Valve Seat Materials: Metal Seats vs Soft Seats and Application Selection

The valve seat is the sealing surface against which the valve closure element (ball, disc, plug, or wedge) presses to achieve shutoff. Seat design and material selection have a profound effect on achievable shutoff tightness, seat durability, temperature capability, compatibility with the process fluid, and resistance to damage from particles in the flow stream. The fundamental choice between metal seats and soft (resilient) seats influences all of these performance attributes and must be carefully matched to the service requirements.

Metal Seat Designs

Metal-seated valves use hard metal seating surfaces on both the closure element and the seat ring. The sealing mechanism depends on the controlled contact of two machined metal surfaces, with sealing quality determined by the surface finish, flatness, and the contact stress generated by the closing force. Metal seats are inherently more durable than soft seats at elevated temperatures, in abrasive services, and in applications where fire safety requires that the valve maintain some degree of shutoff after fire damage to soft seal components. Common metal seat materials include 316 stainless steel, Stellite (cobalt alloy) hardfacing, Inconel hardfacing, and tungsten carbide hard facing for the most abrasive conditions.

• Temperature capability: metal seats rated to 600 degrees Celsius or higher depending on material

• Shutoff class: typically ANSI Class IV (0.01% of rated Cv) for standard metal seats

• Stellite hardfacing: excellent hardness and corrosion resistance, widely specified for steam and gas service

• Metal seats required in fire-safe applications per API 607 and API 6FA standards

• Higher closing forces required to achieve metal-to-metal seal versus soft seat designs

Soft Seat Materials

Soft seats use elastomeric or polymeric insert materials that deform under closing force to conform to the closure element surface, achieving very tight shutoff with lower closing forces than metal seats. PTFE (Teflon) is the most widely used soft seat material for ball valves and butterfly valves due to its excellent chemical resistance, low friction, and temperature capability to approximately 200 degrees Celsius. PEEK (polyetheretherketone) provides better temperature capability and wear resistance than PTFE. Elastomeric seat materials including EPDM, Buna-N (NBR), Viton (FKM), and HNBR are used in butterfly valves and check valves where flexibility and compliance are important.

• PTFE seats: excellent chemical resistance, low friction, temperature to 200 degrees Celsius

• PEEK seats: superior strength and temperature capability compared to PTFE, up to 260 degrees Celsius

• EPDM seats: excellent water and steam resistance, not suitable for hydrocarbon service

• Viton (FKM) seats: excellent chemical and high-temperature resistance to 200 degrees Celsius

• Nylon (PA) seats: good mechanical strength, used in water service butterfly valves

Selection Guidelines

For clean service at moderate temperatures (below 150 degrees Celsius), soft-seated valves are generally preferred because they achieve ANSI Class VI (bubble-tight) shutoff with lower actuator forces, have lower procurement cost, and are less sensitive to small particles in the flow stream that would score metal seats. For high-temperature service, steam service above 200 degrees Celsius, fire-safe applications, and services where abrasive particles are present, metal-seated valves are specified despite their higher cost and higher required closing forces. Many modern valve designs use a combination approach with a primary soft seat and a secondary metal seat that provides backup shutoff in the event of soft seat damage.