Marine Boiler Superheater Tubes Expansion Port Stress And Strain Analysis

The steam superheater is one of the most important components of marine boiler. It holds the extremely important status in the entire steam power installment. The function of superheater is further heating the saturated steam generated in the drum to a higher temperature of superheated steam to meet the needs of marine navigation. The tubes of the superheater and the headers' tube sheet are usually connected by expansion. Sometimes the expansion port of the superheater is finding to be leaky during the operation process.This thesis focused on the study on the causes of expansion port leakage. The main content of this study were as follows:1. On the basis of consulting of plenty of literatures about tube-to-tubesheet joint, using ANSYS finite element analysis software, after appropriate simplification, the 3-D finite element model of marine boiler superheater tube-to-tubesheet joint is established. By using elastic-to-plastic finite element analysis of the process of expansion joints,12Cr1MoVg, 1Cr19Ni11Nb, GH984 these three common materials of superheater tubes'expansion performance to the header box were studied. The results of analysis indicated:the three materials all have good cold expansion performance, grooving on the tubesheet has benefit to the expansion joint, two grooves on the tubesheet can make the joint get better expansion performance.2. Analysis the seal performance of the expansion joint when it is in the high temperature working state. The results indicate:when using 1Cr19Ni11Nb as superheater tubes, the joint can keep in touch and seal. It will leak in the cooling process.Exert temperature and pressure cyclic loads on the joint, then analysis the load cycle number on the impact of sealing performence of joint. The results indicate:with the increase of the load cycle number, the residual contact pressure in the joint decreased slightly in the cooling process. This thesis also inspected high temperature on the impact of sealing performance of the joint, when using 12Cr1MoVg and GH984 as the tube. FEA results show that the higher the working temperature is, the lower the residual contact pressure is after from the high temperature state to the room temperature. 3. The residual contact pressure of the joint decrease after start and shutdown the boiler, and expansion leakage. This mainly caused by the difference of coefficient of thermal expansion between tube and tubesheet. This thesis discussed the cases that tube and tubesheet have different coefficient of thermal expansion. It finds that when the tube's coefficient of thermal expansion is higher than the tubesheet's, the joint will leak during the cooling process. When the tube's coefficient of thermal expansion is lower than the tubesheet's, the joint will leak during the heating process.Through the above theoretical reseach, it gives some advice to avoid expansion port leak, and has some significance for application.