What Is a Butterfly Valve?
A butterfly valve is a quarter-turn rotary valve that uses a disc mounted on a shaft to control flow through a pipe. Rotating the shaft 90 degrees moves the disc from fully open (parallel to flow) to fully closed (perpendicular to flow).
When to Use a Butterfly Valve
Butterfly valves are the preferred choice when compact size, light weight, and low cost matter more than absolute zero-leakage shut-off. They dominate in large-diameter applications (12” to 120”+) where gate valves and ball valves become prohibitively heavy and expensive. Common services include cooling water, fire water, HVAC, and large-diameter process lines.
Specifications
| Feature | Details |
|---|---|
| Function | On/off isolation and flow regulation |
| Operation | Quarter-turn (90 degrees) |
| Disc position (open) | Parallel to flow (some pressure drop remains) |
| Body styles | Wafer, lug, double flanged |
| Offset types | Concentric, double offset, triple offset |
| Standard | API 609 |
| Pressure classes | Class 150-600 (triple offset up to Class 900+) |
| Face-to-face | ASME B16.10, API 609 |
| Body materials | A216 WCB (CS), A351 CF8M (SS), ductile iron (ASTM A395) |
| Sizes | 2” to 120”+ |
| Seat materials | EPDM, NBR, Viton (resilient); Stellite, Inconel (metal) |
Butterfly vs Gate and Ball Valves
| Feature | Butterfly | Gate | Ball |
|---|---|---|---|
| Weight (24”) | ~120 kg | ~1,200 kg | ~800 kg |
| Shut-off | Good (triple offset: bubble-tight) | Good | Excellent |
| Pressure drop (open) | Moderate (disc in flow path) | Very low | Very low |
| Cost (large sizes) | Lowest | High | Highest |
How a Butterfly Valve Works
The disc is mounted on a shaft that passes through the center of the pipe. Turning the shaft 90 degrees rotates the disc between the open and closed positions. Even when fully open, the disc remains in the flow path, creating some pressure drop. This is the main limitation compared to a gate valve or full bore ball valve.
The trade-off is size and weight. A 24” butterfly valve weighs roughly one-tenth of a 24” gate valve and costs a fraction of a 24” ball valve. This advantage grows with size.
Butterfly Valve Body Styles
Wafer: the valve body fits between two pipe flanges, held in place by the flange bolts passing through the valve’s lug holes. Cannot be used as an end-of-line valve because removing one flange releases the valve.
Lug: the body has threaded inserts (lugs) so each flange bolts independently to the valve body. Can be used as an end-of-line shut-off, allowing one side of the piping to be removed for maintenance.
Double flanged: the body has integral flanges that bolt directly to the pipe flanges. Most reliable design, used for high-pressure and critical applications.
Offset Types Overview
| Type | Shaft Position | Disc-Seat Contact | Sealing | Pressure Limit |
|---|---|---|---|---|
| Concentric | Centered in disc | Full 360-degree contact (rubbing) | Resilient seat only | Class 150 |
| Double offset | Offset from disc center and body center | Cam action reduces rubbing | Resilient or metal seat | Class 150-300 |
| Triple offset | Double offset + conical seat geometry | Metal-to-metal, no rubbing at all | Metal seat (bubble-tight) | Class 150-900+ |
Concentric butterfly valves dominate water treatment, HVAC, and low-pressure utility services. Double and triple offset designs are used in oil and gas process piping, where tighter shut-off and higher pressure ratings are required.
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