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

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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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slide1
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
Introduction
  • Liquefied natural gas (LNG) market is increasingly expanding
  • Storage, handling and transportation of large volumes is involved
  • Large-scale hazards ??
general data
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
lng hazards
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
rapid phase transition rpt
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)
lng release on water
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
release dynamics
Release dynamics

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

experimental data
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)
lng composition
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

uncertainties
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
pool boiling
Pool boiling

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

pool boiling12
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)

rpt modelling
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.

rpt modelling14
RPT modelling

Phase envelope for an LNG mixture

Source: SuperChemsExpert v5.7, ioMosaic Corp.

rpt modelling15
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:

rpt modelling16
RPT modelling

Under acoustic assumption:

and in spherical coordinates for radius r:

POTENTIAL WAVE

EQUATION

where co is the ambient speed of sound.

rpt modelling17
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).

rpt modelling18
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

example of application
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

results
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

results21
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

results22
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

alternative model
Alternative model

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

simulation r esults
Simulation results

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

Catastrophic release

simulation r esults25
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
simulation r esults26
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
simulation r esults27
Simulation results

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
Simulation results

Release dynamics from a 27 m diameter tank, almost full

( 10000 m3)

  • 100 cm dia. hole
  • Hole level 2 m
  • No padding
conclusions
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
references
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.
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