Concentric butterfly valve
The compact construction of concentric butter fly valves and the flexible selection of compo nents permit an easy adaptation to varying operation conditions. A synthetic liner material typically protects the valve body. It is both simple and effective and permits the utilisation of inexpensive valve body materials. The use of suitable elastomers or plastics means that every valve can meet the demands of individual applications and branches. ... ...
Structure
A concentric butterfly valve incorporates a body with two symmetrically opposite holes, in which the two shafts are mounted. The closing element consists of a disc and a liner and fills up the inner space of the valve body. De pending on requirements, either one solid or two separate shaft elements operate the closing device. To minimise wear, the shafts can be pro tected by three slide bearings, right above and below the disc and at the top end of the shaft, inside the body mounting flange. Regardless of the shaft design, the peripheral sealing seat pro vides the sealing in the area where the shaft goes through the seat.
Quarter-turn valve
Inside the liner, a concentrically positioned disc is moved through a 90° angle in the media flow. Hence the name 90° valve, or due to the symmetric design, spinning valve. In the open position, the lenticular shape of the flow-effi cient disc profile provides a good Kv value (flow factor). At disc positions between 30° to 70°, the valve displays linear control properties.
Therefore, concentric butterfly valves are often selected to control the media flow.
Sealing System
Most butterfly valves are equipped with liners made from elastomers or plastics, a smaller num ber of metal-seated concentric butterfly valves. The design of metal-seated concentric butterfly valves has an annular gap of at least 0.1 mm be tween the disc and the body. Soft-sealing butter fly valves benefit from the material properties of elastomers and plastics. If a disc turns within this type of liner, its material properties cause a com pression and provide the sealing effect without any gap. The feedthrough of both shaft ends is critical. The compression of disc and liner, plus additional seals around the opposing holes of the body, generate a complex sealing effect. This par ticular design is a crucial criterion for the lifetime of a concentric butterfly valve. Superior butterfly valve designs incorporate vulcanised metal rings around the shaft, improving the sealing effect, so that lantern ring systems or even air purge designs with gas overlay can be used.
Application Area
Concentric butterfly valves are applied to either control or start and stop media such as solids, liquids or gases. A closed position pro vides a balance of pressure to both sides of the disc, so the valve cannot open by itself. Upon opening, the flow moment always acts in the closing direction. Hence, self-locking actuation devices are required for added safety. It is only in very rare cases that the dynamic flow moment has a decisive effect on the torque. Butterfly valve applications depend on the accurate mater ial selection for disc, liner and body. Concentric butterfly valves are typically operated at tem peratures up to 200°C (392°F) and maximum pressures up to 25 bar (362 PSI). Operational parameters beyond these values demand a sig nificantly higher compression ratio for disc, liner and body, so that actuation devices become uneconomically big and expensive.
One-piece or split body
Concentric metallic sealing and elastomer-seated butterfly valve are typically made from a one-piece ring body. Differrent liner materials such as PTFE (polytetrafluorethylene) or one-piece shaft-disc combinations require a split body design, fixing the housing with two bolts from the outside. The properties of the disc (e.g. material selection, design, surface roughness) can be individually adapted to meet specific requirements.
In almost every case, the concentric butterfly valve is the most economical solution compared to any other valve solutions. Some cast-iron but terfly valves offer shaft designs turning directly in the housing to further reduce costs. A lack of suitable bearings, however, can cause corrosion between shaft and housing, with the result that operation becomes more sluggish, or even blocks the disc function.