Fire & Explosion Hazards - Overpressure

Hydrogen Explosions

Hydrogen in sufficient concentrations and quantities can create a harmful overpressure, which may result in direct hazards from the overpressure and indirect hazards from building damage or flying debris. Overpressures can occur because of unignited releases of pressurized gas or because of ignition of a cloud of released flammable gas.

Overpressure from Unignited Releases

Any cryogenic fluid that is warmed and vaporized occupies significantly more space as a gas. Liquid hydrogen expands to about 850 times its volume from liquid to gas phases. This means that a confining vessel, pipeline, or sealed space could easily become over-pressurized when hydrogen vaporization occurs.

If a pressurized gas container is heated, the gas will continue to expand, creating an unsafe condition. This occurs when the pressure exceeds the container design and a mechanical failure happens.

Pressure-relief devices (PRDs) such as rupture disks or relief valves should be installed to prevent overpressure from occurring. The PRD should be vented to a safe location.

Overpressure from Ignited Releases

In addition to overpressure associated with the stored gas, flammable gases like hydrogen can burn or combust. If a cloud of gas is ignited, the rapid combustion of hydrogen can create an overpressure. This is the common perception of an "explosion."

As with gasoline vapors, certain precautions must be taken with respect to limiting the amount of ignition sources (such as lit cigarettes or unclassified electrical equipment) from areas where a release of hydrogen could form a hazardous cloud with sufficient concentration to create an ignited overpressure. Often these areas are referred to as "exclusion zones" or "separation distances."

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