Questions regarding pressure testing of pipe joints when the tube side pressure is higher than the shell side pressure
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I’m quite confused about section 25.14.2 in the Training Manual for Pressure Vessel Design Engineers (by Li Shiyu); I don’t understand it. It deals with pressure testing of pipe joints when the pressure in the tube side is higher than that in the shell side. In such a situation, how should pressure testing of the joints be carried out? GB 151 stipulates that \"when the test pressure on the tube side is higher than that on the shell side, the pressure testing of the joints shall be conducted in accordance with the specifications given in the drawings, or according to a method agreed upon by both the supplier and the buyer.\" This provision specifies that the designer should consider solving this problem before manufacturing. When the above conditions occur, the following treatment methods are generally adopted: (1) Use a stress value of 0.9φReL to calculate the test pressure for the shell side, in order to increase this pressure as much as possible so that it reaches the test pressure of the tube side. However, it is necessary to ensure that other pressure-bearing components on the shell side can also withstand the strength and sealing performance under this test pressure. If the specified test pressure for the tube side cannot be achieved using the above methods: ① If the difference is not significant, considering an appropriate increase in wall thickness may be an option; ② If the difference remains large, testing can only be carried out at the maximum allowable test pressure for the shell side, after which supplementary tests using ammonia leakage, halogen leakage, or helium leakage can be conducted on the shell side. (2) For extractable tube bundles where the heat exchange tubes are arranged in a square pattern, high pressure can be applied to the tube side first, and a sight glass can be used to check for leaks from the back side of the tube sheet.
Recently, we have encountered a basically identical problem. The specific approach to dealing with it is as follows: 1) Calculate the maximum allowable operating pressure for the shell side of the container, and use this value as a basis to determine the test pressure for the shell side (which should be equal to or greater than the test pressure for the tube side). If that is not possible, either increase the thickness of the shell side (taking costs into account), or conduct tests in accordance with the ammonia leakage test method specified in Appendix A of HG/T20584 (relevant testing equipment must be used for this). 2) Consider whether the piping layout and pipe spacing are appropriate; the minimum size for an endoscope lens is generally 4 mm.
This post was last edited by tent80 on 2012-1-13 08:43. I don’t know what the principle is? Weren’t the calculations for the individual tube banks and shell side all satisfactory? Is this different from a container with a jacket?
It’s like a car on a slope: pushing it from below may not make it move, while pushing it from above might work. To ensure the car stays still, it’s necessary to try both methods. If the same force cannot push it from above, then it won’t move either if pushed from below.
This post was last edited by tent80 on 2012-1-13 at 23:13. I’m still a bit confused – isn’t this tube sheet already considered acceptable for both the tube bank and the shell side? Can’t hydraulic tests be conducted separately on the tube bank and the shell side?
When inspecting a heat exchanger, the key is to check the tube ends. The most effective inspection of the pipe ends should be carried out while pressurizing the shell side. At this point, the front side of the pipe fitting is exposed, making it easy to inspect. When the pressure in the tube side is higher than that in the shell side, the fact that there is no leakage in the shell side does not mean there will be no leakage either during testing of the tube side. The test pressure on the shell side cannot be increased indefinitely either. Thus, the solution proposed in the \"Training Manual for Pressure Vessel Design Engineers\" was put forward. Once you understand this principle, everything makes sense.
Reply to 5# tent80: Because the tube sheet and pipe fittings are subjected to forces on both sides
This post was last edited by tent80 on 2012-1-14 at 21:31. Force is applied to both sides; the hydrostatic test pressure for the tube side is high, which indicates that the operating pressure on that side is also high. During operation, the tube sheet on the tube side is subjected to high pressure. Can’t the hydrostatic test be conducted on just one side? The operating pressure on the shell side is low; is it still necessary to increase the hydrostatic test pressure for the shell side? Or I would like to ask, is a hydrostatic test required on both sides of the welding joint between the heat exchange tube and the tube sheet?
I found 2 opinions on the forum; please help refer to them. 1. Has anyone thought about why this requirement was put forward (the design pressure of the shell side is lower than that of the tube side, and the hydrostatic test is carried out based on the test pressure of the shell side = test pressure of the tube side)? The main consideration is that during use, if the heat exchange tubes are damaged due to abrasion or corrosion, it may lead to leakage of the fluid on the tube side to the shell side. If the shell side is part of a closed fluid circulation system, the pressure on that side will increase gradually or rapidly. In the absence of safety valves or in the event that such valves fail, the strength of the shell and the pipe fittings attached to it will be put under severe stress, with the possibility of the container even exploding. To eliminate this risk, this provision is considered in advance. If the design pressure of the tube side exceeds the maximum allowable test pressure on the shell side, but it is necessary to take the above requirements into account, the main pressure-bearing components such as the shell and tube sheet must be redesigned, usually by increasing their thickness. 2. The main purpose is to check whether the welding at the pipe ends is proper. When the pressure in the tube side is higher than that in the shell side, the pressure in the shell side is not increased; therefore, when testing the tube side, even if it is known that there is a leak at some pipe end, it is difficult to determine exactly where the leak occurs. The first perspective is to prevent explosions in the shell side due to low pressure caused by leaks at the joints, while the second perspective is to check whether the welding at the pipe ends is adequate. It’s not clear which of these two perspectives is correct
After thinking it over, I believe what Fei Yan Ta Xue said makes a lot of sense – the most effective way to inspect the pipe ends is by applying pressure to the shell side. At this point, the front side of the pipe fitting is exposed, making it easy to inspect. There is no need to apply pressure for testing on both sides of the welding joint of the heat exchange tube.
Increasing the test pressure in the shell side to the value of the test pressure in the tube side actually serves as a way to check whether there are leaks at the pipe joints during the tube side testing (using shell side pressure testing to fulfill this function, as it is inconvenient to conduct inspections in the shell side during tube side testing; this is the basis for considering this approach)