Reference Manual

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actuator and operate it much like a spring-and-
diaphragm. These designs use a single-acting
positioner that loads the piston chamber to move
the actuator and compress the spring. As air is
unloaded, the spring forces the piston back. These
designs use large, high output springs that are
capable of overcoming the fluid forces in the valve.
The alternative design uses a much smaller spring
and relies on valve fluid forces to help provide the
fail-safe action. In normal operation they act like a
double action piston. In a fail-safe situation the
spring initiates movement and is helped by
unbalance forces on the valve plug. These
actuators can be sized and set up to provide full
spring closure action without process assistance.
An alternative to springs is a pneumatic trip
system which often proves to be complex in
design, difficult to maintain and costly. While a trip
system is completely safe, any fail-safe
requirement consideration should be given first to
spring-and-diaphragm operators if they are
feasible.
Special care should be given during the selection
of throttling piston actuators to specify a design
that has minimal hysteresis and deadband. As the
number of linkage points in the actuator increases,
so does the deadband. As the number of sliding
parts increases, so does the hysteresis. An
actuator with high hysteresis and deadband can
be quite suitable for on-off service; however,
caution is necessary when attempting to adapt this
actuator to throttling service by merely bolting on a
positioner.
The cost of a spring-and-diaphragm actuator is
generally less than a comparable piston actuator.
Part of this cost saving is a result of the ability to
use instrument output air directly, thereby
eliminating the need for a positioner. The inherent
provision for fail-safe action in the spring-and-
diaphragm actuator is also a consideration.
Rack and Pinion Actuators
Rack and pinion actuators may come in a
double-acting design, or spring return, and are a
compact and economical solution for rotary shaft
valves. They provide high torque outputs and are
typically used for on-off applications with high
cycle life. They may also be used in processes
where higher variability is not a concern.
Figure 2-7. The FieldQt actuator is a quarter
turn pneumatic rack and pinion actuator. It comes
with an integrated module combining the solenoid
and switchbox into a low profile, compact
package.
W9479
Electric Actuators
Electric actuators can be applied successfully in
many situations. Most electric operators consist of
motors and gear trains and are available in a wide
range of torque outputs, travels, and capabilities.
They are suited for remote mounting where no
other power source is available, for use where
there are specialized thrust or stiffness
requirements, or when highly precise control is
required.
Electric operators are economical versus
pneumatic actuators for applications in small size
ranges only. Larger units operate slowly and weigh
considerably more than pneumatic equivalents.
Available fail action is typically lock in last position.
One key consideration in choosing an electric
actuator is its capability for continuous closed-loop
control. In applications where frequent changes
are made in control-valve position, the electric
actuator must have a suitable duty cycle.
High performance electric actuators using
continuous rated DC motors and ball screw output
devices are capable of precise control and 100%
duty cycles.
Compared to other actuator designs, the electric
actuator generally provides the highest output