What is explosion overpressure?
Explosion overpressure is the transient pressure rise above ambient generated when a flammable cloud burns rapidly in a congested or confined region. It is quantified in bar or kPa and, together with impulse and duration, defines the blast load used for building damage, escalation and occupied-building risk assessment.
Overpressure damage thresholds
- 0.02 bar (2 kPa) — window glass breakage; the usual nuisance and minor-injury threshold.
- 0.07 bar (7 kPa) — minor structural damage to buildings, doors and cladding.
- 0.14 bar (14 kPa) — partial demolition of unreinforced buildings; common occupied-building screening level.
- 0.35 bar (35 kPa) — near-total destruction of ordinary buildings and serious damage to process equipment.
Why congestion matters more than cloud mass
A flammable cloud burning in the open produces very little overpressure. Overpressure comes from flame acceleration, which is driven by repeated turbulence generation as the flame front passes obstacles — pipe racks, structures, vessels, cable trays. This is why the congested volume, obstacle blockage ratio and confinement are the controlling parameters, not simply the total cloud size.
Only the part of the cloud inside congestion contributes meaningfully to the blast.
Blast modelling methods
- TNT equivalence — simple, based on an explosion yield fraction; poor at representing the pressure-time history in the near field.
- TNO Multi-Energy — treats each congested region as a separate blast source with a strength class; widely used in QRA.
- Baker-Strehlow-Tang — links flame speed to reactivity, congestion and confinement.
- CFD explosion modelling — resolves geometry explicitly and is the reference approach for complex offshore modules.
How results are used
Overpressure results feed occupied-building siting studies, blast-resistant design requirements, control-room location, and escalation checks on pressurised equipment. Both the peak overpressure and the impulse matter: a short high-pressure pulse and a long lower-pressure one can cause very different damage to the same structure.
Frequently asked questions
What is the difference between a VCE and a BLEVE?
A vapour cloud explosion is the rapid combustion of a dispersed flammable cloud. A BLEVE is the catastrophic failure of a vessel holding liquid above its atmospheric boiling point, producing a blast from flashing liquid plus, for flammable contents, a fireball.
Why is deflagration distinguished from detonation?
A deflagration propagates below the speed of sound in the unburned gas and produces a relatively slow pressure rise; a detonation propagates supersonically and produces much higher, sharper peak pressures. Most process vapour cloud explosions are deflagrations, but strong congestion can drive transition.
Does overpressure kill directly?
Direct blast lung injury requires very high overpressure. Most fatalities in vapour cloud explosions come from building collapse and flying debris, which is why occupied-building assessment focuses on structural response.
Calculators for this topic
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases