Several industrial applications involving liquids and gases rely on valves to control fluid flow and ensure efficient system operation. Pneumatic butterfly valves use compressed air to control fluid flow and they find applications in different sectors. We discuss how they work, the different types, and how they are used in different industries.

Working Principle of a Pneumatic Butterfly Valve

To understand how this type of valve works, we’ll need to look at the basic operation of a butterfly valve. Ideally, a butterfly valve consists of a disc positioned at the center of a stem and a pipe. The stem is connected to a handle (for manually operated valves) or an actuator (for automatically actuated valves). In our case, the stem will be connected to a pneumatic actuator that uses compressed air to move a diaphragm or a piston to either close or open the valve.

The disc sits perpendicular to the fluid flow and is held securely by a valve seat when the valve is closed. An O-ring is also used to seal the stem, preventing any leakages and ensuring a tight yet flexible fit. When the pneumatic actuator rotates the stem at a 90-degree angle, the disc moves such that it’s positioned parallel to the fluid flow. If the stem is rotated partially, the flow becomes proportional or throttled, and the valve could be used for modulating services. 

Pneumatic butterfly valves used for modulating control are designed to have either a linear or an equal percentage characteristic. Here is a classification of the two.

• Linear – with this design, the amount or angle at which the disk travels is directly proportional to the flow rate. For instance, if the disc is rotated ¼ of a turn (22.5 degrees), the flow rate will be 25% of the maximum.
  
• Equal – an equal percentage characteristic means that an equal increment of valve travel results in a similar percentage change in the flow rate. For instance, if a valve traveling from 40 to 50 degrees opening increases the flow rate from 100 to 180 m3/h (80%), then a travel from 50 to 60 degrees will result in a flow rate from 180 to 324 m3/h (80%). In other words, the relationship between the flow rate and disc travel is logarithmic. Most butterfly valves with this equal characteristic design operates within a 20 to 90-degree angle range. 

Types of Pneumatic Butterfly Valves and Applications

Butterfly valves are generally categorized based on three factors: the disc closure design, actuation method, and connection design. Under the disc closure design, we have concentric and eccentric valve types, and under the connection design, there are three types: wafer-style, lug-style, and flange connection. All these valves require actuation, which could be manual or automatic. Pneumatic butterfly valves, like their electrical and hydraulic counterparts, are automatically actuated. 

Ideally, pneumatic butterfly valves can be concentric or eccentric, plus the connection design could be wafer-style, lug-style, or flanged. Here’s a quick overview.

Concentric Type

With this design, the stem passes through the disc’s centerline, and the seat is located inside the valve body. This means that the disc and the seat come in contact at around 85-90 degree rotation. Concentric butterfly valves are often used for low-pressure ranges.

Eccentric Types

Eccentric butterfly valves have their stems behind the disc’s centerline, unlike the concentric types. There are three designs of eccentric valves: single-offset, double-offset, and triple-offset. By taking the stem behind the disc’s centerline, there’s reduced disc contact with the seal, i.e., before the valve fully closes. Eccentric butterfly valves have a higher-pressure rating and are less susceptible to wear. A triple offset is highly efficient and is used in critical fluid control applications. 

Connection Design 

The most common connection designs are wafer-style and lug-style. Wafer-style design is commonly used to prevent backflow and bi-directional differential pressures. On the other hand, Lug style features threaded inserts outside the valve body. This allows for easy disconnection of one side for easy servicing and maintenance. Lug-style valves for dead-end service have reduced pressure rating. They also carry the weight of the pipe through the valve body. 

Applications 

Pneumatic butterfly valves find applications in various industries, such as chemical, pharmaceutical, fuel handling, gas supply, etc. They are available in large sizes and are ideal for most liquid types, including slurries. That said, pneumatic actuators are more reliable and are often used to control larger valves than electric actuators at a relatively lower cost. This makes pneumatic butterfly valves quite common in the market.

Summary  

Pneumatic butterfly valves are automatically actuated, allowing you to control fluid flow remotely. They use compressed air to control the rotation of the disc that opens and closes the valve. These valves can have a wafer or lug connection style, and you can choose one based on your needs and system requirements. 

Additionally, there are various seals, housings, and sizes of pneumatic butterfly valves. The right choice for you will depend on the fluid you want to control, material compatibility, and the operating temperate and pressures. Other critical aspects to consider are the valve’s sealing properties, reliability, and durability. 

Author Name: Charles Kolstad
Title: Marketing Manager
Company Link: https://tameson.com
Bio: Charles Kolstad has been working at Tameson since 2017 and is from the United States of America. He has his Mechanical Engineering degree from the University of St. Thomas in Minnesota, USA. He works remotely while traveling throughout Europe, Asia, and the Americas. However, he does visit Tameson’s HQ from time to time to meet the new members of the team and to work in the office.