Sharing on the classification of pressure vessels

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I received a set of drawings for a pressure vessel yesterday; it is a sewage tank with a design pressure of 6.3 MPa and an operating pressure of 5.5 MPa. The material contained within the tank is sewage, the volume of the tank is 7.2 m³, the inner diameter of the tank is 1600 mm, and the wall thickness of the tank body is 36 mm. But the container category on the container drawing is “/” – no classification? ! My first instinct was that something was wrong – such a large container and such high pressure; how could it not be classified? ! I contacted the designer, and the reply was that according to clause 1.3 of the TSG21-2016 regulations, it does not meet requirement (3), which stipulates that the medium contained must be a gas, a liquefied gas, or a liquid whose maximum operating temperature is equal to or higher than its standard boiling point. Literally speaking, this is true as the medium in question is sewage, so it indeed does not meet this requirement; therefore it falls outside the scope of application of TSG21-2016. I also consulted DP, Doubao, and others via mobile phone, but the responses all indicated that it was a Class II pressure vessel, without any explanation for why. Today, I received the replacement drawings for this container; its category has been changed to Class II. Why? I took out TSG21-2016 again and studied the section \"1.3 Scope of Application\" word by word; finally I found the reason. In item (3) there is \"Note 1-4\": \"When the medium in the container is a liquid whose maximum operating temperature is below its standard boiling point, if the volume of the gas space is greater than or equal to 0.03 m³, it also falls within the scope of this regulation.\" In other words, for this sewage tank to meet the requirements of this clause, it is necessary to ensure that it is filled with sewage and that the volume of the gas space (i.e., air) is less than 0.03 m³. I had read TSG21 many times before and paid attention to this annotation as well, but I never really kept it in mind; when it came time to use it, I couldn’t recall it. It seems that one needs to read the standards more often, repeatedly, and while reading, one must also use one’s brain – not just memorize the standards, but also learn to understand them. I gained some insights from this experience, so I’m writing them down and sharing them on the forum.