Pressure piling

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Pressure piling happens when burning gas in a tube or long vessel compresses the gas in front of the flame and heats it up. As the flame moves, the trapped gas increases in pressure, which can rise to about two to eight times the starting pressure depending on the shape and layout of the system.

If several vessels are connected by pipes, starting a fire in one vessel can push gas into the others. This can turn a slow flame (deflagration) into a much more powerful explosion (detonation), creating a pressure spike far larger than a single vessel would.

In hazardous electrical equipment, if two enclosures are connected by a conduit, an explosion in one compartment can travel through the conduit into the next. The combined pressure can be huge, and heat or sparks from the equipment can ignite surrounding gas or vapour.

To reduce risk, avoid linking classified equipment with conduits. Use barrier glands on cables to keep compartments separate and limit the spread of flames and pressure.