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Vacuum technology puts great demands on the functioning and reliability of the valves, which are often needed in large numbers in a plant. The demands are fulfilled only if correct shut-off devices are installed for each application, depending on the method of construction, method of operation, and size. Moreover, in the construction and operation of vacuum plants, factors such as the flow conductance and leak-tightness of valves are of great importance.
Valves are constructed so that they will not throttle pumping speed. Hence, when opened fully, their conductance in the rough and medium vacuum regions equals that of corresponding tube components. For example, the conductance of a right-angle valve will equal the conductance of a bent tube of the same nominal bore and angle. Similarly, the conductance of the valve for molecular flow (i.e., in the high and ultrahigh vacuum regions), is so high that no significant throttling occurs. Actual values for the conductance of various components are given in the catalog.
To meet stringent leak-tightness demands, high-quality vacuum valves are designed so that gas molecules adhering to the surface of the valve shaft are not transferred from the outer atmosphere into the vacuum during operation. Such valves are therefore equipped with metal bellows for isolating the valve shaft from the atmosphere, or alternatively, they are fully encapsulated, that is, only static seals exist between atmosphere and vacuum. This group is comprised of all medium and high vacuum valves from Leybold that are operated either manually or electropneumatically (Fig. 2.80) and (Fig. 2.79). The leak rate of these valves is less than 10-9 mbar · l/s.
Valves sealed with oil or grease can be used for highly stringent demands. Their leakage rate is also about 10-9 mbar · l/s. However, a special case is the pendulum-type gate valve. Despite its grease covered seal, the leak rate between vacuum and external atmosphere is virtually the same as for bellows-sealed valves because when the valve is in operation the shaft carries out only a rotary motion so that no gas molecules are transferred into the vacuum.
For working pressures down to 10-7 mbar, valves of standard design suffice because their seals and the housing materials are such that permeation and outgassing are insignificant to the actual process. If pressures down to 10-9 mbar are required, baking up to 392°F (200°C) is usually necessary, which requires heat resistant sealing materials (e.g., VITILANh) and materials of high mechanical strength, with prepared (inner) surfaces and a low outgassing rate. Such valves are usually made of stainless steel. Flange connections are sealed with aluminum gaskets, so permeation problems of elastomer seals are avoided. In the UHV range these issues are of special significance so that mainly metallic seals must be used. The gas molecules bonded to the surface of the materials have, at pressures below 10-9 mbar, a very great influence. They can only be pumped away within a reasonable period of time by simultaneous degassing. Degassing temperatures up to 932°F (500°C) required in UHV systems, pose special requirements on the sealing materials and the entire sealing geometry. Gaskets made of gold or copper must be used.
The various applications require valves with different drives, that is, valves that are manually operated, electropneumatically or magnetically operated, and motor driven, such as variable-leak valves. The variety is even more enhanced by the various housing designs. In addition to the various materials used, right-angle and straight-through valves are required. Depending on their nominal width and intended application, flanges fitted to valves may be small (KF), clamp (ISO-K), bolted (ISO-F), or UHV (CF).
In addition to the vacuum valves, which perform solely an isolation function (fully open – fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10-10 cm3/s (NTP) up to 1.6 · 103 cm3/s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored.
When aggressive gases or vapors have to be pumped, valves made of stainless steel and sealed with VITILAN ® sealant are usually used. For nuclear technology, valves have been developed that are sealed with special elastomer or metal gaskets. If you would like to know more about a specific application, please contact us. We will be pleased to provide further design information for your area of application.