Butterfly vas are part of a group that uses quarter-turn valves to regulate fluid media flow. They function in a similar way to ball valves but have different operational efficiency and performance. The butterfly valve has a disc attached to a rod. This rod rotates and controls the valve’s opening and closing.
Butterfly valves are used in many industries due to their light weight, low cost, small footprint and large size. You can operate them manually with handles or gears, or automatically using actuators. More information is available below.
How Butterfly Valves Work
Butterfly Valves have a simple housing, body, seal, stem and disc. In a closed position with the valve seat, the disc is perpendicularly to the flow direction. The disc is usually located in the middle of the stem and connected pipe. An o-ring seals any stem that is connected to a handle, actuator or other device.
Figure 2: Actuator/handle (A), Stem (B), Stem O-ring (C), Seal (D), Disc (E), Valve body (F).
When the handle or actuator rotates the stem by 90 degrees, the disc moves away from the valve seat, positioning itself parallel to the flow. The flow can be throttled or proportional by rotating the stem part-way.
If butterfly valves are used to modulate services, they can either be linear or have an equally percentage characteristic. This means that valve travel in equal increments will result in a proportional change of flow rate.
In other words, there is a logarithmic relationship of the flow rate to the disc travel. With the linear design, the flow rate is directly proportional with the distance traveled by the disc. For instance, if the disc is opened 1/8 of a turn (i.e., 11. 25 degrees), then the valve’s flow rate is 12.5%.
Butterfly Valves Types
Butterfly vales can be classified based on their disc closure design, actuation methods, and connection designs. Here is a brief overview.
Disc Closure Design
There are two types of valves in this category: concentric and eccentric. This classification is based on the position of the stem relative to the disc, and the angle at which the disc closes.
With the concentric type, seat is within the body’s diameter and stem runs through the centerline of the disc. This valve design has a zero offset. The seal’s efficiency is determined by the seal’s flexibility when the valve is closed. These valves are used in low-pressure applications.
An eccentric butterfly valve’s stem doesn’t go through the disc centerline, but rather passes behind it. There are three types of eccentric valves: single-offset (double-offset), double-offset (triple-offset) These three types are intended to reduce disc contact with seals until the valve is completely closed. This is done to prolong the service life of the valve.
The triple offset type is more efficient and is often used in critical applications. The eccentric valves are generally more resistant to wear and have a higher pressure rating.
A butterfly valve can be connected to a piping system in many ways. Based on the type of connection, there are three types of butterfly valves. There are three types of common valves: lug, wafer, and flange connections. The wafer type is the most cost-effective and it also ensures efficient sealing against backflows and bi-directional pressure differences.
The lug style has threaded inserts that are located outside of the valve body. This allows you to disconnect one side for dead-end services. These valves are usually designed for low pressure applications and carry the weight of the pipe through them.
As mentioned earlier, butterfly valves are operated using handles, gears or automatic actuators. Manually actuated valves can be very simple and cost-effective. The smaller butterfly valves have a hand lever while the larger ones can be actuated by a gear system. These valves can not be back-driven, but can be fitted with position indicators to ensure efficient operation.
Power-operated actuators are used to control butterfly valves in demanding and sensitive applications. These actuators are classified as
- Electric actuators – use an electric motor to close and open the valve stem.
- Hydraulic actuators – use hydraulic pressure to move a diaphragm or piston to close and open the valve.
- Pneumatic actuators – use compressed air to move a diaphragm or piston to close and open the valve.
Butterfly vales are one of the most widely-used valves on the market. This is due to their versatility, cost-efficiency and ease of use. Butterfly valves are almost always an option to replace ball valves, except in high-pressure situations and where flow must be maintained at a certain level.
There are many industries that butterfly valves can be used in, including pharma, food and water supply operations, fuel handling operations, and chemical processing. They are ideal for handling low-pressure liquids containing large quantities of solids due to their larger size.
When choosing a butterfly valve for your application, be sure to consider your needs. To avoid making mistakes in your selections, it is a good idea to seek professional assistance when you navigate the market.