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Figure 1: Butterfly Valves
Butterfly valves are among the family of quarter-turn valves and work very similar to ball valves. The “butterfly” is a disk connected to a rod. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the flow direction (right image in Figure 1). When the valve opens, the disk is rotated back to allow the flow (left image in Figure 1).
Butterfly valves are used for on-off or modulating services and are popular due to their light weight, small installation footprint, lower costs, quick operation and availability in very large sizes. These valves can be operated by handles, gears or automatic actuators.
Table of Contents
- Principle of Operation
- Types of Butterfly Valves
- Actuation Method
- Ball Valve vs Butterfly Valve
- Additional Information
Butterfly valves have a relatively simple construction. The main components of a butterfly valve are the body, seal, disc and stem (Figure 2). A typical butterfly valve has the disc positioned in the center of the connected pipe and a stem that is connected to an actuator or handle on the outside of the valve. In the closed position, the disc is perpendicular to the flow, as shown in Figure 2, and is sealed by the valve seat. The stem is also sealed by the use of an o-ring. When the actuator or handle rotates the stem back 90 degrees, the disc moves away from the valve seat and positions itself parallel to the flow. Partial rotation allows for the flow to be throttled or proportional.
Figure 2: Anatomy of a typical butterfly valve
Butterfly valves used for modulating services can be designed to have a linear or an equal percentage characteristic.
- Linear: When the flow rate is in a linear relationship with the amount the disk travels, it means that at X% of opening of the disc the flow rate will be at the same X% of maximum flow rate. Example being that if the disc is opened 1/3 of a turn (30 degrees), then the flow rate will be 33.3% of maximum.
- Equal: If a butterfly valve has an equal percentage characteristic, that means equal increments of valve travel produces equal percentage changes in the flow rate. As an example, if travelling from 30 to 40 degrees opening increased the flow rate from 100 to 170 m3/h (by 70%), then a travel from 40 to 50 degrees will increase the flow rate from 170 to 289 m3/h (by 70%). This results in a logarithmic relationship between disc travel and flow rate. Advances in butterfly valve design has made the equal percentage characteristic possible for angles of opening from 20 to 90 degrees.
Butterfly valves can come in diverse designs, each of which serve specific applications and pressure ranges. Butterfly valves can be categorised based on their disc closure design, connection design, and actuation method.
Butterfly valves can be concentric or eccentric depending on the location of the stem in relation to the disc and the seat surface angle on which the disc closes.
The most basic type of butterfly valve design is a centric or concentric butterfly valve. This means that the stem passes through the centerline of the disc which is in the center of the pipe bore and the seat is the inside diameter periphery of the valve body (Figure 3 on the left). This zero-offset valve design is also called resilient-seated because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. In this type of valve, the disc first comes into contact with the seat at around 85° during a 90° rotation. Concentric butterfly valves are commonly used for low pressure ranges.
Figure 3: A zero offset butterfly valve with a lever handle on the left and an eccentric butterfly valve with a hand wheel on the right
Figure 4: Top view schematic of an eccentric butterfly valve
An eccentric butterfly valves means that the stem does not pass through the centerline of the disc, but instead behind it (opposite of flow direction) as seen in Figure 2 on the right. When the stem is located right behind the centerline of the disc, the valve is called single-offset. This design was developed to reduce the disc contact with the seal before full closure of the valve with the aim of improving service life of the valve. Today, single-offset valves have given way to double offset and triple offset butterfly valves.
In a double-offset or doubly eccentric butterfly valve, the stem is located behind the disc with an additional offset to one side (Figure 4). This double eccentricity of the stem enables the rotating disc to rub over the seat for only about one to three degrees.
A triple offset butterfly valve (TOV or TOBV) is often used in critical applications and is designed similar to a double offset butterfly valve. The third offset is the disc-seat contact axis. The seat surface takes a conical shape which coupled with the same shape at the ridge of the disc, results in minimal contact before full closure of the valve. A triple offset butterfly valve is more efficient and allows for less wear. Triple offset valves are often made of metal seats to create a bubble-tight shut-off. The metal seats allow butterfly valves to be used in higher temperature ranges.
High performance butterfly valve designs use the pressure in the pipeline to increase the interference between the seat and the disc edge. These butterfly valves have higher pressure ratings and are prone to less wear.
Butterfly valves can be connected to a piping system in different ways. The most common methods are wafer type or lug type. However, they can also utilize a flange or a tri-clamp connection.
Figure 5: Butterfly valve connection types
A wafer-style butterfly valve is the most economical version and it is sandwiched between two pipe flanges. This valve may or may not have flange holes outside of the valve body as seen in Figure 5. The pipe flanges are connected through long bolts that cross the entire valve body, as seen in Figure 6. The sealing between the valve and pipe flanges is accomplished via gaskets, O-rings, and flat valve faces on both sides of the valve. This type of connection is designed for sealing against bi-directional differential pressures and to prevent backflow in systems designed for universal flow.
The lug-style butterfly valve has threaded inserts (lugs) outside the valve body (Figure 4 & 5). Two sets of bolts connect pipe flanges to each side of the bolt inserts without nuts. This design enables the disconnection of one side without affecting the other for dead-end service. Lug-style butterfly valves used in dead end service generally have a lower pressure rating. The lug-style butterfly valves, unlike the wafer-style, carry the weight of the piping through the valve body.
Figure 6: Lug-style vs. Wafter-style butterfly valve installation
Butterfly valves can be operated manually by handles and gears or automatically by electric, pneumatic or hydraulic actuators. These devices allow precise rotation of the disc to positions ranging from fully open to fully closed. A brief understanding of the different types of actuation methods are below.
Manual actuated butterfly valves are inexpensive and easy to operate. The two common methods are a discussed below:
- Hand Lever: Is common for small butterfly valves and is capable of being locked into a position of open, partially open, or closed. An example can be seen in Figure 3 on the left.
- Gear: These are designed for slightly larger butterfly valves and utilize a gearbox to increase torque at the expense of decreased speed of opening/closing. Gear operated valves are also self-locking (cannot be back driven) and can be equipped with position indicators. An example can be seen in Figure 3 on the right.
Figure 7: An electric actuated butterfly valve
Power operated actuators are a reliable method of controlling valves from a remote location. These actuators also make rapid operation of larger valves possible. Actuators can be designed to fail-open (stay open in case of actuator failure) fail-close (stay closed in case of actuator failure) and often come with a manual actuation method in case of failure (as seen in Figure 7). The three types of automatic actuators are listed below, but more a more thorough understanding read our actuator article.
- Electric: Use an electric motor to turn the valve stem (Figure 7).
- Pneumatic: Requires compressed air to move a piston or diaphragm to open/close the valve.
- Hydraulic: Requires a hydraulic pressure to move a piston or diaphragm to open/close the valve.
Butterfly valves are widespread among diverse industries and applications such as pharmaceutical, chemical and oil, food, water supply, wastewater treatment, fire protection, gas supply, fuel handling. These valves are available in very large sizes and are suitable for handling slurries, and liquids with relatively large amounts of solids at low pressures.
Typically, a butterfly valve with similar characteristics is cheaper, easier to install and has a small installation footprint compared to butterfly valves. However, due to the disc in butterfly valves, they cannot be pigged for cleaning purposes. Ball valves are advantageous for high pressure small diameter applications as they are better suited for higher differential pressures and cause very small pressure drop over the system. Butterfly valves have a relatively uncomplicated design, meaning they have fewer moving parts and fewer pockets/traps for media resulting in easier repairs and cheaper maintenance costs. For small pipe diameters, the torque and material use of ball valves is relatively low. Typically, the torque and cost advantages of butterfly valves start to take over from around DN 50 mm and upwards.
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