TYPES OF VALVES
Butterfly valves are especially well-suited for the handling of large flows of liquids or gases at
relatively low pressures and for the handling of slurries or liquids with large amounts of
Butterfly valves are built on the principle of a pipe damper. The flow control element 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. When the disk lies parallel to the piping run, the valve
is fully opened. When the disk approaches the perpendicular position, the valve is shut.
Intermediate positions, for throttling purposes, can be secured in place by handle-locking
Butterfly Valve Seat Construction
Stoppage of flow is accomplished by the valve disk sealing against 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.
In early designs, a metal disk was used to seal against a metal seat. This arrangement did not
provide a leak-tight closure, but did provide sufficient closure in some applications (i.e., water
Butterfly Valve Body Construction
Butterfly valve body construction varies. The most economical is the wafer type that fits
between two pipeline flanges. Another type, the lug wafer design, 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.
Butterfly Valve Disk and Stem Assemblies
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. Uniform sealing is accomplished and external stem fasteners are eliminated. This
method of assembly is advantageous in the case of covered disks and in corrosive applications.
In order for the disk to be held in the proper position, the stem must extend beyond the bottom
of the disk and fit into a bushing in the bottom of the valve body. One or two similar bushings
are along the upper portion of the stem as well. These bushings must be either resistant to the
media being handled or sealed so that the corrosive media cannot come into contact with them.