3.6 Valves
Valves in vacuum systems can also be subject to special requirements, in addition to the general technical requirements for shut-off elements that are typical of vacuum technology and have to be taken into consideration in engineering the products.
The minimum displaced ultimate pressure and the high flow resistance of components in the molecular flow range must be taken into consideration in configuring and selecting vacuum valves. In addition, minimum leakage rates are required for the valve housing and valve seat. Vacuum-side lubricants for the moving parts in the valves must be suitable for the required pressure and temperature ranges, or avoided entirely, if possible, in high or ultra high vacuum. Minimum dead volumes and high conductivities are important, particularly in the molecular flow range.
The feedthrough for mechanical actuation elements must be designed in such a manner as to satisfy requirements with respect to tightness, as well as the pressure and temperature ranges. Depending on the quality, elastomer sealed feedthroughs (e.g. shaft seals) can be used for lower vacuum requirements in the pressure range greater than 1 · 10-4 hPa. While membran or spring bellows are used for pressure ranges of less than 1 · 10-4 hPa. In addition, valves sealed with a metal bellows can be baked out if appropriately engineered. Valves with elastomer sealed housing, plate or flangs are used for pressures of up to 1 · 10-8 hPa. The installation is generally done in a way that the atmospheric pressure is on the valve plate, in closed position, and thus increases the closing force.
All-metal valves, in which all seals are made of metal, are suitable for UHV applications and higher bake-out temperatures, however they usually require higher closing forces to seal. Soft metals (copper or special alloys) are used as sealing materials. In addition to higher closing forces, shorter seal service life must also be expected.
There are a variety of different types of valves for the various applications in the field of vacuum technology; these valves are named on the basis of their design or function.
There are various ways in which valves can be actuated. Valves with small nominal diameters can be opened electromagnetically by electromagnets resp. coils. They generally close with spring force. For larger valves, the required coils are quite large and produce a lot of heat. After the opening of the solenoid current, the holding current can be reduced by the built-in control electronics to prevent overheating of the drive. Never- theless, valves larger than dimensions DN 40 are rarely powered electromagnetically.
With a pneumatically operated valve, air pressure is used for actuation. The required control pressure is often in the range 0.4 to 0.8 MPa. A pneumatic cylinder transmits its movement to the valve plate. If a direction is operated by compressed air and the opposite direction reset with a spring, the drive is called “single acting pneumatic”. If compressed air is required for both directions, it is called “double acting pneumatic”. If there is an electromagnetic control valve for the inlet and outlet of the compressed air directly on the pneumatic drive head, there is an electro-pneumatic drive. Here, air pressure must applied and must be controlled by switching the control valve with its control voltage (often 24 V DC). For the regular pneumatic drive, an electromagnetic valve is also generally used for controlling the compressed air. But it is, for example, located in the cabinet or if many valves are connected, it is on a so-called valve terminal that houses the control valves. Should the control fail, it is often advantageous when the valves with return springs fall in a defined valve position. A distinction is made between the designs “normally closed” and “normally open“, where the standard is the closed position. In addition to that, valves can be operated by electric motors. Intermediate positions are also possible if the valve design permits.
Most valves have and “open / close” optical position indicator, which indicates the valve position. For automated processes, it is useful or even necessary to get feedback about the actual valve position, which is independent of the switching state. For that purpose, valves are equipped with valve position indicators that directly indicate the position of the valve plate. It indicates malfunctions, such as a failure of the compressed air or malfunction of the control valve.
