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Dynamic Behavior of

Closed-Loop Control

Systems

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Control System Instrumentation

Figure 9.3 A typical process transducer.

Transducers and Transmitters

Figure 9.3 illustrates the general configuration of a measurement

transducer; it typically consists of a sensing element combined

with a driving element (transmitter).

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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 adjustableinput 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.

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Instrument Selection Criteria

solid/gas/liquid, corrosive fluidnature of signal, speed of responseaccuracy, measurement rangecostsprevious plant practiceavailable spacemaintenance, reliabilitymaterials of constructioninvasive/non-invasive

environmental/safety (enclosures, fugitive emissions)


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Transmitter/Controller

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May need additional transducers for Gm if its output is in

mA or psi. In the above case, Gc is dimensionless (volts/volts).


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Figure 9.15 Nonideal instrument behavior: (a) hysteresis,

(b) deadband.

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Measurement / Transmission Lags

Temperature sensor

make as small as possible (location, materials forthermowell)

Pneumatic transmission lines

usually pure time delay, measure experimentally (no

time delays for electronic lines); less common today

compared to electronic transmissions.

ss

ssM

AU

Cm=

1+s

1

)s(T

)s(T

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from Riggs, J.B., Chemical Process Control

Numbers in table above correspond to Cvf(l), dp in psi, q in gal/min, andgs is

specific gravity:


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Three valve characteristics determined by plug shape:

(1) Quick Opening (square root trim)

(2) Linear Trim

(3) Equal Percentage

must take other flow obstructions into account for actual valve

performance

1)s(0valvetheofopenfractions sf

sf

50-20f~slope1 sf

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See Example 9.2


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Suppose valve has linear trim and flow must be

changed. Ifp through exchanger does not change,

valve would behave linearly (true for low flow rates),since it takes most ofp . For lower flow rates, p

through exchanger will be reduced, changing effective

valve characteristics (valve must close more than

expected

nonlinear behavior).

Equal % in this case behaves more like linear valve.

Size pvalve = 25% total p , at s=50% (p$)

valves need to operate between 5% and 95%,

valveP~flow

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Pneumatic control valves are to be specified for the

applications listed below. State whether an A-O or A-C

valve should be specified for the following manipulatedvariables:

(a) Steam pressure in a reactor heating coil.

(b) Flow rate of reactants into a polymerization reactor.(c) Flow of effluent from a wastewater treatment

holding tank into a river.

(d) Flow of cooling water to a distillation condenser.

Failure philosophy: Keep process pressure low,

protect environment (equipment and engineers)

A-O F / C

A-C F / O

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