Analysis of the causes of air preheater leakage in purification plants
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I. Basic Overview: The air preheater in our plant was manufactured and installed in 2020; it uses medium-pressure saturated steam to preheat compressed air to 230°C before sending it to the combustion furnace. During a major maintenance session in September 2022, a pressure test conducted according to the design parameters for the shell side revealed leaks in the 1st/3rd tube bundles. The equipment has only been in use for 2 years. The basic parameters of the heat exchanger are shown in the attached diagrams. II. Preliminary Cause Analysis: 1. Corrosion factors: The media in the tube and shell sides are steam/compressed air respectively, so the possibility of corrosion is low. At the same time, since the medium is not highly corrosive, it is also acceptable to use Q245R (for the shell side)/20# (for the tube side). 2. Operational factors: By examining the historical trend of steam consumption, it was found that after the operation of this heat exchanger, the maximum steam consumption was approximately 2.8 t/h. The designed steam flow rate is 3.85 t/h; the actual steam flow rate is lower than this designed value, so there is no risk of tube bundle damage due to excessive steam flow. 3. Comparison of relevant heat exchangers: the comparison table is shown in the attached figure. When compared with the primary and secondary reheaters, the main difference lies in the temperature of the fluid flowing through the shell side; the inlet temperature of the fluid on the shell side of the air preheater is 60 degrees Celsius. The minimum inlet temperature of the shell-side fluid in the primary and secondary reheaters is 187 degrees Celsius. The maximum temperature difference of the medium in the shell and tube sides of the air preheater is 193 degrees Celsius; whether such a large temperature difference could cause leaks requires further analysis and discussion with relevant professionals. 4. Manufacturing quality factors: Spectral analysis results show that the material and design of the tube sheet and tube bundle are consistent, ruling out material-related issues. Therefore, the quality of the heat exchange tubes purchased or mechanical damage during installation are possible factors that caused the leakage in this tube bundle. I’m not sure if the above analysis is appropriate; I would appreciate your guidance, thank you!
In shell-and-tube heat exchangers with U-shaped tube bundles, there is no thermal stress between the tubes and the shell; therefore, a large temperature difference is not the main factor contributing to equipment failure. The original poster did not specify whether the leakage in the pipe fitting occurred at the tube sheet near the tube box end or at the end near the U-bend, nor whether the inlet for compressed air was located at the tube box end or at the U-bend end. If the leakage occurs at the end of the U-bend, it is likely that the pipe at that U-bend lacks a support plate or the support plate is not installed properly ； If the leakage in the tube bundle is located near the tube sheet end, and the inlet for compressed air is also at the tube sheet end, it is necessary to check whether the anti-erosion plates or rods at the shell-side inlet are properly installed, whether the unsupported span of the heat exchange tubes in the end region is too large (vibration analysis indicates a risk of vibration in these tubes), and whether reliable support plates are in place in that end region.