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 as a result of unignited releases of pressurized gas or as a result of ignition of a cloud of released flammable gas.
Overpressure from Unignited Releases
- As with any cryogenic fluid, if it is warmed and vaporizes into a gaseous state, it occupies significantly more space. Liquid hydrogen expands to about 850 times from liquid to gas phases. Therefore a confining vessel, pipeline or sealed space could easily become over pressurized.
- If a pressurized gas container is heated the gas will expand creating an unsafe condition. This occurs when the pressure exceeds the container design and a mechanical failure occurs.
- Pressure-relief devices (PRDs) such as rupture disks or relief valves should be installed to prevent an overpressure from occurring. The PRD should be vented to a safe location.
Overpressure from Ignited Releases
- Beyond 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 at the fueling station, 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'.