Modelling shock waves from BLEVEs - · PDF file• Loss of containment causes sudden boiling and flashing, ... accident/experiment data ... • Sudden expansion of vaporized liquid

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Modelling shock waves from BLEVEs

FLUG meeting, June 1st 2016

©Lloyd’s Register Consulting

Title

• Intro

• BLEVE

• Consequence modelling

• Modelling the pressure waves in FLACS

• Comparison with experiments


Boiling Liquid Expanding Vapour Explosion

• Explosion resulting from failure of a vessel containing a liquid at temperatures far above its normal boiling point at atmospheric conditions.

• Loss of containment causes sudden boiling and flashing, generating shock waves

• If liquid is above its superheat level, the reaction is more violent

• If combustible, ignition means added danger in fireball radiation and explosion overpressures


Consequence modelling

Empirical methods (TNO Yellow Book, Lee’s Loss prevention):

• Pressure waves: energy calculated and put in TNT-equivalent methods:

• Genova (2008) – energy estimated from thermal head cp x (Tinit – Tboil) x 0.07

• Birk (2007) – energy from isentropic expansion of vapour cap (ignores liquid boiling)

• Casal (2006) – energy based on level of superheat

• Prugh (1991) – isentropic expansion from vapour+flashed liquid

• Projectiles: accident/experiment data

Ignited BLEVEs:

• Fireball radiation: empirical formulae available

• Blast waves from ignition: currently not estimated (?), even though blast strength on par or may even be larger than shock waves from rupture/boiling


Modelling pressure waves

• Two main sources of shock waves:

• Sudden release of vapour overpressure

• Sudden expansion of vaporized liquid

Liquid

Vapour


Modelling pressure waves – vapour cap

• Vapour cap:

• Straightforward to model, physics captured ok by FLACS.

• The vapour cap is added as a high-pressure region

• Finite opening times can be achieved using panels


Modelling pressure waves – liquid expansion

• Physics not captured by FLACS – using empirical pseudo-source to release appropriate amount of energy.

• Based on analogy to analytical/empirical consequence models based on one-dimensional energy-equivalent methods. E.g. Genova (2008) uses 7% of the thermal energy available – we use a similar approach.

• Pseudo-source in FLAGS: a high-pressure, temperature at boiling point, region with 20% of the estimated flashed gas.


FLACS setup

• Initial state, two high-pressure regions

• KEYS “PS1=01” - to allow supersonic flow

• Grid size 5-10 cm, stretch as little as possible…

• CFL on the order 0.1

• Running time ~1,000 CPU-hours per second of simulation time with 5 million cells

Example to illustrate concept


Experiments

To test the method, the model was compared with British Gas experiments (Johnson, 1999, add ref). Experiments consisted of 5+1 tests, where one of the tests was repeated:

• Propane or butane

• Controlled amount of vapour and liquid

• Release by rupturing vessels with shaped charges to ensure repeated releases

Experiment Mass

(t)

Pressure

(barg)

Gas Liquid fill

(-), by volume

1R 2 15.1 Butane 77%

2 2 15.2 Butane 39%

3 1 7.7 Butane 68%

4 2 15.1 Butane 40%

5 2 15.2 Propane 80%


Simulation of British Gas experiments

• 0.1 m resolution around vessel, stretched to 1 m in x and y.

• Measurement points corresponding to experiment at 25 and 50 m in the perpendicular and axial directions

MP 1 MP 2

MP 10

MP 11

25 m

25 m

25 m 25 m



10 ms 20 ms



30 ms 60 ms


Comparison with experiments

• BLEVE 1

Perpendicular

Axial


Experiments

• Generally conservative with vapour and liquid (simulated pressures at least 100% –200% of measured pressures)

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 1 2 3 4 5

Pre

ssure

Pre

ssure

Pre

ssure

Pre

ssure

rati

ora

tio

rati

ora

tio

sim

ula

tion/e

xperim

en

tsi

mu

lati

on/e

xperim

en

tsi

mu

lati

on/e

xperim

en

tsi

mu

lati

on/e

xperim

en

t

BLEVE BLEVE BLEVE BLEVE experimentexperimentexperimentexperiment numbernumbernumbernumber

Vapour only

Vapour and liquid


BLEVE in the open vs in a tunnel

Same BLEVE, placed in the open and in a tunnel












































Conclusions

Vapour space simulated well, physics captured.

Liquid evaporation must be accounted for the using a pseudo-source. Since there are many unpredictable process involved, especially regarding the rupturing process (rupture time, vessel weakness and rupture pressure/temperature), a good deal of conservatism must be used.

Simulations of shock waves from BLEVEs can be captured at least as accurately as 1D energy-methods, with added bonuses:

• 3D-effects

• Realistic geometries may be used (plants, railways, tunnels, platforms, inside buildings and so on)

• Non-symmetric effects due to vessel shape captured


Conclusions, continued

Some particular issues, in comparison with the typical simulations that are performed with FLACS - many related to that the solver in FLACS is not specialized to solve this type of wave problems:

• More work than normal required when setting up simulations

• Crashes more often due to large gradients

• Cartesian grid not optimal

• Short time-steps, fine grids and long simulation times are required, limiting use for smaller case studies (the Euler solver available in FLACS could probably reduce running time without compromising results)

• Huge rd and r3-files requires lots of disc space, but necessary for proper checking

Despite this, results are in acceptable agreement with experiments and with a developed workflow, FLACS it well suitable to use for BLEVE case studies in real settings, in particular as part of larger studies. There might also be potential to account for effect from ignited BLEVEs (blast waves and radiation?).


Thank you for your attention!

Lloyd’s Register and variants of it are trading names of Lloyd’s Register Group Limited, its subsidiaries and affiliates.Copyright © Lloyd’s Register Consulting. 2014. A member of the Lloyd’s Register group.

Working togetherfor a safer world

Malte KjellanderSenior ConsultantLR Consulting BergenE [email protected]

Lloyd’s Register Consultingwww.lr.org/consulting

Documents

Modelling shock waves from BLEVEs - · PDF file• Loss of containment causes sudden boiling and flashing, ... accident/experiment data ... • Sudden expansion of vaporized liquid