BUTTERFLY VALVES are typically simple and reliable valve structures that are particularly designed for operation in a low pressure service condition. Sealing of a butterfly valve is generally accomplished by an interference fit between a rotatable plate like valve element and a stationary valve seat that generally is composed of an elastomeric substance that will yield to define an interference fit with the rotatable valve. Because the seal is of interference nature and because the elastomeric seat material will yield quite readily to excessive pressures, it is considered difficult to provide a butterfly valve construction that will withstand pressures greatly in excess of 150-200 psi. As fluid pressures increase, therefore, it is generally deemed necessary to provide a valve of different operating capability and capable of withstanding higher pressures.

Another disadvantage of Butterfly Valves typically available is the restriction created by the valve element in the cross-sectional area of the flow passages of the valves. Since the butterfly element of a butterfly valve is frequently disc-like and tapers from a fairly thick center to relatively thin edges, the butterfly element will typically be of substantial cross-sectional area and will adversely affect the flow of fluid through the valve. The cross-sectional area of the flow passages of a butterfly valve therefore frequently must be quite large in order to provide sufficient cross-sectional area for purposes of fluid flow and the size of the resulting valve is consequently quite large.

The butterfly valve is typically a valve of relatively thin silhouette and is generally quite structurally stable. Butterfly valve structures are frequently designed to be received between the flanges of ordinary pipe thereby providing a valve structure that is generally of lower cost as compared to other types of valves.

The load applied to a closed Butterfly Valves element by pressurized fluid is typically transmitted directly to the trunnions of the valve, thereby causing the trunnions to be highly stressed when the fluid is maintained under relatively high pressure. When valves are used to control fluids containing a high concentration of hydrogen sulfide, the trunnions, being highly stressed, will be subjected to conditions which make the trunnions susceptible to stress corrosion, frequently referred to as hydrogen embrittlement. It would therefore be desirable to restrict the magnitude of forces applied to the trunnion and drive stem portions of the valve construction especially when the valve is to be employed to control the flow of fluids of high hydrogen sulfide content.

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