Butterfly valves belong to a family of quarter-turn valves that work very similarly to ball valves. When the lever rotates the disc a quarter turn to a position perpendicular to the direction of water flow, it closes. When the valve is opened, the disc is rotated back to allow flow.

Butterfly valves are used for switching or regulating services and are popular because they are lightweight, have a small installation area, are inexpensive, quick to operate, and are available in very large sizes. These valves can be operated by handle, gear or automatic actuator.

Operating Principle

The structure of a butterfly valve is relatively simple. The main components of a butterfly valve are the valve body, seals, valve flaps and stem. A typical butterfly valve has a valve flap located in the center of the connecting pipe and a stem connected to an actuator or handle on the outside of the valve. In the closed position, the valve flap is perpendicular to the water flow and is sealed by the valve seat. The stem is also sealed with an O-ring. When the actuator or handle rotates the stem back 90 degrees, the valve flap leaves the seat and brings itself parallel to the fluid. The partial rotation allows the flow to be throttled or proportional.

Butterfly valves for regulating service can be designed to have either linear or equal percentage characteristics.

Linear. When the flow rate is linear with the amount of movement of the valve disc, this means that at X% of the disc opening, the flow rate is the same as X% of the maximum flow rate. For example, if the disc is open 1/3 turn (30 degrees), the flow rate will be 33.3% of the maximum flow rate.

Equal: If the butterfly valve has an equal percentage characteristic, this means that equal increments of valve travel will produce an equal percentage change in flow. For example, if opening the stroke from 30 to 40 degrees increases the flow from 100 to 170 cfm (a 70% increase), then the stroke from 40 to 50 degrees increases the flow from 170 to 289 cfm (70%). This results in a logarithmic relationship between butterfly valve stroke and flow rate. Advances in butterfly valve design have made the equal percentage characteristic possible with opening angles from 20 to 90 degrees.

Butterfly valves can come in a variety of designs, each suitable for a specific application and pressure range. Butterfly valves can be classified according to their valve closure design, connection design, and actuation method.

Disc Closure Design

Butterfly valves can be concentric or eccentric, depending on the position of the stem relative to the valve flap and the seat surface angle at which the valve flap closes.

Concentric

The most basic type of butterfly valve design is the concentric or concentric butterfly valve. This means that the stem passes through the centerline of the valve plate, which is in the center of the bore, and the seat is around the inside diameter of the valve body. This zero-offset valve design is also known as a resilient seat because it relies on the flexibility of the seat rubber to effectively seal the fluid when closed. In this type of valve, the valve plate first makes contact with the seat at approximately 85° during a 90° rotation. Concentric butterfly valves are typically used in the low-pressure range.

Eccentric

Eccentric butterfly valves mean that the stem does not pass through the centerline of the valve plate, but is behind it (opposite the direction of flow).  When the stem is located directly behind the centerline of the valve plate, the valve is called single offset. This design was developed to reduce the contact between the valve flap and the seal before the valve is fully closed, with the goal of increasing the life of the valve. Single eccentric valves have given way to double and triple eccentric butterfly valves.

In a double eccentric or double eccentric butterfly valve, the stem is located behind the valve plate and is additionally offset on one side. This double eccentricity of the stem allows the rotating disc to rub only about 1 to 3 degrees on the valve seat.

Triple eccentric butterfly valves (TOV or TOBV) are typically used in critical applications and are similar in design to double eccentric butterfly valves. The third offset is the disc seat contact axis. The seat surface is tapered and combines with the same shape of the valve ridge to create minimal contact before the valve is fully closed. Triple eccentric butterfly valves are more efficient and have less wear. Triple eccentric valves are typically made with metal seats to create a gas-tight shutoff. Metal seats allow butterfly valves to be used in higher temperature ranges.

High performance butterfly valve designs use pressure in the pipeline to increase interference between the seat and the edge of the valve plate. These butterfly valves have a higher pressure rating and are easy to wear.

Connection Design

Butterfly valves can be connected to piping systems in a variety of ways. The most common methods are butt, lug, and flange connections.

Wafer Type

The wafer type butterfly valve is the most economical version and is sandwiched between two piping flanges. The valve may or may not have a flange hole on the outside of the valve body. The pipe flanges are connected by long bolts that span the entire valve body. Sealing between the valve and the pipe flanges is achieved by gaskets, O-rings, and flat valve faces on both sides of the valve. This type of connection is designed to seal differential pressures in both directions and to prevent backflow in systems designed for general purpose flow. 

Lug Type

Lug type butterfly valves have threaded inserts (lugs) on the outside of the valve body. Two sets of bolts connect the pipe flange to each side of the bolted insert without a nut. This design allows one side to be disconnected without affecting the dead-end service on the other side. Lug type butterfly valves for dead-end service typically have a lower pressure rating. Unlike a counterclip, a lug type butterfly valve carries the weight of the piping through the valve body.

Actuation Method

Butterfly valves can be operated manually by handle and gear, or automatically by electric, pneumatic or hydraulic actuators. These devices allow the disc to be rotated precisely from a fully open to fully closed position. Below is a brief look at the different types of actuation methods.

Manual Actuation

Manual butterfly valves are inexpensive and easy to operate. Two common methods are discussed below.

Handle: Commonly found on small butterfly valves, they are capable of being locked in the open, partially open, or closed position.

Gears: These are designed for slightly larger butterfly valves and utilize a gearbox to increase torque, but will reduce the speed of opening/closing. Gear operated valves are also self-locking (not reverse actuated) and can be equipped with a position indicator.

Automatic actuation

Electric actuators are a reliable way to control valves from a remote location. These actuators also make it possible to operate large valves quickly. Actuators can be designed to fail open (remain open in the event of an actuator failure) fail closed (remain closed in the event of an actuator failure) and are typically manually actuated in the event of a failure.

Electric: Uses an electric motor to turn the valve stem.

Pneumatic: requires compressed air to move a piston or diaphragm to open/close the valve.

Hydraulic: requires hydraulic pressure to move the piston or diaphragm to open/close the valve.

Applications

Butterfly valves are used in a wide variety of industries and applications such as pharmaceutical, chemical and petroleum, food, water supply, wastewater treatment, fire protection, gas supply, fuel handling, etc. These valves are available in very large sizes and are suitable for handling slurries, as well as liquids with relatively large amounts of solids at low pressures.

Ball Valves vs. Butterfly Valves (Click for more detailed technical article)

Typically, butterfly valves with similar characteristics are cheaper, easier to install and have a smaller installation footprint than butterfly valves. However, because of the valve flaps in butterfly valves, they cannot be purged for cleaning. Ball valves are advantageous for high-pressure, small-diameter applications because they are better suited for higher differential pressures and produce very small pressure drops in the system. The relatively simple design of butterfly valves means they have fewer moving parts and fewer media pockets/catchers, which makes repairs easier and maintenance costs lower. For small pipe diameters, ball valves have relatively low torque and material usage. Typically, the torque and cost advantages of butterfly valves begin to emerge from around DN 50 mm and up.

FAQ

What are butterfly valves used for?

Butterfly valves are a series of quarter-turn rotary motion valves used to shut off flow in pipelines. It is often said that butterfly valves can be used to regulate flow. However, we do not recommend this as it can damage the valve disc and negatively affect the sealing performance.

What are the disadvantages of butterfly valves?

Even when fully open, a portion of the valve plate will be present in the fluid. Because of this, the butterfly valve will always create a pressure switch on the valve, regardless of the setting. Thicker fluids may also prevent the movement of the discs.

What are the advantages of Butterfly Valves?

The valve flap is lighter than a ball valve, and the valve requires less structural support than a ball valve of comparable diameter. Butterfly valves are very precise, which gives them an advantage in industrial applications. They are very reliable and require little maintenance.

Do butterfly valves restrict flow?

Butterfly valves are used where space is limited. Unlike gate valves, butterfly valves can be used to throttle or regulate flow as well as in the fully open and fully closed positions.

How much can a butterfly valve open?

They can be opened anywhere from 0 to 90 degrees. The farther they open, the more fluid they pass. While some piping systems do not require partial flow, many others do.