Liquid System Pressure Relief
The ullage volume (headspace) specified for a cryogenic vessel should always be respected. With the addition of heat, liquid hydrogen will expand at a rate 23 times greater than water. Overfilling a cryogenic vessel can lead to entrainment of liquid in the pressure relief system and the possibility of over pressurization.
LH2 storage and handling facilities must be equipped with pressure relief devices (PRDs). The PRD protects piping and equipment from exceeding the maximum allowable working pressure. PRDs should be designed to discharge upward and unobstructed to the open air to prevent any impingement of escaping gas upon the vessel, adjacent structures, or personnel. See Venting for more detail about vent stacks.
Note that an LH2 cryogenic vessel will vent as part of normal operation whenever the rate of consumption of hydrogen by the process being supplied is less than the rate at which LH2 evaporates, or “boils off” in the vessel.
PRDs and vent piping must be designed and located so that moisture cannot collect, freeze, or interfere with proper operation of the relief device.
Types of PRDs include:
- rupture disks
- pressure relief valves
- breaking pin devices
Since the PRD will cycle many times as part of the normal operation of a cryogenic vessel, it must be designed specifically for repeated cycles in a hydrogen environment.
If the PRD fails to function as designed, an overpressure condition could occur, possibly resulting in equipment failure.
Pressure relief valve settings should be checked and verified to be correct for the installation and may be required to be periodically tested.
Redundant pressure relief protection is recommended for liquid storage tanks.
CGA H-3, Standard for Cryogenic Hydrogen Storage provides guidance for design of pressure relief systems.
If a liquid storage tank has been empty for a year or more, safety features such as pressure relief devices should be thoroughly checked before the tank is filled. Tanks that are warm need special filling procedures regardless of the length of inactivity.