Is there a difference in the principles of leakage between vacuum systems and pressure systems?
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Are the leakage principles of vacuum systems and pressure systems the same? Why is the leakage rate of a flange at 2.0 MPa lower under a positive pressure of 1.0 MPa (a pressure difference of 1.0 MPa) compared to that of a flange in a vacuum system (a pressure difference of 0.1 MPa)? (Normal situation)
The leakage principles of vacuum systems and pressure systems are different. In a pressure system, since the pressure inside the system is greater than the ambient pressure, the medium will leak outward from inside the system through leakage points. Factors such as the size of the leak site and the properties of the sealing material will affect the leakage rate. Furthermore, within a certain range, the greater the pressure difference, the greater the leakage rate. In a vacuum system, since the pressure inside the system is lower than the ambient pressure, ambient air will enter the system through leakage points. The leakage of a vacuum system is influenced not only by the size of the leakage point and the properties of the sealing material, but also by the average free path of gas molecules inside and outside the system. Within a certain range, the smaller the pressure difference, the longer the average free path of the gas molecules, and the greater the leakage rate. The flange with a pressure rating of 2.0 MPa that you mentioned will have a lower leakage rate at a positive pressure of 1.0 MPa (a pressure difference of 1.0 MPa) compared to flanges in vacuum systems (a pressure difference of 0.1 MPa). The reason for this is likely that, in vacuum systems, the longer average free path of the molecules in the surrounding gas allows them to more easily enter the system, resulting in higher leakage rates in such systems. In a pressure system, although the pressure difference is large, the leakage amount remains relatively small due to the short average free path of gas molecules and the significant pressure difference between the inside and outside of the system. .
It should be related to the structure of the flange. For example, the flange connection surface may have a ribbed pattern. As for the leakage amount, it is better to measure it under the condition of no leakage.
How did you come to this conclusion?
According to the chemical process design manual, the flange leakage rate for vacuum systems with a diameter of DN150 or less is 0.227 KG/H. In positive pressure systems, this leakage rate is too high and considered abnormal.