One known arrangement utilizes two circular seats where the pressure forces cancel. These valves are relatively larger and more expensive than the standard gate and plug valves. Further, it is often difficult to ensure proper mechanical closure of both seats.

A second known arrangement utilizes one circular seat with a balancing chamber connected to the upstream pressure with a movable piston tied to a valve stem.GLOBE VALVE These valves are complex, and again more expensive to manufacture.

As an alternative to the aforementioned larger and more costly balanced valves, it is known to create a balanced valve where the flow passes through a balanced plug that is typically in the shape of a cylinder. The cylindrical or other closed perimeter shaped plug that allows fluid to pass through is known as a balanced plug and is a key element in forming a balanced plug valve.Globe Valves The cylinder method successfully eliminates the friction and back pressure forces, thus forming a balanced valve. However, the known cylinder type balanced valves have their own shortcomings. These include the fact that they have poor capability for flow modulation or for tight shut-offs.

In view of the foregoing limitations and shortcomings of the above noted devices, as well as other disadvantages not specifically mentioned, a balanced plug valve with the ability to predictably modulate flow and also provide for tight shut-off of flow was developed by the assignee of the instant invention. This a balanced plug valve with a contour shaped wall is described and illustrated in U.S. Pat. No. 6,394,135, issued on May 28, 2002, the teachings and disclosure of which are incorporated herein in their entireties by reference thereto. The contour shaped wall of this balanced plug valve forms a gap with an edge of a balanced plug. Fluid is able to flow through an input port, through the balanced plug, through the gap, and out an output port. The shape of the contour and the relative position of the balanced plug to the contour shaped wall affect the modulation of the rate of the fluid flow through the gap, and thus, through the valve. Multiple possible variations of the dimensions of the contour shaped wall make possible a multitude of flow rate verses valve stroke relationships. Further, the use of a balanced valve decreases friction forces on the plug which allows for smaller, more efficient, and more economical valve actuators.

While this balanced plug valve presents significant advantages to the art and provides fully balanced operation on a two-way valve with an external contour, operation with an internal contour, such as shown in FIG. 6 of the Erickson et al. '135 patent, is not quite balanced. That is, when the valve is closed, fluid pressure acts on one end of the plug, while the internal edge of the plug that sealing engages the contoured edge is isolated from this fluid pressure. Cast Gate, Globe, Check Valves As a result, there is a pressure differential across the plug, resulting in unbalanced forces.

Therefore, there exists a need in the art for a fully balanced plug valve that includes an internal seat flow control contour that governs the flow modulation through the valve, and that provides for tight shut-off of flow.

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