The pressure in the tube side of the heat exchanger is greater than the pressure in the shell side
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I have just designed a shell-and-tube heat exchanger with fixed tube sheets. The design pressure for the shell side is 0.6 MPa at a design temperature of 50°C, while the design pressure for the tube side is 1.5 MPa at a design temperature of 200°C. The cylinder dimensions are 159*4; the length of the heat exchange tubes is 400 mm, with a spacing of 16 mm between them. The specifications for the heat exchange tubes are 3/8”*0.049”. Expansion joints have been installed on the shell side, specifically of the type ZDL159-1.0-1x1.5(1.3)x1. According to standard design practices, since the pressure on the tube side is higher than that on the shell side, an ammonia leak test is required for the shell side. However, there is another opinion suggesting that it is sufficient to set the test pressure on the shell side to match that of the tube side, and then conduct the pressure test. I followed the second method in my calculations, and found that no matter what value was used for the test pressure, the strength of the expansion joints could still meet the required standards. Upon checking the calculation documents, it was found that the strength of the expansion joints was evaluated based on the design pressure, without taking the test pressure into account. Later, I adjusted the design pressure on the shell side to match that of the tube side, and as a result, the expansion joints no longer met the required standards. My question now is: how should the verification be carried out using the second method? In SW6, should the design pressure be increased for the verification, or should the test pressure be raised? !
The first pressure testing method (ammonia leakage in the shell side) should still be used. It is unsafe to increase the test pressure on the shell side in order to check the welding quality of the tube ends in the tube sheet, and increasing the wall thickness of the shell side cylinder and expansion joints as a result is wasteful and uneconomical.
Generally, a gas-tightness test is sufficient for the shell side, and this can also be used to detect leaks in cans. Helium leak detection is generally required in situations where the pressure in the tube side is high, the pressure in the shell side is also high, and the medium in the tube side is harsh.
May I ask where the specific regulations can be found?
With such a small heat exchanger, it’s better to simply increase the pressure in the shell side, thus avoiding the hassle of ammonia penetration.
The key issue is that ignoring the pulling force at the tube ends in the shell side due to the absence of expansion joints isn’t acceptable; on the other hand, using expansion joints to increase the design pressure also isn’t viable. This is how we handle it now: we conduct a airtightness test, which is essentially a leakage test as well, and it should meet the relevant standards and specifications. It’s just that if there’s no mandatory requirement for ammonia leak testing, then it won’t work.