Butterfly valve
An ellipse with a diagonal disc inside represents a butterfly valve. Compact, low-cost, suitable for large diameters where a globe or ball would be unwieldy.
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How it’s drawn.
A round or elliptical body with a disc line across it, the disc that swings a quarter turn to open or close. It is compact for large diameters where a globe or ball body would be bulky. An actuator drawn on top, a hand lever, a motor, or a spring, shows how it is driven.
Variants.
- Lugged body. Threaded inserts let either flange be removed without taking the valve apart.
- Wafer body. Clamped between flanges with through-bolts. Lower cost, both flanges must be removed for service.
- Resilient-seated, rubber-lined. Water and low-pressure utility, soft shutoff to bubble-tight on clean fluids.
- High-performance, double-offset. Disc rotates off-axis, reduces seat wear, suitable for higher pressure and modulating service.
- Triple-offset metal-seated. Tight shutoff in steam and refinery service, meets API 598, API 6D Class VI.
- Actuator options. Hand lever, gear operator, electric, pneumatic scotch-yoke, hydraulic.
Typical usage.
Cooling water headers, large-diameter air or low-pressure service, HVAC-style isolation. Tight shutoff at high differential pressure is harder than with a ball valve, so butterflies usually aren't picked for ESD service.
Telling it apart.
- Butterfly and ball are both quarter-turn. The body outline and the disc line distinguish the butterfly.
- Lugged and wafer bodies look similar on the drawing. The difference is in how the body mounts between flanges, often carried as a note.
- A butterfly used for control carries a positioner and actuator, the same as any modulating valve.
FAQ.
When is a butterfly valve preferred over a ball or gate valve.
Large diameters from about 8 inch upward, where compact face-to-face and weight matter. Low to mid pressure utility service. Partial-throttling duty where a ball would erode. And any large air, water, or HVAC isolation point. Below 4 inch a ball valve is usually cheaper and tighter.
Is the symbol the same for hand-actuated and automated butterfly valves.
The body symbol, ellipse with disc is identical. The actuator drawn above the body is what changes. A circle with M for motor, a rectangle with diagonal lines for diaphragm, a long stem with a handwheel for manual gear, or a small circle with C, O for cylinder.
How does a P&ID indicate whether a butterfly valve is open or closed.
ISA 5.1 does not mandate a default fill convention for the disc. Project standards typically use a shaded disc to mean closed for normally-closed isolation duty, and an unfilled disc to mean open. The fail position is annotated on the actuator, FC, FO, FL, not on the body symbol.
Are butterfly valves rated for SIS, ESD service.
Soft-seated butterflies generally do not meet API 6D, Class V shutoff for emergency block service. Triple-offset metal-seated butterflies with certified actuator packages do meet API 598, API 607 and are used as SIS-final-elements in refinery and gas service. The P&ID tag changes from HV, XV to SDV when SIS-rated.