Application of Sub-Modeling in the Finite Element Analysis of a Large Fixed Tubesheet Heat Exchanger

Abstract

There are enormous tubes in large fixed tubesheet heat exchangers making it difficult to model all the tubes and tubesheet connections in detail for a finite element (FE) analysis. Alternatively, an equivalent solid plate with various simplified tube connections are employed in the FE modeling. But this fails to yield an accurate stress field around the connecting region of tubes and the tubesheet. Given that the maximum stress of the equivalent solid plate generally occurs adjacent to the solid tubesheet, a novel finite element modeling methodology is proposed in this paper. A two part process is used. The first part is a coarse FE model and the second part is a more detailed FE model. In the coarse FE model, the equivalent solid plate is employed in the central region of the tubesheet with simplified tube connections such as equivalent cylinder by multi-points contacting. And quasi detailed tube and tubesheet connections are used in the coarse FE model for the region adjacent to the solid plate, in which the tube and the tubesheet are simply connected with same nodes. This means that both tubesheet and tubes are established in this quasi detailed FE modeling region. Although both the weld and contacting condition between the tube and the tubesheet are not included, the coarse model is enough to yield a believable stress field for the determination of the maximum stress point of the quasi detailed FE modeling region. In the second part, the sub-model methodology is utilized in the predetermined maximum stress point, in which the detailed connecting structure of the tube and the tubesheet is included, such as the weld and the contact condition. The proposed modeling methodology is helpful to have an insight into the stress around the connecting region of the tube and tubesheet for the effective evaluation of the tubesheet and the connection.