Failure analysis of flow transmitter (FT) tapping line

The pipe of a flow transmitter (FT) tapping line has failed by sulfide stress cracking (SSC). Analysis revealed numerous cracks on the pipe. The cracks were found to have nucleated from several pits located on the outer surface of the pipe. The nature of cracking indicates that the pipe has failed under hoop stress. Chemical analysis revealed significant amount of sulphur in the crack propagation front. Though chemical composition of the pipe material conforms to the specification, the hardness was found to be considerably higher (~251 HV) against the specification (~124 HV). Such a high hardness would promote sulfide stress cracking of the pipe, if sulphur is present in the environment. It is therefore, recommended to identify the source of sulphur and incorporate corrective measures. It is also suggested to maintain the hardness of pipe material according to the specification