Valve Comparison Guide
Soft Seat vs Metal Seat Butterfly Valve: Sealing, Temperature & Service Selection
Soft seat vs metal seat butterfly valve comparison: seat materials, shutoff class (Class IV/V vs VI), temperature limits, fire safety, cost, and selection guide for clean fluids, steam, cryogenic, and abrasive service.
Overview
A soft seat butterfly valve uses an elastomeric liner or seat ring (typically EPDM, NBR, or PTFE) to create a bubble-tight seal when the disc contacts the seat. The elastomer deforms to conform to the disc edge, achieving ASME Class VI (bubble-tight) shutoff. Concentric and double-offset body geometries are the most common soft seat designs. The seat material limits the maximum temperature and chemical compatibility of the valve.
DN50–DN2000, PN6–PN16 or Class 150, WCB or SS 316 disc, EPDM/NBR/PTFE seat, API 609 Category A
A metal seat butterfly valve uses metallic seating surfaces — either a machined SS 316, Inconel, or Stellite-6-faced seat ring and matching disc edge — to create the shutoff seal. The triple-offset geometry (three geometric offsets in the disc and seat design) ensures that the disc lifts off the seat on opening without rubbing, eliminating wear and allowing the hard metallic contact to provide reliable sealing. Metal seat butterfly valves achieve ASME Class IV or Class V shutoff (not Class VI as a rule) but are rated for much higher temperature, pressure, and fire-safe service.
DN50–DN2000, Class 150–600, WCB/CF8M/LCC disc and body, Stellite-6-faced seat ring, API 609 Category B/C, fire-safe per API 607
Pros & Cons
Soft Seat (Elastomeric) Butterfly Valve
Metal Seat (Triple-Offset) Butterfly Valve
Soft Seat (Elastomeric) Butterfly Valve vs Metal Seat (Triple-Offset) Butterfly Valve — Specification Comparison
| Parameter | Soft Seat (Elastomeric) Butterfly Valve | Metal Seat (Triple-Offset) Butterfly Valve |
|---|---|---|
| Seat Material | EPDM, NBR, PTFE, or Hypalon elastomeric seat/liner | SS 316, Inconel, or Stellite-6-faced metallic seat ring |
| Shutoff Class (ASME) | Class VI — bubble-tight zero leakage | Class IV or Class V (near-zero leakage, not bubble-tight as standard) |
| Maximum Temperature | EPDM: 130°C; NBR: 100°C; PTFE: 200°C | 600°C+ with Stellite 6-faced seats and alloy body/disc |
| Maximum Pressure | PN16 / Class 150 (standard concentric/double-offset) | Class 150–600 (and Class 900 in some triple-offset designs) |
| Fire-Safe Qualification | Not fire-safe — elastomeric seats destroyed by fire | Fire-safe per API 607 / ISO 10497 — metallic secondary seal survives fire |
| Abrasion Resistance | Poor — elastomeric liners eroded by abrasive slurries | Excellent — hard Stellite-faced metallic seats resist abrasion |
| Cryogenic Service | Limited — some PTFE designs to −60°C | Yes — CF8M or LCC body with extended bonnet per BS 6364 to −196°C |
| Steam Service | EPDM to 130°C (low-pressure steam only) | Yes — Class 150–600 steam to 538°C with Stellite trim |
| Body Geometry | Concentric (zero offset) or double-offset | Triple-offset (three geometric offsets in disc and seat design) |
| Cost | Lower — elastomeric seats, simpler geometry | Higher — precision CNC-machined triple-offset geometry and hard-faced seats |
When to Use Each
Use Soft Seat (Elastomeric) Butterfly Valve when:
Use Metal Seat (Triple-Offset) Butterfly Valve when:
Decision Guide
Choose a soft seat (elastomeric) butterfly valve when: (1) the medium is clean water, air, nitrogen, or mild chemicals at temperatures below 130°C (EPDM) or 200°C (PTFE); (2) bubble-tight Class VI shutoff is required and the service conditions are within the elastomeric seat temperature and chemical compatibility range; (3) cost and weight are primary drivers (large-diameter water and HVAC systems); (4) the service does not require fire-safe qualification (non-flammable media). Choose a metal seat (triple-offset) butterfly valve when: (1) the operating temperature exceeds 200°C — steam, hot oil, thermal fluid, refinery hydrocarbon vapour; (2) fire-safe qualification per API 607 is required — flammable hydrocarbon service in refineries and chemical plants; (3) the medium is abrasive (slurry, fly ash, coal dust, abrasive powders) and elastomeric liners would suffer rapid wear; (4) cryogenic service (LNG, liquefied gases at −50°C to −196°C) with extended bonnet design; (5) Class 300 or above pressure rating is required — most concentric soft seat designs are limited to Class 150. Rule of thumb: soft seat for utility and clean process up to 130°C and Class 150; metal seat (triple-offset) for high-temperature, high-pressure, fire-safe, abrasive, or cryogenic service.
Frequently Asked Questions
Why can't a soft seat butterfly valve be fire-safe?
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