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Comparison and analysis of various stresses before and after tubesheet corrosion

Study notes, limited to the ability of time and energy, did not carry out a rigorous derivation of the argument, please correct any errors.

SW6 from version 3.0 onwards, the new fixed tube plate heat exchanger tube plate corrosion before the calculation. This has aroused everyone’s “curiosity”, the forum also has a lot of discussion. In fact, we can use SW6 before and after the plate corrosion calculation results, to analyze why “previously did not calculate the use of good, but now it is necessary to do more”.
First of all, as the teacher has been stressed to us, with no problem, does not mean that the design is complete and accurate, because the actual use, unlike laboratory verification, can “completely” test the correctness of the theory.
GB/T151-2014 did not find the need for the tube plate corrosion before the calculation of the provisions. But ASMEVIII-1 UHX-13.4(c) states, “Becauseany increase of tubesheet thicknessmay lead to overstresses in the tubes, shell, channel, or tube -to tubesheet joint,a final check shall be performed, using in the equations the nominal thicknessof tubesheet, tubes, shell, and channel,in both corrodedand uncorroded conditions.”
SW6 calculations were performed before and after corrosion of the tubesheet and shell process barrel. But the tube box (i.e., channel) is not considered, the calculation book “tube box cylinder constant” and other values before and after corrosion is the same, in fact, the stiffness of the tube box flange, before and after corrosion is not the same, this is also the impact on the shell flange. As for the heat exchanger tube, although the standard does not take into account the corrosion margin, but the difference in thickness before and after the heat exchanger tube corrosion is actually there, but the impact is small, SW6 did not consider this understandable.
For the heat exchanger we examined, there are the following conclusions.
1. The calculation results show that after superimposing the temperature difference stress, the stress level of the heat exchanger is significantly higher than the case where only the pressure effect is considered. In fact, without expansion joints, the temperature difference load is decisive for the strength of the heat exchanger [2].
2. The stresses in the shell flange before corrosion are more than 50% higher than after corrosion. This is similar to the flange, corrosion before the tube plate and shell process cylinder thickness is greater, the stiffness is also greater, the shell flange moment coefficient is also greater, that is, the greater the load assigned to get.
Shell flange stress calculation formula, only the shell flange moment coefficient and effective pressure Pa corrosion before and after the change, the rest is unchanged. But Pa change is relatively small.
3. Heat exchanger tube axial force, from the “free state” under the stress, deformation coordination process in the boundary force and boundary bending moment caused by the stress of three parts [1]. Before corrosion, due to the shell process cylinder stiffness is larger, heat exchanger tube bundle and shell stiffness ratio Qex is smaller. Qex decreases, the stress caused by the boundary force will become larger, and then the heat exchanger tube axial stress increases.
If the shell process temperature is higher than the tube bundle, the free expansion of the shell process barrel will be constrained by the tube bundle, and this “constraint” will produce compressive stresses in the shell process barrel and tensile stresses in the heat exchanger tube around the outside of the tube bundle. If the shell process temperature is low, the opposite is true.
It should be noted that the stresses in the tube bundle, either by pressure or by temperature difference, are distributed in a decaying wave from the edge to the center, with the maximum stresses occurring at the periphery of the tube bundle [2].
4. The heat exchanger tube and tube plate connection pull-off stress is proportional to the axial stress of the heat exchanger tube, which is clearer.
5. The shear stress around the tube sheeting area is proportional to the shear stress coefficient there, and the coefficient is inversely proportional to Qex. If Qex is small, that is, the shell stiffness is larger, the shear stress (absolute value) around the tube plate cloth tube area is larger.
6. Tube plate radial stress and the square of the thickness of the tube plate is inversely proportional, so the radial stress of the tube plate before corrosion is less than after corrosion.
7. Shell process barrel axial stress, also inversely proportional to Qex, but mainly by the shell process barrel of the metal cross-section As, As the larger, the smaller the stress. Before corrosion is less than after corrosion.
8. Finally, if you do not count the temperature difference stress, the stresses in general only before corrosion of the shell flange stress is greater than after corrosion.
References

  • [1] Sang Ru Pod. Fixed tube plate heat exchanger shell and tube stress analysis and its dangerous working conditions of the test (a). Petrochemical Equipment Technology, 1996,17(6).
  • [2] Rufus Sang. Force analysis of fixed tube plate heat exchanger shell and tube and its dangerous working conditions (II). Petrochemical Equipment Technology, 1997,18(1).
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