During normal operations, three out of four cap head screws securing a socket cap on a pressure switch failed, resulting in a release of gas. The fractured cap screws had been fitted in the flanges of the process entry end of the stainless steel pressure switch which was being used in the fuel gas supply line to a turbine. Nearby gas detectors for the turbine combustion air intake ducting detected the gas and initiated the GPA. A shutdown and blowdown was then performed.
Subsequent investigations have revealed that the cap screws fractured as a result of chloride stress corrosion cracking. The primary cause of fracture was the combination of marine environment (possibly warm), and a susceptible material coupled with the stresses on the cap screw.
Pressure switch details: Pyropress Titan Flame Proof Pressure Switch, Model: PF261S1B/7W10F75P/22S1DB. The socket head cap screws were stamped with the markings “THE A2-70”, and had the dimensions M5x40mm. The screws had a threaded shank with a short unthreaded section under the cap screw head.
Water ingress can find a path between flange and pressure switch bodies, running onto threaded stud connections, creating a corrosion cell resulting in chloride stress corrosion cracking. If the bolts are a lower grade stainless steel and/or the temperature in the area is high, this can accelerate the problem.
It is recommended that the cap screws are replaced with a material more resistant to chloride stress corrosion.
Stainless steel pressure switch being used in the fuel gas supply line to a turbine.