Concentric vs. Double Offset Butterfly Valveb
Butterfly valves serve a broad range of applications throughout fluid processing industries. Among their positive attributes, quarter-turn operation and compact design make this valve type a primary selection for many shutoff applications.
Within the butterfly valve family, their are several basic design subdivisions that are substantial to the point of limiting or expanding the potential application of the valve. One of those subtypes is the double offset butterfly valve.
A general-purpose (concentric) butterfly valve, often referred to as a resilient seated butterfly valve, has a disc connected to a rotating shaft. The rotational movement of the shaft positions the disc to fully obstruct the flow path (closed) or provide continuing degrees of open space in the fluid path cross-section (open). The rotational movement of the shaft from fully open to fully closed is 90 degrees. The shaft is centered in the pipe bore and is also centered on the disc. This shaft orientation will cause the disc to contact the sealing surface throughout a considerable portion of the rotation nearing closure. This contact tends to accentuate wear on the resilient seat and limits the pressure rating for this type of valve.
A double offset butterfly valve is also known by two other common names, "double eccentric" and "high performance". The "offsets" refer to the position of the shaft relative to the pipe bore and the centerline of the disc. In this valve design, the shaft is behind the centerline of the disc and slightly to one side of the pipe bore centerline. This results in a different travel path for the disc as it moves from the open to close positions. The cam action movement of the disc limits its contact with the seat until the final few degrees of travel. The limited wiping contact of the seat tends to extend its useful life. Additionally, this type of closure movement affords the double offset butterfly valve a higher pressure rating than the concentric design.
Valve selection can be a challenging task, with numerous types and variants from which to choose. Share your industrial valve requirements and challenges with a valve specialist, combining your process knowledge and experience with their product application expertise to develop an effective solution.
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