Analysis of Bolted Joints with Nonlinear Gasket Behavior

Design methods for full face gaskets in bolted pressure vessel joints have received little attention in the literature. Such gasketed joints play a prominent role in attaching rectangular plan from water boxes to rectangular tubesheets in condenser water boxes. With higher cooling water pressures becoming evident due to cooling tower circuits, the water box-tubesheet structure, and its bolted joint connection requires rigorous analysis for both structural integrity and leak tightness. Although it is well known that gasket material has a highly nonlinear stress strain behavior, very few analyses are available to calculate and evaluate the effect of the nonlinear gasket behavior in a bolted joint connection. In this paper, an approximate method for simultaneously analyzing structural integrity and leak tightness of typical bolted flange connections with nonlinear gasket material is developed. The flange is modeled as an elastic element, the bolt is simulated by a linear spring with bending and extensional resistance, and the gasket is modeled by a series of nonlinear compression springs. A simple nonlinear stress-strain relation for initial loading and unloading of the gasket is developed based on experimental data. The analysis technique employs an incremental procedure which follows the configuration through preloading and pressurization and checks structural integrity and gasket leakage. To illustrate the method, a typical full face gasket and flange construction is studied, and the effect of gasket properties on the final state is investigated. A series of simulation results are obtained which illustrate clearly the effect of gasket prestrain, undersizing of bolts, and wall rotational resistance. Of particular importance is a simulation comparing results obtained using actual nonlinear gasket stress-strain data with results obtained using linear models for the gasket. It is demonstrated that for full face gasket configurations, simulation of the nonlinear behavior is required to achieve accurate results. The procedure developed in this work is ideal for optimization of flange gasket configurations because of its cost effectiveness while simultaneously evaluating the interaction between structural integrity and joint leak tightness.