Questions regarding the design pressure of vacuum vessels
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According to the manuals and standards, the design external pressure for vacuum vessels without a jacket or safety relief device is set at 0.1 MPaG. In a full vacuum state, isn’t the maximum difference between internal and external pressures 0.1 MPaA? Then why is the design external pressure set at 0.1 MPaG (0.2 MPaA)? Additionally, for vessels without a jacket but equipped with a safety relief device, the design external pressure is taken as either 1.25 times the maximum difference between internal and external pressures or 0.1 MPaG, with the smaller of these two values being used. Isn’t the maximum difference between internal and external pressures still 0.1 MPaA? Even if multiplied by 1.25, it would still be only 0.125 MPaA, which is definitely less than 0.1 MPaG This post was last edited by liming730 on 2009-3-17 12:13]
Don’t confuse gauge pressure with absolute pressure; it becomes clear when comparing them within a system.
Agree with the person above. Additionally, as a clarification: the vacuum container is not in a completely vacuum state; rather, when determining the design pressure, a value of -0.1 MPa is used, and this value represents the design external pressure. If the vacuum level is low, then the pressure difference with the outside environment is also small. For example, if the vacuum level is -0.05 MPa, the pressure difference with the outside environment is 0.05 MPa. Multiplying this by 1.25 gives 0.75; therefore, it is sufficient to use this lower value, and there is no need to design for the maximum external pressure.