Failure analysis of a gate valve bonnet at wellhead facilities in sour gas service

Abstract A catastrophic failure of a gate valve bonnet at wellhead facilities has been presented and investigated in the current research. One crack was found in bonnet after about 48240 h service in sour gas. The chemical analysis, mechanical tests such as tensile, micro-hardness, and impact tests in addition to microstructural analysis such as optical microscopy and Scanning Electron Microscopy (SEM) have been conducted on the failed part. While chemical composition, tensile strength, and impact energy conform to the corresponding standard, the hardness was higher than the acceptance criteria in NACE MR0175/ISO 15156 requirements for sour gas service. Microstructural investigations revealed that the failure mechanism of the bonnet is hydrogen-induced cracking (HIC) due to the presence of hydrogen, stress, high hardness and presence of corrosive medium containing chloride and sulfide simultaneously. In order to decrease the bonnet cracking susceptibility, tempering as a heat treatment was recommended for decreasing the hardness and preventing premature failure.