The correct answer is D (2–5%) because, according to CCPS guidance on vapor cloud explosions (VCEs) , only a small fraction of the total thermodynamic energy released during combustion is converted into blast overpressure in unconfined and uncongested conditions.
In such scenarios, the combustion process occurs relatively slowly (deflagration rather than detonation), and much of the energy is dissipated as heat, light, and expansion of gases , rather than contributing to pressure waves. As a result, only about 2–5% of the total energy is typically available to generate damaging overpressure effects.
Higher energy conversion efficiencies (such as 10% or more) are generally associated with congested or confined environments , where obstacles (pipes, equipment, structures) promote turbulence and flame acceleration, significantly increasing overpressure. In contrast, an open and uncongested area limits turbulence, reducing explosion severity.
Options A , B , and C significantly overestimate the fraction of energy contributing to blast effects, while E (0%) is incorrect because some overpressure is always generated.
CCPS emphasizes understanding explosion efficiency when performing consequence analysis, as it is a key factor in predicting blast damage and designing appropriate safeguards.
