What is a Butterfly valve?
A butterfly valve is a valve that isolates or regulates the flow of a fluid. The closing mechanism is a disk that rotates.
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. When the valve opens, the disk is rotated back to allow the flow.
Butterfly valves are used for on-off or modulating services and are popular due to their lightweight, small installation footprint, lower costs, quick operation, and availability in very large sizes. These valves can be operated by handles, gears, or automatic actuators.
Principle of Operation
Operation is similar to that of a ball valve, which allows for quick shut-off. Butterfly valves are generally favored because they cost less than other valve designs, and are a lighter weight so they need less support. The disc is positioned in the center of the pipe.
A rod passes through the disc to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow, so it induces a pressure drop, even when open.
A butterfly valve is from a family of valves called quarter-turn valves. In operation, the valve is fully open or closed when the disc is rotated a quarter turn. The “butterfly” is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway.
When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.
There are different kinds of butterfly valves, each adapted for different pressures and different usage. The zero-offset butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high-performance double offset butterfly valve, used in slightly higher-pressure systems, is offset from the centerline of the disc seat and body seal (offset one), and the centerline of the bore (offset two).
This creates a cam action during operation to lift the seat out of the seal resulting in less friction than is created in the zero offset design and decreasing its tendency to wear. The valve best suited for high-pressure systems is the triple offset butterfly valve.
In this valve, the disc seat contact axis is offset, which acts to virtually eliminate sliding contact between disc and seat. In the case of triple offset valves, the seat is made of metal so that it can be machined such as to achieve a bubble-tight shut-off when in contact with the disc.
Parts of the Butterfly Valve
Butterfly valves are quarter-turn rotational motion valves that can start, stop, and regulate flow. They are simple to use, which makes them a popular choice. Below are the parts of a butterfly valve and how they contribute to the function of the valve.
The body of the butterfly valve is the part that fits between the two pipes. This slim valve comes in two major body styles: wafer and lug. The wafer-style body is the most economical of the two. It is connected to the pipes with bolt holes that run through the body of the valve.
This design is easy to install and primarily used to prevent backflow in universal flow systems. However, it should be noted that they are not recommended for use in systems that require regulation.
Lug-style butterfly valves are slightly more expensive than their counterpart but offer a few distinct advantages. Lug butterfly valves are connected to the pipe through protruding lug holes on the outside of the body of the valve. With this design, the valve actually helps carry the weight of the piping throughout its body, unlike the wafer valve. This allows for removing piping from one side of the valve for dead-end servicing.
Both wafer and lug-style butterfly valves have a disk inside, and this disk is what controls the flow of media through the valve, much like a gate in a gate valve or the ball in a ball valve.
The disc of the valve can be either concentric or eccentric. The simplest of these two is the concentric disk. In the concentric design, the steam passes through the centerline of the disk. Concentric valves are commonly used in low-pressure applications.
The eccentric disk design, however, has a stem that doesn’t pass through the centerline, but rather behind it, in the opposite direction of the flow. The eccentric disk was first designed to help prevent contact of the disk and seal before it was fully closed to help prolong and improve the life of the valve.
The stem of the butterfly valve may seem like a small part, but it plays an important role. The stem is what connects the disk inside to the handle outside. Without it, the valve would not be able to open or close, and what’s a valve if it can’t open or close?
The stem has two different designs: wetted or non-wetted, which refers to the protection on the stem. Non-wetted stems are protected against any potential corrosive or erosive media in the line. Wetted stems are not protected against these and, thus, must be made of material that is compatible with the media flowing through the line.
The seat of a butterfly valve is made of different elastomers or polymers and is either bonded to the body or pressed or locked down. The seat runs along the inner diameter of the valve. When the disk presses along with the seat, they seal together to stop the flow.
When butterfly valves were first designed the metal of the disk sat against the metal body, which did not provide a tight, leak-free seal. The addition of elastomers or polymers to the design of the valve is what helps provide the leak-proof seal.
Types of Butterfly Valves
There are 3 main categories of butterfly valves: Rubber-lined, Plastic-lined, and Metal.
- Rubber-Lined Valves: Less expensive valves for non-demanding, non-abrasive, lower temperature applications. Normally for 150-300# (PN10-25) flanges
- Plastic-Lined Valves: For use with harsh chemicals, ultrapure applications, sanitary, or abrasive applications. Normally for 150# (PN10/16) flanges
- Metal Valves: For high-pressure or high-temperature applications, or those requiring fire resistance. Can be used in 150-2500# (PN10-160) flanges. Can be Single Offset, Double Offset, or Triple Offset
Types of Butterfly Valves Based on Body Construction
1. Wafer Types
The wafer-style butterfly valve is designed to maintain a seal against bi-directional pressure differential to prevent any backflow in systems designed for unidirectional flow. It accomplishes this with a tightly fitting seal; i.e., gasket, o-ring, precision machined, and a flat valve face on the upstream and downstream sides of the valve.
2. Lug Style
Lug-style valves have threaded inserts at both sides of the valve body. This allows them to be installed into a system using two sets of bolts and no nuts. The valve is installed between two flanges using a separate set of bolts for each flange. This setup permits either side of the piping system to be disconnected without disturbing the other side.
A lug-style butterfly valve used in dead-end service generally has a reduced pressure rating. For example, a lug-style butterfly valve mounted between two flanges has a 1,000 kPa (150 psi) pressure rating. The same valve mounted with one flange, in dead-end service, has a 520 kPa (75 psi) rating. Lugged valves are extremely resistant to chemicals and solvents and can handle temperatures up to 200 °C, which makes it a versatile solution.
3. Flanged Type
It has a flange face on both sides of the valve. Popular in the very large size valves. Some companies use a ‘U’ shaped valve–this is cheaper; but the two flanges are not always parallel, which can cause problems with bolting against the pipe flanges.
4. Butt Welded Ends Types
These types of ends are used in high-pressure services and it directly welded to the pipe. Because most plastic-lined valves are limited to PN10/16 (150#) flanges, this type is rarely seen with plastic-lined valves.
There are three main ways a disc can be aligned in a valve: zero offsets, double offset, or triple offset.
Zero offset Butterfly Valve
Zero offset design is used for the valve that is 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 a zero offset valve.
Double Offset Butterfly Valve
In Double Offset, the disk is offset from the valve center line and also from the valve body centerline. 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 is created by the geometry design of the seating surface. The seat is machined into an offset conical profile resulting in a right-angled cone.
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; the Metal seated valve uses a triple offset design.
Uses of Butterfly Valve
A butterfly valve regulates flow by starting, slowing, or stopping media. The disk opens and closes with a low-torque rotation of 90 degrees and works for any compatible application. Because they cost less and are lighter weight, the butterfly valve is often preferred over other types of valves.
Butterfly valves can be used across a wide range of applications. They perform well in large volume water applications and slurry applications. The following are some typical applications of Butterfly valves:
- Cooling water, air, gases, fire protection etc.
- Slurry and similar services
- Vacuum service
- High-pressure and high-temperature water and steam services
- Compressed Air or Gas Applications
Manual Actuation vs. Automatic Actuation
One major advantage of butterfly valves is that they can be operated either manually or automatically to fully rotate the disc inside the valve.
Manual actuation for butterfly valves involves either a hand lever or gears. Hand levers are used primarily on smaller valves, and much like their name implies, are a lever on top that is rotated up to 90 degrees to open, close or regulate flow.
Gear-operated valves are used on bigger valves and utilize a gearbox to help open and close the valve. While the gearbox does make it easier for the operator to open the valve, it does come at the price of a decreased speed.
The ability of butterfly valves to be operated automatically allows them to be controlled remotely. Automatic actuation occurs by the use of either an electric motor, hydraulic pressure, or pneumatic pressure.
Automatic butterfly valves are extremely helpful with larger valves (butterfly valves have been produced as large as 200inches!), and come with fail-safe options like fail-open (open in case of failure) or fail-close (close in case of failure).
Advantages of a Butterfly Valve
Butterfly valves are similar to ball valves but have several advantages.
- They are small and, when actuated pneumatically, open and close very quickly.
- The disc is lighter than a ball, and the valve requires less structural support than a ball valve of comparable diameter.
- Butterfly valves are very precise, which makes them advantageous in industrial applications.
- They are quite reliable and require very little maintenance.
- Can install/remove without pipe system dislocation
- Ability to throttle flow
- Variety of actuation methods available
- Available in large diameters
- A variety of seat and disc materials
- Full stainless-steel bodies available
- Lightweight aluminum bodies available
Disadvantages of a Butterfly Valve
One disadvantage of butterfly valves is that some portion of the disc is always presented to the flow, even when fully opened. The use of a butterfly valve therefore always results in a pressure switch across the valve, regardless of the setting.
- A certain part of the disc always faces the flow, even when completely opened. The operation of a butterfly valve thus always leads to a pressure switch over the valve irrespective of the setting.
- It has a poor sealing function.
- Throttling by employing butterfly valves is restricted to low-pressure drop systems.
- Possible cavitation and choked flow is a worry when butterfly valves perform.