Monday, September 24, 2012

Butterfly Valve Basics

What is a Butterfly Valve

A butterfly valve, is a quarter turn ,rotary motion valve that is used to stop, regulate, and start fluid (air,gas,water,steam) flow in pipes. Butterfly valves are easily and quickly operated because of 90o rotation of disk from a fully closed to fully opened position.That,s why butterfly valves are also called quick turn valves.

Butterfly Valve How It works

Butterfly valves are built on the principle of a pipe damper.These are also called butterfly damper valves.The flow control element in butterfly valve is a disk of approximately the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis. 

butterfly valve how it works
When the disk lies parallel to the piping run, the valve is fully opened. When the disk of this rotary valve approaches the perpendicular position, the valve is closed.In butterfly valves intermediate positions can also be attained for throttling purposes.Intermediate position in butterfly valves are usually achieved by handle-locking devices.
Larger butterfly valves are actuated by hand wheels connected to the stem through gears that provide mechanical advantage at the expense of speed.Butterfly valves may be motor operated or motorized ,air operated or pneumatically actuated also.

Advantages of Butterfly Valves

Butterfly valves have many advantages over gate, globe, plug, and ball valves, especially for large industrial applications of valve.
  • Butterfly valves present savings in weight, space, and cost.
  • The maintenance costs of butterfly valves are usually low because there are a minimal number of moving parts .
  • Also in butterfly valves there are no narrow spaces or pockets where fluid can be trapped.
  • Butterfly valves are well-suited for the handling of large fluid flows of liquids or gases that are at relatively low pressures .
  • Butterfly valves are also good for the handling of slurries or liquids which have large amounts of suspended solids.
Disadvantages of Butterfly Valves

  • Butterfly valves  have limited throttling range .
  • Butterfly valves  have although ability to handle slurries but it is limited.
  • Butterfly valves  require special seal for tight shuttoff.
  • Butterfly valves require high torque required for control.

Types of Butterfly Valves
  • The most common and less expensive or standard type is the wafer type butterfly valve  that fits between two pipeline flanges also called flanged butterfly valves.This type is also called general butterfly valve.
  • Another type, the lug wafer design, or lug style butterfly valve,is held in place between two pipe  flanges by bolts that join the two flanges and pass through holes in the valve's outer casing.
Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded end construction.

Mechanical assembly of Disk ,Seat and Stem in Butterfly Valve

Butterfly valves stop the flow of fluid by the valve disk that rests on a seat that is on the inside diameter periphery of the valve body. Many butterfly valves have an elastomeric seat against which the disk seals. Other butterfly valves have a seal ring arrangement that uses a clamp-ring and backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings.Metal disk on metal seat (in metal seated butterfly valves) has failed to provide tight seal in butterfly valves.

The stem and disk for a butterfly valve are separate pieces. The disk is bored to receive the stem. Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned. In the first method, the disk is bored through and secured to the stem with bolts or pins. The alternate method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-shaped stem. This method allows the disk to "float" and seek its center in the seat.

Sealing (Packing or O-ring) in butterfly valves

O-ring seals or conventional stuffing box packing are used in butterfly valves to prevent the leakage or accomplish the valve sealing.If a stuffing box or external O-ring is employed, the fluid passing through the valve will come into contact with the valve stem.
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