Friday, October 4, 2013

What is a Butterfly Valve?

high performance butterfly valve
RENCOR actuated HPBV
A butterfly valve is used for stopping or controlling flow of liquids or materials through pipes. The "butterfly" refers to the round, flat disk that allows for flow through the valve. Butterfly valves are a member of the "quarter-turn" valve family, meaning fully open to fully closed in 90 degrees rotation. They are opened and closed via a lever, manual gear operator, pneumatic actuator, or electric actuator. Butterfly valves can be used for on-off service and some varieties are used as control valves. Butterfly valves are generally less expensive than other high flow valves, lighter in weight, and take up less piping length. Since the disk is always in the flow path, butterfly valves always have a pressure drop across the valve.

There are two primary types of butterfly classifications: 1) so called "rubber lined" butterfly valves (resilient seated) which are best suited for lower pressure, lower temperature, general purpose applications, and 2) High Performance Butterfly Valves (HPBV) which are designed to ANSI pressure classifications and are suited for more robust industrial applications.

rubber seated butterfly valve
Wafer body, rubber lined
Rubber lined (resilient seated) butterfly valves come in a variety of seating materials, where the disk "jams" in to the seat. This provides a tight closure, but also causes wear on the seat. Seat replacement is part of normal maintenance on these valves. Another characteristic of rubber lined butterfly valves are higher starting and ending torques due to the "jamming" and "unjamming" of the disk in the seat.

Dezurik HPBV
Dezurik High Performance
High performance butterfly valves have precision machined teflon, or metal seats, and are slightly offset as to lessen the amount of seat-to-disk interference. The disc still is pushed against he seat, but in a much more controlled and measured manner. Seat wear is still an issue, but not like rubber lined valves. Seating and unseating torque effects are much less as well.

Velan Torqseal
Velan triple eccentric HPBV
A variation of the HPBV is the triple eccentric butterfly valve which uses a metal seat and a "triple-offset" design, best described as the disk coming "off and away" from the seat. This assures that the disk only contacts the seat at time of full closure, reducing wear on the metal seat. These valves provide excellent shut off in critical applications.

Butterfly valves come in three body styles; 1) wafer body, whereby the valve is "sandwiched" and held in place between two pipe flanges and are suitable for lower pressure applications; 2) lug style bodies that have threaded "lugs" cast in to the body of the valve and bolts are used to secure the valve for end of line service or keeping the valve in place when the piping is disassembled; and 3) ASME flanged butterfly valves where the valve body is in-between two ASME flanges.