Download Control System Instrumentation

Control System Instrumentation
Transducers and Transmitters
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Figure 9.3 illustrates the general configuration of a measurement transducer; it
typically consists of a sensing element combined with a driving element
(transmitter).
Since about 1960, electronic instrumentation has come into widespread use.
Sensors
The book briefly discusses commonly used sensors for the most important process
variables. (See text.)
Transmitters
• A transmitter usually converts the sensor output to a signal level appropriate for
input to a controller, such as 4 to 20 mA.
• Transmitters are generally designed to be direct acting.
• In addition, most commercial transmitters have an adjustable input range (or span).
• For example, a temperature transmitter might be adjusted so that the input range of
a platinum resistance element (the sensor) is 50 to 150 °C.
Instrument Selection Criteria
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solid/gas/liquid, corrosive fluid
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nature of signal, speed of response
accuracy, measurement range
costs
previous plant practice
available space
maintenance, reliability
materials of construction
invasive/non-invasive
environmental/safety (enclosures, fugitive emissions)
Transmitter/Controller
May need additional transducers for Gm if its output is in mA or psi. In
the above case, Gc is dimensionless (volts/volts).
Figure 9.15 Nonideal instrument behavior: (a) hysteresis, (b) deadband.
Measurement / Transmission Lags
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Temperature sensor
TM ( s )
1

T ( s )  s+1
=
m s Cs
U s As
make  as small as possible (location, materials for thermowell)
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Pneumatic transmission lines
usually pure time delay, measure experimentally (no time delays for electronic lines); less
common today compared to electronic transmissions.
(1) Quick Opening (square root trim)
f  s
s  fraction open of the valve (0  s  1)
(2) Linear Trim
f s
(3) Equal Percentage
f   s 1
slope~f
  20-50
must take other flow obstructions into account for actual valve performance
\
Suppose valve has linear trim and flow must be increased. If p through exchanger did
not change, valve would behave linearly (true for low flow rates), since it takes most of
p. For higher flow rates, p through exchanger will be important, changing effective
valve characteristics (valve must open more than expected  nonlinear behavior).
Equal % in this case behaves more like linear valve.
Size pvalve = 25% total p, at s=50%
valves need to operate between 5% and 95%,
flow~ ΔPvalve
Specify pneumatic control valve to be A-O or A-C:
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Steam pressure in a reactor heating coil.
Flow rate of reactants into a polymerization reactor.
Flow of effluent from a wastewater treatment holding tank into a river.
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Flow of cooling water to a distillation condenser.