Butterfly Valve Offset Types
Butterfly valves are classified by the number of offsets between the shaft, disc, and seat. Each offset reduces friction and wear, extending seat life and expanding the valve’s pressure and temperature range.
| Type | Max Pressure | Max Temp | Seat | Cost |
|---|---|---|---|---|
| Concentric | Class 150 | ~120 degC | Resilient only | Lowest |
| Double offset | Class 150-300 | ~400 degC | Resilient or metal | 2-3x concentric |
| Triple offset | Class 150-900+ | ~815 degC | Metal (Stellite, Inconel) | 5-10x concentric |
Offset Explained
Each “offset” refers to a geometric shift of the shaft relative to the disc and seat:
- First offset: the shaft is moved behind the plane of the seat (eccentric), so the disc lifts away from the seat as it opens.
- Second offset: the shaft is moved off the centerline of the pipe bore, creating a cam action that produces a wiping seal on closure.
- Third offset: the seating surface is machined to a conical (not flat) profile, so the disc and seat contact only at the moment of final closure. No friction during the first ~85 degrees of rotation.
Detailed Comparison
| Parameter | Concentric (Zero Offset) | Double Offset | Triple Offset |
|---|---|---|---|
| Shaft position | Center of disc and pipe | Behind seat + off pipe center | Behind seat + off pipe center + conical seat |
| Seat contact | 360-degree rubbing (continuous) | Reduced rubbing (cam action) | Contact only at final closure (zero rubbing) |
| Seat type | Resilient (EPDM, NBR, Viton) | Resilient or metal | Metal (Stellite, Inconel, SS) |
| Sealing class | Soft seat: bubble-tight | Soft seat: bubble-tight; Metal: Class IV-V | Metal: Class VI (bubble-tight) |
| Max pressure class | Class 150 | Class 150-300 | Class 150-900 (up to 1500 in special designs) |
| Max temperature | ~120 degC (resilient seat limit) | ~250 degC (resilient) / 400 degC (metal) | Up to 815 degC (metal seat) |
| Seat life | Shortest (continuous friction) | Moderate | Longest (friction-free operation) |
| Operating torque | Moderate | Lower (cam effect) | Lowest (no friction except at seating) |
| Standard | API 609 Cat. A | API 609 Cat. A/B | API 609 Cat. B |
| Typical application | Water, HVAC, low-pressure utility | Process water, cooling systems | Hydrocarbon, steam, high-temp process |
| Cost | Lowest | 2-3x concentric | 5-10x concentric |
Concentric (Zero Offset)
The simplest design. The shaft passes through the center of the disc. The resilient (rubber or elastomer) seat is in constant contact with the disc edge during rotation, which causes friction and seat wear over time. Adequate for water treatment, HVAC, fire protection, and general utility below 150 psi and 120 degC.
Double Offset
Shifting the shaft creates a cam-like motion that lifts the disc off the seat shortly after opening begins. This reduces the 360-degree rubbing of concentric designs to a brief wiping contact. Seat life improves significantly. Available with resilient or metal seats. Used in cooling water, moderate-pressure process, and services up to Class 300.
Triple Offset
The addition of a conical (funnel-shaped) seating surface means the disc only contacts the seat at the final few degrees of closure. Through the entire operating range, the disc moves freely without touching the seat. This eliminates wear and allows metal-to-metal sealing that achieves Class VI shut-off per API 598.
Triple offset butterfly valves have replaced gate valves and plug valves in many high-temperature and high-pressure applications because they are lighter, more compact, and require lower actuator torque.
All three types comply with ASME B16.34 pressure-temperature ratings and ASME B16.10 face-to-face dimensions.
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