A high-pressure relief valve (400 psig) failed at a weld while in service, resulting in a flammable gas leak and autorefrigeration of the valve. The leak caused a plant shutdown.

Laboratory testing showed that the fracture surface of the failed weld was typical of cleavage. Hydrogem embrittlement was considered; however, the softness of the weld, appeared to preclude that failure mechanism. It was believed that the most logical explanation for the PSV valve failure was CO, CO₂, H₂0 stress corrosion cracking. This little recognized cracking mechanism occurs in a high pressure CO, CO₂, H₂0 environments. PPM levels of oxygen act as a cracking accelerator. Critical levels of CO are neccesary for cracking to occur. Stress relieving will slow cracking but will not eliminate it. Cracks can also occur in stress relieved base metal, depending on pressure stresses.