Butterfly valve basics
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.
A butterfly valve control flows through a pipe using a single disc mount
ed on a central shaft. The disc creates a variable orifice (see Figure 1). Butterfly valves are grouped into three basic types:
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.
s of a butterfly damper valve:
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 shows components of a typical resilient seat butterfly valve. They include:
Dust seal (packing)
Bushings or bearings
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 dove tail 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.
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.
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 polytetrafluoroethylene (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.
Components of a high performance butterfly valve include:
Seal ring (triple-offset design only)
Bushings or bearings