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Q: |
How do manual, electric, pneumatic, and
hydraulic valve actuation methods differ? In
what sort of application environments is each
actuation method typically a good fit? |
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A: |
 Let’s
start with the most often used actuators - the
pneumatics. The automated and control valves in
process plants are usually operated by pneumatic
actuators. They are, by far, the most commonly
used actuators due to their being light weight
and rugged. Their low cost and high reliability
make them very popular. Pneumatic actuators are
commonly used in a potentially explosive
atmosphere. The failsafe action of single-acting
actuators is system-inherent.
Manual shutoff valves are used to isolate or
bypass control valves. They are only
occasionally operated and used for on/off
applications, when the response time is not
critical.
Electric actuators are mainly used in
applications where compressed air is not
available; for example, in small systems for
building automation, or in some areas of power
plants. However, the implementation of failsafe
action (fail open or fail close) upon power
supply failure or emergency shutdown in large
electric actuators requires a technically
complex construction.
Hydraulic actuators are used in applications
where the required force and/or stroke cannot be
accommodated by pneumatic or electric actuators.
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Q: |
What is the difference between a rotary valve
actuator and a linear valve actuator? What are
some common applications/examples of each
actuation method? |
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A: |
Linear actuators are used to actuate linear
valves such as globe, three-way (diverting or
mixing) and angle valves. Pneumatic linear
actuators have the simplest construction and
provide very precise control of flow.
Rotary actuators, with shafts performing up to
90-degree rotary motion, are mounted on rotary
valves such as ball, butterfly, rotary plug and
segmented ball valves. These actuators are
derived from linear actuators, and use a gearbox
or lever system to convert the linear into a
rotary motion. The actuator construction, where
compressed air acts on a rotary vane instead of
a piston, has not established itself on the
market.
In
contrast to the pneumatic version of rotary
actuators, the rotary motion of electric
actuators is inherent. A complex linear unit is
needed to transform the rotary motion into a
linear stroke. It would be beyond the scope of
this interview to go into details of the
individual construction of these actuators.
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Q: |
What role do valve positioners play in valve
actuation and control applications? |
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A: |
 We
very often describe the positioner as the brain
and heart of the control valve, because its
functions and capabilities are so comparable.
Analog positioners are used to precisely convert
the input signal from a controller or control
system and control the valve position
correspondingly, despite various actuator bench
ranges and disturbance variables, such as
varying supply air, dynamic forces affecting the
valve plug, and valve stem friction. In
addition, positioners are responsible for
adapting the inherent characteristic of
butterfly and ball valves to the required
characteristic form.
At
the present time, digital positioners are
expected to perform additional tasks, such as
automatic startup and self-diagnostics, as well
as many asset management functions, which enable
predictive, status-oriented maintenance of
control valves.
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Q: |
What are the key specifications users must
evaluate when considering valve actuation? |
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A: |
Generally, the user decides whether manual,
pneumatic, electric or hydraulic actuation is to
be used to operate the actuator. Manual
actuators can only be used for on/off purposes
where the response time is not critical and the
valve is accessible. For all other applications,
it is important to decide which auxiliary power
is available. The valve supplier then chooses
the actuator to match the valve and operating
conditions such as shutoff pressure,
differential pressure, flowrate, leakage class
and closing time at minimum, normal and maximum
load.
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Q: |
What are some of the common pitfalls end-users
need to be aware of when specifying valve
actuation and control technology for a given
application? |
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A: |
As
already mentioned, the valve suppliers specify
the actuator. They calculate the necessary
nominal forces over the entire travel range or
angle of rotation, and also take into account
the actuator stiffness, for example, to operate
the valve in the flow-to-close direction. For
processes that require fast responses, the valve
supplier specifies the tube size connecting the
positioner with the actuator and, if necessary,
installs a booster to increase the air capacity
of the positioner.
For
the control valve technology, flashing and
cavitation are two troublesome conditions that
can quickly destroy the valve. Depending on the
cavitation level, several methods can be
employed to reduce or eliminate its effects.
Flashing can be prevented by adjusting the
process conditions. Special materials can also
be used to prolong the life of a flashing or
cavitating valve.
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Q: |
What are some key steps end-users can take to
ensure they are employing the appropriate valve
actuation solution for a given application? |
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A: |
For
end-users, the question focuses on which type of
actuator (pneumatic, electric or hydraulic)
works most cost-efficiently and reliably for the
given process. After carefully weighing all the
pros and cons, end-users will surely reach the
conclusion that pneumatic actuators are the best
solution in large processing plants, in which a
compressed-air supply network already exists for
other tasks. These actuators are compact,
exceptionally rugged, reliable, require little
maintenance and are excellent value for the
money. They can be used in hazardous areas
without any complications and have an inherent
fail safe action.
In
places where a compressed-air supply network
does not exist and just a few control valves are
to be actuated, the electric actuator is
probably the most cost-effective solution.
The
scope of use for hydraulic actuators is
relatively small. They find use in applications
where extremely high positioning forces and
long, fast stroking are required.
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Q: |
How do you envision valve actuation and control
technology evolving of the next 10-20 years? How
will the valve actuation and control technology
of tomorrow be better than the valve actuation
and control technology of today? |
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A: |
The
fundamentals of the control valve and actuator
technology will not see dramatic changes; they
will continue to develop in an evolutionary way,
fine-tuning the areas of design, production and
application of new materials.
Major
innovations are to be anticipated in the field
of valve positioners. More powerful sensors will
be used in combination with positioners to
precisely detect the valve faults and failures
that, up to now, can only be identified
indirectly.
There
are two other areas where future development
will have major impacts on control technology -
wireless communication and Safety Instrumented
Systems (SIS). More and more systems that are
being built take advantage of wireless
technology. Also, SIS is quickly gaining in
importance with end-users. These new areas are
developing very rapidly.
The
valve actuation and control technology of
tomorrow will be safer and more economically
viable.
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Q: |
How has valve actuation and control technology
evolved over the past 10-20 years? How is the
valve actuation technology of today better than
previous generations of valve actuation
technology? |
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A: |
The
fundamentals of control valve and actuator
technology have developed in an evolutionary way
during the last 20 years. New design ideas,
improved production methods, new materials and
extensive use of computer systems have led to
significant improvements. Present-day valves are
more compact, have higher shutoff capability,
and more advanced sealing technology, especially
with the bellows design. Their resistance to
erosion and corrosion has also improved, and the
anti-cavitation and noise-reducing trims are
more advanced. As a result, modern valves are
more reliable, have longer service life, and
provide for lower cost-of-ownership than the
valves of the previous generations.
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