Chapter%205%20:%20Control%20Systems%20and%20Their%20Components%20(Instrumentation)

1
Chapter 5 Control Systems and Their Components
(Instrumentation)

• Professor Shi-Shang Jang
• Department of Chemical Engineering
• National Tsing-Hua University
• Hsinchu, Taiwan
• March, 2013

2
Overview

• Need of Instrumentation.
• Signals and Signal Levels.
• Sensing Element.
• What are Final Control Elements?
• Characteristics of some transducers and
  transmitters.
• Transmission Signals and representation.
• Accuracy of Instrumentation.
• What are Transducers and Transmitters?

3
Need of Instrumentation

• As already discussed in earlier chapter, we need
  to measure some parameters to control the
  process.
• Instrumentation is a methodology through which we
  obtain value of a desired parameter.
• Type of instrument to be used is very vital issue
  and is decided by-
• Type of the process.
• Instruments dead-time velocity lag.
• Accuracy in measurement.
• Sampling rate (for digital systems).
• Measuring range.
• Safety and hazards associated with it.

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An Example- Blending Process (Instrumentation)
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Gas Process Control
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Temperature Control of Non-isothermal CSTR
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3-1 Signals and Signal Levels

• Any Data or instruction carrying entity is called
  a signal.
• Signals could be characterized by the nature of
  information it transport and the medium of
  Transport.
• On the basis of nature of information, the signal
  could be a continuous signal or a
  Discrete/Digital Signal.
• On the basis of medium of transport, the signal
  could be an Electric Signal, Light/Laser Signal,
  Sound Signal, Radio Signal, Pneumatic Signal etc.

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3-1 Signals and Signal Levels cont..

• Signal level is a physical range within which an
  information is transmitted as a signal.
• If the signal is continuous, the signal level are
  generally continuous.
• If the signal is digital, the signal level is a
  set of discrete values.
• Signal levels are an industry standard and may
  change time to time.
• Signal range/level used in industry are-

• 05 V DC
• 10 V DC
• 3-15 psig

• 15 mA
• 420 mA
• 1050 mA

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3-1 Sensors/Sensing Element

• Sensing Elements may be in physical contact with
  the system and are responsible for determining
  the parameter. For example-
• Temperature-Thermocouples, Filled-bulbs,
  Resistance (RTD).
• Pressure-Bourdon tubes, Strain gauges.
• Level- Float position, Difference Pressure,
  Ultra-sonic level meters.
• Flow rate- Orifice plates, Venturi meters,
  Rotameter.
• Composition- Gas chromograph (GC), pH meter,
  Conductivity, IR absorption, UV absorption.

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3-1 Sensors/Sensing Element
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Sensors/Sensing Element

• Characteristics of a linear temperature-current
  sensing element.

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Example A Typical Experiment
Time (second) Y(temperature,oC,70-100oC) Y(temperature,mA,4-20mA) Y (temperature, ) ln(1-Y)
0 70 4 0. 0
1 71.74 4.928 0.058 -0.0598
2 76.51 7.472 0.217 -0.2446
3 80.8 9.76 0.360 -0.4463
4 84.64 11.808 0.488 -0.6694
5 88 13.6 0.600 -0.9163
6 90.76 15.072 0.692 -1.1777
7 93.16 16.352 0.772 -1.4784
8 94.99 17.328 0.833 -1.7898
9 96.64 18.208 0.888 -2.1893
10 97.75 18.8 0.925 -2.5903
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Temp.?C
Time, sec.
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3-2 Final-control Element

• After the data for the control variable (CV) and
  other parameters is processed by the designed
  controller, signals are sent to Final-control
  element which manipulates other variables like
  flow-rate etc. for the system.
• Generally, control is done by changing flow rates
  for the inlet/outlet of material and energy to
  achieve control and control valves are widely
  used for it.
• Control Valves are of different kinds like-

• Ball Valve.
• Butterfly Valve.

• Pneumatic.
• Electro-mechanical.
• Manual.

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3-2 Control Valve- Final Control Element

• There are generally two type of valve-
• On/Off Control Valve
• Proportional

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Control Valve Action
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3-2 Control Valve- Final Control Element Cont..

• Air to Close(AC)
• - Valve action to close as the pressure
  increases.
• Air to Open(AO)
• - Valve action to open as the pressure increases
  as the previous figure.
• The selection of AC or AO control valve is based
  on the consideration of the emergent need, for
  instance, the emergent shut down.
• A transducer is needed to convert the electronic
  signal to the pneumatic signal and thus finally
  controlling the flow rate.

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3-2 Control Valve- Final Control Element Cont..
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Control Valve- Final Control Element Cont.. Sizing
For all valves, the liquid flow rate going
through is following the equation below
(5-1)
where, Fflow rategal/min. ?PPressure
droppsi. Gf specific gravity of the
fluid. Cv Size, choose the valve size
such that at normal operation,
the valve is nearly half opened, i.e.
vp0.6?0.7 vp is the fractional area of
the valve that allows the
fluid going through, if vp1, then all area
is available (valve fully open), if
vp0, the valve is fully
closed.
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Control Valve- Final Control Element Cont.. Valve
Characteristics

• The graph shows the flow-rate as the function of
  opening of the valve.

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Control Valve- Final Control Element Cont..

• Valve Characteristics
• In most practical cases, equal percentages valves
  are selected to make sure that the flow rate
  through the control valve is proportional to the
  signal vp by choosing correct values of ?.
• This is due to the friction of the fluid through
  the pipe lines, and in most cases this is
  non-negligible.
• ?pLkLGf f2
  (5-2)

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Control Valve- Final Control Element Cont..
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Control Valve- Final Control Element Cont..
Example

• Figure 5-2.3 shows a process for transferring an
  oil from a strage tank to a separation tower.
  Nominal oil flow is 700 gpm, friction pressure
  drop is 6 psi, available pressure drop for the
  control valve is 5 psi.

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Control Valve- Final Control Element Cont..
Example
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Control Valve- Final Control Element Cont..
Example
Matlab code gtgt alpha50 vplinspace(0,1) for
i1100 f(i)640alpha(vp(i)-1)/(sqrt(113e-6(64
0alpha(vp(i)-1))2))sqrt(11/0.94) f1(i)640vp
(i)/(sqrt(113e-6(640vp(i))2))sqrt(11/0.94) e
nd gtgt plot(vp,f1,vp,f)
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Control Valve- Final Control Element Cont..
Transfer Function
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What are Transducer and Transmitter?

• Transducer is a device which convert one type of
  signals into another. In other worlds, it may
  convert one form of energy to another form.
• Eg 1 A Digital thermometers Transducer convert
  thermal energy into equivalent electrical
  signals.
• A typical Transducer consist of a sensing element
  combined with a driving element (transmitter).
• Transducers for process control measurements
  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.

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What are Transducer and Transmitter? Cont..

• Transducer is a device which convert one type of
  signals into another. In other worlds, it may
  convert one form of energy to another form.
• A transmitter is usually required to convert
  sensor output compatible with the controller
  input and to drive the transmission lines
  connecting the two.
• Pneumatic (air pressure) signals were used
  extensively up till 1960s but currently Digital
  instrumentation is widely used.

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An Example- Blending Process (Instrumentation)
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Block Diagram
Assume ?m is small
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3-3. Conventional Feedback Controllers -
Proportional Controllers
R(t) errorR(t)-C(t)

• Kc is called controller gain

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3-3 Conventional Feedback Controllers
-Proportional Controllers Cont..
control valve saturation
100
Kc100/502
control valve saturation
Proportional Band PB50/10050
PB100/Kc
0
50
Kclt0 Direct Acting Control ?p increases with y
increases Kcgt0 Reverse Acting Control ?p
decreases with y increases
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Conventional Feedback Controllers

• Proportional-Integral Controllers

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Conventional Feedback Controllers

• Proportional-Integral Controllers Cont..

• The integral actions contribute positive or
  negative as the signs appeared shown above.
  However further build up of integral term becomes
  quite large and the controller is saturated is
  referred to as reset windup.

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Conventional Feedback Controllers

• Proportional-Integral Controllers Cont..

• Reset windup occurs when a PI or PID controller
  encounters a sustained error. In this situation,
  a physical limitation prevents the controller
  from reducing the error signal to zero.
• It is undesirable to have integral term continue
  to build up after the controller output
  saturates.
• Commercial controllers available provides
  anti-reset windup.

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Conventional Feedback Controllers

• Proportional-Integral-Derivative Controllers

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Conventional Feedback Controllers

• Proportional-Integral-Derivative Controllers
  Cont..

• The direct implementation of the derivative term
  of a PID controller is basically undesirable due
  to
• Noisy signal is normally received. The
  derivatives of these signals are meaningless.
• The derivative element is physically
  unrealizable.

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3-3 Typical Responses of Feedback Control Systems
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3-3 Typical Responses of Feedback Control Systems
- Continued
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3-3 Typical Responses of Feedback Control Systems
- Continued
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3-4 Process Control Applications

• Applications of process control are mostly in the
  areas of
• Flow rate control
• Level control
• Air pressure control
• Temperature control
• Composition control

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3-4 Process Control Applications- continued

• Flow rate control
• Applications inlet flow control, outlet flow
  control of processes, reflux flow rate, pipeline
  flow rate,,etc.
• Implementation
• Remarks
• (i) Due to the effect of turbulence and pressure
  fluctuation, the measurement is noisy.
• (ii) no offset is allowed in flow rate control
  integral action is necessary.
• (iii) flow rate process is fast no need for
  derivative actions.
• Conclusion PI control is needed with low gain
  and ?I?10-20 seconds

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3-4 Process Control Applications- continued

• Liquid level control
• Applications reactor volume control, buffer tank
  level control, reboiler level control accumulator
  level control, steam generator level control,
  ,etc.
• Implementation

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3-4 Process Control Applications- continued

• Remarks
• (i) the level process is basically noisy
  fluctuation of the liquid level low controller
  gain
• (ii) in case of important levels such as reboiler
  level, accumulator, integral action is needed,
• (iii) the level processes are basically a first
  order system.
• Conclusions The tank level control is basically
  loose, for instance, to maintain the tank is not
  completely empty at low inlet flow rate and to
  maintain tank is not full at high inlet flow
  rate. Thus, a low gain P controller is
  frequently implemented. However, in case of
  important level system such as reboiler level,
  accumulator, PI controllers should be used.
• Other tips If the outlet of the tank is very
  important for example, the flow rate to the
  reactor. Then, the level controller should not
  influence the flow rate. The controller gain of
  the P- controller should be tuned to very low.

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3-4 Process Control Applications- continued

• Air pressure control
• Applications Gas storage tank, air phase
  reactors.
• Remarks
• (i) vapor pressure control is not this case, it
  should be considered as temperature control in
  the next slide,
• (ii) Gas pressure system is fast no derivative
  action is needed,
• (iii) the measurement is not noisy.
• Conclusions Use high gain P-only controllers.

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3-4 Process Control Applications- continued

• Temperature Control
• Applications reactor temperature control, heat
  exchanger temperature control, temperature
  control of pre-heaters, vapor pressure
  control,,etc.
• Manipulated variables cooling water flow rate,
  steam flow rate.
• Remarks
• (i) the quality of sensor is crucial for this
  type of control, for instance, sensor noise and
  time lag will influence the control quality,
• (ii) the system is quite slow (heat transfer
  mechanism), derivative action is needed,
• (iii) off set of the temperature is not allowed
  integral action is needed,
• (iv) there exists an inherent upper bound of the
  controller gain, process stability is an issue.
• Conclusion A typical PID control situation.

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3-4 Process Control Applications- continued

• Composition control
• Applications pH control, reactor composition
  control, distillation composition control, ,etc.
• Remarks
• (i) in some cases, the measurements are noisy
  with time lag (e.g. GC) makes the control very
  difficult,
• (ii) the process is typically slow derivative
  action,
• (iii) no offset is allowed integral action.
• Conclusion PID control situation, PI may be
  implemented in some cases, controller settings
  are case by case.

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3-5 Summary

• Instrumentation is a part of the manufacturing
  process.
• The sensors and transmitters are introduced
• The sizing and characteristics of control valves
  are essential for instrumentations
• The conventional controllers are derived and
  analyzed
• The applications of the controllers are introduced

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Homework

• Text p192
• 5-3, 5-5, 5-10, 5-16, 5-18

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Supplemental Material
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Control Valve- Final Control Element Cont..
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Pressure drop vs flow rate
Matlab code k30/(200)2 flinspace(0,300)
for i1100 dp(i)kf(i)2 end plot(f,dp) dp_valv
e100-dp plot(f,dp_valve)
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• Example
• A pump furnishes a constant head of 40 psig, the
  heat exchanger pressure drop is 30 psig at 200
  gal/min. Select a Cv of the valve and plot the
  installed characteristic for
• A linear valve that is half open at the design
  flow rate.
• An equal percentage valve (R50) that is sized to
  be completely open at 110 of the design flow
  rate.

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Area vs valve position
xlinspace(0,1) R50 for i1100 fl(i)R(x(i)-1
)100 end plot(x,fl)
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• Solution

Given any flow rate q, pressure drop across the
heat exchanger
Pressure drop across the valve
(a) Calculate rated Cv
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• Solution

(b) Calculate the rated Cv at 110 of qd
To plot for q over l, (i) set q, (ii) get ?Ps,
(iii) get ?Pv, then get l cv115R50
for i111 q(i)20i
dps30(q(i)/200)2
dpv40-dps l(i)1log(q(i)/(cvsqr
t(dpv)))/log(R) end
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Example

• The temperature of a CSTR is controlled by a
  pneumatic feedback control system containing
• (1) a 100 to 200oF temperature transmitter,
• (2) a PI controller with integral time set at 3
  minutes/repeat and proportional band a 25, and
• (3) a control valve with linear trim, air to
  close action, and a Cv4 through which cooling
  water flows. The pressure drop across the valve
  is a constant 25 psi.
• If the steady-state controller output
  pressure is 9 psig, how much cooling water is
  going through the valve? If a sudden disturbance
  increases reactor temperature by 5oF, what will
  be the immediate effect on the controller output
  pressure and the water flow rate?

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• Solution