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 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.

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Instrument Selection Criteria

•solid/gas/liquid, corrosive fluid•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)

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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 for thermowell)

• 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 ControlNumbers in table above correspond to  Cvf(l), dp in psi, q in gal/min, and gs is

specific gravity:

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(0 valve theofopen fraction s sf

sf

50-20 f~slope 1 sf

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See Example 9.2

Suppose valve has linear trim and flow must be changed. If p through exchanger does not change, valve would behave linearly (true for low flow rates), since it takes most of p . 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% (Δp→$)

valves need to operate between 5% and 95%,

valveΔP~flow

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Pneumatic control valves are to be specified for theapplications listed below. State whether an A-O or A-Cvalve 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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