Butterfly Valve Selection Guide: Wafer, Lug, Double Offset, Triple Offset

Butterfly Valve Selection Guide: Wafer, Lug, Double Offset, Triple Offset

Butterfly valves are among the most widely used isolation and throttling valves in modern industry. Their compact design, lightweight construction, fast operation, and relatively low cost make them ideal for water treatment, HVAC, power generation, oil & gas, and chemical processing. However, selecting the right butterfly valve requires careful evaluation of valve type (concentric vs offset), end connection (wafer vs lug), seat material, pressure class, and actuator compatibility. This comprehensive guide explains every critical selection parameter and helps you avoid costly mistakes.

Butterfly Valve Basic Design

A butterfly valve consists of a disc mounted on a rotating shaft (stem). When the stem rotates 90 degrees, the disc moves from a position parallel to the flow (fully open) to perpendicular (fully closed). The compact design provides several advantages: Short face-to-face dimension (saves space and pipe support cost), Low weight compared to gate or ball valves, Quick 90-degree operation, and Availability in very large diameters (up to 120 inch).

Concentric (Zero-Offset) Butterfly Valves

Design: The stem passes through the center of the disc. The disc centers on the stem. Sealing occurs by squeezing the disc against the seat. Applications: Water distribution, HVAC, low-pressure air, and non-critical services. Advantages: Simple, low-cost, Reliable for on-off isolation. Limitations: High operating torque (disc rubs against seat throughout travel), Not suitable for throttling in some services, Limited temperature/pressure range.

Double Offset (Double-Eccentric) Butterfly Valves

Design: The stem is offset from the disc center (first offset) and from the valve centerline (second offset). This reduces contact between the disc and seat during operation, lowering torque and seat wear. Applications: Power plant cooling water, large-diameter water mains, and moderate-pressure hydrocarbon service. Advantages: Lower operating torque than concentric design, Improved seat life, Suitable for larger sizes and higher pressures. Limitations: More expensive than concentric design, Not suitable for cryogenic or high-temperature service.

Triple Offset (Triple-Eccentric) Butterfly Valves

Design: Adds a third offset — the seating surface is conical (tapered) rather than flat. The disc contacts the seat only at the final closed position, providing friction-free opening/closing and bubble-tight shutoff. Applications: High-pressure, high-temperature, and critical isolation (refinery, LNG, steam). Advantages: Bubble-tight shutoff (metal seat), Fire-safe design available, Suitable for high-pressure Class 150-600, Long cycle life. Limitations: Higher cost, More complex manufacturing.

End Connection Types: Wafer vs Lug vs Double Flanged

Wafer Type: The valve is sandwiched between two flanges using long bolts that pass through the valve body. Advantages: Low cost, Lightweight. Limitations: Cannot be used at the end of a line (cannot be disconnected on one side without removing the entire valve).

Lug Type: The valve has threaded lugs (inserts) around the circumference. Each flange is bolted independently. Advantages: Can be used at the end of a line (dead-end service), Easier maintenance (one side can be disconnected). Limitations: Higher cost than wafer type, Slightly heavier.

Double Flanged: The valve has flanges on both ends, bolted directly to the pipe flanges. Advantages: Easiest installation, Suitable for high-pressure large diameters. Limitations: Highest cost and weight.

Seat Material Selection

Seat material determines chemical compatibility, temperature range, and shutoff capability. Common seat materials: EPDM (Ethylene Propylene Diene Monomer): Excellent for water, steam (up to 180°C), and mild chemicals. Not suitable for hydrocarbons or oils. NBR (Nitrile): Good for oil, gasoline, and water. Temperature range -20°C to 100°C. PTFE (Teflon): Excellent chemical resistance, suitable for acids, alkalis, and solvents. Temperature range -200°C to 200°C. RPTFE (Reinforced PTFE): Contains glass or carbon filler for improved mechanical strength. Metal Seat (Stellite, Inconel): For high-temperature, high-pressure, and abrasive services. Bubble-tight shutoff achievable with proper lapping.

Pressure-Temperature Rating

Butterfly valves are available in Class 150, 300, and 600 (ASME B16.34). Large-diameter water valves are often rated to PN 10/16 (approximately Class 125/150). Always verify the pressure-temperature derating curve — the allowable pressure decreases as temperature increases.

Actuator Selection for Butterfly Valves

Butterfly valves are highly actuator-friendly due to their quarter-turn operation. Common actuators: Manual lever (small sizes, infrequent operation), Gear operator (larger sizes, high torque), Pneumatic actuator (fast operation, fail-safe options), Electric actuator (precise positioning, remote control), and Hydraulic actuator (very large diameters).

Selection Checklist: 10 Critical Questions

1. What is the fluid type (water, steam, oil, gas, chemical)? 2. What are the design pressure and temperature? 3. Is the service on-off or throttling? 4. What end connection is compatible with existing piping (wafer, lug, flanged)? 5. What seat material is chemically compatible? 6. Is fire-safe certification required? 7. What is the required shutoff class (ANSI/FCI 70-2)? 8. What actuator type is preferred (manual, electric, pneumatic)? 9. Is the valve accessible for operation and maintenance? 10. What is the budget vs lifecycle cost consideration?

Frequently Asked Questions

Q: Can I use a concentric butterfly valve for throttling? A: Yes, but only for moderate throttling. For severe throttling, use a double-offset or triple-offset design with characterized disc.

Q: What is the difference between wafer and lug type? A: Wafer type is sandwiched between two flanges; lug type has threaded inserts allowing independent flange removal.

Q: Are triple-offset butterfly valves fire-safe? A: Yes, most triple-offset designs are fire-tested to API 607 or API 6FA.

Contact Us

Wenzhou Wofer Valve Co., Ltd. manufactures a complete range of butterfly valves: concentric (wafer/lug), double-offset, and triple-offset (metal-seated). All valves comply with API 609, ASME B16.34, and EN 593. Contact our technical team for detailed selection assistance, CAD drawings, and competitive quotations:

Ted Wang

Wechat/Whatsapp: +86 18267833722

Email: sales@wofervalve.com

Web: www.wofervalve.com

Wenzhou Wofer Valve Co., Ltd.