Metallurgical examination of the two failed disks by light optical microscopy (LOM), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopic analysis (EDS) found them to be fabricated from pure nickel with evidence of extensive fracture. Each of the 24 tubes in the system is protected by a burst disk. Examination of another disk in the system that had not given way found that it, too, possessed surface fracture features, and they extended around the entire periphery of the rupture disk. Such defects are indicative of hydrogen embrittlement. An inspection of all vent circuits found that each of the 24 disks in service was made from nickel. Nickel is a material not recommended for hydrogen service in rupture disks.

Prior to the attempted use of the tube bank for hydrogen service, the vessel had been employed for helium service. The pressure vessel documentation accompanying the system indicated that the burst disks were made of stainless steel and rated to 10,000 psig. Careful physical inspection of system hardware is recommended on any system being adapted to hydrogen service. In this instance, inspection conducted prior to the transfer in service could have alerted operators to the need to install disks with the proper material, and therefore, have prevented the incident.

Relief of hydrogen gas should not lead to movement of the vent line sufficient to cause system damage. Corrective actions included increasing the line diameter and adding bracing between the lines and the system bulkhead to strengthen the components should other releases occur. The hardware that failed was of a commercial origin. Caution should be exercised to insure that all hardware is adequate for its designed purpose, even when procured from a commercial source.

More information on management of change can be found in the Lessons Learned Corner and also in the Hydrogen Safety Best Practices Manual. A web-based resource developed by Sandia National Laboratories to provide data on hydrogen embrittlement of various materials is available at Technical Reference for Hydrogen Compatibility of Materials.