Structural Classification: Four Basic Designs
Butterfly valves are described in the first instance by where the shaft sits relative to the disc and the body, because that geometry decides friction, sealing performance and service life. The four common designs are concentric, single eccentric, double eccentric and triple eccentric.
| Design | Geometry | Typical performance range | Typical duty |
|---|---|---|---|
| Concentric, centre seal | Shaft, disc and body centres coincide | Low pressure, ambient temperature, roughly 100,000 cycles | Water and air lines, municipal supply and drainage |
| Single eccentric | Shaft offset from the disc centre | Moderate pressure, rubber seats, zero visible leakage | Clean process pipework, food and pharmaceutical lines |
| Double eccentric | Shaft offset from both disc and body centres | Higher pressure, metal or graphite seats, upper limit around 425 degrees Celsius, over 500,000 cycles | High-temperature steam and gas lines in petrochemical and metallurgical plants |
| Triple eccentric | Double eccentric design with an inclined sealing face | Metal-to-metal seal certified to API 609 Class VI, high pressure and temperature extremes, wide turndown | Cryogenic storage, main steam and demanding shutoff duty |
In the concentric design the disc and seat rub over their full circumference during every stroke, so the seat wears and the sealing life is short; the design is nevertheless cheap and perfectly adequate for low-pressure water and air. Offsetting the shaft reduces the rubbing, because the disc leaves the seat earlier in the stroke. In the double eccentric design the disc disengages completely at the start of opening, which removes most of the friction and makes metal seats practical, allowing much higher temperatures. The triple eccentric design adds an inclined conical sealing surface so that the seal is made by mechanical wedging of two metal surfaces rather than by sliding contact, which is how a metal-seated valve achieves a Class VI leakage rate while still giving stable flow control across the full quarter turn.
Actuation: Manual, Pneumatic, Electric and Hydraulic
Butterfly valves are actuated in four ways, and the choice usually follows the size of the valve and the speed of operation required. Manual operation with a handle, worm gear or handwheel suits small valves of DN200 and below that are operated infrequently. Pneumatic actuation, either double acting or spring return, is fast, completing a stroke in a few seconds, and is preferred for rapid shutoff and for hazardous areas where explosion-proof certification is required. Electric actuation offers remote control and accurate modulating duty through a position signal, which suits central monitoring and precision regulation, although its explosion protection is less robust than that of a pneumatic unit. Hydraulic actuation delivers high torque from a hydraulic power supply and is used on very large valves and in harsh environments such as hydropower tailrace and offshore platforms.
Sealing Type: Soft Seat and Metal Seat
Soft-seated valves use elastomer or polymer seats, typically nitrile rubber, ethylene propylene rubber or polytetrafluoroethylene, and they seal tightly at moderate pressure and temperature. Their temperature capability is the limiting factor, generally up to about 150 degrees Celsius, but they are inexpensive and are the correct choice for water, oil and many chemical duties. Metal-seated valves use a stainless steel seat, a hardened or carbide-coated disc and graphite packing, and they tolerate high temperatures, high pressures and abrasive or particulate media, at the cost of higher machining precision and a much higher price than a soft seat.
Special Function Designs
Beyond the standard catalogue, valves are built for particular service conditions. High-temperature designs use heat-resistant cast alloys and allow clearance between shaft and bearing so that thermal expansion cannot jam the disc. Cryogenic designs use austenitic stainless steel and nickel-based materials that keep their toughness at very low temperature, with a polymeric seat that does not become brittle, and are used on liquefied gas storage. Wear-resistant designs add a carbide coating on the disc and use a full-bore flow path to limit particle erosion in slurry and ash handling. Vacuum designs combine a metal bellows with an elastomer seal to prevent leakage into semiconductor, coating and vacuum heat treatment equipment.
Connection Types and Selection
The body connection decides how the valve is installed. Wafer valves are clamped between two pipe flanges and save considerable face-to-face space, but they are not suitable for high-pressure or high-vibration lines. Flanged valves are stronger, tolerate higher pressure and are easier to remove for maintenance. Clamp-ended valves allow rapid assembly and are common on hygienic process lines, and fully welded bodies are used where absolute leak tightness is required. Selection should start from the medium: corrosive fluids call for a lined or high-alloy body, and particulate media for a hardened, eccentric design. Then come the operating parameters, since pressure, temperature and vacuum each point to a specific seat and design, followed by the automation requirement and the cost or life expectation of the application. Pressure testing of the finished valve is carried out to recognised valve test standards, and butterfly valve design and face-to-face dimensions are covered by standards such as API 609, ISO 10631 and, in China, GB/T 12238 for resilient-seated and JB/T 8527 for metal-seated valves.
FAQ
Q: How many main types of butterfly valve are there?
Four structural types are normally recognised: concentric or centre-seal, single eccentric, double eccentric and triple eccentric. Each type is available with different seals, actuation and body connections.
Q: What is the difference between a double eccentric and a triple eccentric butterfly valve?
The double eccentric design offsets the shaft from both the disc and body centres so the disc clears the seat on opening. The triple eccentric design adds an inclined sealing surface, so the seal is formed by wedging two metal surfaces, giving a tighter and more durable metal seat.
Q: When should a soft-seated valve be used instead of a metal-seated valve?
Use a soft seat for water, oil and general chemical duty within its temperature limit, where tight shutoff and low cost are the priorities. Use a metal seat for high temperature, high pressure, abrasive media or where a fire-safe design is required.
Q: Which actuation type is best for hazardous areas?
Pneumatic actuation is usually preferred, because spring-return and double-acting cylinders are available with explosion-proof certification and give fast, reliable operation. Electric actuators can also be supplied for hazardous areas but need additional protection.
Q: How is a butterfly valve connected to the pipeline?
Wafer, flanged, clamp and welded connections are the common choices. Wafer bodies are compact and economical, flanged bodies handle higher pressure and are easier to service, and welded bodies are used where no leakage can be tolerated.
