A facility replaced the copper tubing used for hydrogen distribution, with stainless steel tubing. This was done to address a fire protection concern related to the solder on the copper tubing being susceptible to heat, melting, and releasing a flammable gas. The facility maintenance personnel completed the replacement, noted the pressure on the hydrogen bottle, and left the building. When the maintenance person returned on the following day, s/he noticed the pressure on the hydrogen bottle had dropped 500 psi overnight, indicating a leak in the system. S/he notified the appropriate facility personnel and together they began to determine why the hydrogen had dropped 500 psi overnight. The hydrogen line originates at a manifold, which is part of a glove box atmosphere purification system that also includes an argon line. The onsite personnel experimented with the valves associated with the hydrogen and argon lines and determined that the check valve associated with the argon line was defective. As a result, the hydrogen was back flowing through the argon check valve into the argon supply line.
The direct cause of this occurrence was attributed to a failed argon check valve that allowed hydrogen to escape from the hydrogen supply manifold and enter the argon system. The contributing and root cause was identified as a design problem. The check valve installed to prevent the back flow of hydrogen into the argon system, designed to open with an upstream pressure of 1/3 psig. The product catalog states that valves with nominal cracking pressure of 8 psig or lower may require back pressure to reseal bubble-tight. The back pressure in the system was not sufficient to reseal a 1/3 psig valve. Product data suggests that a the valve, having a cracking pressure of 10 psig and not requiring back pressure to reseal, should have been chosen for this application. Also, double check valves (i.e., two check valves in series) should have been installed to provide double isolation between the two gas systems. However, it is doubtful in this case if two valves would have been adequate to prevent the backflow of hydrogen since the wrong sized valve (i.e., 1/3 psi) was initially chosen.
The hydrogen was sufficiently diluted with argon in the argon system and did not pose a safety, fire, or explosion hazard.
This occurrence underscores the importance of appropriate design, equipment selection, and design review; the potential drawbacks of sharing utilities with other facilities; and the need to simplify [in this case, glove box atmosphere purification equipment/process] operations as much as possible.