It’s not possible to apply pressure to the floating head of a stainless steel heat exchanger; the manufacturer believes that the operating pressure is too low and wonders if it’s feasible to reduce the test pressure
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This post was last edited by HSLJHZ on 2019-12-16 at 16:02. The heater has a diameter of 1500, a design pressure of 1.6 Mpa, and is made of material S31603; the floating head bolts are specified by the owner to be made of S30408. The hydrostatic test pressure for the floating head is 2.0 Mpa. The manufacturer reported that it was not possible to achieve this pressure, suggesting that the allowable stress of the stainless steel bolts was too low. They proposed that since the operating pressure on the tube side is only 0.45 Mpa, it might be possible to reduce the test pressure to 1.6 Mpa. According to our standard specifications, it is definitely not allowed to reduce the test pressure in order to pass the hydrostatic test; I have replied that the tests must be carried out using the values specified in the drawings. I searched the forum and didn’t see any similar posts before. I would like to ask my colleagues: 1. In cases where the design pressure is much higher than the operating pressure, is it feasible in practice to reduce the pressure value of the heat exchanger’s floating head in order to pass the hydraulic test? Are there any similar experiences or standard references available? 2. Are there any other solutions for situations like this, where the stainless steel floating head cannot be pressurized? I hope my colleagues will not hesitate to share their insights.
If the shell material must meet the design pressure testing requirements. During the pressure testing, the bolts can be replaced with high-strength bolts that meet the requirements of the test, and then the testing can be conducted again. Replace with the original bolts after the test is passed. This results in slightly higher pressure testing costs, but it meets the design requirements.
In this case, the bolts actually used in operation have lower strength than those used during pressure testing – is this in compliance with the standards?
Since the operating pressure of the equipment is not high, there is no need to artificially increase the design pressure, as it is uneconomical. If the operating pressure of the equipment is high, do not set it at a low value. Product design parameters should be reasonable. If the operating pressure of the equipment is not that high, reduce the test pressure accordingly based on actual calculations. If high pressure does occur, it’s better not to reduce it, as the equipment may need to be rebuilt.
The discussion is about pressure testing, and the key is to consider the purpose of such testing. The purpose is to verify the strength of the pipe schedule. Besides bolts, there are other possible reasons for failed tests, such as tube sheet leakage, etc. Therefore, high-strength bolts were used for the test. If it meets the criteria, it is deemed qualified; if not, it is deemed unqualified. After passing the inspection, replace it with the specified bolts and put it into operation.
Regarding question 1, I personally think it’s not allowed! If the pressure is reduced, it is considered unqualified.
150.1 stipulates that it is necessary to calculate the test stress for each stressed component prior to the pressure test; the values of 0.9 for hydraulic pressure and 0.8 for pneumatic pressure refer to this very thing. In other words, if these requirements are not met, the design needs to be adjusted. It is recommended to calculate the test stress of the bolts; if it is close to the yield limit, then the design needs to be adjusted ； If the bolts are defective, it means the problem is detected early. Using high-strength bolts to withstand pressure tests – is that just trying to deceive oneself?
The bolt material is incorrect; replace it with a new bolt
The poster made it clear: it’s a floating flange with a sealing surface, and the reason for the leakage is that the stainless steel bolts do not have sufficient strength. . . It depends on whether the solution is to replace the bolts or to reduce the test pressure. If, as you suggest, the specified bolts are used again, is it still necessary to conduct a pressure test? That just brings us back to the same old problem. Is that how you give advice to people?
Reducing pressure is definitely not advisable. Firstly, the S30408 fasteners are low-strength fasteners, and using them under such pressure is clearly inappropriate. The specified parameters require the use of medium to high-strength fasteners; strain-hardened or other types of stainless steel fasteners may be used, and reference can be made to HG20613 for pipe flange fasteners. Be sure not to lower the blood pressure! ! !