Analysis of Stop Valve Leaks - Environmental Stress Cracking of Styrene Copolymer Valve Stems

Numerous claims of water damage to residential properties were related to the failures of one particular stop valve model. An engineering evaluation of three representative leaking stop valves was initiated to determine the cause of the stop valve failures. The subject stop valves contained a polystyrene-based copolymer (e.g., ABS or SAN) valve stem that provided a leak path for the valve upon failure through its hollow interior. Each leaking valve exhibited brittle fracture of the polymer valve stem. The mechanism for the brittle fractures was environmental-stress cracking (ESC). The likely source of the sustained stresses for the ESC failures was residual stresses from manufacturing. The incompatible chemical environment for the ESC failures was silicone-containing sealants or lubricants within the valve body interior that were introduced during valve manufacturing. Avoiding the use of silicone-containing oils or greases that could come into contact with styrenic polymers, such as ABS and SAN, was recommended to prevent future similar valve failures.