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How Pressure Relief Valves Prevent Major Tank Explosions

There are certain events that only seem to happen in films, TV shows and computer games, and a surprising number of them are thwarted in reality by vacuum relief valves.

One of these is the propensity in action films for heroes and villains respectively to shoot at propane tanks and other pressurised gas tanks and cylinders, causing them to explode. It was even part of a particularly popular segment of the television show Mythbusters.

However, propane tanks actually exploding is exceptionally rare, with a 1981 US Department of Energy study claiming there is roughly the same chance of dying in a propane explosion as dying in an aeroplane crash (one in 37 million).

There are many reasons for this, and all pressurised gas tanks are equipped with safety mechanisms designed to stop this exact situation.

The biggest, and most important, of these is the relief valve, which opens when the tank’s pressure reaches an unsafe level and vents out gas, allowing the tank to compensate and avoid potential catastrophe.

Tanks are also exceptionally hardy and are designed to avoid ruptures caused by both internal and external causes as much as possible.

Ultimately, there are only two main ways that propane explosions happen, both of which are either the result of misuse or unlikely circumstances.

The first is a gas leak, which is when excessive gas leaks from the tank, finds an ignition source and catches fire, which spreads to the pressurised gas and causes a rapid expansion and inevitable rupture of the tank.

The alternative is a “boiling liquid expanding vapour explosion” (BLEVE). This is caused by exceptionally unlikely situations where the tank’s pressure is higher than the valve can vent safely.

This almost exclusively happens due to exposure to exceptionally high temperatures which heats the compressed gases and leads to expansion too rapid to vent safely and a rupture of the tank.