A butterfly valve is a quarter-turn valve where a disc mounted on a shaft rotates within the flow path. Rotate the disc 90°: from fully blocking the pipe (closed) to lying edge-on with the flow (open). It's the workhorse valve for large-diameter, low-to-moderate-pressure service — water treatment plants, HVAC chilled water loops, fire mains, and any place a gate valve would be enormous and expensive.

Why butterflies dominate large pipes: Compare a 12-inch gate valve (heavy, tall, expensive, slow to actuate) with a 12-inch butterfly (thin wafer body sandwiched between flanges, light, cheap, opens in a quarter turn). The butterfly's face-to-face dimension is often 1/5 that of a gate valve, and it weighs a fraction as much.

Three main types:

• Concentric (resilient-seated): Shaft centered in the disc, disc centered in the bore. Elastomer seat (EPDM, Buna-N) does the sealing. Cheap, good for water, limited to ~200 psi and ~250°F.
• Double-offset (high-performance): Shaft offset from disc centerline and from pipe centerline. The disc cams away from the seat as it opens, reducing seat wear. Good to ~600 psi.
• Triple-offset: Adds a third offset to the seat cone angle, creating a metal-to-metal seal with zero rubbing. Used for steam, high temperatures, and tight shutoff on critical service. Expensive.

The flow characteristic problem: Butterfly valves have a highly nonlinear flow curve. At small openings (0–20°), almost no flow passes — the disc is still nearly blocking. Between 60–90°, the flow barely changes with angle. The useful control range is roughly 25°–70°. Trying to throttle near closed causes the disc to flutter and erode the seat.

Rule of thumb — Cv sizing: A fully open butterfly valve typically has a flow coefficient roughly Cv ≈ 20 × D², where D is nominal diameter in inches. So a 6-inch valve has Cv ≈ 720. Flow rate in GPM ≈ Cv × √(ΔP), where ΔP is pressure drop in psi. A 6-inch valve with 4 psi drop passes: 720 × √4 = 1,440 GPM.

Watch out for:

• Disc protrusion: In the open position, the disc still sits in the flow path. You cannot pig the line through a butterfly valve, and a swing check downstream can collide with the disc if spaced too close.
• Dynamic torque: Flow exerts a torque trying to slam the disc closed (or open). Actuators must be sized for the worst-case dynamic torque, not just seating torque — often 3–5× higher.
• Cavitation at partial opening: High pressure drop across a partially closed disc accelerates fluid past vapor pressure on the downstream face. Trim erodes fast.