingDue to the temperature difference between LNG Due to the temperature difference between LNG

and water (about 180 °C) film boiling will result:and water (about 180 °C) film boiling will result:

Pool boilingPool boiling• At high methane concentrations (initial At high methane concentrations (initial

stages):stages):High temperature differenceHigh temperature difference

Film boiling / lower heat transfer ratesFilm boiling / lower heat transfer rates(Vapour film acts as an insulator)(Vapour film acts as an insulator)

• At later stages:At later stages:Lower temperature differenceLower temperature difference

Nucleate boiling / higher heat transfer ratesNucleate boiling / higher heat transfer rates(Very fast evaporation and (Very fast evaporation and RPTRPT))

RPT modellingRPT modellingPrevalent theory for RTP explosion is based on the Prevalent theory for RTP explosion is based on the

superheat temperature Tsuperheat temperature TSHSH::

TTSHSH ( ( 170 K for methane; 326 K for propane) < 170 K for methane; 326 K for propane) < TTwaterwater

Source: SuperChemsExpert v5.7, ioMosaic Corp.

RPT modellingRPT modellingPhase envelope for an LNG mixturePhase envelope for an LNG mixture

Source: SuperChemsExpert v5.7, ioMosaic Corp.

RPT modellingRPT modellingThe propagation of blast wave may be reproduced The propagation of blast wave may be reproduced

by the acoustic analysis from conservation by the acoustic analysis from conservation equations of mass and momentum:equations of mass and momentum:

0udt

d

0Pudt

d

and by the definition of potential and by the definition of potential φφ as: as:

dr

dgradu

0dt

rdrc

2

222

o

where cwhere coo is the ambient speed of sound. is the ambient speed of sound.

POTENTIAL WAVE

EQUATION

RPT modellingRPT modellingUnder acoustic assumption:Under acoustic assumption:

dt

dP

and in spherical coordinates for radius r:and in spherical coordinates for radius r:

where where is the ratio of specific heats, c is the ratio of specific heats, coo is the ambient speed is the ambient speed of sound, R is the distance from source and Φ is the volume of sound, R is the distance from source and Φ is the volume source strength (msource strength (m33/s)./s).

dt

d

c

1

R2P

2o

RPT modellingRPT modellingThe potential wave equation has been solved The potential wave equation has been solved to give the peak overpressure to give the peak overpressure P as a function P as a function of the distance R from the acoustic far-field of the distance R from the acoustic far-field source point (considering a ground explosion source point (considering a ground explosion in open atmosphere) as:in open atmosphere) as:

22o t

VF2

c

1

R2P

P

VFdt

d

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

integrationintegration

RPT modellingRPT modelling

Conservative option (worst-worst case analysis):

V = 10000 mV = 10000 m33 (Moss sphere) (Moss sphere)

Time to release = 1 s – 10 s (instantaneous Time to release = 1 s – 10 s (instantaneous release)release)

Flashing ratio F = 1Flashing ratio F = 1

LNG composition = methane 100%LNG composition = methane 100%

liquid density ρ = 423 kg/mliquid density ρ = 423 kg/m33 (at ambient (at ambient temperature) temperature)

vapour density ρ = 1.819 kg/mvapour density ρ = 1.819 kg/m33 at boiling point at boiling point

vapour density ρ = 0.68 kg/mvapour density ρ = 0.68 kg/m33 (at ambient (at ambient temperature) temperature)

expansion ratio expansion ratio 620620

These equations have been applied to LNG phase These equations have been applied to LNG phase transition after release on sea leveltransition after release on sea level..

Example of applicationExample of application

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

0.00

0.10

0.20

0.30

0.40

0.50

0 100 200 300 400

Distance [m]

P [

bar

]ResultsResults

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

ResultsResults

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 20 40 60 80 100

Distance [m]

P [

bar]

Acoustic model: overpressure profiles at different release time

Dashed lines: 0.08 and 0.3 bar Discharge time: Red = 2 s; Green = 1 s

ResultsResults

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 50 100 150 200 250 300

Distance [m]

P [

bar]

Alternative modelAlternative model

1

)( 01

VPPE

By adopting Brode’s equation with PBy adopting Brode’s equation with P11=24.6 bar =24.6 bar (corresponding to T(corresponding to TSH SH for methane), and Pfor methane), and P00=1.01 bar:=1.01 bar:

P (kPa) D (m)

8 325

30 115

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Simulation rSimulation resultsesults

Catastrophic release

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Simulation rSimulation resultsesults

•100 cm dia. hole

•Hole level 2 m

•Pin = 1.5 bar

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Simulation rSimulation resultsesults

•100 cm dia. hole

•Hole level 2 m

•Pin = 1.5 bar

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Simulation rSimulation resultsesults

•100 cm dia. hole

•Hole level 2 m

•No padding

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Simulation rSimulation resultsesults

•100 cm dia. hole

•Hole level 2 m

•No padding

ConclusionsConclusions• LNG presents various sources of LNG presents various sources of

hazardshazards• RPT explosions do not generate large RPT explosions do not generate large

distance impact areasdistance impact areas• Thus RPTs don’t seem to represent a Thus RPTs don’t seem to represent a

main hazard to public safetymain hazard to public safety• However, they still can generate However, they still can generate

further damages close to the spill further damages close to the spill location, due to:location, due to:– Brittle fractureBrittle fracture– Thermal effectsThermal effects– OverpressureOverpressure

ReferencesReferences• W.E. Baker, P.A. Cox, P.S. Westine, J.J. Kulesz, R.A. Strehlow, W.E. Baker, P.A. Cox, P.S. Westine, J.J. Kulesz, R.A. Strehlow,

Explosion hazards and evaluation, Elsevier, Amsterdam, 1983.Explosion hazards and evaluation, Elsevier, Amsterdam, 1983.• G.B. Whitham, On the propagation of weak shock waves, Journal of G.B. Whitham, On the propagation of weak shock waves, Journal of

Fluid Mechanics 1 (1956) 290.Fluid Mechanics 1 (1956) 290.• A.C. van den Berg , M.M. van der Voort, J. Weerheijm, N.H.A. A.C. van den Berg , M.M. van der Voort, J. Weerheijm, N.H.A.

Versloot Expansion-controlled evaporation: a safe approach to Versloot Expansion-controlled evaporation: a safe approach to BLEVE blast, Journal of Loss Prevention in the Process Industries 17 BLEVE blast, Journal of Loss Prevention in the Process Industries 17 (2004) 397–405(2004) 397–405

• Lighthill, J.(1978). Waves in fluids.Cambridge : Cambridge Lighthill, J.(1978). Waves in fluids.Cambridge : Cambridge University Press.University Press.

• Reid, R.C.(1976).Superheated liquids. American Scientist, 64, 146–Reid, R.C.(1976).Superheated liquids. American Scientist, 64, 146–156.156.

• Reid, R.C.(1979). Possible mechanisms for pressurized-liquid tank Reid, R.C.(1979). Possible mechanisms for pressurized-liquid tank explosions or BLEVE’s. Science, 203, 3.explosions or BLEVE’s. Science, 203, 3.

• Strehlow, R.A. (1981).Blast wave from deflagrative explosions: an Strehlow, R.A. (1981).Blast wave from deflagrative explosions: an acoustic approach. 13th AIChE loss prevention symposium, acoustic approach. 13th AIChE loss prevention symposium, Philadelphia (PA).Philadelphia (PA).

• A. Luketa-Hanlin, A review of large-scale LNG spills: Experiments A. Luketa-Hanlin, A review of large-scale LNG spills: Experiments and modeling, Journal of Hazardous Materials A132 (2006) 119–and modeling, Journal of Hazardous Materials A132 (2006) 119–140140

• C. Conrado, V. Vesovic, The influence of chemical composition on C. Conrado, V. Vesovic, The influence of chemical composition on vaporization of LNG and LPG on unconfined water surfaces, Chem. vaporization of LNG and LPG on unconfined water surfaces, Chem. Eng. Sci. 5 (2000) 4549–4562.Eng. Sci. 5 (2000) 4549–4562.

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