The functional operation of the valve is very simple. The entire body and pressure boundary is formed by three parts: the main body, the bushing, and the head (Figure 1). In the first concept, these are all held together by bolts or studs squeezing the three parts together, with the bushing in the middle and everything sealed with O-rings.
The floating piston is positioned inside the body assembly and seals to the body assembly in four places with circular seals such as O-rings, or pressure-energized seals such as omni-seals. The piston and body shape, combined with the four seals, forms three annular chambers on the outside of the piston.
The first chamber is connected to the downstream face of the piston through a series of holes drilled through the piston. This connection between the chamber and the downstream face can be done externally through the body if desired, but as shown in Figure 2, it is through the piston. The annular area of the downstream face of the piston is exactly equal to the connected face area of the piston in the cavity. Because the two faces have the same area and see the same pressure, no force is generated to try to move the piston by either of these areas, no matter what the pressure is inside the valve.
In similar engineering, the upstream face of the piston is equal in area to the expanding area of the flowpath inside the piston. So again, the pressure acting on both of these faces will be equal, and thus the forces generated by these pressures will cancel each other out and not generate a motive force trying to move the piston.
The remaining two chambers are equal in size and connected to the outside of the valve body through a drill passage for each. These chambers are the operating chambers and provide the force to open and close the valve. The working fluid to operate the valve is up to the valve designer and user. The force required can be generated by gas of any pressure by varying the outer diameter of the chamber. The working fluid can come from an external source or from the inside of the valve. The operation of the valve is simple — one chamber is pressurized and the other chamber is vented. The piston moves in the direction of the vented chamber until it is physically stopped by either the seat on closing or the main body surface on opening. The speed of the piston is easily controlled by controlling the rate at which the venting chamber bleeds off.
The most significant challenge associated with this valve concept is that there is no obvious way to see the position of the piston in the valve. Another challenge is that the valve has to be removed from the line in order to be serviced. Many ball, globe, and gate valves permit the bonnet to be removed and the valve serviced while the body remains installed in the field. Designers generally design the wear parts in a way that they are accessible and replaceable in the field. That may not be possible with this valve concept; however, the lower part count and predicted increased reliability may make that a minimal factor.
The balanced, floating piston valve design has a wide range of potential applications of all sizes and pressure ranges. The extremely simple design and few parts makes the concept inherently reliable, simple to manufacture, and easy to service. The valve concept works with soft or hard metal seats, and the closing force is easily adjustable so that any closing force desired can be created. The valve will consistently close with exactly the same force as long as the driving medium is at the same pressure as designed. The fact that no adjustment is required in the design will ensure valve performance throughout its life.
The one moving part with incorporated simple seals in a well-protected configuration — along with the short travel of the piston — should add to the life of the valve and reduce associated maintenance. In addition, the reduction in commodities needed to operate the valve will reduce overall lifecycle cost and enable designs that operate longer on smaller accumulators in case of loss of the operating medium or in power failures.
This article was written by Bruce Farner of NASA's Stennis Space Center, Mississippi. For more information, visit here.