Honeywell AUTOMATIC CONTROL Engineering Manual page 445

STEM
SEATS
various temperature ratings.
Fig. 3. Ball Valve.
Ball valves provide tight shut-off, while full port
models have low flow resistance, and reduced port
models can be selected for modulating applications.
Butterfly valve: A valve with a cylindrical body, a shaft, and a
rotating disc (Fig. 4). The disc rotates 90 degrees from
open to closed. The disc seats against a resilient body
liner and may be manufactured for tight shut-off or
made smaller for reduced operating torque but without
tight close-off. Butterfly valves are inherently for two-
way operation. For three-way applications, two
butterfly valves are assembled to a pipe tee with
linkage for simultaneous operation.
Fig. 4. Butterfly Valve.
Double-seated valve: A valve with two seats, plugs, and discs.
Double-seated valves are suitable for applications
where fluid pressure is too high to permit a single-
seated valve to close. The discs in a double-seated
valve are arranged so that in the closed position there
is minimal fluid pressure forcing the stem toward the
open or closed position; the pressure on the discs is
essentially balanced. For a valve of given size and port
area, the double-seated valve requires less force to
operate than the single-seated valve so the double-
seated valve can use a smaller actuator than a single-
seated valve. Also, double-seated valves often have a
BODY
BALL
PORT
Flanged-end connections: A valve that connects to a pipe by
M12228
Globe valve: A valve which controls flow by moving a circular
Pilot-operated valve: A valve which uses the differential
STEM
Reduced-Port valve: A valve with a capacity less than the
BODY
RESILIENT
Screwed-end connection: A valve with threaded pipe
SEAL
DISC
Single-seated valve: A valve with one seat, plug, and disc.
M12247
Three-way valve: A valve with three ports. The internal design
Two-way valve: A valve with one inlet port and one outlet
435
VALVE SELECTION AND SIZING
larger port area for a given pipe size. A limitation of
double-seated valves is that they do not provide tight
shut-off. Since both discs rigidly connect together and
changes in fluid temperature can cause either the disc
or the valve body to expand or contract, one disc may
seat before the other and prevent the other disc from
seating tightly.
bolting a flange on the valve to a flange screwed onto
the pipe. Flanged connections are typically used on
large valves only.
disk against or away from a seat. When used in
throttling control a contoured plug (throttling plug)
extends from the center of circular disk through the
center of the seat for precise control (Fig. 1).
between upstream and downstream pressure acting on
a diaphragm or piston to move the valve plug. Pilot-
operated valves are suitable for two-position control
only. The valve actuator exerts only the force necessary
to open or close the small pilot port valve that admits
fluid flow into the diaphragm or piston chamber.
maximum for the valve body. Ball, butterfly, and
smaller globe valves are available with reduced ports
to allow correct sizing for good control.
connections. Valve threads are usually female, but male
connections are available for special applications.
Some valves have an integral union for easier
installation.
Single-seated valves are suitable for applications
requiring tight shut-off. Since a single-seated valve
has nothing to balance the force of the fluid pressure
exerted on the plug, it requires more closing force than
a double-seated valve of the same size and therefore
requires more actuator force than a double-seated
valve.
of a three-way valve classifies it as a mixing or
diverting valve. Three-way valves control liquid in
modulating or two-position applications and do not
provide tight shut-off.
port. Two-way valves control water or steam in two-
position or modulating applications and provide tight
shut-off in both straight through and angle patterns.
ENGINEERING MANUAL OF AUTOMATIC CONTROL