To avoid creating an explosive mixture of air and hydrogen inside any part of a hydrogen system, the air, oxygen, and any other oxidizers must be purged from the system prior to introducing the hydrogen. Similarly, when preparing a system for maintenance, the hydrogen must be purged from the system prior to opening the piping or equipment in order to avoid releasing hydrogen into the air where it could create a flammable mixture. Inert gas subsystems are typically used for these purging functions. They are also used to pressurize the system to check for leak tightness.
The inert gases are typically nitrogen for hydrogen gas systems and helium for liquid hydrogen systems. Helium is used because at liquid hydrogen temperatures nitrogen becomes a solid.
There are three general approaches to purging a system:
- Flowing gas purge uses an inert gas flowing into one part of the system and out of another part of the system. The success of this technique is dependent on the system geometry, e.g., it is more difficult to apply to a multi-branched system. Vent gases are directed to a safe location, e.g. a vent stack, to eliminate asphyxiation potential.
- Pressurizing-venting cycle purge uses alternating pressurizing with inert gas and venting to atmospheric pressure. This procedure stepwise dilutes the contents of a volume until the desired mixture concentration is obtained. This method can be used in systems that have long dead ends but requires pausing the purge when pressurized to allow the gases to mix. Pressurizing-venting cycle purge is typically used for purging Type IV cylinders and other components which cannot tolerate Vacuum Purging.
- Vacuum purging involves 1) venting the system to atmospheric pressure, then 2) pumping to a relatively low pressure with a vacuum pump, then 3) re-pressurized with inert gas to a positive pressure and 4) vented to atmospheric pressure. Depending on the goal of the purge and the capability of the vacuum pump, more than one cycle may be required. The vacuum pump must be suitable for the gases being evacuated, typically hydrogen, air, and the inert gas.