Stress Analysis and Qualification of Non-standard Gasket Joint of Heavy Water Heat Exchanger

Abstract

Dhruva is a 100 MWth Research Reactor having heavy water as primary coolant and demineralized water as secondary coolant. There are three shell and tube type vertical heat exchangers in the primary coolant system. The heat exchanger is having single-pass construction on both sides, with heavy water on the tube side and demineralized light water on the shell side. Due to identifying a minor leak from tube side to shell side, one of the heat exchangers had to be taken out for repairs, which would eventually require the replacement of cover joint gaskets. The dished end covers of the heat exchanger are bolted to the shell with a double tongue and groove joint having one inner ring gasket and an outer ring gasket to provide the sealing. The gaskets used in these grooves are spiral wound metallic gasket with graphite as the filler medium. Because of the inherent design of the joint, there is a limitation in tightening torque of the flanged bolts. The inner and outer gaskets are compressed by the 60 numbers of flange bolts. The allowable torque on the bolts is limited due to the configuration. Excess tightening will result in flange rotation and induce undue stresses as well as leakage from the inner gasket. Hence to optimize the bolt loading, a stress analysis of the joint was carried out. To validate the results, a mock set-up simulating the site condition and configuration of the joint was fabricated. New gaskets were procured and tested in the mock set-up for various bolt load conditions and the results were found to be in agreement with the results of analysis. Subsequently, the heat exchanger was removed and after repair and leak rectification, the dished end covers were assembled with new gaskets as per the test results. The heat exchanger is performing satisfactorily without any leaks.