Investigation of Bolt Failures on a Gate Valve From a Natural Gas Pipeline

This paper describes the results of an investigation of bolt failures that occurred in a gate valve from a natural gas pipeline. Failures of this type are not common; therefore, it was important for the operator to know the failure cause so that an effective mitigation strategy could be implemented. Accordingly, a standard failure analysis consisting of optical photography, metallography, and scanning electron microscopy examination of the fracture surfaces was performed. It was concluded that the failures were a result of environmentally assisted cracking (EAC). Coating the bolts was chosen as the simplest method for mitigation. Corrosion tests were carried out on stressed bolts with and without the coating to demonstrate its ability to mitigate the EAC.