Depending upon the application in question, shut-off elements (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 upon the quality required, elastomer-sealed feedthroughs (e.g. shaft seal rings) can be used for lower vacuum requirements in the pressure range of over 10-4 mbar, while diaphragm or spring bellows are used for pressure ranges of less than 10-4 mbar. In addition, valves sealed with a metal bellows can be baked out if appropriately engineered. Valves with elastomer housing, plate and flange seals are used for pressures of up to 10-8 mbar.
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 (gold on a stainless steel substrate, 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 also various ways in which valves can be actuated. Valves can be actuated manually, electromagnetically, pneumatically or electropneumatically, and even by means of electric motors. Depending on the requirement and version in question, visual and / or electrical position indicators (limit switches) are available for most valves.
Angle valves consist of a valve housing having an angled configuration. The valve plate is forced onto the valve seat to close the valve. The valve plate is sealed with either a trapezoid or o-ring elastomer seal.
Figure 6.8 shows the design of an angle valve that is sealed with a metal bellows. Since the mechanical activation elements are located outside the vacuum chamber, they can also be lubricated without any problem. These types of valves are available with either manual actuation or with electromagnetic or pneumatic drives.
Rubber plugs or small plates that seal against blade-shaped valve seats (solenoid valves) are also used for extremely small valves.
In principle, inline valves are designed the same as the above-described angle valves, however they differ from them in that the connection flanges are located on one axis. Due to their design, the flow resistance of inline valves is usually higher than that of comparable angle valves.
Gate valves are used for large nominal diameters (> DN 100). They are characterized by their low flow resistance and short physical height. Valve plates, usually of double design, move back and forth to open and close these valves. In the closed position, both elements are forced apart and against the sealing surfaces by means of balls. Depending upon the direction of movement of the valve gate, a distinction is made between rebound valves, shuttle valves and rotary vane valves. While most gate valves can seal against a differential pressure of 1 mbar on the valve plate due to their special design, they can only open in the presence of a low differential pressure on the valve plate.
Plate and butterfly valves
In valves of this type, the sealing valve plate is swung open by a lever (plate valve) or tilted open by means of a simple rotary motion (butterfly valve), with the valve plate remaining in the valve opening. Plate valves, in particular, are used to close larger nominal diameters.
Stopcocks are shut-off elements in which the sealing and shut-off element has a hole through it, and the flow is shut off or released by rotating this element. Ball valves have a proven track record in the fine and medium vacuum ranges. A ball with a hole through it is rotatably supported and sealed on both sides by means of universal ball joints (usually made of PTFE), which also have holes through them. When the hole is in the direction of flow, the entire cross section is released. Ball valves are actuated either manually by means of a rotary feedthrough or by means of pneumatic swivel actuators for larger nominal diameters. It should be noted that ball valves contain an enclosed volume when closed.
In addition to the above-described types of valves, there are also numerous special valves in differing configurations for differing applications:
- Venting valves for slow, dosed venting of a vacuum system
- Gas-dosing valves, some with a control unit, for manual or automatic pressure or flow regulation
- Needle valves for admitting minute volumes of gas that can be very precisely and reproducibly set
- Fast-action valves, e.g. to quickly close a vacuum system in the event of a malfunction
- Pressure relief and differential pressure valves that open and close automatically under certain pressure conditions
- Bakable UHV dosing valves that seal against a metal seat by means of a ceramic plate (Figure 6.13)
Pfeiffer Vacuum offers all popular types of valves. Please refer to the appropriate sections in the catalog in this connection.
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