Flash fire

What is a Flash Fire?

A Flash Fire is characterised by the combustion of a flammable vapour and air mixture, in which the flame passes through the mixture at less than sonic velocity, resulting in negligible damaging overpressure. In simpler terms, a Flash Fire involves the burning of a vapour cloud with a low flame intensity.

How does a Flash Fire arise?

A Flash Fire typically occurs following the release of flammable gases or vapours under calm or low wind speed conditions, leading to the formation of a flammable cloud at rest. Additionally, the flammable cloud can be formed from pool evaporation or the release of pressurised liquid undergoing flash vaporisation. If the flammable cloud disperses into the air and the mass of the flammable cloud falling within the flammability limits comes into contact with a source of ignition, a Flash Fire occurs. Upon ignition, the flame front moves from the ignited region in various directions across the cloud. Common substances involved in Flash Fires include LPG (Liquefied Petroleum Gas) and LNG (Liquefied Natural Gas).

The video below from Gexcon’s Fire and Explosion Testing facility shows an example of a refrigerant gas release close to the ground and later ignition resulting in a Flash Fire.

As the flame propagates further from the ignition point, the expansion from combustion products causes a laminar flow ahead of the flame. Typically, the flame speed under laminar or near-laminar conditions is too low to cause significant blast overpressure. In such scenarios, the combustion rate does not intensify, and the vapour cloud simply burns. The burning velocity is highly dependent on the type of fuel and wind speed. The duration of a Flash Fire is brief and primarily affects the area covered by the flammable mixture. Conversely, the thermal radiation’s impact on the area outside the flammable cloud is significantly smaller and can be considered negligible.

When analysing the effects of a Flash Fire on equipment, it is often assumed that the damage to equipment within the cloud area is minimal, except for floating roof tanks where a Tank Fire may arise. Therefore, the damage caused by equipment and subsequent domino effects is insignificant in relation to a Flash Fire. However, flames frequently reach the source of the released flammable mass, leading to subsequent events such as a Jet Fire or Pool Fire. The impact on the population outside the cloud area is negligible, but within the cloud area, it can be fatal. Consequently, the effects calculation primarily focuses on predicting the area affected by the Flash Fire. The size and location of the flammable cloud can be determined by using dispersion models.

In contrast to a Flash Fire, ignition of a large confined flammable mass leads to explosive combustion accompanied by a blast effect. In such cases, the accident is classified as an explosion (Vapour Cloud Explosion) rather than a Flash Fire (Bosch, 2005).

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.