Two questions about the expansion jointing of heat exchange tubes
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A fixed-tube-sheet heat exchanger is now being designed, in which the tube-sheet connection is achieved through strength welding combined with expansion gaskets. The design temperature is 600°, and the design life is 20 years; therefore, a verification calculation for its tube joints is being carried out. There are two questions: First, when evaluating the ability of pipe fittings to withstand axial forces at the design temperature, can the contribution of expansion joining be taken into account? Could it be considered to increase the expansion degree of the bonding to a strength-expansion level? II. After the heat exchange tubes are expanded and joined to the tube sheet, the tubes undergo plastic deformation, while the tube sheet experiences elastic deformation in the area of the tube holes; the tube sheet thus clamps around the tubes, generating a clamping force. However, this heat exchanger is designed for an operating temperature of 600°, and stress relaxation may occur in the area where the tubes are bonded to the tube sheet. The elastic deformation that occurs during bonding in that area gradually transforms into plastic deformation, resulting in a decrease in the clamping force. I would like to know approximately how long it takes for this stress relaxation to occur; that is, after how much time does the clamping force drop to a level at which it hardly changes over time? Hundreds, thousands, or tens of thousands of hours? If you have any of the above questions, I kindly ask for guidance from those who are more experienced. If there are any errors in my description, please feel free to point them out!
This post was last edited by wanlirn on 2017-8-6 at 15:46. Regarding fixed tube sheets at 600 degrees, I would like to ask what materials are used for the heat exchange tubes, tube sheets, shell, etc. Are they nickel-based? I’d like to learn more about it
Applying a value merely fills in the gap; it cannot be used in strength calculations, and the strength value at 600 degrees will also decrease. It is recommended not to include it in the calculations. There are many factors that affect stress relaxation, making it difficult to determine the time involved; it is still different from creep
A temperature of 600 degrees is already quite high. Moreover, you are using strength welding combined with expansion fitting; the expansion fitting is employed to prevent gap corrosion or to reduce vibrations, and it does not contribute to the strength calculations. According to GB151 standards, strength expansion fitting is no longer applicable at temperatures above 300 degrees, and this is due to considerations such as the potential failure of the expansion fitting under such conditions.
The temperature is too high; expansion jointing isn’t feasible. What the person above said is correct – expansion jointing is no longer suitable. I wonder what the pressure level is for the person who posted this question?