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Butterfly valves come in a variety of shapes and sizes, each suited to particular pressures and purposes. There are two kind of butterfly valves: concentric (zero offset) and eccentric type. The eccentric type can be single, double, or triple offset type.
Zero offset (concentric) butterfly valve
The zero offset (concentric) butterfly valve is a standard or a general use butterfly valve. The shaft is located in the center of the disc. During opening or closing, there are some parts of the disc that always in-contact or rubbing the seat.
This arrangement will make the seat experience friction each time the valve is operating. In a typical application, this concentric butterfly valve is limited to class 150 due to its seat design.
Double offset (double eccentric) butterfly valve
The double offset butterfly valve is a valve that has 2 offset compare with the concentric butterfly valve. The first offset is the shaft located not in the center of the disk but located on slightly behind the disc. This offset will make the valve has a continuous sealing surface on the disc.
The second offset is the shaft located not in the pipe center-line but located on slightly in the right side of the center. This offset will make the valve didn’t touch the seat at all when it is fully opens (see illustration above). This arrangement will make the seat experience less friction than the concentric typebutterfly valveand thus will prolong its lifetime. In a typical application, this double-offset butterfly valve is limited to class 600 only.
Triple offset (triple eccentric) butterfly valve
The triple offset butterfly valve has 3 offset compare with the concentric butterfly valve. Just like the double-offset butterfly valve that has 2 offsets, this triple offset also has the same offset except it has one additional offset which is a conical shape seat. This conical-shaped seat will match with the disc that also fabricated to match the seat.
This arrangement will make the seat has no friction or rubbing while closing or opening. This arrangement will also make the valve has a bubble-tight in the high-pressure application and much longer lifetime rather than a double-offset butterfly valve. In typical application just like the double-offset butterfly valve, this triple-offset butterfly valve is limited to class 600 only.
To sum up, the zero-offset butterfly valve, which relies on the flexibility of rubber, has the lowest pressure rating. The high-performance double offset butterfly valve, which is utilized in somewhat higher-pressure systems, is offset from the centerline of the body seal and the disc seat (offset one), as well as the centerline of the bore (offset two); this provides a cam motion during operation to elevate the seat out of the seal, leading to reduced friction and a lower wear tendency than the zero offset design. The triple offset butterfly valve is the optimum valve for high-pressure systems. The disc seat contact axis is offset in this valve, effectively eliminating sliding contact between the disc and the seat. The seat is a triple offset valve is made of metal so that it may be machined to create a bubble tight shut-off when it comes into contact with the disc.
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