How to calculate the bottom plate stress of a pressure-welded vessel under negative pressure at normal pressure
Thread Content
There is a water tank with a flat arch roof, with a diameter of 2000 mm and a height of 2600 mm; the operating pressure is -0.09 MPa. How can the thickness of the bottom plate be determined? Logically, if the gravity of the water is greater than the intensity of the negative pressure, there should be no problem; it seems however that the gravity of the liquid is not strong enough to counteract the internal pressure.
Refer to relevant standards! There are standards even at atmospheric pressure!
It won’t work either if the gravity of water is greater than the negative pressure; generally, the gravity of water acts at the lowest point of the container, while the operating pressure is the pressure at the highest point of the container, right? In other words, your design pressure is the pressure at the highest point of the container. Also, the water doesn’t get filled up immediately; in your operation, does the water first need to be filled before negative pressure is applied? Is there negative pressure as soon as water starts to be filled? According to your theory, wouldn’t the calculated pressure of the equipment be the design pressure minus the weight of the water? So far, in the standards, only the static pressure of the liquid column has been considered; there is no indication yet of a need to subtract anything
For negative pressure, the bottom plate can be calculated according to GBT150.3 or NBT47003, taking into account the addition of reinforcement ribs to the bottom plate
According to the standards, the most severe operating conditions must be taken into account; it is not recommended to use the static pressure of a liquid column to counteract the working pressure. Although this approach may work in practice, the safety factors specified by the standards are relatively conservative. What does not meet the standard calculations might still be acceptable based on engineering experience, just like standard pipe flanges and equipment flanges
It should be calculated according to GB150; it’s not low either.
Negative pressure no longer makes the use of normal-pressure containers appropriate, and a flat-bottomed or vaulted structure for the equipment is also unsuitable. It is recommended to use a structure with upper and lower elliptical end caps.
Initially, I also considered using an elliptical head in accordance with GB150, but the user said that they had checked with the design institute and that this type of structure was acceptable.
If the weight of the fluid and the shell is not offset, I can reinforce this pressure cylinder section; the top and bottom definitely won’t be able to handle it. If reinforcement is added as well, the calculations become more complex.