Why is a hydrostatic test still required when the MAWP has already been determined? What exactly is being verified by expanding the hydrostatic test stress to 0.9Rel?
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Dear teachers, I have several questions regarding the relationship between MAWP and hydrostatic testing: 1) Why is a hydrostatic test required even after the maximum allowable working pressure MAWP has been determined? What is the purpose of this hydrostatic test? What exactly is the overpressure verification meant to verify? Why is it set to 1.25 times P? 2) The allowable stress for hydraulic pressure testing is 0.9RelxΦ, which is very close to the yield limit; it seems that hydraulic pressure is used to determine the critical point at which the equipment begins to undergo plastic deformation. —— Why is a conventional safety factor applied to determine the allowable stress for calculating pressures and MAWP, while a higher value of 0.9RelxΦ can be used for the allowable stress in hydraulic testing? The two verification processes use different allowable stresses; what are the purposes of these two verifications? When calculating the MAWP conventionally, the allowable stress used remains the minimum value among (Rm/2.7, Rel/1.5, Rdt/1.5…); the minimum value is taken into account for tensile strength, yield strength, high-temperature creep, and fatigue limit. Why, then, is only 0.9ReL considered for hydrostatic testing, without taking other mechanical properties and operating conditions into account? 3) Is multiplying the hydrostatic test pressure by the allowable stress ratio [σ]/[σ]t the operating condition conversion factor? The hydraulic test was initially intended to determine whether, at the design temperature and with a pressure of 1.25p, the pressure resistance of the equipment could be assessed But at high temperatures, water evaporates and it’s not possible to conduct pressure tests; however, when the test is carried out at room temperature, the allowable stress of the material is no longer the same as that at high temperatures. Therefore, in order to achieve an equivalent conversion from the design temperature to room temperature, since the allowable stress changes at room temperature, this value must be increased by the same factor as the increase in allowable stress from the design temperature to room temperature. By increasing 1.25P by the same factor, an equivalent conversion from the design temperature to room temperature can be achieved, and that factor is 【σ】/【σ】t (This is a personal speculation.) 4) For the hydrostatic test at 1.25P, when calculating for multiple chambers, should the value of P be the calculated pressure or the design pressure of that specific chamber? For multi-chamber jacketed vessels, the hydrostatic test pressure calculated by SW6 is defaulted to 1.25x(p_calculated). As for the pressure values for the inner cylinder, p_calculated is determined as follows: 1) P_calculated = p_inner – p_outer; 2) P_calculated = p_inner; 3) P_calculated = -p_outer. In all three cases, SW6 uses p_calculated to determine the hydrostatic test pressure, and the maximum value among these results is then selected. However, there are also opinions suggesting that there is an issue with the SW6 calculation; in 1.25P, P should not be considered based on operating conditions to determine the pressure separately, and instead 1.25xP should be used for design purposes. So, which calculation method is correct?
1) The main purpose of the hydrostatic test is to verify the structural integrity of the equipment and its leak-free sealing during the manufacturing process. Although the maximum allowable working pressure (MAWP) has been calculated, theoretical calculations can only ensure safety on paper; in actual production, issues such as welding defects and material defects may arise. Hydrostatic testing is typically carried out at 1.25 times the MAWP, in order to simulate conditions exceeding normal operating pressure and ensure that no danger arises even if pressure exceeds limits occasionally in actual use. 2) The allowable stress for the hydrostatic test is set at 0.9 times the yield strength Rel multiplied by the efficiency factor Φ, as the hydrostatic test is a brief process aimed at checking the load-bearing capacity and leakiness of the equipment under critical or near-plastic deformation conditions. Lower allowable stresses are used in conventional MAWP calculations to take into account factors such as fatigue and creep during long-term safe operation. The hydrostatic test, on the other hand, focuses on whether the equipment can withstand higher test pressures in the short term without suffering any permanent deformation; as a result, relatively high allowable stress values can be used. 3) Your speculation is basically correct. The hydrostatic test converts the conditions at the design temperature to normal temperature for testing, and by adjusting the allowable stress and pressure, it simulates and verifies the safety under design conditions. The material properties at the design temperature typically change due to temperature effects, and hydrostatic testing simulates the actual operating conditions under design by adjusting the allowable stress and pressure at room temperature. 4) Regarding the calculation of the hydrostatic test pressure for multi-chamber systems, in theory, the actual operating conditions and the design pressure of each chamber should be taken into account. In practical applications, it may be more intuitive and conservative to use a design pressure of 1.25 times (i.e., the design pressure for each chamber individually) for calculations. However, the specific calculation method should be determined based on specific engineering standards and safety regulations. If there are any doubts or if the results obtained from the software are disputed, it is recommended to consult relevant professionals or check the latest engineering specification guidelines. In practice, ensuring safety is always the top priority; therefore, it is often advisable to adopt a more conservative calculation method. .
You’ve put a lot of effort into this. It seems that there’s no one in your company who truly understands containers; no one can answer such basic questions for you. I suggest you do the design during midday, otherwise problems are bound to arise later on. Containers are subject to lifetime responsibility; if no one is accompanying them, extra care must be taken. There are too many points to cover. Let me answer your first question: Besides verifying strength, stiffness, stability, and sealing performance, hydrostatic testing also helps to relieve stress, eliminate peak stresses at locations of local discontinuities, and has a closing effect on micro-cracks. This is why the test pressure needs to be close to the yield point; otherwise, these effects won’t occur.
Hello, could you explain in detail why it is necessary to approach the yield strength in order to eliminate stress and local peak stresses? Stress-relief heat treatment takes advantage of the decrease in a material’s yield strength at high temperatures, allowing the residual stresses to exceed this yield strength and resulting in plastic deformation that releases the elastic deformation and thus eliminates the residual stresses. Does a hydrostatic test work in a similar manner? However, to release stress through plastic deformation, the stress must first reach the yield strength; only then can plastic deformation occur and the stress be released ； Limiting film stress during hydrostatic testing
Put simply, to design rules one only needs to know how to apply formulas and ensure that they comply with various rules; there’s no need to delve into the underlying principles of those formulas and rules. Over time, with careful attention, one will gradually come to understand them. The standard formulas and rules are generally the results achieved by large enterprises or **institutes through cost-unconcerned investments; alternatively, they are lessons learned from painful experiences.
1. Regarding stress-relief heat treatment • Stress-relief heat treatment primarily utilizes the property that the yield strength of materials decreases at high temperatures. When the residual stress exceeds the yield strength at high temperatures, the material undergoes localized plastic deformation. Since residual stress is generated by elastic deformation, this kind of plastic deformation can release the elastic strain energy within the material, thereby reducing the residual stress. This process indeed requires stress to reach the yield strength at the prevailing temperature in order for the material to undergo plastic deformation and thereby release the stress. 2. Regarding hydrostatic testing and stress relief • The main purpose of hydrostatic testing is not to eliminate residual stresses directly, as stress-relief heat treatment does. A hydrostatic test is primarily used to verify the strength and sealing performance of a container. Limiting film stress
Due to the additional wall thickness and rounding effects, the stress level during the pressure testing of the new equipment does not even reach the designed stress level; this seems to be a problem, as it fails to achieve the purpose of testing the equipment’s pressure-bearing capacity and eliminating local peak stresses.