Seat and trim materials for chemical service

EPDM (ethylene propylene diene monomer) is the default seat material for water service, HVAC, steam (at moderate temperatures), and many general-purpose applications. It has excellent resistance to water, ozone, weathering, alcohols, ketones and dilute acids. It is also approved for potable water contact under WRAS, ACS and other national drinking water standards.

EPDM is not suitable for hydrocarbon service. Oils, petroleum products, fuel and solvents will cause EPDM to swell and soften rapidly, destroying the seat seal. The temperature range is typically -40°C to +130°C in standard formulations, with higher-temperature grades available to 150°C.

Butterfly valves, ball valves and gate valves for water treatment and distribution are almost universally supplied with EPDM seats as standard. Specify NBR or FKM if there is any possibility of hydrocarbon contamination, even intermittently.

NBR (nitrile butadiene rubber, also known as Buna-N) offers good resistance to petroleum oils, fuels, hydraulic fluids and some acids. It is the standard seat material for valves in fuel oil systems, hydraulic circuits and general oily water service.

NBR is not resistant to ozone, strong oxidising acids, ketones or aromatic hydrocarbons. Its temperature range is approximately -30°C to +100°C, narrower than EPDM. For higher temperatures in oil service, FKM is the preferred alternative.

In practice, NBR is often specified as the seat material for valves in utility services where there is occasional contact with light oil or lubricants, even if the primary service is water. It gives broader chemical compatibility than EPDM at modest additional cost.

FKM (fluorocarbon rubber, commercially known by the trade name Viton among others) offers excellent resistance to a wide range of aggressive chemicals including aromatic hydrocarbons, chlorinated solvents, fuels, hydraulic fluids and many acids. Its working temperature range extends to approximately 200°C, making it the preferred soft seat for high-temperature process valves.

FKM is significantly more expensive than EPDM or NBR. It is specified when the service conditions push beyond what NBR can handle: high temperature, aggressive chemicals, or a combination of both. FKM is the standard seat material for process valves in refinery, chemical and petrochemical service.

Not all FKM compounds are equivalent. The standard FKM grade (Viton A) suits most process applications. Low-temperature grades (Viton GF) extend the lower limit to approximately -20°C. Ultra-low-temperature grades are required for cryogenic service.

PTFE (polytetrafluoroethylene) is chemically resistant to almost all industrial fluids: acids, alkalis, solvents, oxidising agents, fuels and water. It is used as a seat liner, seat disc or ball seat insert in applications where no elastomer would survive. PTFE ball valve seats are standard across pharmaceutical, chemical and food-grade applications.

PTFE has a high deformation rate under sustained loading (cold flow), which limits its use in high-pressure applications where the seat would be compressed for long periods. Filled PTFE (with glass fibre, carbon or graphite reinforcement) reduces cold flow and extends the pressure range.

The operating temperature range for PTFE seats is approximately -200°C to +200°C, making PTFE suitable for cryogenic applications as well as high temperature. Below -50°C, PTFE becomes brittle and must be handled carefully. Expanded PTFE (ePTFE) sheet is used as a gasket material for the same chemical compatibility reasons.

Metal-to-metal seats are used where temperatures exceed the limits of soft seats, where the service involves steam or hot oil above 200°C, where abrasive media would erode a soft seat rapidly, or where the standard requires fire-safe shutoff capability after seat destruction by fire.

Carbon steel seats (on carbon steel valves) are the basic metal seat, suitable for steam and hot water service to the limits of the body material. Stainless steel hard-faced seats with Stellite (a cobalt-chromium alloy) overlay are specified for high-temperature steam valves and high-pressure gate and globe valves where erosion resistance is required.

Metal-to-metal shutoff is inherently less tight than a soft seat under low differential pressure, because both mating surfaces must be machined to fine tolerances for the valve to seal. API 598 and EN 12266 both acknowledge this with a higher permissible leakage rate for metal-seated valves. If bubble-tight shutoff at low differential pressure is required, specify a soft-seated valve or an independent check on the leakage rate requirement for the service.