Chemical Engineering June 2022 - 38

energy into mechanical motion and
can be used in locations with no
electricity. Pneumatic actuators are
of two types: single-acting or doubleacting.
Single-acting pneumatic actuators
use a single compressed-air
source to turn the valve with a spring
to return the valve to the normal position.
A double-acting pneumatic
actuator has two compressed-air
sources that turn the valve and return
it to the original position, otherwise
known as a fail position.
Pneumatically
controlled
valves
are relatively simple when compared
to electric actuators - they are easy
to install and maintain, and have a
very fast operating speed. There is a
cost benefit of using pneumatic actuators,
but it only applies in valves
up to a certain size.
Pneumatic actuators often use
a cylinder with a mechanism that
converts the linear motion from the
compressed air into rotational motion.
The most common mechanism
is the rack and pinion, but it can also
be a diaphragm, piston or scotch
yoke. Most pneumatic actuators are
used for quarter-turn valves. The
mechanism can be spring-loaded to
return to a normal shut-down position
in emergencies.
Solenoid valves are used to regulate
airflow into the actuator. Electrical
signals from a controller energize the
solenoid valve position to either open
or closed, allowing compressed air
to flow through to the pneumatic actuator's
sides. It is important to note
that, in order to actuate a valve with
a pneumatic actuator, there must be
a supply of clean, instrument-quality
air, normally at 60 or 80 psi.
Hydraulic actuator. Hydraulic actuators
convert hydraulic power to
achieve mechanical work. They can
be used for quarter-turn valves, such
as ball valves, or multi-turn valves,
such as globe valves. Hydraulic actuators
consist of a cylinder and a
mechanism for converting linear motion
to rotational motion, such as a
scotch yoke mechanism. Hydraulic
actuators use high-pressure oil from
a hydraulic pump to drive the valve.
Like pneumatic actuators, hydraulic
actuators can be single-acting,
with a spring as a fail-safe, or double-acting.
They are relatively small
compared to pneumatic actuators,
but with thicker parts due to the high38
pressure
operation. They are also
more precise than the pneumatic
actuators because oil is incompressible.
Hydraulic actuators are commonly
used in large valve sizes that
require a large turning force.
Actuator selection criteria
Before purchasing an actuator, several
basic parameters should be
considered. These parameters are
based on the function for which you
will use the actuator, as well as the
environment. However, some actuators
have unique features in addition
to the basic parameters, which
makes them unique. Always read
the manufacturer's documentation
for recommendations and features
of each actuator.
Presented below is a list of important
parameters that should be
considered when deciding on which
actuator would be most suitable for
an application.
Operating conditions. The actuator's
operating conditions and environment
go a long way toward determining
what type will fit best for
your application. Operating conditions
can include the following:
Temperature: Electric actuators
can overheat if the operating temperature
is too high. Pneumatic actuators
are more commonly used
and best suited for high-temperature
operations.
Pressure: While all actuator types
can operate at high pressure, consider
the pressure differential across
the valve, which will determine the
amount of required torque for the
actuator to turn the valve.
Hazardous environment. Make
sure to select the actuator and actuator
accessories with the correct
IP code (ingress
protection code
against intrusions) if your environment
has dust or moisture. Electric actuators
with IP 67 and below are vulnerable
to damage from moisture and
condensation. Pneumatic actuators
are preferred in wet environments.
If the actuator operates in an explosive
environment, one must consider
whether the subject actuator meets
a specific explosion protection standard,
such as those that carry an
explosion-proof NEMA rating
or a
flameproof ATEX classification. Another
thing that must be considered
is the duty cycle. Establishing an acFIGURE
2. The pneumatic actuator shown here is
attached to a two-way ball valve made of brass
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curate duty cycle can help to decide
which actuation mechanism should
be used for a specific application. An
electrically actuated valve can provide
reliable service for a piping system
that operates a few times a day.
However, as the frequency of operations
increases, and with it, the duty
cycle, the electric actuator may suffer
from burnout due to motor coils heating
up. Pneumatic actuators are the
most suitable choice for applications
that require frequent valve operations
as they can handle high-frequency
duty cycles without failure.
Connection type. Consider that
actuators have different connection
types based on various standards.
In order for the actuator to be connected
to the valve, specific adapters
must be used so that it may be
mounted on the valve stem properly
and perform its function as expected.
Consistency. While sometimes not
obvious, the simplest way to decide
which actuator is suitable for your
application is by checking to see
what type of actuators are already in
use in the process.
Control functions. The type of control
the process requires, either on/
off or modulating, will determine
whether the actuator requires a positioner
or end switches. Consider
the type of signal that will be sent to
achieve this control. There are digital
signals for on/off controls and various
types of analog signals for modulating
flow control.
Sizing. The actuator should be sized
according to the torque requirement.
It is quite common that manufacturers
supply both the actuator and
valve as one unit. When you already
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