Butterfly valve basics

Inside Process: Like other types of valves butterfly valves interrupt product flow within processes. Typically used in pharmaceutical, chemical, and food industries, they have generally replaced ball valves due to lower cost and ease of installation.

11/01/2016


Figure 1: Butterfly valves control the flow of materials through a pipe within processes using a disc mounted on a central shaft. Courtesy: Cross Company Process Instrumentation & ControlsA butterfly valve controls flow through a pipe using a single disc mounted on a central shaft. The disc creates a variable orifice (see Figure 1).

Butterfly valves are grouped into three basic types:

  • Damper valve
  • Resilient seated
  • High-performance. 

Damper valves

A butterfly damper valve essentially is a butterfly valve without a seat (see Figure 2). The disc in the valve does not touch the inside wall of the valve. It does not shut off flow; it only slows down the flow of media in a pipe or duct because there is a gap between the outer diameter of the disc and the inner wall of the valve body. Damper valves are used primarily for controlling air or gaseous flow and are intended for low-pressure applications. Common applications include heating and air ducts, exhaust vents, and chimney flues.

Figure 2: A damper valve does not have a valve seat, and the disc does not touch the inside wall of the valve. Courtesy: Cross Company Process Instrumentation & Controls

Components of a butterfly damper valve:

  • Body
  • Disc
  • Stem
  • Operator.

Resilient seated valves

A resilient seated butterfly valve includes a rubber seat between the outer diameter of the disc and the inner wall of the valve. This seat is mechanically attached to the valve body, and the disc has an interference fit with the seat in the closed position. These valves do have a positive shutoff and are capable of handling higher pressures. Common ways that the seat is attached to the body include dovetail joint seats, cartridge seats, and bonded seats.

Figure 3: A resilient seated butterfly valve has a rubber seat between the outer diameter of the disc and the inner wall of the valve. The photo shows components of a typical resilient-seat butterfly valve. Courtesy: Cross Company Process Instrumentation & Controls

Figure 3 shows components of a typical resilient-seat butterfly valve. They include:

  • Body
  • Neck
  • Disc
  • Seat
  • Stem
  • Dust seal (packing)
  • Bushings or bearings 
  • Operator.

Dovetail seats

The dovetail connection uses a male dovetail that is cast or machined into the inner wall of the valve body. The seat has a female dovetail molded into its inner diameter that fits over the male dovetail on the body. When compressed between two pipe flanges, the seat is locked into place. This seat design is never to be used in vacuum applications because the seat is not bonded to the body and can be sucked away from it. This can allow the disc to pinch or cut the seat when closing the valve. 

Cartridge seats

The cartridge design seats have a rigid metal or phenolic resin ring molded inside the rubber seat. The inside wall of the valve and the outer diameter of the seat are both smooth. The inside wall diameter is slightly smaller than the outside diameter of the seat. The seat is pressed into the body and is held in place by this interference fit. Cartridge seats tend to handle much higher pressures than dovetail seats and will also handle vacuum applications. 

Figure 4: A double-offset design relies on the offset shaft and a bevel machined into the edge of the disc. The seat is the sealing component when in contact with the bevel on the disc. Courtesy: Cross Company Process Instrumentation & ControlsBonded seats

The third type of seat is a bonded seat. These seats are glued or injection-molded to the body of the valve and are not repairable if the seat becomes worn or damaged. These valves tend to handle the most pressure and vacuum of any rubber-seated valves. Many times, these are also the least expensive resilient-seated butterfly valves.

This seat is slightly wider than the valve body on all of the seat designs so that when the valve is installed between two pipe flanges, no gaskets are needed. The stem, or shaft, of the valve passes through the seat and is sealed by the seat. Inside the neck of the valve, there is a bushing that keeps the shaft aligned and prevents metal-to-metal contact on the stem. Because of this seat and stem design, there is no need for a packing that holds pressure from inside the valve. The packing in these valves simply prevents dirt and dust from entering from outside the valve. 

High-performance seated valve

A high performance butterfly valve uses a rigid PTFE, or metal seat, that is narrower than the valve body. This seat is mechanically retained inside the valve body. High-performance butterfly valves are usually a double- or triple-offset disc design.

The valve shaft does not pass through the seat on this design, and the shaft does not pass through the centerline of the disc. This allows the disc to cam into the seat, which reduces the torque required to open or close the valve. A double-offset design relies on the offset shaft and a bevel machined into the edge of the disc (see Figure 4). The seat is the sealing component when in contact with the bevel on the disc. A triple-offset valve adds an elliptical seal ring to the disc and uses the offset shaft along with a beveled seat (see Figure 5). The seal ring on the disc is the sealing component on a triple offset when it is in contact with the beveled seat. High-performance butterfly valves require pipe flange gaskets when installed between pipe flanges.

High-performance butterfly valves handle the highest temperatures and pressures of any butterfly designs.

Figure 5: A triple-offset valve adds an elliptical seal ring to the disc and uses the offset shaft along with a beveled seat as shown in the cut-away photo. The seal ring on the disc is the sealing component on a triple offset butterfly valve when it is in contact with the beveled seat. Courtesy: Cross Company Process Instrumentation & Controls

Components of a high performance butterfly valve include:

  • Body
  • Disc
  • Seat
  • Seal ring (triple-offset design only)
  • Seat retainer
  • Stem
  • Stem packing
  • Bushings or bearings
  • Operator.

Ronnie Moore is Cross Company's resident valve expert working in inside sales and support. A Knoxville native, Ronnie spent time in the Air Force as crew chief during the first Gulf War. He studied Aerospace Engineering at the University of Tennessee-Knoxville, as well as Mechanical Engineering at Pellissippi State. When not problem-solving valve applications, Ronnie is an active participant in his two sons' Boy Scouts troop. Cross Company is a CFE Media content partner and a CSIA member as of Oct. 3, 2015. Edited by Jack Smith, content manager, CFE Media, Control Engineering, jsmith@cfemedia.com.

MORE ADVICE

Key concepts

  • A butterfly valve controls flow through a pipe using a single disc mounted on a central shaft.
  • A resilient seated butterfly valve includes a rubber seat between the outer diameter of the disc and the inner wall of the valve.
  • High-performance butterfly valves handle the highest temperatures and pressures of any butterfly designs.

Consider this

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