Pressure piling is a phenomenon related to combustion of gases in a tube or long vessel.
The amount of compression varies depending on the geometry and can range from twice to eight times the initial pressure.
Where multiple vessels are connected by piping, ignition of gases in one vessel and pressure piling may result in a deflagration to detonation transition and very large explosion pressure.
Heat, arcs or sparks escape from the equipment and ignite any gas or vapour that may be around.
Operators avoid this by not using conduits to join classified equipment together and by using barrier glands on cables going into the enclosure.