Chapter 9 - Department of Chemical Engineering

Chapter 9
Dynamic Behavior of
Closed-Loop Control
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).
• Since about 1960, electronic instrumentation has come into
widespread use.
Chapter 9
The book briefly discusses commonly used sensors for the most
important process variables. (See text.)
• 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
Chapter 9
Instrument Selection Criteria
•solid/gas/liquid, corrosive fluid
•nature of signal, speed of response
•accuracy, measurement range
•previous plant practice
•available space
•maintenance, reliability
•materials of construction
•environmental/safety (enclosures, fugitive emissions)
Chapter 9
Chapter 9
May need additional transducers for Gm if its output is in
mA or psi. In the above case, Gc is dimensionless (volts/volts).
Chapter 9
Chapter 9
Figure 9.15 Nonideal instrument behavior: (a) hysteresis,
(b) deadband.
Chapter 9
Chapter 9
Chapter 9
Measurement / Transmission Lags
Chapter 9
• Temperature sensor
TM (s)
T(s) s + 1
m s Cs
Us As
make  as small as possible (location, materials for
• Pneumatic transmission lines
usually pure time delay, measure experimentally (no
time delays for electronic lines); less common today
compared to electronic transmissions.
Chapter 9
Chapter 9
Chapter 9
Chapter 9
from Riggs, J.B., Chemical Process Control
Numbers 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)
Chapter 9
f  s
s  fractionopenof thevalve(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
Chapter 9
See Example 9.2
Chapter 9
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%,
flow ~ ΔPvalve
Chapter 9
Chapter 9
Failure philosophy: Keep process pressure low,
protect environment (equipment and engineers)
Chapter 9
A-O  F / C
A-C  F / O
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
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.
Flow of cooling water to a distillation condenser.
Chapter 9
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