Hydrogen released in sufficient quantities can create a harmful overpressure which may result in direct hazards 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
- When any cryogenic liquid 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.
- Gas containers can be over-pressurized by unintended operation, exposure to heat such as fire and other causes
- Pressure-relief devices (PRDs) such as rupture disks or relief valves must be installed in any hydrogen equipment 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 released into the air is ignited, the rapid combustion of hydrogen can create an overpressure or explosion.
- Precautions should be taken to eliminate ignition sources from areas where vented or released hydrogen can form a cloud with sufficient concentration to create an ignited overpressure.
- Examples of ignition sources: spark producing electrical equipment and an electrostatic discharge.
- Areas where hydrogen clouds can form are often known as ‘exclusion zones’ and are sometimes defined by separation distances that stipulate how far away structures, vehicles, and other equipment must be located.
Types of hydrogen combustion - Deflagration versus Detonation
- Deflagrations are combustion explosions in which there is subsonic flame propagation through the hydrogen-oxidant (typically hydrogen-air) mixture.
- Detonations are combustion explosions in which there is supersonic flame propagation through the hydrogen-oxidant (typically hydrogen-air) mixture, such that shock waves are generated. Detonations are frequently more destructive than deflagrations.
- Deflagrations can sometimes accelerate, e.g., when the flame propagates across repeated small obstacles or through long pipes, to produce deflagration-to-detonation transition (DDT). DDT does not occur in hydrogen concentrations near the flammable limits and is more likely to occur in large equipment or piping, or in very large hydrogen releases in a partially confined area.
- Deflagration venting per NFPA 68 Explosion Protection by Deflagration Venting is not an effective explosion protection measure when DDT occurs; it is effective for deflagrations.
- Detonation prevention and detonation pressure loads for gas mixtures in piping are described in NFPA 67 Explosion Protection for Gaseous Mixtures in Pipe Systems.
- Either a Deflagration or a Detonation would likely be perceived as an explosion by a bystander.