A pressure relief device (frangible burst disk) on one of a hydrogen delivery tube trailer's 26 tubes failed prematurely and released hydrogen while filling a hydrogen storage tank at a government facility (see Attachment 1). Prior to the filling process, all procedures and safety checks, including connection to the facility's regulator/distribution control system with leak checking and follow-up verification of leak checking by facility personnel, were completed (see Attachment 2 for more details). During the filling process, a person walking near the facility heard the noise of escaping gas that included occasional popping sounds typical of bursts of gas release. Facility personnel were alerted and the tube trailer vendor's incident response team was dispatched to the incident location.

The hydrogen leak was safely contained and mitigated by the tube trailer vendor's incident response team. On arrival, the response team first noted a decrease in pressure from the shipped pressure of 2100 to 1700 psi (145 to 117 bar) at the tube trailer's common supply manifold. The hydrogen leak was located to a single tube on the tube trailer coming from a burst disk (over-pressure-protection rupture disk). The hydrogen gas from the tube trailer to the common supply manifold was shut off by closing the isolation valve on each of the 26 tubes. The leaking tube was vented to atmosphere by attaching a 10-ft (3-meter) vertically oriented pipe downstream and opening the leaking tube's isolation valve. After the leaking tube was completely vented, the leaking tube's isolation valve was closed and the vent pipe was removed. The final step was to disconnect the tube trailer from the facility's regulator distribution control and move it outside of the facility's designated hydrogen storage area. No property damage or injuries occurred from this incident.

The over-pressure-protection burst disks on each of the tube trailer's 26 tubes had a rated burst pressure of 4000 psi (275 bar). The purpose of the pressure relief device is to operate in accordance with 49 CFR §173.301(f). The pressure relief device must be capable of preventing rupture of the normally filled cylinder when subject to a fire test conducted in accordance with CGA C-14. Based on CGA C-4, the pressure relief device is designed to operate with a frangible burst disk in combination with a eutectic material, thereby minimizing the incidence of unintentional release by only operating in the event of a fire. The frangible (bursting) disk is designed to burst at 5/3 the container's pressure and is backed by eutectic material that melts at 100 °C (212 °F). The frangible burst disk material used (Inconel #600 series) had been the predominant material at the time the tube trailer was last requalified.

Post-event analysis of the incident included removal of the tube trailer's affected over-pressure-protection frangible disk. Inspection of the disk determined that it had ruptured and the disk opening was separated from the rupture disk holder. No visible indication or presence of corrosion or contamination was found. Subsequent testing of other pressure relief devices also showed disk rupture at lower-than-expected pressures.

The probable cause of this event was that the burst disk was degraded by the synergic events of exposure to hydrogen and stress associated with the working pressure involved. A contributing factor in this incident was the burst disk Inconel #600 material that has a high hardness/tensile strength, making it more susceptible to stress corrosion cracking (SCC). It should also be noted that the leak detection step prior to starting the hydrogen fill process is useful for finding leaks in threaded connections, but it could not have detected or predicted the premature failure of the burst disk.