A rotary plug valve having a plug member rotatable in a valve housing member between an open position and a closed position, PLUG VALVESthe plug member carrying a sealing ring, such as an O-ring seal or the like, in a groove provided on its surface between its port openings, the O-ring seal being adapted to encircle one of the ports of the housing member when the plug member is in the closed position. Reducing ValvesThe rotary plug valve is capable of two-way flow therethrough, and this is realized by providing means for equalizing the pressure across the unsupported portion or segment of the O-ring seal when the same is passing across one of the ports of the valve housing. By such an arrangement, the pressure across the O-ring seal may be equalized in certain situations such as when the plug member is rotated towards and approaches a position for closing the valve and when the plug member is rotated towards and approaches the full open position and this prevents clipping or shearing of the O-ring seal. Cast Steel Valves This particular arrangement of pressure equalizing means for the O-ring seal of the rotary plug valve makes the valve capable of two-way flow in that it may be inserted into a fluid pressure line so that the O-ring seal surrounds the upstream port of the housing member when in closed position, or the downstream port of the housing member when in closed position.

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