The Valve seat or seat ring is used to support the valve plug or disc in a fully closed position and to form a sealing part. Usually, the valve seat diameter is the maximum flow diameter of the valve. The seat material is very wide, all kinds of rubber, plastic, and metal materials can be used as seat material, such as EPDM, NBR, NR, PTF.E, PEEK, PFA, SS315, STELLITE, and so on

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For the butterfly valve, the seat material usually are rubbers, such as EPDM, NBR, NR (Natural hard rubber)

For the ball valve and plug valve the seat material usually is plastics, such as PTFE, PEEK, and so on.

For the globe valve and gate valve the seat material usually is metal to metal.

For valves using soft seat material, their shut-off performance is great and even can reach zero bubble seat tightness. while the disadvantaged part is they can not resist high temperature and high-pressure applications.

On the other hand, those valve’s seat material is metal to metal therefore they can stand with high-pressure ratings up to 2500LB.

Types of Seat Rings

Soft Seat (Nonmetallic Seat Ring)

The majority of valves have soft seat inserts and elastomer or polymer seals. The nonmetallic seat ring is made from e.g. Teflon. The ring is held against the ball by an insert. For fire-safe service, a secondary metal-to-metal seating arrangement is required to omit leakage in the event that the soft seat material is lost during a fire. Soft seated valves are recommended for clean service only and are unsuitable for dirty/abrasive service or high temperatures.

Double Beveled Sealing Ring

The structure design of a double-beveled sealing ring has been adopted to reduce the friction between the ball and the sealing ring to achieve lower operating torque. When the medium pressure is low, the ball has a smaller contacting surface with a sealing ring, which introduces higher sealing pressure to ensure the sealing reliability. When the medium pressure gets higher, the contacting area between the ball and the sealing ring increases accordingly. A larger contact area reduces the sealing pressure to avoid the deformation of the sealing ring.

Cantilevered lips seat rings

Seat rings with cantilevered lips are designed so that the ball contacts initially only the tip of the lip. As the upstream and downstream seats are pre-stressed on an assembly against the ball, the lips deflect and put the seat rings into a torsion. When the valve is being closed against the line pressure, the lip of the downstream seat deflects still further until finally the entire seat surface matches the ball. By this design, the seats have some spring action that promotes good sealing action also at low fluid pressures. Furthermore, the resilient construction keeps the seats from being crushed at high fluid loads

Pressure equalizing slots

The seat ring shown is provided with peripheral slots, which are known as pressure-equalizing slots. During the closing of the valve, the maximum surge pressure occurs, during which the downstream seat can be forced to intrude into the ball port and the valve can become inoperative. The pressure equalizing slots prevents this potential failure. When pressure causes the upstream seat to move against the ball and the ball moves downstream, the pressure simply leaks into the valve body cavity through the relief slots. The upstream seat ring becomes pressure balanced. Downstream sealing prevents seat damage and lowers operating torque while providing full bidirectional capability. The seats with pressure equalizing slots also permit the valve to operate at a higher pressure differential with lower torques than double sealing valves in which there is sealing between both seats and the ball.

Soft Seat (Metallic Seat Ring)

The metallic seat ring, containing a nonmetallic insert, usually requires spring force to achieve a tight seal. Due to springs, this design is not used in corrosive service. Even though this seat has a metal seat ring, it is categorized as a soft seated ball valve due to its use of soft material insert, which is in contact with the ball.

Metal to Metal Seats

For services unsuitable for soft seating, metal and ceramic seating are being used. In general practice, hard metal seated ball valves (typically stellite) are only used in severe service conditions such as high temperature, high erosion, corrosion or abrasion, slurry, etc. Metal seats are generally not bubble tight. Metal seats and balls are typically lapped in matching sets resulting in perfect roundness and fitness to ensure smooth operation and tight shut-off and achieving ‘Zero Leakage’. Metal seats come in different classes of Shut Off including Class IV, V, and VI. Metal Seat and Ball are fabricated from base metals coated with hard chrome, tungsten carbide, and Stellite.