The short answer
Soft-seated butterfly valves aim for "zero leakage" (ISO 5208 Rate A), but operating temperature and medium are limited by the non-metallic seat; metal-seated (triple-offset, metal-to-metal) valves aim for high temperature resistance, erosion resistance, fire safety and long service life, with leakage typically at Rate C/D. Choose on temperature and medium first, then check leakage and fire-safety requirements.
The fundamental difference between the two designs
- Soft seated (resilient): the seat is a non-metallic material such as EPDM, NBR, PTFE, RPTFE, PEEK, Viton or PCTFE, usually in a centered or double-offset design, sealing through the elasticity of the rubber or polymer.
- Metal seated: metal-to-metal sealing surfaces, usually a triple-offset design with hardfacing of Stellite 6 / 13Cr / tungsten carbide / 316 / duplex steel, sealing where the conical face of the disc meets the body seat ring.
Key comparison
- Temperature: soft seat approximately −196 to +200/260 °C (depending on material); metal seat up to 425–600 °C and above.
- Leakage class: soft seat can reach ISO 5208 Rate A (zero leakage); metal seat is typically Rate C/D.
- Particle and erosion resistance: soft seat limited; metal seat good.
- Fire safety (API 607): soft seat needs a dedicated fire-safe design; metal seat provides a natural metal secondary seal.
- Operating torque: low for soft seat; the triple-offset design has zero friction during travel, so torque is low and stable.
- Cost and service life: soft seat costs less; metal seat costs more but offers longer life and more cycles.
Selection by operating conditions
- Temperature ≤ 200 °C, clean medium (water, air, general chemicals), zero leakage required → prefer soft seated, which offers the best value. Common in water treatment, HVAC and large-diameter FGD slurry service.
- Temperature > 200 °C, steam or hot flue gas, solids or erosive medium, fire-risk areas, long life with low maintenance → choose triple-offset metal seated. Typical applications are main steam in power plants, high-temperature chemical lines, solids-laden slurry and fire-safe duty.
- Frequent cycling with stable tightness required → the triple-offset characteristic of "no friction during travel, contact only at closure" is an advantage.
Common misconceptions
- Using a soft seat for high temperature or particle-laden medium — rubber and PTFE will age, wear and fail, and internal leakage rises sharply.
- Assuming that "a metal seat always leaks more" — with a triple-offset design, metal sealing can also achieve very low leakage and remains stable after precision lapping.
- Overlooking fire-safety requirements: if a soft-seated valve is not given a fire-safe design to API 607/6FA (metal secondary seal after seat burn-out, graphite packing, anti-blowout stem), fire-risk service carries exposure.
Vexval supplies both soft-seated centered and double-offset butterfly valves and triple-offset metal-seated butterfly valves, and can select by the combination of temperature, medium and leakage class.