Case Study of a FCC Overhead Line Crack Failure

Pipe stress analysis (PSA) is a common method for evaluating the ductwork in Fluid Catalytic Cracking (FCC) units. However, the typical approach only considers global loads and stresses in the piping system. Localized stresses must be evaluated using other methods. This can result in a PSA that produces acceptable results but fails after years of operation due to a local constraint or stress concentration. This paper presents a case study of a failure in a FCC Overhead Line constructed of 304H stainless steel material and operates at approximately 25 psig (0.172 MPa) and 1,175 °F (635 °C). The line passes stress limits per ASME B31.3 using a PSA but failed via a crack at a conical transition to long radius elbow joint weld after approximately fifteen years of operation due to a localized stress concentration. The local transition on the line is evaluated for Creep damage using a MPC OMEGA material/damage model which is incorporated into the finite element analysis (FEA) via a user subroutine. The evaluations indicated that the large half-apex angle of the transition combined with the elbow location resulted in significant Creep damage at the joint. Subsequent destructive testing confirmed that the elbow joint developed sigma phase formations which reduced the Creep life significantly.