Triple-Offset Butterfly Valve (Metal-Seated Butterfly Valve)
General / GeneralDefinition
A triple-offset butterfly valve (TOV) adds a third geometric offset — the cone angle of the sealing surfaces — to the double-offset design. The seat and disc contact along a conical path rather than a flat plane, and the offset cone geometry ensures that the disc contacts the seat only at the final moment of closure, with zero sliding friction. This frictionless metal-to-metal seating achieves zero leakage (API 598 / FCI Class VI) that was previously only possible with soft seats, while handling temperatures above 1000°F and pressures to Class 1500. The TOV has become the preferred isolation valve for many high-duty applications that once required gate or ball valves.
Working Principle
The three offsets work together: (1) Shaft offset behind the disc face — disc cams away from the seat on opening. (2) Shaft offset from the pipe centerline — elliptical rotation path reduces interference. (3) Conical seat geometry — the sealing surface is a truncated cone, not a flat circle. The third offset means the disc and seat contact at a single line along the cone, and the contact angle is such that the disc closes with a 'wiping' motion rather than sliding. Because there is no sliding contact, metal seats achieve zero leakage without the wear, galling, or high actuator forces that plague flat metal-seat designs.
Key Components
| Disc | Conical sealing edge; Stellite, tungsten carbide, or 316SS overlay |
| Seat ring | Conical matching surface; lapped to the disc for line contact seal |
| Shaft | Offset in two planes; splined or keyed to the disc |
| Body | Wafer, lug, or fully flanged; high-alloy steel for severe service |
| Packing | Graphite rings or live-loaded packing for fugitive emission control |
| Retaining ring | Holds the seat ring in the body with spring force |
| Actuator | Gear, pneumatic, or electric — sized for the offset geometry torque curve |
Types & Variants
| Feature | Triple-Offset (TOV) | Double-Offset (HPBV) | Concentric |
|---|---|---|---|
| Seating | Metal, conical, zero-friction | Metal or soft, some friction | Soft only, full sliding contact |
| Leakage class | FCI Class VI (zero) | FCI Class IV/V (soft=VI) | FCI Class VI (soft seat) |
| Temperature | -320 to 1200°F+ | -50 to 1000°F | -20 to 400°F |
| Pressure class | To Class 1500+ | To Class 600 | To Class 300 |
| Seat life | Very long (no friction wear) | Moderate | Limited by sliding wear |
Materials
High-temperature service: A217 C12/C12A (9Cr-1Mo) or A182 F91 forgings for supercritical steam and refinery duties above 800°F. General petrochemical: A216 WCB or A351 CF8M. Seat and disc sealing edges: Stellite 6 or 21 overlay (40–45 HRC) for most services; tungsten carbide (70+ HRC) for abrasive catalyst or slurry duties. Shaft: 17-4PH precipitation-hardened stainless for strength and corrosion resistance. Packing: flexible graphite, live-loaded with Belleville washers for ISO 15848-1 compliance.
Applicable Standards
API 609 Category B covers double- and triple-offset butterfly valves. Some manufacturers reference MSS SP-68 for high-pressure butterfly valves. Fire testing per API 607/6FA. ASME B16.34 for pressure ratings. ISO 15848-1 for fugitive emissions. API 598 for seat leakage testing — triple-offset valves routinely pass the zero-leakage criteria.
Applications
Refinery main process isolation: crude distillation column feeds, vacuum tower bottoms, FCC catalyst lines at 800–1100°F where gate valves are being replaced by TOVs for weight savings and maintenance reduction. Power generation: high-pressure steam isolation on main steam and hot reheat (Class 600–1500) where the compact TOV envelope fits where gate valves cannot. LNG: cryogenic isolation at -320°F with 316SS or 9% nickel steel bodies. Seawater desalination: large-diameter (48