Recent Advancements in Vapor Cloud Explosion Modeling for

Recent Advancements in Vapor Cloud Explosion Modeling for Onshore Installations

Vijay Raghunathan, DNV Energy25 Oct 2006

Version Slide 208 November 2006

Introduction

Facility Siting Issue

Facility Siting Standards- API RP 752, CCPS

Confinement and CongestionRef:Hoorelbelke, P., Izatt, C.;Brewerton, R.W. “Vapor Cloud Explosion

Analysis of Onshore Petrochmeical Facilities” ASSE, 2006.


Vapor Cloud Explosions (VCE)

Vapor Cloud Formation- Rapid Release - Mixing- Ignition- Flame propagation

Vapor Cloud Explosion- Turbulence- Partial Confinement

Ref: http://www.tworzywa.com.pl/zagadnienia/foto/awarie_chemiczne_3.jpg


VCE Incidents

Explosion at the Flixborough Plant (1974)

Piper Alpha Explosion (1988)

Ludwigshaven Rail Car Incident (1943)

Texas City Explosion (2005)


VCE Modeling

Empirical ModelsEmpirical models also referred to as quasi-theoritical and are based on limited experimental data and can be considered the most simplified method for estimating the blast effects from vapor cloud explosions.

Phenomenological ModelsPhenomenological models are simplified physical models, which seek to represent only the essential physics of explosions.

Computational Fluid Dynamics (CFD) ModelsComputational Fluid Dynamics (CFD) model solves the partial differential equations governing the explosion process


Empirical Models

TNT Equivalency MethodThe TNT equivalency method is based on the assumption that explosive power of a vapor cloud explosion can be related to the mass of the TNT that can produce the same explosive power.

)(*)(*)(

TNTd

gascgasTNT H

EHWW

−

−

∆

∆=

TNTW

gasW

gascH −∆

E

TNTdH −∆

- Equivalent Mass of TNT, kg

- Mass of Gas involved in VCE, kg

- Heat of Combustion of Gas, J/kg

- Heat of detonation of TNT, J/kg

- Efficiency Factor (dimensionless)


Empirical Models

Limitations of TNT Model

- Weak Gas Explosions are not well represented

- Selection of yield factor

- Geometry of vapor cloud is taken into consideration

- Blast wave produced by hydrocarbons does not resemble TNT explosion


Empirical Models

TNO Multi Energy Model

This model is based on the assumption that vapor cloud explosion can occur only within that portion of a flammable vapor cloud that is partially confined.

33.00 )/(* EPRR =

−

−

R

R

0P

E

- Scaled Distance from the charge, (dimensionless)

- Distance from the charge, m

- Ambient Pressure (Pa)

- Charge Combustion Energy, Joules


Empirical Models

Limitations of TNO Model

- The selection of the severity factor or charge strength is to be done based on the confinement and little guidance is provided for partial confinement geometries.

- Not clear how to deal with several congested regions

- Difficult to represent complicated geometries.


Empirical Models

Baker-Strehlow ModelThe model is quite similar to the TNO model with the premise that a vapor cloud explosion can occur only within that portion of a flammable vapor cloud that is congested.

The steps followed in this model are very similar to the TNO model and the only critical difference is the selection of an appropriate flame speed (Mach number) instead of the charge strength.

- Model can handle multi ignition points.

- Accounts for confinement in the model.

- Easy and fast to use

- Overpressure results on the conservative side.


Phenomenological Models

SCOPE (Shell Code for Over-pressure Prediction in gas Explosions)

- Developed at Shell's Thornton Research Centre, March 1994.

- The model is one-dimensional in nature and is based on the idealized geometry of a vented vessel containing a series of obstacle grids.

- SCOPE 3 replaced SCOPE 2 and was improvised to handle mixed scale objects.

- SCOPE 3 has been validated against more than 300 experiments.

- Future developments include modeling with effect of water deluge.



Limitations of SCOPE Model

- Does not provide the great level of geometric detail as the CFD Models.

- Can deal with single enclosures only.



CLICHE (Confined LInked CHamber Explosion)

- Developed by Advantica Technologies Ltd

- CLICHE code represents a generalization of the vented vessel explosion concept to a sequence of interlinked explosion chambers.

- Simple combustion model, based on a mixture of some fundamental physics and empirical correlations

- Can generate its own input parameters from an obstacle database

- Very short model run times


CFD Models

FLACS (FLame ACceleration Simulator)

- Originally developed at the Christian Michelsen Research Institute in Norway, now GEXCON.

- CFD codes provide great depth of information about flow field.

- Uses advance user interface like CASD( Computer Aided Scenario Design) and Flowvis.

- Can be applied to congested,but unconfined geometries.

- Extensive validation against small , medium and large scale experiments.

- Model supports CAD data input


CFD Models

AutoReaGasCouples the features of the REAGAS and BLAST codes developed by TNO and have been integrated into an interactive environment based on the AUTODYN-3D code developed by Century Dynamics Ltd.

Advanced CFD Models : REACFLOW, CFX-4 and COBRA produce more accurate results.


Conclusion

Empirical Models: Screening level

Phenomenological Models: Good substitute for CFD code

CFD Models: Detailed representation of the real scenario

Recent Advancements in Vapor Cloud Explosion Modeling for

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