Minimal criteria for Rapid Phase Transition explosion of cryogenic gases
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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 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


Introduction l.jpg
Introduction cryogenic gases

  • Liquefied natural gas (LNG) market is increasingly expanding

  • Storage, handling and transportation of large volumes is involved

  • Large-scale hazards ??


General data l.jpg
General data cryogenic gases

  • 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


Lng hazards l.jpg
LNG hazards cryogenic gases

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


Rapid phase transition rpt l.jpg
Rapid Phase Transition RPT cryogenic gases

  • 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)


Lng release on water l.jpg
LNG release on water cryogenic gases

  • 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


Release dynamics l.jpg
Release dynamics cryogenic gases

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


Experimental data l.jpg
Experimental data cryogenic gases

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)


Lng composition l.jpg
LNG composition cryogenic gases

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


Uncertainties l.jpg
Uncertainties cryogenic gases

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


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Pool boiling cryogenic gases

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


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Pool boiling cryogenic gases

  • 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)


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RPT modelling cryogenic gases

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.


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RPT modelling cryogenic gases

Phase envelope for an LNG mixture

Source: SuperChemsExpert v5.7, ioMosaic Corp.


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RPT modelling cryogenic gases

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:


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RPT modelling cryogenic gases

Under acoustic assumption:

and in spherical coordinates for radius r:

POTENTIAL WAVE

EQUATION

where co is the ambient speed of sound.


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RPT modelling cryogenic gases

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).


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RPT modelling cryogenic gases

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


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Example of application cryogenic gases

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


Results l.jpg
Results cryogenic gases

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


Results21 l.jpg
Results cryogenic gases

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


Results22 l.jpg
Results cryogenic gases

Acoustic model: overpressure profiles at different release time

Dashed lines: 0.08 and 0.3 bar

Discharge time: Red = 2 s; Green = 1 s


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Alternative model cryogenic gases

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


Simulation r esults l.jpg
Simulation r cryogenic gasesesults

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Catastrophic release


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Simulation r cryogenic gasesesults

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

  • 100 cm dia. hole

  • Hole level 2 m

  • Pin = 1.5 bar


Simulation r esults26 l.jpg
Simulation r cryogenic gasesesults

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

  • 100 cm dia. hole

  • Hole level 2 m

  • Pin = 1.5 bar


Simulation r esults27 l.jpg
Simulation r cryogenic gasesesults

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

  • 100 cm dia. hole

  • Hole level 2 m

  • No padding


Simulation r esults28 l.jpg
Simulation r cryogenic gasesesults

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

  • 100 cm dia. hole

  • Hole level 2 m

  • No padding


Conclusions l.jpg
Conclusions cryogenic gases

  • 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


References l.jpg
References cryogenic gases

  • 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.



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