Why are containers with liquid as a medium subject to regulatory controls?

Such situations do occur. For example, in a methanol flash tank where the operating pressure is 0.4 MPa and the operating temperature is 35 degrees Celsius, a level gauge and a self-acting valve are used to control the liquid level. The upper part of the tank contains the gas phase, while the lower part contains liquid at room temperature. If the volume of the gas phase exceeds 0.025 cubic meters, it falls under the regulations governing pressure vessels.

Let me share my personal opinion: if the liquid here isn’t in a liquid state. When the medium is in a liquid state: first of all, there is no such thing as a gas phase space in shell-and-tube heat exchangers; a storage container does not experience any pressure during normal operation (experts often use this example, and I really want to hit them with bricks), only separate containers may have pressure. Even if the separating container is damaged and the medium is in liquid form, there is no danger. So I think this aspect of the tolerance regulations is a bit of a waste of **resources, and it goes against the original intent of those regulations

In my understanding, take water and steam as an example: below is condensed water at 90 degrees Celsius, and above that is steam; moreover, the minimum volume of steam that can be controlled is 0.025 M3. (A flash tank is an example of such a device.) The pressure is 0.8 Mpa. Of course, this type of container falls under regulatory oversight.

Safety regulations are established to ensure the protection of lives and property. The energy stored in the gaseous phase of pressurized liquid gases is also considerable; in actual production, various issues can arise, leading to dangerous operating conditions. According to Note 3 at the end of Article 9 of the regulations, pressure vessels in such high-risk conditions should be classified accordingly, making it necessary to oversee their design, manufacturing, and use.

This is how I understand it: although the main medium inside this container is a liquid with a boiling point below standard levels, there is still a certain amount of gaseous medium present. If, under normal conditions, the volume of this gaseous medium is 0.025 m3 or more, and the operating pressure of the container is above 0.1 MPa, then this container falls under the regulations governing pressure vessels. For example, in the case of a water storage tank at room temperature with an operating pressure of 0.5 MPa, if there is some air inside and its volume is 0.025 m3 or more, then this container is subject to these regulations; if the volume of air is less than 0.025 m3, it is not covered by these regulations.

The filling factor is different; especially in some reaction vessels, they aren’t completely filled, so there must be a gas phase space present. That’s easy to understand

It seems that in training programs everywhere, hot water at several dozen degrees is often used as an example. So let’s discuss where the pressure in the container used to store hot water comes from; the pressure in such a storage container should refer to the pressure under storage conditions, right? Can a thermos bottle containing a little over half a bottle of hot water generate such high pressure? Even if there is pressure, what extent of damage could occur if the container holding the water breaks? If hot water at that temperature splashes on someone’s body, it will at most cause redness; there might be slight burns on the skin surface, and the range of damage is extremely limited – 2-3 meters is the maximum distance affected. In fact, a container with moderate pressure but smaller capacity can cause greater damage. Don’t blindly believe that standards and experts are always correct; obviously, they have set regulations that aren’t entirely accurate, leaving us no choice but to make excuses for them. The responses of the pressure vessel industry to the same issues in the 1990s and the 21st century were often completely opposite. Taking the hot water storage tank as an example, I am convinced that it won’t appear again in a few years