Butterfly Valve - How They Work

Butterfly valve

Figure 1: Butterfly valve

Butterfly valves are a family of quarter-turn valves and work very similar to ball valves. The ‘butterfly’ is a disk connected to a rod. When the valve opens, the disk rotates to allow fluid to pass through. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the flow direction. Butterfly valves are popular for on-off or modulating services due to their lightweight, small installation footprint, lower costs, quick operation, and availability in huge sizes. These valves can be operated by handles, gears, or automatic actuators.

Table of contents

Principle of operation

Butterfly valves have a relatively simple construction. The main components of a butterfly valve are the body, seal, disc, and stem, as seen in figure 2. A typical butterfly valve has the disc (Figure 2 labeled A) positioned in the center of the connected pipe and a stem (Figure 2 labeled B) that is connected to an actuator or handle (Figure 2 labeled C) 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 (Figure 2 labeled D). The stem is also sealed by using an o-ring (Figure 2 labeled E). When the actuator or handle rotates the butterfly valve stem back 90 degrees, the disc moves away from the butterfly valve seat and positions itself parallel to the flow. Partial rotation allows for the flow to be throttled or proportional.

Parts of a butterfly valve; A: disk, B: stem, C: handle, D: seal, E: o-ring, F: valve body

Figure 2: Parts of a butterfly valve; A: disk, B: stem, C: handle, D: seal, E: o-ring, F: valve body

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 the disc's opening, the flow rate will be at the same X% of maximum flow rate. For example, if the disc is opened 1/3 of a turn (30 degrees), the flow rate will be 33.3% of maximum.
  • Equal: If a butterfly valve has an equal percentage characteristic, equal increments of valve travel produce equal percentage changes in the flow rate. For example, if traveling from 30 to 40 degrees opening increased the flow rate from 100 to 170 m3/h (by 70%), then traveling 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 have made the equal percentage characteristic possible for angles of opening from 20 to 90 degrees.

The symbol for a butterfly valve is shown below:

Butterfly valve symbol

Figure 3: Butterfly valve symbol

Types of butterfly valves

Butterfly valves come in diverse designs, each serving specific applications and pressure ranges. Butterfly valves can be categorized based on their disc closure design, connection design, and actuation method.

Disc closure design

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.

Concentric

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 4 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.

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: A zero offset butterfly valve with a lever handle on the left and an eccentric butterfly valve with a hand wheel on the right

Eccentric

An eccentric butterfly valve 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 4 (right). When the stem is located right behind the centerline of the disc, the valve is called a single offset. This design was developed to reduce the disc contact with the seal before the full closure of the valve to improve the 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 5). 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 similarly 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.

Top view schematic of an eccentric butterfly valve

Figure 5: Top view schematic of an eccentric butterfly valve

Connection design

Butterfly valves can be connected to a piping system in different ways. The most common methods are wafer type, lug type, and flange connection.

Butterfly valve connection types; A: Wafer type butterfly valve, B: Flanged butterfly valve, and C: lug-style butterfly valve

Figure 6: Butterfly valve connection types; A: Wafer type butterfly valve, B: Flanged butterfly valve, and C: lug-style butterfly valve

Wafer-style

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 the valve body (Figure 6 labeled A). The pipe flanges are connected through long bolts that cross the entire valve body. 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 preventing backflow in systems designed for universal flow.

Lug-style

The lug-style butterfly valve has threaded inserts (lugs) outside the valve body (Figure 6 labeled C). 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.

Actuation method

Butterfly valves can be operated manually by handles and gears or automatically by electric, pneumatic or hydraulic actuators. These devices allow precise disc rotation to positions ranging from fully open to fully closed. A brief understanding of the different types of actuation methods is below.

Manual butterfly valve

Manual actuated butterfly valves are inexpensive and easy to operate. The two common methods are a discussed below:

  • Hand lever: A butterfly valve with a hand lever Is common for small butterfly valves. The handle of the butterfly valve enables the valve to be locked into an open, partially open, or closed position. An example can be seen in Figure 4 (left). Extended spindle butterfly valves have long stems allowing the remote operation of the valve when located underground or within pits.
  • 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 4 (right).
Manual butterfly valves

Figure 7: Manual butterfly valves

Actuated butterfly valve

Power-operated actuators are a reliable method of controlling butterfly valves from a remote location. The butterfly valve actuators also make the rapid operation of larger valves possible. These actuators can be designed to fail-open (stay open in case of actuator failure), fail-close (remain closed in case of actuator failure), and often come with a manual actuation method in case of failure (as seen in Figure 8). The three types of automatic actuators are listed below, but more a more thorough understanding, read our actuator article.

  • Electric (motorized) butterfly valve: Use an electric motor to turn the butterfly valve stem.
  • Pneumatic butterfly valve: Requires compressed air to move a piston or diaphragm to open/close the valve, as seen in Figure 8.
  • Hydraulic butterfly valve: Requires a hydraulic pressure to move a piston or diaphragm to open/close the valve.
Pneumatic butterfly valves with a handwheel

Figure 8: Pneumatic butterfly valves with a handwheel

Applications

Butterfly valves are used in diverse industries and applications such as pharmaceutical, chemical and oil, food, water supply, wastewater treatment, fire protection, gas supply, fuel handling, and sanitary fittings. Butterfly valves for water are used as control valves in pipelines to shut off water flow. These valves are available in huge sizes and are suitable for handling slurries and liquids with relatively large amounts of solids at low pressures. Stainless steel butterfly valves are used in corrosive and marine environmental applications as the material is highly durable and resistant to corrosion.

Ball valve vs butterfly valve

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 minimal 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. Read our article on ball valves vs. butterfly valves for a detailed comparison between both valve types.

Butterfly valve (left) and ball valve (right)

Figure 9: Butterfly valve (left) and ball valve (right)

FAQs

What is a butterfly valve used for?

Butterfly valves are used for on-off or modulated fluid control.

Can you use a butterfly valve for gas?

Yes, butterfly valves can be used for liquids and gasses, but not bulk solids.

What is the advantage of a butterfly valve?

Butterfly valves are lightweight and can be used for both on-off and modulated fluid control.


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