Failure Analysis of a Cracked Pipe Clamp

Following the in-service failure of a pipe clamp on one of two combustion turbines (CT-2, for reference) at a natural gas fired combined cycle unit, station personnel found three other pipe clamps in CT-1 that were similarly damaged. One of the cracked pipe clamps from CT-1 was submitted to the metals laboratory for analysis to determine the cause of the failure. Subsequently, a piping segment and one additional clamp from CT-2 was submitted to the lab for analysis of the external deposit and were found to contain similar cracks in the clamp. The clamps are used to secure the steam piping to the outlet portion of the transition pieces within the CT. Analysis of the cracks identified them as intergranular, initiating on the ID surface of the clamp (where the applied stresses of the clamp are in tension) but which would be external to the pipe itself. Analysis of the actual deposits showed them to be very high in sulfur (soluble anion analysis identified them as sulfates) and minor amounts of fluorides, with no detectable chlorides either by SEM/EDS or by soluble anion analysis. The area is washed down during shut-down and an analysis of the washwater additive showed very low levels of chloride (7ppm) and sulfate (4 ppm). Concentration of chlorides could be occurring as the units heat up, but none could be identified on the actual fracture surface or the deposit with any certainty. The cracking is consistent with intergranular stress-corrosion cracking, although a specific causative agent/environment could not be identified with any certainty. Given the operating temperatures, any SCC that would be occurring would likely be during shut-down or start-up in order for an aqueous environment to be present. The final disposition of these clamps is for regular replacement during the routine overhauls.