Chemical Engineering June 2022 - 39

have a valve and want to select an
actuator, check for the size range
to fit the mounting flange and the
valve's minimum torque requirement.
This information is available in the
valve documentation.
Torque. When deciding the valve's
torque, consider the minimum torque
required to start motion (otherwise
known as breakaway torque) when
the valve is at rest on the valve seat
and the torque needed to close the
valve into the valve seat completely.
The actuator torque should be some
percentage - usually 10-30% -
higher than the minimum torque required
from the valve.
Frequency of use. Some actuators
are designed for on/off positions
with
limited use frequency,
while
others are designed for continuous
modulation throughout their working
life. Consider the control function in
your process before deciding what
type of actuator will be the best fit.
The frequency of use directly affects
an actuator's durability as the actuator
is a mechanical device that wears
with use. This is particularly crucial
for modulating valves that may be
constantly operated and could overheat
or fail.
Operating speed. Actuators can
be fast-acting, closing a valve from
a fully open position in a few seconds,
or slow-acting, which takes
several minutes. The particular process
application will determine what
kind of actuator speed you need. For
example, a ventilation actuator in a
building will be slow-acting because
its thermal mass will prevent quick
temperature changes. On the other
hand, a hot-water supply line in a
brewery will be fast-acting to ensure
precise flowrate and volume is distributed
for the brewing process.
The speed of an actuator is also
directly related to the power used by
the actuator during operation. The
more the speed that is required, the
higher the power rating. For electric
actuators, this defines the electric
motor power and gear system,
while for pneumatic and hydraulic
actuators, it defines the actuator's
operating pressure and size. When
choosing the operating speed of an
actuator, it is crucial to make sure
that the valve is not closing too fast.
Closing the valve too quickly may
lead to a hydraulic shock to the system,
which may result in damage to
the connected pipes and equipment.
Ease of operation and maintainability.
Pneumatic actuators are
the easiest to install and maintain
because they have very few parts
and a simple operation mechanism.
Electric actuators can be sophisticated
and may be the most difficult
to troubleshoot when they do not
work. Pneumatic and hydraulic actuators
are also very durable when
compared with electric actuators.
Available power source. Sometimes,
the choice of an actuator type
is strictly determined by the available
power source within the valve
environment. Electric actuators can
be powered by 24-V, 110-V, 230-V,
400-V, single- or three-phase power,
and by direct or alternating current.
Pneumatic actuators require
compressed air between 60 to 150
psi (4-10 bars), while hydraulic actuators
could have the oil operating
pressure at 2,900 psi (200 bars).
Failure mode. If the valve is required
to be at a particular position
when there is a sudden loss of
power source, a fail-safe mechanism
is needed. Pneumatic and hydraulic
valves can have a spring-loaded actuator
that returns the valve to a default
position, while electric actuators
have a battery. They may also have
springs for fail-safe operation. This
adds to weight, size and cost. The
fail-safe functions can be one of the
following: close at no power, close
at no control, open at no power and
open at no control. When adding a
fail position in a pneumatic or hydraulic
actuator, the actuator must be upsized.
Now, the actuator is overcoming
not just the torque of the valve,
but the spring's pressure as well.
Available space. A pneumatic actuator
can be significantly bigger than
the valve it controls, especially for low
compressed-air pressure
applications.
Check that available space can
accommodate a particular actuator
type before final decisions are made.
Actuator cost. Regardless of an
actuator's suitability to your application,
the cost is often a determinant
of the type of actuator selected. The
higher the torque, power, size, extra
features like ATEX, positioner, and
so on, the higher the actuator's cost
will be. While certain features can be
avoided to reduce cost, others, such
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2022
as torque and power requirements,
cannot be compromised.
Selection process
The starting point for the actuator
selection is always the question,
" What function will the actuator
perform in your process? " . After
this is clearly determined, take care
to go through the selection criteria
one after the other to ensure that
all have been considered before selecting
the type of actuator, as well
as its size, and the features applicable
within your budget.
The selection process for the right
valve actuator can be complicated.
Sometimes, existing infrastructure,
such as power sources, communication
modules, distance to the control
room, as well as the experience
of the engineer, can play a significant
role in the decision-making process.
In other situations, valve-actuator selection
is done first with few desired
specifications and other parameters
have to be created to support the
operation of the actuator.
The ultimate purpose of a valve
actuator is to perform a certain operation,
or series of operations, in
the most cost-effective way possible,
while automating the process.
There are many features to consider
when selecting a valve actuator, but
remember that these features will affect
the quality and cost of the actuator.
Any additional torque capacity
beyond the specification, or an ATEX
approval or a positioner, may not be
needed for your application. Leaving
them out can save you quite a bit
of money. If needed, consult a valve
engineer to assure the sizing and selection
has been done correctly for
your specific application.
n
Edited by Scott Jenkins
Author
Gilbert Welsford Jr. is the
founder of ValveMan.com (310
Commerce Drive, Exton, PA 19341;
Phone: 484-713- 0065; Email:
editor@valveman.com) and a
third-generation valve entrepreneur.
Welsford has learned valves
since a young age and has brought
his entrepreneurial ingenuity to the
family business in 2011 by creating
the online valve store ValveMan.com. Welsford's focus
is building on the valve business legacy his grandfather
started, his father grew, and he has amplified. Welsford
holds a B.A. in business management from James Madison
University. He has over 15 years of experience selling
valves, and is a published writer in industry-leading publications
focused around the valve industry.
39
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