TYPES OF VALVESDOE-HDBK-1018/2-93ValvesButterfly valves are especially well-suited for the handling of large flows of liquids or gases atrelatively low pressures and for the handling of slurries or liquids with large amounts ofsuspended solids.Butterfly valves are built on the principle of a pipe damper. The flow control element is a diskof approximately the same diameter as the inside diameter of the adjoining pipe, which rotateson either a vertical or horizontal axis. When the disk lies parallel to the piping run, the valveis fully opened. When the disk approaches the perpendicular position, the valve is shut.Intermediate positions, for throttling purposes, can be secured in place by handle-lockingdevices.ButterflyValveSeatConstructionStoppage of flow is accomplished by the valve disk sealing against a seat that is on the insidediameter periphery of the valve body. Many butterfly valves have an elastomeric seat againstwhich the disk seals. Other butterfly valves have a seal ring arrangement that uses a clamp-ringand backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings.In early designs, a metal disk was used to seal against a metal seat. This arrangement did notprovide a leak-tight closure, but did provide sufficient closure in some applications (i.e., waterdistribution lines). ButterflyValveBodyConstructionButterfly valve body construction varies. The most economical is the wafer type that fitsbetween two pipeline flanges. Another type, the lug wafer design, is held in place between twopipe flanges by bolts that join the two flanges and pass through holes in the valve's outer casing.Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and ina threaded end construction. ButterflyValveDiskand StemAssembliesThe stem and disk for a butterfly valve are separate pieces. The disk is bored to receive thestem. Two methods are used to secure the disk to the stem so that the disk rotates as the stemis turned. In the first method, the disk is bored through and secured to the stem with bolts orpins. The alternate method involves boring the disk as before, then shaping the upper stem boreto fit a squared or hex-shaped stem. This method allows the disk to "float" and seek its centerin the seat. Uniform sealing is accomplished and external stem fasteners are eliminated. Thismethod of assembly is advantageous in the case of covered disks and in corrosive applications.In order for the disk to be held in the proper position, the stem must extend beyond the bottomof the disk and fit into a bushing in the bottom of the valve body. One or two similar bushingsare along the upper portion of the stem as well. These bushings must be either resistant to themedia being handled or sealed so that the corrosive media cannot come into contact with them.ME-04Rev.0Page 32
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