Single Piston Effect Ball Valve

Single Piston Effect Ball Valves refer to Trunnion mounted ball valves with two unidirectional (self-relieving) seats are designed for blocking the fluid in both directions. Each seat is sealing in one direction, from the valve ends to the valve body cavity.

 

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Key Specifications / Features

Single Piston Effect Ball Valves refer to Trunnion mounted ball valves with two unidirectional (self-relieving) seats are designed for blocking the fluid in both directions. Each seat is sealing in one direction, from the valve ends to the valve body cavity. The difference in area (D=A-B) times the line pressure creates a “piston effect” force which pushes the seat against the ball surface resulting in a tight effect seal. Spring loaded seats provide a low pressure seal behind the seat by keeping the seat in contact with the ball.

Single Piston Effect Ball Valves  Seat Design

When the ball valves are in closed position with the pressure trapped in the ball, fluid in the space between the seats when closed. If the liquid heats up, it will increase the pressure in this cavity as a result of fluid expansion. When the pressure in the valve cavity exceeds the upstream line pressure and/or the spring load on the seat, the seat will be pushed away from the ball to relief the valve cavity pressure. This eliminates the need for a body relief valve. The single piston design permits the automatic release of any over pressure in the body cavity when the valve is in fully open or fully closed position. The pressure can be relieved to the pressure source or to the low pressure side, generally downstream. The relieving pressure in the body cavity shall not exceed 1.33 times the fully rated valve flange rated pressure. The single piston effect seat design ball valve in the API 6D is also called the Double-Block-and-Bleed (DBB) ball Valve.

Notes:

A bidirectional seat is a valve seat designed to seal against pressure source in either direction.
A unidirectional seat is a valve seat designed to seal the pressure source in one direction only.

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