Dynamic Behavior of Closed-Loop Control Systems Chapter 9

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). Chapter 9

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. Chapter 9

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 Chapter 9 May need additional transducers for G m if its output is in mA or psi. In the above case, G c is dimensionless (volts/volts).

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Figure 9.15 Nonideal instrument behavior: (a) hysteresis, (b) deadband. Chapter 9

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. Chapter 9

from Riggs, J.B., Chemical Process Control Numbers in table above correspond to C v f(l), dp in psi, q in gal/min, and g s 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 Chapter 9

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  p valve = 25% total  p, at s=50% (Δp→$) valves need to operate between 5% and 95%, Chapter 9

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 manipulated variables: (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) Chapter 9

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