A Butterfly Valve is a types of valve that isolates and regulates the flow of a fluid by a disc provided across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted.
It comes among the family of quarter turn valve and working is same as ball valve.
The valve is quick acting if required,as only a quarter of a turn of the spindle is required to move the valve from the fully open to the fully closed positions.
A “butterfly” is a disk attached to a rod. It closes when the rod rotates the disc a quarter turn to a perpendicular position to the flow direction.
When the valve is opened, the disk rotates back to allow flow.
Figure
Butterfly valves are popular for on-off or modulating services due to their light weight, small installation footprint, lower costs, quick operation, and availability in extremely large sizes (Sizes range from 6 mm to over 1000 mm bore ). Handles, gears, or automatic actuators can be used to operate these valves.
Large Butterfly valves are typically equipped with a so-called gearbox, which connects the handwheel to the stem via gears. This simplifies the valve’s operation, but at the expense of speed.
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Principle of Operations of Butterfly Valve
The principle of operation of butterfly valve is isolating and regulating the flow of fluid by a disc pivoted a cross the bore of a ring body.
The disc is positioned in the pipe’s center. A rod runs through the disc to an actuator on the valve’s outside. Rotating the actuator causes the disc to move in either a parallel or perpendicular direction to the flow. Unlike a ball valve, the disc is always present in the flow, causing a pressure drop even when closed.
A butterfly valve, known for its quarter-turn functionality, relies on a disc mechanism positioned within the pipeline to manage the flow of liquids or gases. This disc, firmly attached to a shaft, resides at the heart of the valve assembly.
The operation of a butterfly valve involves a 90-degree rotation of the disc. This rotation is responsible for either permitting unimpeded flow when the valve is open or obstructing the flow when the valve is closed. This is where rotary actuators come into play. The rotary actuators serves as the driving force behind the disc’s movement.
In industrial contexts, butterfly valves often incorporate pneumatic and hydraulic actuators due to their adeptness at handling diverse pressure and flow conditions.
Why is it called a Butterfly Valve ?
The “butterfly” name comes from the fact that the disc, or “wings,” moves around a central axis, which acts like the body of a butterfly.
Butterfly valve parts
Parts of butterfly and it’s function
1.Butterfly valve body:-Butterfly valves typically have bodies that fit between two pipe flanges, with lug and wafer body designs being the most common.
A.The wafer-style body is the more cost-effective of the two. It is connected to the pipes via bolt holes that run through the valve’s body. This design is simple to install and is primarily used in universal flow systems to prevent backflow. It should be noted, however, that they are not recommended for use in regulated systems.
B.Lug style butterfly valves are slightly more expensive than their counterparts, but they provide several distinct advantages. Lug butterfly valves connect to the pipe via protruding lug holes on the outside of the valve’s body. Unlike the wafer valve, the valve in this design actually helps carry the weight of the piping throughout its body. This enables dead-end servicing by removing piping from one side of the valve.
2.Disk:-Both wafer and lug style butterfly valves have a disk inside that controls the flow of media through the valve, similar to a gate or a ball in a ball valve.
The valve disc can be either concentric or eccentric. The concentric disk is the simplest of the two. The stem in the concentric design passes through the centerline of the disk. In low-pressure applications, concentratic valves are commonly used.
The eccentric disk design, on the other hand, has a stem that runs behind the center line, in the opposite direction of the flow. The eccentric disk was initially designed to help prevent contact between the disk and the seal before the valve was fully closed, thereby extending and improving the valve’s life.
3.Stem :-The butterfly valve stem may appear to be a small component, but it plays an important role. The stem is what connects the disk on the inside to the handle on the outside. Without it, the valve would be unable to open or close, and what good is a valve if it cannot open or close?
The stem has two designs: wetted and non-wetted, which refer to the protection on the stem.
A.Non-wetted stems are protected from corrosive or erosive media in the line.
B.Wetted stems are not protected from these and must therefore be made of a material compatible with the media flowing through the line.
4.Seat :-A butterfly valve’s seat is made of various elastomers or polymers and is either bonded to the body or pressed or locked down. The seat runs the length of the valve’s inner diameter. When the disk presses against the seat, they seal together, effectively stopping the flow. When butterfly valves were first designed, the metal of the disk sat against the metal body, resulting in a loose, leaky seal. The addition of elastomers or polymers to the valve design is what contributes to the leak-proof seal.
Actuation method :-
Butterfly valves can be controlled manually with handles and gears or automatically with electric, pneumatic, or hydraulic actuators. These devices allow the disc to be precisely rotated to positions ranging from fully open to fully closed. A summary of the various types of actuation methods is provided below.
Manually actuated butterfly valves are inexpensive and simple to use. The following are the two most common methods:
1.Hand Lever: This type of valve is common for small butterfly valves and can be locked into an open, partially open, or closed position. Figure 3 on the left shows an example.
2.Gear: These are intended for slightly larger butterfly valves and use a gearbox to increase torque at the expense of slower opening/closing speed. Gear operated valves are also self-locking (cannot be back driven) and can have position indicators. Figure 3 on the right shows an example.
Automatic Actuations
Power operated actuators are a reliable way of controlling valves from a remote location. These actuators also allow for the rapid operation of larger valves. Actuators can be designed to fail-open (stay open in the event of an actuator failure) or fail-close (stay closed in the event of an actuator failure), and they frequently include a manual actuation method in the event of a failure (as seen in Figure 7). The three types of automatic actuators are listed below :-
A.Electric: Turn the valve stem with an electric motor.
B.Pneumatic: A piston or diaphragm is moved by compressed air to open/close the valve.
C.Hydraulic: A hydraulic pressure is required to move a piston or diaphragm to open/close the valve.
Butterfly Valve Types
On the basis of types of end of the bod, butterfly valve are available in following types :-
A.Wafer Type Ends
B.Lug Type Ends
C.Both Flanged Ends
D.Butt Welded Types Ends
A.Wafer type ends :-A water type end butterfly valve is most economical version, which is sandwiched between two pipe flanges and surrounded by flange bolts. The pipe flanges are held together by long bolts that span the width of the valve body.
A tightly fitted seal, such as an O-ring or precision machined gasket, along with a flat valve face on the downstream and upstream sections of the valve, is used to seal the valve and pipe flanges. This connection is intended to seal against bidirectional differential pressures and to prevent backward flow in universal flow systems.
2.The lug-style butterfly valve has threaded lugs on the outside of the valve body. Two sets of bolts connect pipe flanges to each side of the bolt inserts without the use of nuts.
For dead-end service, this design allows for the disconnection of one side without affecting the other. Lug-style butterfly valves used in dead end service typically have a lower pressure rating. Unlike wafer-style butterfly valves, lug-style valves carry the weight of the piping through the valve body.
3.The Double Flange Butterfly Valve is designed with flanged ends on both sides of the valve. Bolts connect the valve to the piping flanges. The double flange butterfly valve body configuration ensures precise installation in piping and, if necessary, can also be used as a piping line end. The adaptability of this valve for lined pipes is due to the negligible disc protrusion beyond the body laying length. This also ensures that the pipe fitting is installed close to other pipe fittings.
4.This types of ends valves are used in high-pressure services and it directly welded to the pipe.
On the basis of disc closure design
A.Concentric (Zero offset )
B. Eccentric
A.Concentric :-The most basic type of butterfly valve design is the Concentric Butterfly Valve, also known as the Zero Offset Butterfly Valve. The stem passes through the center-line of the disc, which is then placed in the center of the pipe bore, and the seat is the inside diameter of the valve body’s periphery. The disc is constantly making contact with the seat.
This zero-offset valve design is also known as “Resilient Seated” or “Rubber Seated” because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. Concentric butterfly valves are commonly used for low pressure ranges. Because the seat is built as a sleeve or cartridge inside the body, the media passing through the valve does not come into contact with the body.
B.Eccentric :-Eccentric Butterfly Valves, also known as High Performance Butterfly Valves, have a stem that does not pass through the center-line of the disc, but rather is offset from the center according to the double and triple offset valve categories.
When the stem is directly behind the center line of the disc, the valve is referred to as a Single-Offset Butterfly Valve. This design was created to reduce disc contact with the seal prior to full valve closure, with the goal of increasing the valve’s service life. Butterfly valves with a single offset have given way to double offset and triple offset butterfly valves.
The stem of a Double-Offset Butterfly Valve is located behind the disc, with an additional offset to one side. Because of the stem’s double eccentricity, the rotating disc can only rub against the seat for about one to three degrees.
A Triple-Offset Butterfly Valve (TOV or TOBV) is a type of butterfly valve that is similar to a double offset butterfly valve but has one more offset. The disc-seat contact axis is the third offset. The seat surface has a conical shape, which, when combined with the same shape at the disc’s ridge, results in minimal contact before the valve fully closes, allowing the disc to seal against the seat with no friction.
A triple offset butterfly valve is more efficient and wear-resistant. To achieve a bubble-tight shut-off, triple offset butterfly valves are frequently made with metal seats. Because of the metal seats, butterfly valves can be used in higher temperature ranges.
High-performance butterfly valve designs use pipeline pressure to increase interference between the seat and the disc edge. These butterfly valves have higher pressure ratings and are less prone to wear.
Applications of butterfly valve
1.Cooling water, air, gases, and other similar applications such as fire protection, water circulation, and so on.
2. Corrosive services that necessitate lined valves
3. Services in food processing, chemical and pharmaceuticals
4. Slurry and related services
5. Services involving high-pressure and high-temperature water and steam
6. Low differential pressure throttling service, such as in cooling water or air supply systems
7. Vacuum services
Advantages and disadvantages of butterfly valve
Advantages
A.Compact design requires considerably less space, compared to other valves
B.Quick operation requires less time to open or close
C.Light in weight
D.Low-pressure drop and high-pressure recovery
E.Available in very large sizes
F.Require less maintenance
G.Required less space compare to other valves.
H.Easy and fast to open or shut off.
Disadvantages
A.Cavitation and choked flow are two potential concerns
B.Throttling service is limited to low differential pressure
C.Disc movement is unguided and affected by flow turbulence
D.Poor sealing functions
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Source :- wikipedia
