Determination of the maximum pressure during design
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According to the specifications, the maximum operating pressure of a container is the highest pressure that the top of the container may reach during normal operation. Why is the maximum operating pressure defined as the greatest pressure that can be reached at the top of the container, rather than at the bottom? If there is liquid inside the container, then the pressure at the bottom of the container equals the pressure of the gas phase at the top plus the hydrostatic pressure of the liquid within the container. From this perspective, shouldn’t the maximum operating pressure be at the bottom rather than at the top?
I think the vapor is at the top; the maximum operating pressure and the maximum pressure tolerance are determined in order to establish the operating pressure for the safety valve. This is done to prevent the container from exploding – at least when the safety valve opens, the pressure is relieved, and thus the container remains safe.
In high-pressure vessels, the pressure exerted by the material inside the vessel at the bottom can be practically ignored.
I agree with the opinion above. Ordinary containers are designed to have sufficient margin to withstand pressure. If the liquid contained is water with a density of 1000 kg/m3 and the container height is 1 m, the resulting pressure will be P=1000×9.81×1=9.81 kPa. Generally, containers are capable of withstanding pressures up to 10 MPa; there’s a difference of three orders of magnitude here
Oh, I see, you’re an expert indeed:victory:
What about medium and low-pressure vessels? For example, if the pressure is 0.5 MPa, could the water level at the bottom reach 2 meters?
For some pressure vessels, it is necessary to take into account the static pressure at the top as well as the pressure generated by the height of the liquid column inside the vessel; this applies, for example, to the design of safety accessories located at the bottom. Relevant standards seem to provide guidelines on this matter
The maximum operating pressure of a container refers to the pressure in the gas space at the top of the container.
From a physical perspective, the bottom part is in contact with the ground; when static pressure is present, the ground exerts a reaction force. These forces balance each other out, canceling one another out. It’s like when you pierce paper with your finger – the paper gets torn. But if you press the paper against a wall, no matter how much force you apply, the paper won’t tear. When the static head is too high, the pressure at the bottom must be taken into account (for example, replacing paper with tofu will cause it to break even when placed against a wall); both air pressure and static pressure need to be considered. Is what I said reasonable? I don’t know much about this aspect. My apologies.
What was said upstairs makes sense. But it’s not completely correct. Many pressure vessels are elevated. For those very tall tower-shaped containers, it is necessary to take into account the hydrostatic head of the liquid inside. In the case of containers with lower pressures, ordinary materials can easily withstand pressures that are much higher than the operating pressure. In high-pressure vessels, the static head of the liquid phase can be ignored.
I think it’s mainly the pressure in the gas phase that matters, because the gas phase is compressible while the liquid phase is not; therefore, the container pressure needs to take into account the gas phase.
Inspired by this, it can be seen that the density of ordinary liquids is generally low; for example, in the case of water, one atmosphere of pressure corresponds to the hydrostatic pressure of 10 meters of water. Gas pressures, on the other hand, are often very high. The density of liquids used in the petrochemical industry is usually lower than that of water, so hydrostatic pressure can indeed be ignored compared to gas pressure.