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How Pressure Relief Valves Make Action Films Impossible

Most people know that films, particularly more conventional action films, take some quite major liberties with reality.

Most people know, for example, that if you fall from a great height into a body of water at a speed too fast for the liquid to displace properly and in the wrong position you can break bones, suffer concussions or even immediately die.

However, one piece of fiction that catches a surprising number of people out is the belief that if a fuel tank suffers a strong enough impact, either from a crash or a bullet piercing it, the fuel tank would explode and take the rest of the car with it.

Thanks to careful design, a pressure relief valve and the nature of most fuels, this is unlikely to be the case, however.

An explosion by definition requires a volume of rapidly expanding gas in an exceptionally short length of time in a small, pressurised container.

A relief valve, vent valve or breather valve, as well as vapour recovery systems and sensors, will ensure that a car’s fuel tank is never pressurised to a dangerous degree. Because of this, a car with a working fuel tank ventilation system will not explode by itself.

Even if that system was blocked, petroleum fuels such as petrol or diesel are flammable but not explosive, and the tanks themselves are far more likely to melt or crack before an explosion can occur.

However, that is not to say a superheated fuel tank is not dangerous; most people assume that fuel tanks do explode in no small part because once they catch fire they burn very quickly, which can look very similar to an explosion.

Similarly, an electric car with a malfunctioning battery would explode due to the nature of the chemical reactions used to generate the electricity needed to power the car.