While valves of the so-called "butterfly" or "disc" type are widely known and used industrially, they have certain inherent operational limitations which curtail their utility.

For example, in such valves a metal-to-metal or metal-to-resilient material sealing means is generally provided. In order to ensure that the valve will be effective against leakage, it is necessary that contact between the sealing edges of the closure member and the valve seat, i.e., the sealing means, be maintained against operating pressures.

When a resilient seating member is employed, which is usually the case in such valves, sealing is accomplished at least partially by the radially outward deformation of the seat adjacent the periphery of the closure member with the consequence that the seat member exerts a sealing pressure against the edges of the disc. Since deformation of the resilient seating member is accomplished through sliding contact with the closure, there is substantial drag or friction between the edges of the closure member and the seating member as the former is moved to and from the closed position.Reducing Valves The principal contact characteristic between the rotatable valve closure member and the valve seat is what is known as the contact angle.Cast Steel Valves Contact angle may be defined as the angle, taken in the plane of a disc radius drawn through the center of the disc, which lies between the valve seat and a line drawn tangentially to the disc at the point of contact between the disc edge and the seat. A large contact angle reduces required torsional forces and a small angle increases torsional forces.

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2011-12-05

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