Failure analysis of a large knife gate valve subjected to multiaxial loading

Knife gate valves are common in process piping plants in the petrochemical industry. In this study a failure analysis is undertaken on a large knife gate valve attached on top of a Ypiece as part of a purge bin outlet in a polyethylene production facility. A large crack was discovered on the knife gate valve shortly after its installation and operation of the facility. The study outlines a methodology, based on the finite element method, to analyze failure of such units. It starts out by determining the stress intensification factors for a Y-piece by conducting an FEA, which are used in CAESER II piping software to determine the operational loads on the valve. The loads are used as inputs for a non-linear FEA model of the valve accounting for material failure based on continuum damage mechanics approach. The FEA results suggest that the operational loads are not the root cause to the observed failure on the gate valve. It is rather attributed to internal casting defects and the lack of post-heat treatment of the valve.