Control Valve Handbook | Valve and Actuator Types
3.8.1 Diaphragm Actuators
Pneumatically-operated diaphragm
actuators use air supply from
controllers, positioners, or other
sources.
Various styles include: direct-acting,
in which the increasing air pressure
pushes the diaphragm down and
extends the actuator stem (Figure
3.37); reverse-acting, in which the
increasing air pressure pushes the
diaphragm up and retracts the
actuator stem (Figure 3.37);
3.8 Actuators reversible, in which actuators can be
Pneumatically-operated control valve assembled for either direct or reverse
actuators are the most popular type in action (Figure 3.38); direct-acting
use, but electric, hydraulic, and manual unit for rotary valves, in which the
actuators are also widely used. The increasing air pressure pushes down
spring-and-diaphragm pneumatic on the diaphragm, which, depending
actuator is most commonly specified on orientation of the actuator lever
due to its dependability and simplicity on the valve shaft, may either open
of design. Pneumatically-operated or close the valve (see Figure 3.39).
piston actuators provide high stem Net output thrust is the difference
force output for demanding service between diaphragm force and
conditions. Adaptations of both spring- opposing spring force.
and-diaphragm and pneumatic piston
Molded diaphragms provide linear
actuators are available for direct
installation on rotary control valves. performance and increased travels.
Output thrust required and supply
Direct-Acting Reverse-Acting
Figure 3.37 Diaphragm Actuators
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Control Valve Handbook | Chapter 3: Valve and Actuator Types
air pressure available dictate size. Other versions for service on rotary
Diaphragm actuators are simple,
control valves include a sliding seal in
the lower end of the cylinder. This
dependable, and economical.
permits the actuator stem to move
laterally, as well as up and down
without leakage of cylinder pressure.
This feature permits direct
connection of the actuator stem to
the actuator lever mounted on the
rotary valve shaft, thus eliminating
one joint or source of lost motion.
Figure 3.38 Field-Reversible Multi-Spring Actuator
Figure 3.40 Control Valve with Double-Acting
Piston Actuator
Figure 3.39 Diaphragm Actuator for Rotary Valve
3.8.2 Piston Actuators
Figure 3.41 Control Valve with Scotch-Yoke
Piston actuators are pneumatically-
Piston Actuator
operated using high-pressure plant
air up to 150 psig (10.3 bar), often
eliminating the need for a supply 3.8.3 Manual Actuators
pressure regulator. Manual actuators are useful where
Piston actuators furnish maximum automatic control is not required,
but where ease of operation and
thrust output and fast stroking speeds.
good manual control is still necessary
Piston actuators are double-acting to (Figures 3.42 and 3.43). They are
give maximum force in both often used to actuate the bypass
directions, or spring-return to valve in a three-valve bypass loop
provide fail-open or fail-closed around control valves for manual
operation (Figure 3.40). control of the process during
Various accessories can be
maintenance or shut down of the
automatic system.
incorporated to position a double-
Manual actuators are available in
acting piston in the event of supply
pressure failure, including pneumatic various sizes for both globe-style and
rotary valves.
trip valves and lock-up systems.
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Control Valve Handbook | Valve and Actuator Types
Dial-indicating devices are available 3.8.5 Electric Actuators
for some models to permit accurate
Electric actuator designs use an electric
repositioning of the valve plug or disk.
motor and some form of gear reduction
Manual actuators are much less to move the valve plug (Figures 3.45 and
expensive than automatic actuators. 3.46). While electric actuators have
traditionally been limited to on/off
operation, some are now capable of
continuous control. The use of brushless
motors in electric actuators can reduce
or eliminate motor burnout associated
with turning the motor on and off rapidly.
The initial purchase price still tends to
remain above that of pneumatic
actuation. The primary usage of electric
actuation is in areas where instrument air
is not readily available or where an
insufficient quantity of valves exist to
justify the cost of a compressor system. n
Figure 3.42 Manual Actuator for Sliding-Stem Valves
Figure 3.43 Manual Actuator for Rotary Valves
3.8.4 Rack-and-Pinion Actuators Figure 3.45 Electric Actuator for Sliding-Stem Valve
Rack-and-pinion designs provide a
compact and economical solution for
rotary valves (Figure 3.44). Because of
backlash, they are typically used for on/
off applications or where process
variability is not a concern.
Figure 3.46 Electric Actuator for Rotary Valve
Figure 3.44 Rack-and-Pinion Actuator
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