Reference Manual

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Chapter 2
Actuator Selection
The actuator is the distinguishing element that
differentiates control valves from other types of
valves. The first actuated valves were designed in
the late 19th century. Today, they would be better
described as regulators since they operated
directly from the process fluid. These “automatic
valves” were the mainstay of industry through the
early 1930s.
It was at this time that the first pneumatic
controllers were used. Development of valve
controllers and the adaptation of standardized
control signals stimulated design of the first, true,
control valve actuators.
The control valve industry has evolved to fill a
variety of needs and desires. Actuators are
available with an array of designs, power sources
and capabilities. Proper selection involves process
knowledge, valve knowledge, and actuator
knowledge.
A control valve can perform its function only as
well as the actuator can handle the static and
dynamic loads placed on it by the valve.
Therefore, proper selection and sizing are very
important. Since the actuator can represent a
significant portion of the total control valve price,
careful selection of actuator and accessory options
can lead to significant dollar savings.
The range of actuator types and sizes on the
market today is so great that it seems the selection
process might be highly complex. With a few rules
in mind and knowledge of fundamental needs, the
selection process can be simple.
The following parameters are key as they quickly
narrow the actuator choices:
D Power source availability
D Fail-safe requirements
D Torque or thrust requirements
D Control functions
Power Source Availability
The power source available at the location of a
valve can often point directly to what type of
actuator to choose. Typically, valve actuators are
powered either by compressed air or by electricity.
However, in some cases water pressure, hydraulic
fluid, or even pipeline pressure can be used.
Since most plants have both electricity and
compressed air readily available, the selection
depends upon the ease and cost of furnishing
either power source to the actuator location.
Reliability and maintenance requirements of the
power system must also be considered.
Consideration should also be given to providing
backup operating power to critical plant loops.
Fail-safe Requirements
The overall reliability of power sources is quite
high. However, many loops demand specific valve
action should the power source ever fail. Desired
action upon a signal failure may be required for
safety reasons or for protection of equipment.
Fail-safe systems store energy, either
mechanically in springs, pneumatically in volume
tanks, or in hydraulic accumulators. When power
fails, the fail-safe systems are triggered to drive
the valves to the required position and to then
maintain this position until returned to normal
operation. In many cases, the process pressure is
used to ensure or enhance this action.