CASE STUDIES ON THE USE OF THERMAL-MECHANICAL FINITE ELEMENT ANALYSIS TO EVALUATE WELD RING GASKET AND DIAPHRAGM SEAL DESIGNS

摘要

Weld ring type gaskets are relatively common in the refining and petrochemical industries. These gasket configurations usually consist of two steel rings, fillet-welded to each of the mating flanges with another seal weld between the lips of the two rings. These seal welds or lip seals are sometimes prone to in-service cracking that can eventually lead to leakage and costly equipment downtime. One particular design feature that can significantly influence the propensity for leakage is the shape of the weld rings; flat-lip weld rings or hollow-lip (Omega-seal) weld rings are the two most commonly used designs. The hollow-lip designs inherently offer more flexibility and can generally accommodate more differential radial thermal expansion between mating flanges. This paper highlights a case study of a high-pressure heat exchanger where the shell side-to-tube side girth flange joint is fitted with a weld ring gasket configuration. Comparisons are made using detailed finite element analysis (FEA) that include bolt pretension and pressure-temperature loading. Sensitivity to lip seal design, temperature profile, and assumed friction coefficient between seating surfaces is also investigated. Furthermore, general commentary on flat and hollow lip weld ring gasket geometries is provided with design recommendations for different applications. In summary, the flat-lip seal weld experiences high shear stress, even with small amounts of differential temperature between flanges, which makes it prone to cracking. The added compliance of the hollow-lip seal design can minimize stresses in the seal weld, but understanding operating temperature differentials between mating flanges represents a key aspect of ensuring leak-free operation. The design of a diaphragm seal on a flat heat exchanger channel cover is also evaluated using FEA techniques, with sensitivity analysis on the size of the external fillet weld and diaphragm material specification. The analysis techniques presented in this paper offer valuable insight into establishing damage tolerant weld ring gasket and diaphragm seal designs to minimize the potential for leakage and to optimize critical gasket and bolting parameters.