(43ad) Effect of Flammable Cloud Treatment On Predicted Blast Loads

Effect
of Flammable Cloud Treatment on Predicted Blast Loads

J. Kelly Thomas and Jihui Geng

Baker Engineering and Risk
Consultants

3330 Oakwell Court, Suite
100

San Antonio, TX  78218-3024

(210) 824-5960

KThomas@BakerRisk.com

Computational fluid dynamic (CFD) analysis of flammable
gas dispersion can provide a realistic evaluation of the resulting flammable
gas cloud.  CFD dispersion analysis can
provide a more realistic assessment of the resulting gas cloud in heavily
congested and/or confined environments relative to simplified dispersion
analysis methods.  In contrast to
simplified methods, the gas cloud predicted using CFD analysis in
congested/confined environments is typically complex, both in terms of shape
and concentration gradients.  The blast
loads predicted using CFD vapor cloud explosion (VCE) analysis for the resulting
flammable gas cloud are dependent upon the representation chosen for the gas
cloud.  Directly utilizing the predicted
dispersed gas cloud, with the corresponding complexities in shape and
concentration gradients, requires an extended series of analyses, since both
ignition location within the cloud and the cloud location can have a strong
influence on the predicted blast load.  A
common approach is to replace the dispersed gas cloud with a simplified
representation for the purposes of performing a CFD VCE blast load assessment,
such as a cubic shaped cloud with a uniform near-stoichiometric fuel
concentration.  This approach is very
attractive since it greatly simplifies the blast load assessment.  However, the relationship between the blast
loads predicted using a simplified cloud representation versus those predicted
for the actual dispersed cloud is not well defined.

This paper presents a comparison of the blast loads predicted
using the dispersed flammable gas cloud resulting from CFD dispersion analysis
versus a simplified representation of the gas cloud.  The FLACS CFD code was employed for this
purpose.  The simplified flammable gas
cloud representations used included the ?equivalent stoichiometric? gas cloud
parameters developed by GexCon (i.e., the Q9 approach) and utilizing the entire
flammable gas volume (i.e., from the lower flammability limit up to the upper
flammability limit).  Several typical
congested volume geometries were employed for this purpose.  Blast loads were assessed at locations both
within and external to the flammable gas cloud. 
A range of potential ignition locations were evaluated in order to
define the impact of ignition location on the predicted blast load relative to
that obtained with the simplified gas cloud representations.  It was found that, in general, the blast
loads predicted using the equivalent stoichiometric cloud are conservative
relative to those predicted for the dispersed cloud.  The blast loads predicted using the entire
flammable gas cloud volume were much larger than those
for the dispersed cloud for the cases evaluated.