Design and Material Considerations of Ball Valves

by:FLOS     2020-07-03
Ball valves are widely used for controlling the flow of fluids in industrial and commercial piping systems. The tight shut-off feature of these valves makes them particularly suited for flow control applications in high pressure and high temperature conditions. They are commonly used in catalytic reforming, refining, power, marine, petrochemical, and process industries. Design engineers developing valves for demanding applications should match the performance of the valve's component elements with the required flow, temperature, and pressure conditions. Carefully designed high pressure and high temperature ball valves can sustain temperatures as high as 400A�F and elevated pressures up to 10,000 psi. Ball Design Port size and the position in which the ball is held in the valve body are two significant aspects of ball design. The hole diameter of a ball valve is called its port size. Reduced, regular, and full are some common port sizes for ball valves. Common ball materials include brass, stainless steel, copper, bronze, and cast iron. Full Port Design The port size of a full port ball valve is approximately same as the pipeline's inner diameter. These designs are best suited for applications that warrant minimal resistance to flow of fluids. Regular Port Design Compared to full port designs, the hole diameter of regular port ball valves is small and exhibits increased resistance to fluid flow. They can be used as alternatives to gate valves in several industrial fluid flow control applications. Reduced Port Design The hole diameter of a reduced port ball valve is approximately equal to 60% of the valve's inner diameter. This ball valve design is suited for small, end-loaded valves that require small-diameter balls. A ball valve's flow control sphere element can be assembled to the ball housing by two ways: the trunnion-mounted ball design and the floating-ball design. The Floating-Ball Design Two seat rings are used to support the ball in a floating-ball design. In floating-ball design, the flow control shaft is attached to the ball at a slotted right angle to the ball valve's port. This arrangement makes the ball to float or move in the direction of the seat ring on top, when the shaft is rotated. When the ball valve is closed, fluid pressure makes the ball to push against the seat ring at the bottom. This design is suited for all differential ball valve body styles. The Trunnion Design In trunnion design, the ball is supported by two trunnions or short-shaft extensions at the ball's top and bottom ends. These trunnions are attached to bearings, which are in turn assembled to the valve body. In high-pressure applications, the ball is held in place by these trunnions. A slot in the top short-shaft extension accepts the flow control shaft. This ball support method is used mainly in ball valves with spilt body style. Seat Design The proper design of ball valves for high pressure and high temperature applications depends greatly on the material used to make the seal ring. Seat rings are typically made from two classes of materials: soft seats and metal seats. Soft Seats Soft seats are made from resilient materials such as PEEK, PFA, and TFE. Through proper design, soft seals can also be used in leak-tight flow control applications. However, the efficiency of soft seat rings is constrained by their service temperatures. Soft seat rings can be manufactured in jam and flexible seat designs. In jam seat design, as the name suggests, the seat ring is compressed by the flow control ball during assembly. This design enhances sealing efficiency, but the design is not suited for wide temperature and pressure fluctuations. The flexible seat design overcomes this limitation by encapsulating the seat with a lip that bends when compressed during assembly. Metal Seats Metal seats are fabricated from base metals coated with tungsten carbide and Stellite. Although metal seat rings withstand high temperature, they are not as leak-tight as soft seat rings. Since metal seats are not as resilient as soft seats, springs are used to sustain contact pressure. In trunnion designs, both the top and bottom seats are backed with two springs, while in floating ball valves only one spring is used to sustain contact pressure. Sealing efficiency of metal seat rings can be enhanced through the installation of a back-up seal. Flexible metal or graphite seat rings are best suited for high temperature, flow control applications. Metal seats are also suitable for applications that use abrasive solids in the process fluid. Top-entry, three-piece, and split body ball valves generally come with metal seats.
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