What is a Butterfly Valve?

Butterfly valve parts

Below are the major butterfly valve components:

• Valve Body
  The valve body fits between the pipe flanges – the most common end connection types being flanged, double lug, and wafer types.
   
• Disc
  Attached to the valve body is the disc that functions as a gate that stops or throttles fluid flow; it can be considered equivalent to a gate in a gate valve, or a ball in a ball valve. The disc is typically bored to receive the stem, or shaft. There are many variations in disc design, orientation, and material in order to improve flow, sealing, and/or operating torque.
   
• Seat
  Lining the internal valve body is a strong elastomer or metal anti-leak seal that secures the disc in place in the closed position in order to achieve complete shutoff. The 9881k series’ stainless steel weld filled and microfinished integral body seat ensures a corrosion and erosion resistant seat face. With this special type of seat design, it is possible to manufacture the valve drop tight according to EN12266 –A requirements.
   
• Stem
  The valve shaft, often also referred to as the stem, is the component that connects the disc to the actuation mechanism and transmits the torque through itself.
   
• Seals
  Seals are present at multiple interfaces within the valve to either ensure a tight seal during operation or to isolate the process media from the valve’s internal components for a more flexible and cost-efficient design.

Sealing on seat face is ensured by a continuous T-profile resilient sealing ring which is held on the periphery of the disc by a retaining ring, preventing the sealing ring from rolling out. In the closed position, the sealing ring is pressed against the seat face, providing a tight seal on both the upstream and downstream ends. In the open position, the sealing ring is completely unstressed due to the double eccentric disc design.

By eliminating interaction between the process media and the stem/shaft, the multiple O-ring shaft sealing system ensures a maintenance-free sealing for the entire life span of the valve. Besides, there are smaller more common components such as the bushings, bearings, and fasteners that are part of most other valve constructions as well. Details on some of the components on the actuation end, such as the handle, lever, gearbox, and handwheel will be covered in the actuation section.

Butterfly valve: working principle

The construction of a butterfly valve is relatively simple, with the rotation of the valve disc controlling the fluid flow. In the closed position, the disc blocks the valve bore while in the open position, the disc is oriented perpendicular to the flow direction to allow flow. Butterfly valves generally provide bi-directional flow and shutoff capability. However, they are not full-bore, which renders them unsuitable for pigging or swabbing. The body material is ductile iron with an epoxy powder coat on both internal and external surfaces. The valves are typically operated by handwheels, gears, or actuators, or a combination thereof, according to the specific application requirement and technical specifications. We will be discussing the actuation methods later in the article.

Butterfly valve actuation

There are multiple ways to operate, or actuate, a butterfly valve – manual, semi-manual, or fully automatic.

• Electric: Uses an electric actuator for controlling the valve with a 4-20mA control signal and are typically used for high precision regulation and time-sensitive application.
   
• Pneumatic: Also used for high precision application and uses compressed air for actuation.
   
• Hydraulic: Typically used for applications requiring large torques for opening and closing the valve, such as high pressure and high viscosity applications.
   
• Manual: Manually operated butter valves make use of a handwheel, crank, or lever to operate the valve. Smaller manual valves are typically fitted with a lever. For larger sizes, handwheels are provided and are accompanied by a gearbox to transfer the torque with a high mechanical advantage. Another way of manual actuation is in use cases where the valve is buried underground without a manhole; in such applications, actuation is done with an extension spindle, adapter / square cap, and a corresponding T-key from the ground level.

Due to inherently lower torque requirements, sizing and installing actuators are less expensive for butterfly valves compared to others, like gate valves.

The 9881k series comes standard with a handwheel actuation and wormwheel gearbox with a self-locking design feature that minimizes gear backlash and guarantees the correct operation and tightness of the end device. It is also compatible with external actuators with an ISO 5210 top flange for mounting of multi-turn actuators.

Advantages of butterfly valves

Depending on the application, butterfly valves can offer significant advantages over other types of valves, especially for dimensions over DN 200 (200 mm) in size:

1. Lightweight and Compact: With a compact design and a smaller face to face dimension, butterfly valves have a considerably less installation footprint and offer savings in the form of lower installation costs including labour cost, equipment, and piping support.
    
2. Low Maintenance Requirements: An inherently simple, economic design that consists of few moving parts, and hence fewer wear points, significantly reduces their maintenance requirements.
    
3. Fast Acting: A 90° rotation of the handle, or the actuation mechanism, provides a complete closure or opening of the valve. However, with larger butterfly valves, a gearbox is often required as part of the actuation mechanism which reduces the operational torque and simplifies the operation of the valve but comes at the expense of speed.
    
4. Low Cost: Owing to their simple design, butterfly valves require less material and are simpler to design and manufacture and are often the more economical choice compared to other valve types. The cost savings are mainly realized in larger valve sizes, typically above DN 300.
    
5. Versatility: Butterfly valves have a wide variety of use cases including the ability to be installed underground.

Double-eccentric MNC butterfly valves

On the application side, butterfly valves are especially well-suited for the handling of large fluid flows of fluid, such as in water distribution systems (pumping stations or water collection and transmission) and water treatment plants (seawater or raw water). This, along with their availability in a large range of sizes including up to DN 1400 (55 inches), makes them ideal for drinking water distribution applications. They are typically used for flow isolation devices and are normally fully open or fully closed. MNC provides butterfly valves geared towards specific application areas and are available on demand. The 9881 Özkan double eccentric double flanged butterfly valves, for example, are offered for high pressure applications upto 40 bar (PN 40) and up to DN 2500.

The MNC 9881k series double eccentric butterfly valves come in sizes ranging from DN 150 up to DN 1600 with pressure ratings of both PN 10 and PN 16. Additionally, design feature optimizations such as the double offset, or double eccentric (e1 and e2 as seen above) disc further reinforces its advantages of achieving a tight seat seal through full circle sealing, and improves the service life of internal components while lowering operating torque requirements.