Why is the hydrostatic test pressure multiplied by the allowable stress ratio?
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Dear teachers, in the formula for the hydrostatic test pressure, what is this allowable stress ratio of /T? Is it a condition conversion factor intended to convert the hydraulic test at the design temperature into a test at normal temperature? Was the purpose of the hydraulic test to verify whether: \"At the design temperature, the equipment can withstand 1.25 times the design pressure\"? But the water has vaporized at high temperatures, making it impossible to conduct a hydraulic test at the designed temperature; therefore, it is necessary to convert the conditions to room temperature instead? In that case, assuming that the hydrostatic test is intended to verify whether the container can withstand 1.25 times the design pressure at the design temperature, for example, if the design temperature is 180°C and the design pressure is P, then the relationship tested by the hydrostatic test is: 1.25P – 180℃ ； At 180°C, water has already vaporized, so a hydraulic test cannot be carried out. Therefore, the objectives of the test have to be adjusted to conditions at room temperature. However, the allowable stress at room temperature is that corresponding to 20°C, not 180°C. If the hydraulic pressure is simply increased by a factor of 1.25 compared to the design pressure for use at room temperature, then using 1.25P to conduct the test at 20°C, it’s clear that 1.25P at 20°C is not equivalent to 1.25P at 180°C. Yet the allowable stress varies linearly; thus, the hydraulic test pressure needs to be increased by the same factor as the decrease in temperature from 180°C to 20°C. Therefore, to convert the hydraulic test pressure to room temperature conditions, it must be multiplied by the factor 20°C/180°C. Hence, “1.25P at 180°C” is equivalent to “1.25P times 20°C/180°C at 20°C”, and so the hydraulic test pressure should be 1.25P times T ； Under this assumption, if the hydraulic test is carried out using hot oil and the temperature during the test is 180°C, then the test pressure can be taken as 1/25P directly, without the need to multiply it by the allowable stress factor? Are the above ideas correct?
Your idea is basically correct. The purpose of the hydraulic test is to ensure that the equipment can withstand 1.25 times the design pressure at the design temperature. However, since water vaporizes at high temperatures, it is necessary to conduct the experiment at room temperature. For equivalent conversion, it is necessary to multiply by a coefficient, which is the allowable stress ratio and can also be understood as the operating condition conversion factor. The allowable stress ratio is established by taking into account the changes in material strength at different temperatures. Therefore, when different liquids or gases are used and the test temperature is changed, the value of the allowable stress ratio also changes, requiring recalculation. If the hydraulic test is conducted using hot oil and the test temperature is 180°C, then the test pressure can be taken as 1/1.25 times the design pressure. Because in this case, the test temperature is the same as the design temperature, so no temperature conversion is required. However, it is still necessary to calculate the allowable stress value based on the actual conditions. .
In addition to testing the strength and sealing performance of equipment, hydrostatic testing also serves the following purposes: 1. By applying short-term overpressure, it is possible to reduce the peak stress in certain local areas, thereby helping to eliminate or reduce residual stresses and making the stress distribution more uniform； 2. According to current fracture mechanics theories, short-term overpressure can induce a closing effect on cracks, blunting their tips and thereby enhancing the safety of the container when operating under normal working pressures ； In short, the method of hydrostatic testing involves short-term overpressure, achieved by increasing the pressure while keeping the allowable stress constant. The author’s view is actually to achieve \"overpressure\" by reducing the allowable stress of the materials, which is fundamentally different from a hydrostatic test
Your idea is quite unique; you have treated the pressure test as a simulation test. Even if you use heat transfer oil this time, what will happen in the future when it’s used at the end user’s site and there is no heat transfer oil available? It will need to be specified separately on the drawings, which doesn’t make any sense. . . It’s unscientific.
The equipment operates at high temperatures, not at room temperature; the stress applied to the materials at the design temperature is low. In order to assess the strength of the container under conditions close to the design temperature, a correction factor is added.
It was determined that the poster sells heat transfer oil furnaces. In the future, both manufacturers and users will need to have a heat transfer oil furnace available for pressure testing, and they must also provide heat protection gear to prevent people from being burned or killed due to leaks of heat transfer oil. :lol:lol:lol:lol:lol
Haha, you really think of everything
I have a container with a design temperature of 523°C; will your heat transfer oil work? If it’s okay, I’ll buy one
I’m not joking – I’m asking this question because someone once expressed this view. The suggestion to use heat transfer oil is based on the idea that hydraulic testing is intended to determine whether the equipment can withstand 1.25P at the design temperature. By raising the question of whether, when using heat transfer oil for hydraulic testing at the design temperature, the allowable stress factor should no longer be applied, it’s not about using heat transfer oil in hydraulic testing itself; rather, it’s an attempt to verify whether the initial idea that hydraulic testing is meant to confirm whether the equipment can withstand 1.25P at the design temperature is correct or not. It’s also a way of checking whether the notion that the allowable stress ratio represents a coefficient for operating condition changes is accurate. Actually, there is a similar stress coefficient in the flange design torque as well. The same question arises: does this stress coefficient for flange torque also depend on the operating conditions?
To be serious, the question raised by the original poster is highly rigorous but lacks feasibility.