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The seat is the sealing interface between the moving closure element – ball, disc or wedge – and the valve body. A perfectly specified body can still leak within hours if the seat material is wrong for the fluid. This guide covers the practical choice between soft polymers, elastomers and metal seats so you can specify with confidence.
Soft seats use a compliant polymer or elastomer that conforms to surface irregularities, giving ANSI/FCI 70-2 Class VI (bubble-tight, zero leakage) shutoff at low actuator force. Their limit is temperature and wear. Metal seats seal by precision-machined metal-to-metal contact and survive 300–800°C+, abrasion and slurry, but typically only reach Class IV–V leakage. The trade-off is the single most important decision in seat selection.
PTFE's weakness is cold flow (creep) under sustained compressive load. Filled PTFE (glass, carbon or graphite) resists this and is the default for most industrial soft-seat valves. PEEK is the choice when PTFE creep or temperature is a problem.
Metal seats are made from stainless steel, duplex alloys, or hard-faced with Stellite / chrome carbide overlays. They are mandatory for superheated steam, slurry, catalyst, abrasive particles and fire-safe duty where a soft seat would burn away. They need higher actuator torque and tolerate minor leakage. In fire-safe designs, the soft seat chars in a fire and a secondary metal land still closes the valve – so specify metal-seated or fire-safe soft-seated valves for oil & gas and hydrocarbon duty.
Soft seats (PTFE, PEEK, elastomers) deform to fill surface gaps and achieve ANSI/FCI Class VI bubble-tight shutoff at low actuator force, but are limited to roughly 200-315C. Metal seats (Stainless, Stellite, Hastelloy) withstand 300-800C+, abrasion and slurry but only reach Class IV-V leakage.
Use EPDM or PTFE. Both are non-toxic and corrosion resistant; EPDM is the standard for potable water and steam service, while PTFE (or FDA-grade PTFE) is preferred where broad chemical resistance or food/pharma hygiene is required.
Virgin PTFE is reliable to about 200C, with short excursions to 260C. Above that it loses mechanical properties. For continuous superheated steam (260C+) specify PEEK or a metal seat.
When service exceeds soft-seat temperature limits, contains abrasive particles or slurry, or needs fire-safe performance where soft-seat destruction must not stop the valve closing. Metal seats also suit cryogenic and high-pressure duty.