Issues related to leak detection in the welding of heat exchanger tube ends and tube sheets
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Regarding the leak detection for the welding quality of heat exchanger tube ends and tube sheets, we know that methods such as hydrostatic testing, airtightness testing, ammonia leak detection, and helium leak detection are generally used to check the welding quality between the tube ends and the tube sheets. Essentially, it involves using the shell side of the heat exchanger as a tool for pressurization, in order to reverse-check the welding quality of the tube ends and tube sheets on the tube side. Of course, if the design pressure of the shell side is not less than that of the tube side, it is reasonable to use this method of applying reverse pressure to the shell side to test the tube side. So the question arises: when the test pressure for the tube side is higher than that for the shell side, the common approaches are as follows: 1) Conduct a hydrostatic test using the test pressure of the shell side to check the welding quality of the pipe ends, and then carry out an ammonia leak test on the shell side to verify the welding quality of those pipe ends ； This is a common method. 2) Appropriately increase the test pressure on the shell side (to a level exceeding the test pressure specified in the drawings) in order to meet the test pressure on the tube side, thereby checking the welding quality of the pipe fittings. So the question is: according to clause 4.6.3 of GB150.1-2011, if a test pressure higher than the specified value is used, it is necessary to verify the stress levels of all stressed components under those test conditions. For example, for shell components, the maximum overall membrane stress δT should be considered; during hydraulic testing, δT must be ≤ 0.9ReLΦ ； During pneumatic testing or a combination of pneumatic and hydraulic testing, δT ≤ 0.8ReLΦ (Note: ReL is the yield strength (or 0.2% offset strength) of the shell material at the test temperature in MPa; Φ is the weld coefficient). ) For ordinary heat exchanger shells (the main pressure-bearing components being the cylinder, head, shell flanges, etc.), the issue at present is this: when performing verification using the methods mentioned above, taking Q245R with a thickness of 10 mm and DN1000 as an example, the pressure tolerance range for the cylinder and head is quite wide; during hydrostatic testing, test pressures up to 4.7 MPa are acceptable. So, the question: How is the flange verified? (Frequent loading is different from short-term test overloads); in this regard, experienced colleagues could share their insights – for example, whether a DN1000, PN2.5 flange can withstand a hydraulic test of 4 MPa?
The flange standards specify that the hydrostatic test pressure can reach 1.5 times the maximum allowable pressure of the flange at the corresponding test temperature; if the pressure used in the test exceeds this value, it is possible to upgrade the flange grade to meet your requirements. If you don’t want to do these, you can only conduct a halogen test after applying water pressure.
Flanges are subject not only to strength issues but also to stiffness issues. If a container flange with a pressure rating of 2.5 MPa is used to withstand a hydraulic test at 4 MPa, calculations need to be done; however, it is likely that the stiffness will be insufficient, meaning that proper sealing cannot be achieved during the hydraulic test. I remember that it is a requirement for pipe flanges that the hydrostatic test pressure can reach 1.5 times the maximum allowable pressure of the flange at the corresponding test temperature; such a requirement does not apply to container flanges, as these flanges are larger in size and have higher safety requirements.
For the equipment flanges, the hydrostatic test pressure (as the design pressure) should be used in the verification calculations.