Assessment of Safety Valve Escape Pipework

Abstract

This paper discusses fitness for purpose (FfP) structural integrity assessments of Safety Relief valve (SRV) vent pipes that were inadequately designed and maintained. The FfP assessments identified several latent errors with the pipework design. The absence of a fault schedule in combination with the latent errors led to a discernable anomaly in the safety case which was finally address but resulted in long outage delays and spiraling costs due to the large number of assessments, inspections and modifications required to achieve and demonstrate integrity. The FfP assessments discussed here consider all failure mechanisms which were identified as being relevant during steam discharge. These include plastic collapse, ratchetting, creep rupture and creep-fatigue and required a series of complex assessments to sentence the SRV pipes for return to service. The Computational Fluid Dynamics (CFD), pipe stress analysis and Finite Element Modeling (FEM) required to demonstrate integrity are discussed. The plant modification and repair solutions required to achieve integrity before the pipes could be returned to service are presented. The method used to apply CFD loads to pipe stress models without double accounting for static pressure stresses in the Finite Element Analyses (FEA), is describe here. Novel analysis techniques used to speed up assessments and the historic plant data reviews that were required to substantiate the claims on historic damage are reviewed.