Vapour cloud explosion overpressure calculator
A vapour cloud explosion occurs when a flammable cloud ignites within congestion and the flame accelerates enough to generate a blast wave. The engineering question is the distance to each overpressure damage level, which depends far more on the congested volume and the confinement than on the total cloud mass.
Open the Vapour Cloud Explosion moduleWhat this calculator returns
- Flammable mass within the congested region
- Blast energy and equivalent charge
- Side-on overpressure versus distance
- Distance to 70, 140, 200 and 350 mbar damage levels
Required inputs
- Flammable cloud mass from the dispersion calculation
- Congested volume and obstacle density
- Confinement geometry and ignition location
- Fuel heat of combustion and reactivity class
Calculation method
Only the portion of the cloud inside the congested volume contributes to blast; the unconfined remainder burns as a flash fire.
Blast strength is selected from the congestion and confinement, then the scaled overpressure is read from the multi-energy blast curves as a function of the Sachs-scaled distance.
A TNT-equivalence check can be run in parallel using a yield factor as a sanity comparison against the multi-energy result.
Governing equations
E = M_congested dHcBlast energy from the flammable mass inside the congested volume.
R_scaled = R / ( E / P0 ) ^ (1/3)Sachs-scaled distance used with the multi-energy curves.
W_TNT = alpha M dHc / 4.68e6TNT-equivalent mass for the comparison method, with yield factor alpha.
Nomenclature
- M_congested
- flammable mass inside the congested region, kg
- dHc
- heat of combustion, J/kg
- E
- blast energy, J
- P0
- ambient pressure, Pa
- alpha
- TNT yield factor, typically 0.03 to 0.10
Assumptions and limitations
- Deflagration rather than detonation unless a highly reactive fuel and severe congestion are specified.
- The congested volume is filled with a stoichiometric cloud at the moment of ignition.
- Ground reflection is included by treating the source as a surface burst.
Reference practice
- Multi-energy method per TNO Yellow Book; damage levels consistent with CCPS explosion guidance and typical facility siting practice.
Worked example
A 500 kg flammable cloud, of which 150 kg lies inside a congested pipe rack, propane heat of combustion 46 MJ/kg.
| Step | Value | Basis |
|---|---|---|
| Contributing mass | 150 kg | Only the congested portion generates blast; the balance burns as a flash fire |
| Blast energy | about 6.9 GJ | E = 150 kg x 46 MJ/kg |
| Blast strength | Multi-energy class for a moderately congested rack | Set from obstacle density and confinement, not from cloud mass |
| Distance to 140 mbar | Read from the scaled curve at the selected class | 140 mbar is the usual threshold for serious building damage |
Doubling the cloud mass moves the overpressure contours by only the cube root; changing the congestion class moves them far more, which is why layout and congestion reduction dominate VCE risk reduction.
Illustrative numbers only — rerun the module with the project basis of design before using any result.
Common questions
What overpressure causes building damage?
About 70 mbar causes minor structural and glazing damage, 140 mbar serious damage to ordinary buildings, 200 to 350 mbar major structural failure and high occupant fatality probability.
Why is the multi-energy method preferred over TNT equivalence?
Because blast strength depends on flame acceleration in congestion, not on total energy. TNT equivalence with a fixed yield ignores that and mis-predicts the near field badly.
Does an unconfined cloud explode?
A genuinely unconfined, uncongested cloud of a low reactivity fuel burns as a flash fire with negligible overpressure. Congestion is what turns it into an explosion.
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