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Valve Control Technology |
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An
Introduction To Valve Positioners |
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 A
Valve Positioner is the critical link between the controlling
elements in a plant and the final control element, the control
valve.
Important considerations when selecting a Valve
Positioner are:
 The
Valve Positioner compares the valve/actuator position to a desired
input signal and influences the valve/actuator so that its position
corresponds with the desired input signal.
Valve
Positioners are often required to operate under difficult conditions
such as high vibration, extreme temperatures, corrosive environments,
poor air quality, and general abuse. Quite often, these products
are expected to perform to exact requirements, and remain maintenance
free, under these rugged conditions.
Design
A simple, clean design has made PMV one of the most accepted positioner
products on the market today. Engineers and maintenance people
have responded positively to the concept that in a positioner,
simpler is better. (back)
Performance
How well does the unit do what it claims it will do? Such characteristics
as gain, hysteresis, repeatability, and linearity are key elements
of the performance equation.
Performance should not only be evaluated with these engineering
terms and numbers, but with the real life world of actual plant
operations. Performance is what brings engineers and maintenance
people back to PMV. (back)
Reliability
How well does the positioner meet the performance specifications,
in the very difficult environment that control valves, actuators,
and positioners often find themselves?
Meeting the performance specifications, under difficult conditions,
helps determine the products reliability. (back)
Durability
How long does the positioner maintain its specifications under
difficult environmental conditions? The life expectancy of all
industrial products is determined by the user.
Under the installed conditions, is the product durable enough
to meet the user's expectations?
(back)
Maintenance
There are no maintenance free industrial products that are installed
on control valve packages. Maintenance should be a function of
frequency and simplicity.
How often are you performing maintenance on your positioners?
How easy are they to maintain?
Two key elements to consider. (back)
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Dynamic Loop Performance - Valve Positioner Issues
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Historically, comparison, and selection of positioners
has been based on performance criteria expressed as a % of full
signal span, i.e. 0-100% stroke of the associated control valve.
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In the last few years engineers have begun to
recognize that specification of traditional performance criteria
is incapable of determining, nor assuring, the best installed
performance of the control valve and entire loop. The reason
for this is simply that the overwhelming majority of control
valves operate either at a steady state, or, when they do make
an adjustment, make small changes in position representing 5%
of signal span, or less.
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While still important to predict a valve's performance
at min., normal and max. flows, full span performance specifications
cannot measure nor predict a valve's/loop performance at the
conditions where they probably spend 99% of their operating
time, either a steady state or small adjustment in the area
of 1-5% of signal span.
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Lack of consideration of the positioners narrow
band performance ultimately denies users the full benefit of
digital process automation, defeating the inherent precision
of digital transmitters and controls.
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Flow
Gain Vs Pressure Gain |
In addition to standard wide band performance criteria
such as accuracy, repeatability, hysteresis, etc., consideration
and specification of both flow and pressure gain can assure optimum
performance in the critical narrow-band control environment. To
evaluate a positioners (control valve) narrow-band potential performance,
it therefore becomes necessary to consider both the flow and the
pressure gain performance.
 As
an example, it can be easily demonstrated that a positioner
with 22 SCFM output and a pressure gain of 800:1 will be less
sensitive and slower in response to small signal changes than
a positioner with a rated output of only 18.5 SCFM, but a
pressure gain of 1000:1. |
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Flow
Gain |
 It
has been, and remains, a common practice to measure a positioners
potential performance by it's maximum rated air delivery at
a given supply pressure. Overlooked, is the fact that a positioners
air delivery is actually proportional to the amount of signal
change whenever the signal change represents 10% of signal
span, or less. The actual air delivery of a positioner is
only a small fraction of the max. rated output when signal
changes represents 5% of span, or less. |
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Pressure
Gain |
 Positioner
pressure gain is essentially it's ability to develop an operating
differential pressure to the valve actuator.
In essence, a positioner with a higher pressure gain will
be more sensitive to small signal changes and provide noticeably
faster valve response in the critical area of 1-5% signal
changes. |
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