Vacuum valves – why do ordinary valves leak when used in vacuum systems?

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The requirements for valves in vacuum systems are completely different from those in ordinary pipelines. A valve that is well-sealed in a normal pipeline may leak badly when used in a vacuum system. Why? Special requirements for vacuum valves: Ordinary valves rely on the pressure of the medium to assist with sealing – the greater the pressure, the tighter the seal. What about the vacuum system? The pressure is negative; the medium does not help to press against the sealing surface – it is entirely dependent on the pre-tightening force of the valve. Therefore, the closing force of the vacuum valve must be greater. Vacuum systems have much higher requirements for leakage rates than ordinary pipelines. Ordinary pipes allow for slight leaks, but vacuum systems cannot; even a tiny leak will destroy the vacuum level. Why can’t ordinary valves be used as substitutes for vacuum valves? The valve body of ordinary valves may contain minor casting pores; it does not leak under positive pressure, but air will be drawn in under vacuum. The packing of ordinary valves is PTFE, which allows for sealing under positive pressure; however, in a vacuum, air will be drawn in through the gaps between the packing and the valve stem. The flange gaskets of ordinary valves are compressed under positive pressure, and may be drawn in and deformed under vacuum, resulting in leakage. The seat sealing surface of a conventional valve is compressed by the pressure of the medium under positive pressure; in a vacuum, there is no such pressure, resulting in insufficient sealing pressure. Structural features of vacuum valves: The valve body of vacuum valves must undergo helium mass spectrometry leak testing to ensure that there are no microholes. The filler should be sealed with a bellows or with a special vacuum filler. The flange should use metal gaskets (copper gaskets, aluminum gaskets) or fluororubber O-rings; ordinary asbestos gaskets cannot be used. The sealing surface must be ground to an extremely high level of smoothness, and sealing is achieved through preload. Types of vacuum valves: Vacuum butterfly valve – simple in structure, suitable for rough vacuum. Vacuum baffle valve – reliable sealing, suitable for high vacuum. Vacuum ball valve – low flow resistance, suitable for applications that require regulation. Vacuum slide valve – large diameter, suitable for ultra-high vacuum. Precautions for installing vacuum valves: Before installation, check whether the valve’s vacuum-sealed packaging is intact. The flange surface must be clean and free of scratches. Use vacuum-specific gaskets, and pay attention to alignment during installation. The bolts should be tightened evenly in a diagonal pattern to prevent the flange from deforming. After installation, a vacuum leak test should be performed to ensure good sealing. Daily maintenance of vacuum valves: The main aspect of maintaining vacuum valves is to keep them clean. Dust and fibers in the valve chamber can affect the sealing, so it should be cleaned regularly. If there are scratches on the sealing surface, it needs to be ground again. The bellows should be replaced promptly if damaged. In one sentence: Vacuum systems are special – positive-pressure valves don’t work well; bellows sealing with metal gaskets is required, and the system can only be used after leak testing is successful.