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Proper selection, application and uses of today's smart positioners, controllers, and valve actuators.

Control Valve

Product Announcement from DFT Inc.

Control Valve-Image

The control valve with pinpoint modulating control characteristics and a 200:1 turndown ratio for high pressure drop applications. This unique design features a venture ball-cage design. See our website for an animation showing how the valve works.

In the full open position, the flow is straight through and operates as a true venturi with inherent high flow and high pressure recovery characteristics. The cage straddles the flow stream and supports the ball on four inclined pads holding it on the edge of the stream. The force holding the ball firmly against the cage is caused by the pressure differential created by the high velocity fluid in the flow stream, compared to the relatively static state of fluid in the valve body. This dynamic characteristic is commonly referred to as the Bernoulli effect. The presence of a pressure differential at full open, and during all positions of the cage and ball, prevents suspended particles in the stream from settling out in the body, thereby keeping the valve clean and free of any material deposits. Also, in the full open position, any suspended particles flowing through this venturi configuration tend to concentrate in the center of the diverging cone and through the orifice. This obviously helps in lessening the erosive action of the fluid on the valve surfaces, in particular the seat face.

In the close throttling position, the ball is supported in three-point suspension by the two forward inclined pads on the cage and the seat face. In the intermediate throttling position, the ball rests on the four cage pads. The seat face acts as a load bearing surface, permitting the ball to cam in and out of the seat. This three-point suspension, with the ever present pressure differential, keeps the ball in a very solid and stable condition during all positions of control, therefore preventing the ball from spinning and/or chattering. This same stable three-point suspension also permits extremely close control throughout the stroke, even at minute openings of the valve, down to only a few thousandths of an inch.

In the closed position, the ball sits freely on a conical seating surface and is held there by the system pressure. The line contact between the ball and the seat yields a high unit loading for exceptionally tight closure. Due to the ball seating with pressure, the higher the pressure, the higher the closing or seating force. This high seating force, along with slight rotation of the ball during the initial opening travel, results in a new seating surface contacting the seat each time the valve is closed. Due to the freedom of movement of the ball in the conical seating surface, temperature changes will not affect a tight shut-off.

In the fully-closed and locked position, a wedging surface inside the cage applies a light mechanical load to the ball, assuring positive shut-off. This function should only be used in low pressure or low flow applications where there is insufficient flow to create an adequate pressure differential. When the valve is operating with a sufficient differential pressure across the valve, the guide pin has no function.

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