Valves
DOE-HDBK-1018/2-93
VALVE ACTUATORS
Pneumatic Actuators
Figure 34 Pneumatic Actuator
Pneumatic
actuators
as
illustrated in Figure 34 provide
for
automatic
or
semi-
automatic
valve
operation.
These actuators translate an air
signal into valve stem motion
by air pressure acting on a
diaphragm or piston connected
to the stem.
Pneumatic
actuators are used in throttle
valves
for
open-close
positioning where fast action is
required. When air pressure
closes the valve and spring
action opens the valve, the
actuator is termed direct-
acting.
When air pressure
opens the valve and spring
action closes the valve, the
actuator is termed reverse-
acting. Duplex actuators have
air supplied to both sides of
the
diaphragm.
The
differential pressure across the
diaphragm positions the valve
stem. Automatic operation is
provided when the air signals
are automatically controlled by
circuitry.
Semi-automatic
operation
is
provided
by
manual
switches
in
the
circuitry to the air control
valves.
Hydraulic Actuators
Hydraulic actuators provide for semi-automatic or automatic positioning of the valve, similar to
the pneumatic actuators. These actuators use a piston to convert a signal pressure into valve
stem motion. Hydraulic fluid is fed to either side of the piston while the other side is drained
or bled. Water or oil is used as the hydraulic fluid. Solenoid valves are typically used for
automatic control of the hydraulic fluid to direct either opening or closing of the valve. Manual
valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic
operation.
Rev. 0
ME-04
Page 47
