DIAPHRAGM CLOSURE ANALYSIS USING NONLINEAR FEA AND CFD

In the previous work PVP2006-93731 “Reinvestigation of Heat Exchanger Flange Leak” (Porter [1]), a series of finite element (FE) models were constructed of a heat exchanger flange. Current FE capabilities were used to further elucidate the reasons for the flange's leakage in-service, reported in a 1994 paper (Porter [2]). The flange leakage was primarily caused by differential thermal expansion causing yield in the flange bolts and gasket scuffing. Correcting the leakage required the implementation of a weld ring gasket. A similar service exchanger was later designed to eliminate the critical differential thermal expansions. This exchanger employed a diaphragm closure method to eliminate the possibility of gasket leakage. This design included an internal pass partition arrangement such that the end closure flanges were exposed to a single process fluid temperature. In the authors’ experience, typically the exchanger vendor provides proprietary calculations verifying the serviceability of the closure design. This prompted the question, “What analysis methodology would be required for an engineer to qualify or verify the design of a welded diaphragm closure configuration?” The authors have used a thorough methodology for the analysis of a diaphragm closure. This was used for verification of the design suitability for design temperature gradients and related thermal expansion. To conduct the analysis, the authors performed a series of computational fluid dynamics (CFD) and non-linear FE analyses on a representative diaphragm closure geometry (under specific service conditions) to determine the closure’s capability to withstand the design load cases. This paper serves to demonstrate how such analyses can be used to qualify a diaphragm closure’s suitability for a specific service. It should be noted that this paper does not represent a complete analysis of a diaphragm closure. Code (ASME [3]) specifies all procedures that shall be employed. The procedures under investigation were applied to the 2 cases analyzed. Complete engineering of the closure may require the analysis of additional cases.