6.3 Detachable joints
The individual components of a vacuum system, e.g. pumps, valves, measurement instruments, vacuum chambers, etc., are connected with one another either directly or by means of hoses or piping. The joints between these components must be vacuum-tight and detachable. In configuring a vacuum system, however, as few detachable joints as possible should be used, as they represent a significantly more frequent source of potential leakage than non-detachable joints.
Piping and piping components made of aluminum, plain steel and stainless steel are used as connection elements. Metal hoses made of stainless steel are preferable to thick-walled rubber or thermoplastics for flexible joints, and they are a strict necessity from the lower medium vacuum range onward. To reduce the wide variety of existing dimensions and shapes, internationally valid series of nominal diameters have been specified. The series of nominal diameters that originated with the PNEUROP manufacturers' association and which were later adopted by the ISO is:
- 10 - 16 - 25 - 40 - 63 - 100 - 160 - 250 - 400 - 630 - 1,000 and is supplemented by the auxiliary series 20 - 32 - 50 - 80 - 125 - 200 - 320 - 500 - 800
The small flange connection (ISO-KF in accordance with ISO 2861 / I / DIN 28403), consisting of two flat flanges having tapered rear sides, a centering ring, a sealing ring and a circlip (clamp), is available in nominal diameters of DN 10, 16, 25, 40 and 50 ISO-KF, whereby DN 50 ISO-KF comes from the auxiliary series and is being used more and more rarely today. An adaptation of components with one of the old nominal widths, 20 and 32, can be adapted by means of a reducing centering ring, or by means of an old DN 25 / 20 ISO-KF or DN 40 / 32 ISO-KF outside centering ring. Small flanges can also be held against flat surfaces by means of claws (Figure 6.5).
Fixed flanges and clamping flanges
Standard ISO 1609.4 / DIN 28404 specifies fixed flanges for nominal diameters of DN 10 – 1,000 ISO-F and clamping flanges for nominal diameters of DN 10 – 630 ISO-K. The small flange components are usually preferred for nominal diameters of DN 10 – 50. As in the case of small flanges, fixed and clamping flanges with centering rings have a seal between the flat mating surfaces of the flanges. The flanges are fastened by means of special clamps (clamping flanges) or by means of screws or pins (fixed flanges). By using a collar flange that is held in a groove on the clamping flange by means of a retaining ring or claws, it is also possible to connect clamping flanges to fixed flanges. When installing turbomolecular pumps with clamping flanges, the manufacturer's special instructions in the operating manuals must be observed for reasons of safety.
Since fixed and clamping flange connections are not designed for overpressure applications and because there are process technologies in which the connections are charged with both vacuum as well as overpressure, DIN flanges continue to be used. DIN 2501 (ND 6) and DIN 2502 (ND 10) specify the flange pattern of these flanges. One flange with o-ring groove and one flange with a smooth mating surface must be paired for each connection. Another disadvantage of these flanges relative to the ISO flanges consists of their significantly higher material and space requirements.
Flanges with cutting edges and copper gaskets, (also referred to as CF or CF flanges) are used for UHV applications. A copper ring is placed between two flanges. The concentric peripheral cutting edges of the flanges penetrate into the copper to form a metallic seal that is characterized by an extremely low leakage and permeation rate, as well as by high temperature resistance. These flanges are standardized in accordance with ISO TS 3669. A sufficient number of screws ensure the necessary high contact pressure. Once used, copper gaskets cannot be reused.
Vacuum systems for glass technology applications employ a glass or ground-in connection, which is used only in connection with small nominal diameters. The connections, consisting of the a tapered male ground joint and the corresponding female ground joint, are held together by the ambient air pressure and are sealed with vacuum grease.
Pfeiffer Vacuum offers all popular standard flanges and other connection elements.
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