A safety research laboratory experienced two similar air-actuated ball valve failures in a 6-month period while performing hydrogen release experiments. The hydrogen release system contains a number of air-actuated ball valves which are sequenced by a Programmable Logic Controller (PLC) in order to obtain the desired release parameters. During an experimental release sequence, a PLC valve command failed to open the valve even though the PLC valve position confirm signal indicated the valve had opened. On further investigation, the valve actuator and valve stem were found to be moving correctly, but the valve was not opening. The system was depressurized and purged with nitrogen, and the valve was removed for inspection. Inspection required dismantling the valve, and in both incidents a sheared valve stem was found. The valve stem failures occurred after 8 to 14 months of continuous hydrogen operation at pressures of 800 to 850 bar (11,603 to 12,328 psi). The two failed hydrogen valves were identical and rated by the valve manufacturer for hydrogen service.
The cause of this incident has not been determined, but valve stem material incompatibility with hydrogen (causing a material weakening) is suspected, although it could also have been a design flaw. Review of the valve manufacturer's drawings showed the valve stem material to be 17-4 precipitation hardening stainless steel that has been reported in the literature (Attachment 1) as having a 90% reduction in fracture toughness in a hydrogen environment. No metallurgical analysis was done on the failed valve stems to confirm a material failure. The valves were repaired with new valve stems made from 316 stainless steel, placed back in service, and have operated satisfactorily for 18 months without failure.
1. Ensure that equipment and materials exposed to hydrogen are compatible with a hydrogen environment (even hydrogen-service-rated equipment).
2. Equipment designs in a hydrogen environment that use the fracture toughness for 17-4 precipitation hardening stainless steel may want to consider a 90% derating in published values (see Attachment 1).
Additional discussion of materials selection for compressed gas storage vessels can be found in the Hydrogen Safety Best Practices Manual. Similar discussion of materials selection for cryogenic liquid storage vessels can be found 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.