What are the different types of flanges and vacuum seals?
In general, demountable joints in metallic vacuum components, pumps, valves, tubulations, and so on, are provided with flanges. Vacuum components for rough, medium, and high vacuum from Leybold are equipped with the following standardized flange systems:
- Small flanges (KF) (quick-action connections to DIN 28 403) of nominal widths 10, 16, 20, 25, 32, 40 and 50 mm. The values 10, 16, 25 and 40 are preferred widths according to the PNEUROP recommendations and the ISO recommendations of the technical committee ISO/TC 112. For a complete connection of two identical flanges one clamping ring and one centering ring are required.
- Clamp flanges (ISO-K) of nominal widths 65, 100, 160, 250, 320, 400, 500 and 630 mm. Also, these flanges correspond to the nominal widths and construction of the PNEUROP and ISO/TC 112 recommendations. Clamp flanges are joined together by clamps or collar rings. Centering rings or gaskets are needed for sealing.
- Bolted flanges (ISO-F) for the same nominal widths as above (according to PNEUROP and ISO/TC 112). In special cases bolted flanges having a smaller nominal width are used. Clamp flanges and bolted flanges are in accordance with DIN 28 404.
The nominal width is approximately equal to the free inner diameter of the flange in millimeters; greater deviations are exceptions, so the clamp flange DN 63 has an inner diameter of 70 mm. See also Table XI).
The nominal internal diameters correspond approximately to the internal diameters of the pipeline components” (DIN 2402 - Feb. 1976). The left-hand column of the nominal internal diameter series is preferred in practice.
Vacuum fittings and their materials
High vacuum components are made of aluminum or stainless steel. Stainless steel is slightly more expensive but offers a variety of advantages: lower degassing rate, corrosion resistant, can be degassed at temperatures up to 392°F (200°C), metal seals are possible and stainless steel is much more resistant to scratching compared to aluminum.
Ultrahigh vacuum components are made of stainless steel and have CF flanges bakeable to a high temperature. These components, including the flanges, are manufactured in a series production, starting with a nominal width of 16 up to 250 mm. CF flanges are available as fixed flanges or also with rotatable collar flanges. They may be linked with CONFLAT flanges from almost all manufacturers. Copper gaskets are used for sealing purposes.
Basically, the flanges should not be smaller than the connecting tubes and the components that are joined to them. When no aggressive gases and vapors are pumped and the vacuum system is not exposed to a temperature above 176°F (80°C), sealing with NBR (Perbunan) or CR (Neoprene) flange O-rings is satisfactory for work in the rough, medium, and high vacuum regions. This is often the case when testing the operation of vacuum systems before they are finally assembled.
All stainless-steel flanges may be degassed at temperatures up to 392°F (200°C) without impairment. However, then Perbunan sealing material is not suitable as a flange sealant. Rather, VITILAN ® (a special FPM) sealing rings and also aluminum seals, which allow heating processes up to 302°F (150°C) and 392°F (200°C) respectively, should be used. After such degassing, pressures down to 10-8 mbar, i.e. down to the UHV range, can be attained in vacuum systems.
Generating pressures below 10-8 mbar requires higher bake-out temperatures. As explained in the page on Ultrahigh Vacuum Techniques, work in the UHV range requires a basically different approach and the use of CF flanges fitted with metallic sealing rings.
Gas locks and seal-off fittings
In many cases it is desirable not only to be able to seal off gas-filled or evacuated vessels, but also to be in a position to check the pressure or the vacuum in these vessels at some later time and to post-evacuate or supplement or exchange the gas filling.
This can be done quite easily with a seal-off fitting from Leybold which is actuated via a corresponding gas lock. The small flange connection of the evacuated or gas-filled vessel is hermetically sealed off within the tube by a small closure piece which forms the actual valve. The gas lock required for actuation is removed after evacuation or filling with gas. Thus, one gas lock will do to actuate any number of seal-off fittings. Shown in Fig. 2.81 is a sectional view of such an arrangement. Gas locks and seal-off fittings are manufactured by Leybold having a nominal width of DN 16 KF, DN 25 KF and DN 40 KF. They are made of stainless steel. The leak rate of the seal-off fittings is less than 1 · 10-9 mbar l/s. They can sustain overpressures up to 2.5 bar, are temperature resistant up to 302°F (150°C) and may be protected against dirt by a standard blank flange.
Typical application examples are double-walled vessels with an insulating vacuum, like Dewar vessels, liquid gas vessels (tanks) or long-distance energy pipelines and many more. They are also used for evacuation or post-evacuation of reference and support vacua in scientific instruments seal-off fittings with gas locks are often used. Previously it was necessary to have a pump permanently connected in order to post-evacuate as required. Through the use of gas locks with seal-off fittings, a vacuum-tight seal is provided for the vessel and the pump is only required from time to time for checking or post-evacuation.