A Critical Review of the Equivalent Stoichiometric Cloud Model Q9 in Gas Explosion Modelling

Vincent Tam,Chris Savvides,Felicia Tan

doi:10.3390/eng2020011

Abstract

Q9 is widely used in industries handling flammable fluids and is central to explosion risk assessment (ERA). Q9 transforms complex flammable clouds from pressurised releases to simple cuboids with uniform stoichiometric concentration, drastically reducing the time and resources needed by ERAs. Q9 is commonly believed in the industry to be conservative but two studies on Q9 gave conflicting conclusions. This efficacy issue is important as impacts of Q9 have real life consequences, such as inadequate engineering design and risk management, risk underestimation, etc. This paper reviews published data and described additional assessment on Q9 using the large-scale experimental dataset from Blast and Fire for Topside Structure joint industry (BFTSS) Phase 3B project which was designed to address this type of scenario. The results in this paper showed that Q9 systematically underpredicts this dataset. Following recognised model evaluation protocol would have avoided confusion and misinterpretation in previous studies. It is recommended that the modelling concept of Equivalent Stoichiometric Cloud behind Q9 should be put on a sound scientific footing. Meanwhile, Q9 should be used with caution; users should take full account of its bias and variance.

Highlights

It is recommended that the modelling concept of Equivalent Stoichiometric Cloud behind Q9 should be put on a sound scientific footing
Gas explosion risk assessment forms a key part of major hazard risk assessments in the oil and gas and petrochemical industries
Simplifications are often made in order to make the assessment tractable within time and resource constraints. This is because a typical explosion risk assessment, say for an offshore facility, could assess thousands of scenarios

Summary

Introduction

Gas explosion risk assessment forms a key part of major hazard risk assessments in the oil and gas and petrochemical industries Central to this assessment is the quantification of consequences of gas explosions, such as loading (i.e., drag and overpressure) impact on structures, equipment and buildings. This assessment includes the calculation of consequences of chains of events prior to gas explosions (examples include failures of containment, formation of flammable gas clouds, their ignitions), as well as those proceeding them (for example: overpressures, blasts, wind, and their impact on equipment, structures, etc.). This is because a typical explosion risk assessment, say for an offshore facility, could assess thousands of scenarios

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