ENGINEERINGNET.EU - Flexible connections and hoses can also act as vibration dampers, and may overcome differences in size of inlet and outlet ports of several vacuum components.
Proper design of the vacuum system may make flexible connections and hoses redundant.
Hydroformed connections are made of a thin-walled, partly straight bead welded and hydroformed tube (Fig. 1). Stainless steel hydroformed flexible connectors are available in different sizes and lengths.
They may resist pressure differentials of 1 bar, without collapsing; and can be used in static and non-static applications, where the number of linear movements and bending is minimal. They are also applied in angle valves.
Edge welded bellows
Edge welded bellows (Fig. 2) consist of a number (depending on the application) of moulded thin metal plates (diaphragms) which are welded together alternately at their inner and outer diameter. Two of these at the inner diameter welded bellows form a convolution. Edge welded bellows are flexible connecting elements between vacuum flanges or arbitrary end fittings.
They counterbalance differences in height or angular offsets and serve as vibration dampers or lifting elements. Furthermore, they are used in linear feedthroughs (Fig. 3), as valve seals, in handling systems and in process automation.
In comparison with hydroformed connections, edge welded bellows can execute significantly larger lateral, axial and angular motions in relation to their size and have a lower spring rate. Edge welded bellows are also stabilized by the vacuum inside. They can be ten times as long as its outside diameter in case of horizontal installation.
In the horizontal installation of long edge welded bellows, it is recommended to split the bellows with intermediate rings into fragment bellows and put up the intermediate rings into a horizontal guidance system. In the vertical installation of long edge welded bellows, the installer must consider that the diaphragm on top always has to carry the weight of the whole edge welded bellow.
Vertical installed edge welded bellows should also be split into fragment bellows by intermediate rings, and should be axially stabilized by a guidance such as rods or wires. The operating pressure, operating temperature and possible torsion influence the life cycle of edge welded bellow connections.
Stainless steel hoses
Stainless steel hoses (Fig. 4) are the perfect solution for applications where standard straight-line vacuum plumbing is not practical, or where vibration isolation is required. These hoses consist of relatively thin wall formed stainless steel bellows enveloped by stainless steel braided sheath that protects the formed bellows from abrasion or impact produced by component movement or vacuum cycling, and from excessive bending.
Both unbraided and braided configurations are offered. The hoses provide great flexibility, strength and durability. Stainless steel hoses are also available in a more rigid medium wall configuration. When installing the thin-walled version, it may not exceed the specified bend radius.
Because all flexible hoses compress when subjected to internal vacuum pressures, equipment connected to either end of a flexible hose must be securely anchored. Atmospheric pressure exerts a substantial force and can move roughing pumps or other light, non-secured components.
The flexible stainless steel hose can be connected to other vacuum components by flange connections, quick couplings, quick flange terminations via stainless steel hose clamps, or welded and permanently bonded to other stainless steel vacuum components.
Rubber and plastic hoses
Wire reinforced PVC hose is a flexible and economical solution for mechanical pump roughing lines. They are suitable for basic vacuum service. The wire reinforced wall prevents tube collapse while under vacuum loads, but yet provides adequate flexibility for convoluted line paths.
PVC’s light weight and flexibility make it simple to assemble in custom configurations right on site. PVC has excellent corrosion resistance and is ideally suited for harsh chemical environments, where exposed metal surfaces are subject to chemical attack.
The PVC hose can be connected to other vacuum components by quick couplings, quick flange terminations via stainless steel hose clamps, or sealed and permanently bonded to other PVC vacuum components by using a PVC primer and cement.
Frank Moerman, European Hygienic Engineering & Design Group - Belgium