How to Select & Size a Butterfly Valve: The Complete Buyer's Guide

Step 1: Choose the Disc Type

The disc type determines the valve's sealing mechanism, temperature capability, and shutoff class:

Concentric (Centric/Resilient Seated)

• Disc centered on stem axis — simplest and most cost-effective design
• Elastomeric seat (EPDM, NBR, Viton, PTFE liner) provides bubble-tight shutoff
• Temperature: -30°C to +120°C (EPDM), up to 200°C (PTFE lined)
• Best for: Water, HVAC, low-pressure chemical, wastewater

Double-Offset (High Performance)

• Stem is offset from disc center and body center — reduces seat wear during operation
• PTFE or graphite-filled RPTFE seat, metal backup seat available
• Temperature: up to 400°C with metal seat
• Best for: Process isolation, moderate throttling, steam, oil & gas utility

Triple-Offset (Metal Seated)

• Three geometric offsets create a cam-action sealing — disc contacts seat only at final degrees of closure
• Metal-to-metal (Stellite/laminated graphite) seat — zero leakage without elastomer
• Temperature: -196°C to +815°C — widest range of any butterfly valve
• Inherently fire-safe — no soft seat to burn
• Best for: Hydrocarbon isolation, high-temperature, cryogenic, fire-safe applications

Step 2: Choose the Body Style

Step 3: Select Seat Material

Step 4: Size the Valve

Butterfly valves are typically line-sized (same nominal size as the pipe). However, for applications requiring precise flow control, the valve can be sized using the Cv (flow coefficient) method:

Cv = Q × √(SG / ΔP)

Where: Q = flow rate (US GPM), SG = specific gravity, ΔP = pressure drop across valve (psi)

Compare the required Cv with the valve manufacturer's Cv table for the selected size. The valve Cv at full open should be at least 1.25× the required Cv.

Step 5: Verify Pressure Class

Match the valve pressure class to the piping system:

• PN10/PN16 (Class 150): Standard water, HVAC, low-pressure process
• PN25/PN40 (Class 300): Medium-pressure process, steam, chemical
• Class 600: High-pressure oil & gas, refinery process (triple-offset only)
• Class 900/1500: Very high pressure — triple-offset metal seated only

Quick Selection Matrix

Frequently Asked Questions

What is the difference between a wafer and lug butterfly valve?

A wafer butterfly valve is clamped between two pipe flanges using through-bolts that pass through both flanges but not the valve body — the valve cannot support dead-end service (downstream pipe removed under pressure). A lug butterfly valve has threaded inserts (lugs) in the body that allow one set of flange bolts to be removed while maintaining pressure on the other side — enabling dead-end service, pump isolation, and downstream pipe removal.

Can butterfly valves be used for throttling?

Yes, but with limitations. Concentric butterfly valves provide poor throttling characteristics — the equal-percentage flow curve is very steep between 0-30° open, making precise control difficult. Double-offset designs offer better throttling in the 20-70° range. For true modulating control, specify a control-characterized butterfly valve (with V-notch disc or profiled disc) or a globe control valve for applications requiring precise Cv control.

How do I choose between butterfly valve and ball valve?

Butterfly valves are preferred for large diameters (6" and above) where they offer significant cost and weight savings over ball valves. Ball valves are preferred for small bore (below 4"), where bubble-tight shutoff is critical, and for dirty/slurry service. For pipeline isolation (API 6D), ball valves are standard. For water transmission, HVAC, and process utility, butterfly valves dominate above 6".