Metal Seat vs Soft Seat Valve

Valve seat material determines shut-off performance, temperature capability, wear resistance, and fire safety. Soft seats (polymer/elastomer) achieve bubble-tight sealing but have temperature limits. Metal seats (hard alloy) survive extreme temperatures and abrasion but allow minor leakage under normal conditions.

Key Difference	Soft Seat	Metal Seat
Shut-off	Class VI (zero leakage)	Class IV-V (minor leakage)
Max temperature	~200-315 degC	~815 degC
Fire resistance	Poor (requires backup)	Inherently fire safe
Cost	Lower	Higher
Best for	Ambient-temperature hydrocarbon, chemical	High-temp, abrasive, fire-critical service

Selection Guidelines

Soft seats provide bubble-tight (zero leakage) shut-off at low operating torque. They are the default for most ambient-temperature services where tight sealing is the priority. However, they have limited temperature range and will degrade or melt in a fire.

Metal seats withstand extreme temperatures, abrasive media, and fire exposure. They do not achieve zero leakage under normal conditions but provide reliable sealing with minimal maintenance over long service lives.

Full Comparison

Parameter	Soft Seat	Metal Seat
Materials	PTFE, RPTFE, PEEK, Nylon, Viton, EPDM, NBR	Stellite 6, Inconel 625, tungsten carbide, 316 SS, Cr2O3 coating
Shut-off class (API 598)	Class VI (bubble-tight, zero leakage)	Class IV or V (measurable but small leakage)
Temperature range	-29 to 200 degC (PTFE); -50 to 315 degC (PEEK)	-196 to 815 degC
Fire resistance	Poor (melts/burns, requires metal backup for fire safe)	Excellent (inherently fire safe)
Abrasion resistance	Low (particles embed in or erode soft material)	High (hard alloy surfaces resist erosion)
Operating torque	Low (low friction polymer)	Higher (metal-to-metal friction)
Sealing longevity	Degrades with cycling and temperature	Long-lasting if properly lapped
Cost	Lower	Higher (hard-facing or special alloys)
Maintenance	Seat replacement more frequent	Less frequent replacement
Typical valve types	Ball, butterfly, plug	Ball, butterfly (triple offset), gate, globe

Temperature Limits by Seat Material

Seat Material	Min Temp	Max Temp	Service
PTFE	-29 degC	200 degC	General hydrocarbon, chemical
RPTFE (reinforced)	-29 degC	260 degC	Improved wear, steam
PEEK	-50 degC	315 degC	High-performance, abrasive
Viton (FKM)	-15 degC	200 degC	O-rings, secondary seals
EPDM	-50 degC	150 degC	Water, steam, utilities
Nylon	-40 degC	120 degC	Low-pressure gas
Stellite 6	-196 degC	815 degC	High-temp, erosive, fire safe
Inconel 625	-196 degC	815 degC	Corrosive + high-temp
Tungsten carbide	-196 degC	500 degC	Extreme erosion (choke valves)

Fire Safety Considerations

Soft-seated valves in hydrocarbon service must be fire safe certified per API 607. The fire safe design includes a metal-to-metal backup seal that engages when the soft seat burns away. Metal-seated valves are inherently fire safe because the sealing surfaces survive fire temperatures.

For critical safety applications (ESD valves, blowdown valves, flare isolation), metal-seated or fire-safe soft-seated valves are mandatory.

Common Mistake

Assuming metal seats provide zero leakage like soft seats. Metal-to-metal contact cannot achieve the same conformability as polymer seats. If zero leakage is required (e.g., toxic or flammable gas isolation), soft seats with fire-safe backup are usually the better choice for temperatures within the polymer’s range.

For related information on valve trim materials and ASME B16.34 pressure-temperature ratings, see the linked guides.

Read the full guide to valve types