Butterfly Valve Seat Design and Sealing Mechanisms

Butterfly valves rely on seat sealing systems to achieve the required shutoff leakage class. Seat design is one of the most critical aspects of butterfly valve engineering and directly affects performance, temperature range, and media compatibility.

Seat Types and Configurations

• Resilient (soft) seats: elastomer liners such as EPDM, NBR, PTFE provide bubble-tight shutoff but are limited by temperature

• Metal seats: used for high-temperature or abrasive applications; typically Class IV to VI per ASME FCI 70-2

• PTFE seats: excellent chemical resistance and low friction, suitable for corrosive media

• Fire-safe seats: combination designs with a primary soft seat and a secondary metal-to-metal seal activated by fire

Sealing Geometry

In concentric (centric) butterfly valves, the disc axis coincides with the seat centerline, causing the seat to be continuously rubbed during operation. Double-offset (double-eccentric) designs move the shaft behind and to the side of the seat plane, creating a cam action that lifts the disc off the seat before rotation, greatly reducing wear.

Triple Offset Design

Triple offset (triple eccentric) butterfly valves add a third geometric offset that creates a conical seat geometry. The disc meets the seat only at the final degree of closure with zero rubbing contact throughout operation. This design achieves metal-to-metal shutoff equivalent to a gate or globe valve.

Seat Material Selection

• EPDM: water, steam up to 150 deg C, not suitable for hydrocarbons

• NBR (Buna-N): petroleum products, oils, fuels

• PTFE/RPTFE: chemicals, acids, wide pH range, up to 200 deg C

• Stellite 6: high-temperature gas and steam, erosive service

• Inconel 625: highly corrosive high-temperature environments

Leakage Class Requirements

Soft-seated butterfly valves typically achieve ASME FCI 70-2 Class VI (bubble-tight) leakage. Metal-seated versions achieve Class IV to V. Triple offset designs with metal seats can meet Class VI in gas service.