Class 19 Control System Instrumentation

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Class 19 Control System Instrumentation

• Chapter 9

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How Do We Implement Our Control Strategies?
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How Do We Implement Our Control Strategies?
It is necessary to measure (at least) the output
variable, transfer the signal, and manipulate one
of the input variables
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How Do We Implement Our Control Strategies?
We need One instrument, one transducer, And one
actuator
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Measurement - Transducer
This is the general configuration of a
measurement transducer This typically consists of
a sensing element combined with a driving element
(transmitter) Sometimes, the whole thing is
called transducer
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Measurement - Transducer
Transducers convert the magnitude of a process
variable (e.g., flow rate, pressure, temperature,
level, or concentration) into a signal that can
be sent directly to the controller The sensing
element is required to convert the measured
process variable into some quantity more
appropriate for mechanical or electrical
processing within the transducer
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Sensors - Selection Criteria
Measurement range (span) Performance Reliability M
aterials of construction Prior use Potential for
releasing substances to the environment Electrical
classification (safety) Invasive or non-invasive
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Sensors - Temperature
Termocouple (widely used for Tlt1000C) Resistance
temperature detector Bimetal thermometer Pyrometer
Surface acoustic wave
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Sensors - Flow
Orifice Venturi Roameter Turbine Coriolis
(becoming popular)
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Sensors - Pressure
Liquid column Elastic element (diaphragm) Piezoele
ctric Optical fiber
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Sensors - Level
Float - activated Head devices Electrical Radiatio
n
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Sensors - Composition
Gas-liquid chromatography Mass spectroscopy Infrar
ed spectroscopy Thermal conductivity Refractive
index Electrophoresis Electrochemical
Note this is the most difficult measurement in
process engineering. One chemical composition
analysis system can cost 100,000. It is often
preferred to do indirect measurements (refractive
index) frequently and on-line, and quantitative
measurements less frequently and off-line.
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Transmitter
A transmitter converts the sensor output to a
signal level appropriate for input to a
controller (usually a computer) This signal tends
to be a current, in the range 4 to 20
mA Transmitters tend to be direct acting the
output signal increases as the measured variable
increases Commercial transmitters have adjustable
input range (span) and zero
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Transmitter - Example

• A temperature transmitter might be adjusted so
  that the input range of a platinum resistance
  element (the sensor) is 50 to 150 C. The
  following correspondence is obtained

• This transducer has a zero of 50 C and a range
  or span of 100 C
• The relation between transducer output and input
  is

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Transmitter - Example
The gain of the measurement element Km is 0.16
mA/C For any linear instrument
Not all transducers are linear Forgetting
non-ideality (which we always do in our examples)
may lead to big problems in practice
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Transmitter - Example
Example of linear instrument calibration
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Final Control Element - Actuator

• Every process control loop contains a final
  control element (actuator), the device that
  enables a process variable to be manipulated
• For most chemical and petroleum processes, the
  final control elements (usually control valves)
  adjust the flow rates of materials, and
  indirectly, the rates of energy transfer to and
  from the process
• The control valve components include valve body,
  trim, seat, and actuator
• We will discuss pneumatic valves, but motors can
  also be used to adjust the trim

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Final Control Element - Valves

• There are many different ways to manipulate the
  flows of material and energy into and out of a
  process for example, the speed of a pump drive,
  screw conveyer, or blower can be adjusted
• However, a simple and widely used method of
  accomplishing this result with fluids is to use a
  control valve, also called an automatic control
  valve
• The the choice of air-to-open (A-O) or
  air-to-close (A-C) valve is based on safety
  considerations
• This consideration depends on the transducer
  when power goes off, the transducer signal goes
  to the lowest value in the range (for
  direct-acting transducers) do we need open or
  close valve when that happens?

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Pneumatic Valves
A pneumatic control valve (air-to-open)
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Pneumatic Valves - Example 1
Pneumatic control valves are to be specified for
the applications listed below. State whether an
A-O or A-C valve should be used for the following
manipulated variables and give reason(s).

• 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.

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Specifying Control Valves
A design equation used for sizing control valves
relates valve lift to the actual flow rate q
by means of the valve coefficient Cv, the
proportionality factor that depends predominantly
on valve size or capacity
When you buy a valve, you need to specify Cv,
together with valve type, material, etc.
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Specifying Control Valves
A design equation used for sizing control valves
relates valve lift to the actual flow rate q
by means of the valve coefficient Cv, the
proportionality factor that depends predominantly
on valve size or capacity

• Here q is the flow rate, is the flow
  characteristic, is the pressure drop
  across the valve, and gs is the specific gravity
  of the fluid.
• This relation is valid for nonflashing fluids.

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Specifying Control Valves

• Specification of the valve size is dependent on
  the so-called valve characteristic f (l).
• Three control valve characteristics are mainly
  used.
• For a fixed pressure drop across the valve, the
  flow characteristic is
  related to the lift , that is,
  the extent of valve opening, by one of the
  following relations

R 20-50
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Control Valves Characteristics
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Control Valves Rangeability
The rangeability of a control valve is defined as
the ratio of maximum to minimum input signal
level. For control valves, rangeability
translates to the need to operate the valve
within the range 0.05 f 0.95 or a
rangeability of 0.95/0.05 19.
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Specifying Control Valves - Example
A pump furnishes a constant head of 40 psi over
the entire flow rate range of interest. The heat
exchanger pressure drop is 30 psig at 200 gal/min
(qd) and can be assumed to be proportional to q2.
Select the rated Cv of the valve and plot the
installed characteristic for the following case

• A linear valve that is half open at the design
  flow rate.

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Solution First we write an expression for the
pressure drop across the heat exchanger
(simplification of the Bernoullis equation)
Because the pump head is constant at 40 psi, the
pressure drop available for the valve is
Figure 9.11 illustrates these relations. Note
that in all four design cases at qd.
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• First calculate the rated Cv.

We will use Cv 125. For a linear characteristic
valve, use the relation between and q from
Eq. 9-2
Using Eq. 9-9 and values of from Eq. 9-7,
the installed valve characteristic curve can be
plotted.
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Accuracy in Instrumentation
Figure 9.13 Analysis of types of error for a flow
instrument whose range is 0 to 4 flow units.
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Accuracy in Instrumentation
We need to distinguish between Error Precision R
esolution Accuracy Repeatability
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Accuracy in Instrumentation
Analysis of instrument error showing the
increased error at low readings
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Accuracy in Instrumentation
Figure 9.15 Nonideal instrument behavior (a)
hysteresis, (b) deadband.