A Butterfly valve is a quarter-turn rotary motion valve, that is used to stop, regulate, and start the flow. Butterfly valves are a quick open type. A 90° rotation of the handle can completely close or open the valve. Normally, they are used in systems where positive shut-off is not required.
Large Butterfly valves are usually equipped with gearbox type actuator, where the handwheel is connected to the stem via a gearbox. This will reduce the force but at the same time reduce the speed of the operation. This type of valve should be installed in the open position. If the valve is closed during installation, the rubber seat will wedge against the valve disc and make it difficult to open.
Butterfly Valve Parts
Typical butterfly valve has a short circular body, a round disk, shaft, and metal or soft seats. You can see the parts in the above image.
Types of Butterfly Valves
Based on the type of ends of the body butterfly valves are available in following types.
- Both Flanged Ends
- Wafer Type Ends
- Lug Type Ends
- Butt Welded Types Ends
Wafer Butterfly Valve
The wafer body is placed between pipe flanges, and the flange bolts surround the valve body. A wafer type butterfly valve is easy to installed but it cannot be used as an isolation valve.
Lug Style Butterfly Valve
The lug body has protruding lugs in the periphery of a body that provides passage to bolt holes that match with those in the flanges.
Flanged Butterfly Valve
In this types, the body has flanged that match with pipe flange dimension.
Butt Welded Ends Types
This types of ends are used in high-pressure services and it directly welded to the pipe.
Butterfly valves can be metal-to-metal seated, soft seated, or with a fully lined body and disc. The first image is of soft seated fully lined body and disc valve. Second, is a soft seat with metal disk and the third is metal to metal seat type valve.
The disk of butterfly valve can be concentric or eccentric with the valve body. Here I have shown three different arrangements of the disk with respect to a center of the valve body.
Zero offset Butterfly Valve
Zero offset design used for the valve that used in low-pressure and temperature services. In this design, the disc and shaft axis is concentric with the valve body. In the open position, the disc divides the flow into two equal halves, with the disc in the middle and parallel to the flow.
This type of valve has a resilient seat. Sealing is achieved when the disc deforms the soft seat. There is friction between the disk and seat during the full operating cycle which is the disadvantage of zero offset valve.
Double Offset Butterfly Valve
In Double Offset, the disk is offset from the valve center line and also from valve body center line. You can see this in the image where one and two are written. This creates a cam action during operation that lifts the seat out of the seal.
Double offset makes opening and closing smooth as friction is applicable only during the first few degrees of opening and final few degrees of closing, approx 10 degrees of opening and closing.
Triple Offset Butterfly Valve
In Triple Offset design the third offset created by the geometry design of the seating surface. The seat is machined into an offset conical profile resulting in a right-angled cone. You can see this in the third image with small enlargement of seating angle.
This ensures frictionless stroking throughout its operating cycle. Contact is only made at the final point of closure with the 90° angle acting as a mechanical stop; Metal seated valve use triple offset design.
Butterfly Valve Applications
- A Butterfly valve is used in many different fluid services and they perform well in slurry applications also. They can be used in liquids, steam, cryogenics, cooling water, air, gasses, firefighting & Vacuum services.
- Butterfly Valve is used in all type of industries application even in High-pressure and temperature services.
Butterfly Valve Advantages
- Butterfly Valve is suitable for large valve applications due to Compact, lightweight design that requires considerably less space, as compared to other valves
- Due to a quick operation, it needs less time to open or close
- The maintenance costs are usually low compared to other valve types
- A pressure drop across a butterfly valve is small
- Valve with Non-metallic seating can be used in chemical or corrosive media.
Butterfly Valve Disadvantages
- Throttling is limited to low differential pressure services and that too with a 30- to 80-degree disc opening.
- There is a chance for cavitation and choke as the disk is always in the flow Turbulence flow can affect the disc movement.