Abstract
To reduce shell and tube longitudinal stresses and tube-to-tubesheet joint stresses in a fixed tubesheet heat exchanger, expansion joints are provided. In addition to providing pressure integrity, the expansion joints provide flexibility for thermal expansion. ASME Sec. VIII Div.1 provides the methodology for evaluation of stresses in the expansion joint in both corroded and non-corroded condition for loading conditions under mechanical loads only, thermally induced displacements only or both. However, when it comes to the design rules, the ASME Sec. VIII Div. 1 leaves it to the user to perform the design calculations by a method (i.e., stress analysis) that can be shown to be appropriate for expansion joints.
For thick bellow expansion joints, it is a common practice for the industry to adopt the methodology described in TEMA for performing the design calculations. The latest TEMA (10th Edition) in comparison to the previous edition has modified the methodology for applying the loading conditions for spring rate and stress determinations.
It is a common practice for the industry to model the half-length symmetry flexible shell element in the Finite Element Analysis for determination of stresses and spring rate. This approach holds good when the heat exchanger comprises only one flexible shell element. However, in the industry owing to complex application requirements at times, it is a requirement to have multiple identical flexible shell elements or multiple non-identical flexible shell elements. The TEMA standard provides rules for FEA when multiple flexible shell elements are provided in an expansion joint. The methodology described in TEMA has certain conditions of applicability and holds good provided those criteria are met. However, when the stipulated criteria are not met, the TEMA standard leaves it to the discretion of the designer to approximately model the geometry for spring rate and stress determination.
The work reported in this paper is an attempt to evaluate the spring rate and determine the stresses for multiple flexible shell elements by means of FEA and verification of Code compliance and review of literature.
