Reference Manual

2−2
Actuator designs are available with a choice of
failure mode between failing open, failing closed,
or holding in the last position. Many actuator
systems incorporate failure modes at no extra
cost. For example, spring-and-diaphragm
actuators are inherently fail open or closed, while
electric operators typically hold their last position.
Torque or Thrust Requirements
An actuator must have sufficient thrust or torque
for the prescribed application. In some cases this
requirement can dictate actuator type as well as
power supply requirements.
For instance, large valves requiring a high thrust
may be limited to only electric or electro-hydraulic
actuators due to a lack of pneumatic actuators with
sufficient thrust capability. Conversely,
electro-hydraulic actuators would be a poor choice
for valves with very low thrust requirements.
The matching of actuator capability with valve
body requirements is best left to the control valve
manufacturer as there are considerable
differences in frictional and fluid forces from valve
to valve.
Control Functions
Knowledge of the required actuator functions will
most clearly define the options available for
selection. These functions include the actuator
signal (pneumatic, electric, etc.), signal range,
ambient temperatures, vibration levels, operating
speed, frequency, and quality of control that is
required.
Signal types are typically grouped as such:
D Two-position (on-off)
D Analog (throttling)
D Digital
Two-position electric, electro-pneumatic, or
pneumatic switches control on-off actuators. This
is the simplest type of automatic control and the
least restrictive in terms of selection.
Throttling actuators have considerably higher
demands put on them from both a compatibility
and performance standpoint. A throttling actuator
receives its input from an electronic or pneumatic
instrument that measures the controlled process
variable. The actuator must then move the final
control element in response to the instrument
signal in an accurate and timely fashion to ensure
effective control. The two primary additional
requirements for throttling actuators include:
D Compatibility with instrument signal
D Better static and dynamic performance to
ensure loop stability
Compatibility with instrument signals is inherent in
many actuator types, or it can be obtained with
add-on equipment. But, the high-performance
characteristics required of a good throttling
actuator cannot be bolted on; instead, low
hysteresis and minimal deadband must be
designed into actuators.
Stroking speed, vibration, and temperature
resistance must also be considered if critical to the
application. For example, on liquid loops
fast-stroking speeds can be detrimental due to the
possibility of water hammer.
Vibration or mounting position can be a potential
problem. The actuator weight, combined with the
weight of the valve, may necessitate bracing.
It is essential to determine the ambient
temperature and humidity that the actuator will
experience. Many actuators contain either
elastomeric or electronic components that can be
subject to degradation by high humidity or
temperature.
Economics
Evaluation of economics in actuator selection is a
combination of the following:
D Cost
D Maintenance
D Reliability
A simple actuator, such as a
spring-and-diaphragm, has few moving parts and
is easy to service. Its initial cost is low, and
maintenance personnel understand and are
comfortable working with them.