Expansion Joints

Abstract

Differential longitudinal expansion between the shell and the tube bundle is a well known problem in fixed tubesheet heat exchanger design. The differential expansion occurs from two sources: (a) temperature; and (b) pressure. Temperature induced differential growth warrants no further explanation. Its presence is directly related to the raison d’ être of the exchanger. The effect of pressure induced differential growth, often overlooked in practical design work in fixed tubesheet exchangers, has been analyzed in Chapter 9. In physical terms, the mismatch in the axial deformation of the shell and tube bundle is caused by the difference in the state of their pressure loadings. The shell is under an internal pressure p„ whereas the tubes are subject to a net external pressure of (p _s − p _i). In simple terms, if p _s >p _t , then the Poisson effect will cause the shell to shrink, and the tubes to expand, resulting in a net differential expansion. A more complete accounting of the pressure induced differential expansion requires consideration of axial forces in the tubes and in the shell, consideration of the deflection profile of the tubesheet, etc. Such a complete analysis has been presented in Chapter 9, where a method for incorporating the presence of an expansion joint is also described. In the scheme of the overall stress analysis, the only quantity required to characterize the behavior of the expansion joint is its “spring rate,” defined as the axial pull per unit circumference of the shell divided by the axial spread of the joint and has the units of force per square linear dimension (e.g., pounds per square inch). The mechanical design of the heat exchanger should, however, not be confined to the evaluation of spring rate alone. The expansion joint forms the pressure boundary, sometimes the most vulnerable one, in a heat exchanger. Its fragility arises from the fact that it must be made “flexible” to alleviate differential expansion induced stresses. It is difficult to build-in flexibility and ruggedness in the same component. Indeed, the first question in the selection of the expansion joint centers around the question of the optimum blend of flexibility, ruggedness, and economy required in the actual application. At present, the governing safety codes provide little guidance in the matter. The ASME Code tackles the issue in Code Case 1177-7 which states that the requirements of U-2 (g) of Section VIII of the Boiler and Pressure Vessel Code be satisfied. In this manner the Code recognizes the use of any rational stress analysis of expansion joints. The above referenced Code case also recognizes that expansion joints cannot normally be designed such that the combination of direct, local membrane and secondary bending stresses is below the Code tabulated allowable stress. An ASME working group [15.1.1] is presently developing a set of design rules for expansion joints.