Failures due to Improper Material Selection

KEI performed a root-cause analysis of main-seat hardfacing cracks in safety-relief valves. The root-cause analysis identified the hardfacing material to be very strong in compression but with a very low ductility. The analysis concluded that the majority of the ultimate strain was consumed by the body-to-seat interference stress, disc-to-seat contact stress, and Welding Residual Stresses (WRS), leaving an unacceptably low margin against cracking. The disc-to-seat impact stresses caused during SRV lift testing and the variability in the estimated WRS can consume the remaining low margin leading to seat cracking.

The current hardfacing material is commonly used in the industry and is available in different grades with higher ductility. Using the same hardfacing material with a different grade and better ductility would increase the margin against the cracking by more than 400%.

Figure 1: Cracks in the main seat of an SRV (left) and maximum strain in the main seat from the elasto-plastic FEA of the main seat (right).

Figure 1: Cracks in the main seat of an SRV (left) and maximum strain in the main seat from the elasto-plastic FEA of the main seat (right).