A 9,000-gallon (34,069-liter) cryogenic liquid hydrogen storage vessel, installed outdoors at a manufacturing plant in an urban area, over-pressurized and released hydrogen into the atmosphere through a safety relief device (burst disk). When the burst disk released pressure, a loud bang was heard by neighbors and reported to the local police. The police investigated and heard the sound of gaseous hydrogen escaping from the vessel's vent stack, which rose approximately 15-20 feet (4.6-6.1 meters) in the air.
Police called the local fire department. Firefighters entered the facility and told employees inside that there was an explosion on the property and they needed to evacuate. As a precautionary measure, some nearby city buildings were also evacuated and the street was blocked off in front of the facility.
A facility representative went to the storage vessel to investigate. This individual saw that hydrogen was escaping from the vent stack, but there was no fire. The individual called the industrial gas company that services the installation to report the incident, and a field service technician was dispatched to the scene.
The technician arrived to find the tank pressure at zero and the burst disk blown. He switched the three-way diverter valve to the other safety relief device and replaced the burst disk when the line defrosted. After rebuilding the pressure, the field service technician leak-checked the lines and did not see any indication of fire at the stack ends.
Because the cryogenic liquid hydrogen storage vessel had experienced a long period of non-use, heating by the ambient air temperature (60°F/16°C) caused the vessel's internal temperature to rise even though it was vacuum-jacketed. The normal storage pressure for this vessel was 150 psi (10.3 bar). There were no injuries or damage from this incident, and inspectors from the manufacturing company and industrial gas company stated that the hydrogen safety venting equipment functioned properly. Normal operations resumed after it was determined that there were no unsafe conditions.
Approximately seven months later, the burst disk that had been replaced in the incident described above ruptured for the same reasons --- back pressure against the burst disk caused premature failure. No outside emergency response was involved with this second hydrogen venting incident, as once again the pressure relief system worked as designed. Following this incident, some modifications were made to the hydrogen piping to eliminate all back pressure on the burst disk.
- Hydrogen Storage Equipment
- Pressure Relief Devices
- Burst Disk
Hydrogen safety training should be provided to local emergency responders.
Liquid hydrogen installations should be inspected by facility personnel on a frequent basis, consistent with NFPA 55, to verify proper operation and inspect for physical damage or leaks. If there are problems, contact the servicing company immediately.
Industrial gas companies that design, install, and maintain liquid hydrogen installations should follow the guidelines set in the Compressed Gas Association (CGA) publications: G-5.4 - Standard for Hydrogen Piping Systems at Consumer Locations, G-5.5 - Hydrogen Vent Systems, H-3 - Cryogenic Hydrogen Storage, and H-5 - Installation Standards for Bulk Hydrogen Supply Systems.
Burst disks are highly sensitive to any form of back pressure.