Plugging criteria for thinned high-pressure feedwater heater tubes considering pressure and thermal loading

When heat exchanger tubes have critical thinning which could cause unsafe conditions, it is general practice to plug the tubes so that they are out of service to prevent rupture failure at the thinnest location. Plugging criteria for the high-pressure feedwater heater tubes of fossil power plant were studied. Tubes in the desuperheating zone were considered due to their high internal pressure and the high temperature difference across the tube wall. Theoretical analysis of uniform thinning and eccentric thinning of the tube was conducted to determine the stress conditions at the thinnest location under internal/external pressure and thermal gradient across the tube. The minimum required tube thickness was determined using the maximum shear stress criterion (Tresca theory). Accuracy and effectiveness of the analytic solutions of the stresses were verified by comparing the results obtained with those obtained from finite element analysis. Using finite element analysis, the stresses for eccentric thinning were determined. Thermal loading due to temperature difference across the wall was considered even though the contribution of pressure loading to the stress of the tube was known to be higher than that of the thermal loading. As case studies, the tube plugging criteria of three high pressure feedwater heaters were determined using actual temperature and design pressure industry data. The procedure used in this study can be applied to different pressure and temperature operating conditions, and different tube materials.