Operator Generic Fundamentals
Components - Controllers and Positioners
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Operator Generic Fundamentals
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Terminal Learning Objective
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of ≥ 80%
on the following area:
1. Describe the arrangement and operation of typical controllers
and positioners within process control systems.
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TLO’s
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TLO 1
TLO 1 – Describe the arrangement and operation of typical controllers
and positioners within process control systems.
1.1 Describe the characteristics of a control system, including process
controllers and position controllers.
1.2 Describe the operation of bistable alarm and control circuits.
1.3 Define the following process control related terms: proportional
band, gain, closed loop system, offset, feedback, deviation,
deadband, direct acting, and reverse acting.
1.4 Describe the operation of an automatic controller, including
proportional control system, proportional-integral (PI) control,
proportional-derivative (PD) control, and proportional-integralderivative (PID) control.
1.5 Describe the operation of a controller in the automatic and manual
modes.
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Enabling Learning Objectives for TLO 1
1.6 Describe the operation of temperature controllers and pressure
controllers.
1.7 Describe the operation of mechanical and electronic speed-control
devices.
1.8 Interpret logic diagrams and determine controller outputs.
1.9 Describe the design and operation of the following types of valve
actuators: pneumatic, hydraulic, solenoid, and electric motor.
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ELOs
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Characteristics of Controllers and
Positioners
ELO 1.1 – Describe the characteristics of a control system including
process controllers and position controllers.
Control Systems
• Designed to maintain a system
– Temperature
– Pressure, etc.
• Use several control elements working together
• Capability for remote and local operation
• Actuator provides precise positioning
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Process Controllers
Sensor
• Detect actual value of controlled parameter
– Temperature
– Pressure
– Flow
• Measured parameter must be converted into usable signal for control
system
Transducer/Transmitter
• Converts sensor signal into pneumatic or electrical signal
• Transmits pneumatic or electrical signal to controller
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Process Controllers
Controller
• Compares value of measured parameter to desired value or setpoint
• Develops error signal
• Sends error signal to final control element
Final Control Element
• Takes controller output signal and manipulates component in
response to error signal
– Open and/or close a valve
– Turn ON or OFF alarm
– Throttle open or closed air-operated valve
– Turn ON or OFF heaters
– Etc.
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Operation of a Simple Controller
Figure: Process Control System Operation
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Bistable Operation
ELO 1.2 – Describe the operation of bistable alarm and control circuits.
• Bistables are two position switches
– They are either on or off, depending on the input variable
• When input reaches setpoint, they are “on”
• When input returns to below setpoint, they are “off”
• May have a reset band above or below the “on” setpoint
– prevent excessive cycling
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Two-Position Controller
• Simplest type of controller
• Device that has two operating conditions:
– Completely ON
– Completely OFF
• Uses Bistable symbol to show how parameter controlled
– Turns ON on an increasing signal
– Turns ON on a decreasing signal
– Turns OFF at same value as ON value
– Turns OFF at different value than ON value
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Two-Position Controller Example 1
Figure: Input/Output Relationship for a Two-Position Controller
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Two-Position Controller Example 2
Figure: Two-Position Control System
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Bistable Symbols
• Which of the four bistable symbols is used for the previous slide
example?(opens valve on low level, closes valve on high level)
Figure: Bistable Symbols
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Bistable Example - Explained
• Consider a set of axes for each
bistable being examined
– ON and OFF – for bistable
setting
– Low and High - for parameter
value that is being controlled
• This symbol used since:
– Valve opens on low level
– Valve closes on different high
level
• Another example of this:
– PZR Backup heaters
o Energize on decreasing
RCS pressure
o Deenergize on different
higher RCS pressure
© Copyright 2016 – Rev 2
Level increases to Reset point
ON
Bistable
Setting
Bistable turns
ON to open
makeup valve
Bistable turns
OFF to close
makeup valve
OFF
Level decreases to Setpoint
Low
Setpoint Reset point
High
Parameter
Value
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Bistable Operation
Knowledge Check
Which of the following bistables energize on an increasing signal and
deenergize on a different signal decreasing?
A.
B.
C.
D.
Correct answer is C.
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Bistable Operation
Knowledge Check – NRC Bank
Refer to the drawing of four bistable symbols (see figure below).
A temperature controller uses a bistable that turns on to actuate a
warning light when the controlled temperature reaches a high setpoint.
The bistable turns off to extinguish the warning light when the
temperature decreases to 5°F below the high setpoint.
Which one of the following bistable symbols indicates the
characteristics of the bistable?
A. 1.
B. 2.
C. 3.
D. 4.
Correct answer is D.
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Process Control Terms
ELO 1.3 – Define the following process control related terms: proportional
band, gain, closed loop system, offset, feedback, deviation, deadband,
direct acting, and reverse acting.
Proportional Band (PB)
• Change in value of controlled variable that results in full travel of the
final control element
– Input/Output
Gain
• Ratio of amount of change in final control element to amount of
change in the controlled variable
– Output/Input
• Factor by which magnitude of error signal will be increased
• Gain is reciprocal to proportional band
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Process Control Terms
Closed-Loop System
• System in which the parameter being controlled feeds into the
controller
– Temperature out of letdown heat exchanger, for example
– Most controller loops are “closed-loop” types
Offset
• Deviation that remains after a process has stabilized
• Difference between setpoint and steady-state value of the controlled
parameter
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Process Control Terms
Feedback
• Information on controlled variable sent back to the controller for finer
control
– For example, Turbine governor control valve position might feed
back to EHC
Deviation
• Difference between setpoint and the actual value
• Also called “error”
Deadband
• Range of values around setpoint of measured variable where no
action occurs
– Recall the tank level bistable example
• Prevents oscillation or hunting in proportional control systems
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Process Control Terms
Direct Acting vs Reverse Acting Controller
• Relationship of controller input to controller output
• Must also consider
– Normal operation of system and fail position of valve
Direct Acting
• Controller input increases, controller output increases
– Input goes from 4 to 20 milliamps
– Output to air-operated valve goes from 3 to 15 psi
Reverse Acting
• Controller input increases, controller output decreases
– Input goes from 4 to 20 milliamps
– Output to air-operated valve goes from 15 to 3 psi
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Process Control Terms - Application
• Consider the following controller face images to apply these definitions:
Image 1
Image 2
Input
95oF
70°F
Controller
Face
0%
50 %
95oF
120°F
120°F
Controller
0%
100 %
Gain =
2
Output/Input
100%/(120°F-70°F)
100/50 = 2
Face
50 %
100 %
Output
Output
What is the GAIN?
70°F
What is the GAIN?
Gain =
Still 2
Output/Input
100%/(120°F-70°F)
100/50 = 2
Direct or Reverse Acting? Direct
Direct or Reverse Acting? Reverse
As temperature increases from 70-120 As temperature decreases from 120-70
Output goes from 0-100%
Output goes from 0-100%
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Process Control Terms
Knowledge Check – NRC Bank
The difference between the setpoint in an automatic controller and the
steady-state value of the controlled parameter is called ________.
A. offset
B. gain
C. deadband
D. feedback
Correct answer is A.
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Process Control Terms
Knowledge Check – NRC Bank
An automatic flow controller is being used to position a valve in a
cooling water system. A signal that is proportional to valve position is
received by the controller. This signal is referred to as...
A. gain
B. bias
C. feedback
D. error
Correct answer is C.
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Operation of an Automatic Controller
ELO 1.4 – Describe the operation of an automatic controller, including
proportional control, proportional-integral (PI) control, proportionalderivative (PD) control, and proportional-integral-derivative control (PID).
Mode of Control – manner in which control system makes corrections
relative to deviation
• Mode of control depends on characteristics of process being
controlled
– Some processes can be operated over wide band
– Others must be maintained very close to setpoint
– Some processes change slowly, while others change almost
immediately
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Modes of Automatic Control
• Four modes of automatic control commonly used:
– Proportional
– Proportional-integral (or proportional-plus-reset) [PI]
– Proportional-derivative (or proportional-plus-rate) [PD]
– Proportional-integral-derivative (or proportional-plus-reset-plusrate) [PID]
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Proportional Controller
Proportional Mode
• Referred to as throttling control
• Controller only matches supply to demand
– Parameter stabilizes at new “Control Point”
– Does not bring parameter back to setpoint
Proportional Control Output
• Proportional controller provides linear stepless output that
– positions valve at intermediate positions,
– as well as "full open" or "full shut”
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Proportional Level Controller Example
• Flow of supply water into tank
controlled to maintain tank
level within narrow band
• Components
– Fulcrum and lever
assembly used as
proportional controller
– Float chamber is level
measuring element
– 4 inch stroke valve is final
control element
Figure: Proportional System Controller
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Proportional Level Controller Example
• Proportional band is input band over which controller provides a
proportional output and is defined as follows:
% 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡
𝑃𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛𝑎𝑙 𝐵𝑎𝑛𝑑 =
× 100%
% 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑜𝑢𝑡𝑝𝑢𝑡
• For this example,
– Fulcrum point is such that full 4 inch change in float height causes
full 4 inch stroke of valve
– Proportional Band = 100%
– Gain = 1
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Proportional Level Controller Example
• If fulcrum setting changed:
– 2 inches, or 50% of input,
causes full 4 inch stroke, or
100% of output
o Proportional band would
become 50%
o Gain is 2
– Recall, the “smaller” the
band, the “larger” the gain
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Figure: Proportional System Controller
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Integral (Reset) Control
• Integral Control - controller in which magnitude of output is
dependent on magnitude of input
– Smaller amplitude input causes slower magnitude of output
– Approximates mathematical function of integration
– Also known as reset control
• Major advantage
– controlled variable returns to setpoint following a disturbance
• Two disadvantages are:
– Slow response to error signal
– Initially allows a large deviation, can lead to system instability and
cyclic operation
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Definition of Integral Control
• Device that performs mathematical function of integration is called
integrator
• Mathematical result of integration is called integral
• Not a function of “how far from setpoint”, but “how long from setpoint”
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Integral Output Example
• Integrator acts to transform step
change of input to 10% into
gradually changing signal
• Constant of integrator causes
output to change 0.2% per
second for each 1% of input
• Input amplitude is repeated in
output every 5 seconds
• As long as input remains
constant at 10%, output will
continue to ramp up every 5
seconds until integrator
saturates
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Figure: Integral Controller Output for a Fixed Input
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Integral Flow Control System Example
• Final control element’s position changes at rate determined by
amplitude of input error signal
𝐸𝑟𝑟𝑜𝑟 = 𝑆𝑒𝑡𝑝𝑜𝑖𝑛𝑡 − 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑉𝑎𝑟𝑖𝑎𝑏𝑙𝑒
– Large error causes final control element to change position
rapidly
– Small error causes final control element to change position slowly
• Magnitude of output of controller:
𝑂𝑢𝑡𝑝𝑢𝑡 𝑀𝑎𝑔𝑛𝑖𝑡𝑢𝑑𝑒 = 𝐼𝑛𝑡𝑒𝑔𝑟𝑎𝑙 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × %𝐸𝑟𝑟𝑜𝑟
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Integral Flow Control System Example –
Controller Operation
• Integral controller maintains
constant flow rate
• System setpoint maintains flow
demand of 50 gpm
– Corresponds to control
valve opening of 50%
• When actual flow is 50 gpm,
zero error signal sent to input of
integral controller
• Controller output is initially set
for 50%, or 9 psi, to position 6in control valve to position of 3in open
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Figure: Integral Flow-Rate Controller
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Integral Flow Control System Example –
Controller Operation
• Measured variable decreases
from 50 gpm to 45 gpm ⇒
positive error of 10% applied to
input of controller
– Controller has a constant of
0.1 seconds-1; controller
output magnitude is 1% per
second
– Controller output increases
from initial point of 50% at 1%
per second
Figure: Integral Controller Response
– Causes control valve to open
further at rate of 1% per
second ⇒ increasing flow
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Integral Flow Control System Example –
Controller Operation
• Controller acts to return process
to setpoint
– Repositions control valve
– Measured variable moves
closer to setpoint
– New error signal is
produced
– Cycle repeats until no error
exists
Figure: Integral Controller Response
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Integral Flow Control System Example –
Controller Operation
• Controller responds to
amplitude and duration of error
signal
– Can cause final control
element to reach "fully
open/shut" position before
error reaches zero
– Final control element could
remain at extreme position
– Error must be reduced by
other means
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Figure: Integral Controller Response
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Proportional Integral Control
• Combination of proportional and integral modes of control
• Combining two modes results in gaining advantages and
compensating for disadvantages of two individual modes
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Proportional-Integral Control
• Advantage of proportional
control
– Output produced as soon as
an error signal exists
– Quickly repositions final
control element
– Compensates for
disadvantage of integral
mode, that an integral
controller does not
immediately respond to new
error signal
Figure: Response of PI Control
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Proportional-Integral Control
• Advantage of integral control
mode
– Output repositions final
control element until error
reaches zero
– Eliminates residual offset
– Compensates for
disadvantage of proportional
control that causes a
residual offset error to exist
for most system conditions
Figure: Response of PI Control
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Proportional-Integral Control Example
• Heat exchanger system - equipped with proportional-integral
controller
Figure: Heat Exchanger Process with PI Control
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Proportional-Integral Control Example
• Response curves illustrate
– Heat demand (cold water
flow)
– Measured variable – hot
water outlet temperature
• Process undergoes demand
disturbance
– Reduces flow of hot water
out of heat exchanger
– Temperature and flow rate
of steam into heat
exchanger remain constant
– Temperature of hot water
out begins to rise
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Figure: Effects of Disturbance on a PI Controller
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Proportional-Integral Control Example
• Proportional action response
– Control valve returns hot
water outlet temp to new
control point
– Residual error remains
(offset)
• Adding integral response
– Produces larger output for
given error signal
– Greater adjustment of
control valve
– Quickly returns to setpoint
– Eliminates offset error
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Figure: Effects of Disturbance on a PI Controller
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Reset Windup
• PI controllers that receive a large error signal can undergo reset
windup
– Large sustained error signal causes controller to drive to its limit
to try and restore system control
– System experiences large oscillations as controller restores
controlled variable to setpoint
– Can be caused by large demand deviation or when initially
starting up system
• PI control mode not well-suited for processes that are frequently shut
down and started up due to this effect
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Proportional-Derivative Control Systems
• Control mode in which derivative section is added to proportional
controller
• Derivative section responds to rate of change of error signal, not
amplitude of error
– Causes controller output to be initially larger in direct relation with
error signal rate of change
o Higher error signal rate of change ⇒ sooner final control
element is positioned to desired value
• Added derivative action reduces initial overshoot of measured
variable
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Definition of Derivative Control
• Differentiator – device that
produces derivative signal
• Provides output directly related
to:
– Rate of change of input
– Derivative constant
• Derivative constant defines
differential controller output
– Expressed in units of
seconds
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Figure: Derivative Output for a Constant Rate of Change
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Definition of Derivative Control
• Differentiator transforms
changing signal to constant
magnitude signal
• Derivative control cannot be
used alone as control mode
– Steady-state input produces
zero output in differentiator
• Derivative action typically
combined with proportional
action such that proportional
section output serves as
derivative section input
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Figure: Derivative-Control Output
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Advantage of Derivative Control
• Proportional action provides an
output proportional to error
– If error is changing slowly
(not step change)
proportional action is slow
• Added rate action provides
quick response to error
Figure: Response of PD Control
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Proportional-Derivative Control Example
• Same heat exchanger system as previously analyzed
• Temperature controller now uses PD controller
Figure: Heat Exchanger Process with PD Control
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Proportional-Derivative Control Example
• Proportional only control mode
responds to decrease in
demand
– Residual offset error
remains
• Adding derivative action
– Only one small overshoot
– Rapid stabilization to new
control point
Figure: Effect of Disturbance on a PD Controller
– Does not eliminate offset
error
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Proportional-Derivative Applications
• Leading action of controller output compensates for processes with
lagging characteristics
– Large capacity
– Slow-responding
– For example, temperature control
• Disadvantage is that derivative action responds to any rate of change
in error signal, including noise
– Not typically used fast responding processes such as flow control
or noisy processes
• PD controllers are useful with processes which are frequently started
up and shut down because they are not susceptible to reset windup
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Proportional-Integral-Derivative
• Proportional-integral-derivative (PID) controllers combine all three
control actions
• Gain benefit from all three modes of control
– Proportional – good stability
– Integral – eliminate offset error
– Derivative – good stability
• Used for processes that cannot tolerate continuous cycling or offset
error, and require good stability
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Proportional-Integral-Derivative
Controller Response
• For example, error is due to
slowly increasing measured
variable
– Proportional action
produces output
proportional to error signal
– Integral action produces
output, changing due to
increasing error
– Derivative action produces
output whose magnitude is
determined by rate of
change
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Figure: PID Control Responses
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Proportional-Integral-Derivative
Controller Response
• Response curves are drawn
assuming no corrective action is
taken by control system
• As soon as output of controller
begins to reposition final control
element, magnitude of error
should begin to decrease
• Controller will bring error to zero
and controlled variable back to
setpoint
Figure: PID Control Responses
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PID Controller Response to Demand
Disturbance
• Now assume action is taken
in response to disturbance
– Proportional action of
controller stabilizes
process
– Reset action combined
with proportional action
causes measured variable
to return to setpoint
Figure: PID Controller Response Curves
– Rate action combined with
proportional action
reduces initial overshoot
and cyclic period
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Operation of an Automatic Controller
Knowledge Check
The water level in a tank is being controlled by an automatic level
controller and is initially at the controller setpoint. A drain valve is then
opened, causing tank level to decrease. The decreasing level causes
the controller to begin to open a makeup water supply valve. After a
few minutes, a new steady-state tank level below the original level is
established, with the supply rate equal to the drain rate. The controller
in this system uses __________ control.
A. proportional, integral, and derivative
B. proportional and integral
C. proportional only
D. bistable
Correct answer is C.
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Operation of an Automatic Controller
Knowledge Check – NRC Bank
Refer to the drawing of a lube oil temperature control system (see
figure below).
If the temperature transmitter fails high (high temperature output
signal), the temperature controller will position the temperature control
valve more __________, causing the actual heat exchanger lube oil
outlet temperature to __________.
A. open; decrease
B. open; increase
C. closed; decrease
D. closed; increase
Correct answer is A.
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Operation of an Automatic Controller
Knowledge Check – NRC Bank
Which one of the following describes the response of a direct acting
proportional-integral controller, operating in automatic mode, to an increase in
the controlled parameter above the controller set point?
A. The controller will develop an output signal that continues to increase
until the controlled parameter equals the controller set point, at which
time the output signal stops increasing.
B. The controller will develop an output signal that will remain directly
proportional to the difference between the controlled parameter and the
controller set point.
C. The controller will develop an output signal that continues to increase
until the controlled parameter equals the controller set point, at which
time the output signal becomes zero.
D. The controller will develop an output signal that will remain directly
proportional to the rate of change of the controlled parameter.
Correct answer is A.
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Automatic and Manual Controller
Operation
ELO 1.5– Describe the operation of a controller in automatic and manual
modes.
• Typical controller
– Many popular controller
types found in industrial
applications
o Extremely versatile
– Can be adapted to control
various types of industrial
equipment and processes
o Pressure, temperature,
valve position, etc.
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Controller Operation
• Operated in either automatic
or manual mode
• Mode depends on:
– complexity of process
being controlled
– specific operational
requirements
Figure: Typical Digital Controller
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Controller Operation
• Pulser knob
• Display pushbutton
• Alphanumeric display
• Auto/manual pushbutton
Figure: Typical Digital Controller
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Automatic Operation
• Controller reacts to control a particular process parameter based on
setpoint
– Automatically responds to any deviation from setpoint
– Adjusts output in order to adjust control element and return
controlled parameter to setpoint
• Adjustment can be made to setpoint
– Operator adjusts setpoint using pulser knob
– Will continue to respond automatically to new setpoint
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Manual Operation
• Controller does not attempt to maintain its programmed setpoint
– Maintains constant output to its control element regardless of
changes in controlled parameter
– Proportional, Integral, and/or Derivative features ALL removed
• Pulser knob must be adjusted by operator in order to change output
of controller
– Requires constant attention by operator
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Controller Transfer Operation
• When transferring control from automatic to manual:
– Normally manual tracks automatic
o Usually no perturbation shifting to Manual
– During instrument failures
o Manual removes failed instrument
o Control of parameter is regained with pulser knob
• When transferring control from Manual to Automatic
– Ensure alternate instrument working correctly
– Ensure parameter back at normal value
– Place controller in Automatic
– Verify no abnormal system perturbation (bumpless transfer)
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System Response to Controller Inputs
A decreasing SG water level will:
• Increase SG level control signal
• Raise control air pressure
• Causing feed control valve to open further
Figure: Pneumatic Control System - PWR
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System Response Practice Question
Knowledge Check
If personnel manually decrease the level control signal, how will the
pneumatic control system affect SG level
A. Level will decrease because the valve positioner will close more,
reducing control air pressure, which causes the feed control valve
to close more.
B. Level will decrease because the valve positioner will open more,
increasing control air pressure, which causes the feed control valve
to close more.
C. Level will increase because the valve positioner will close more,
reducing control air pressure, which causes the feed control valve
to open more.
D. Level will increase because the valve positioner will open more,
increasing control air pressure, which causes the feed control valve
to open more.
Correct answer is A.
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Automatic and Manual Controller
Operation
Knowledge Check – NRC Bank
A flow controller has proportional, integral, and derivative control
features. Which one of the following lists the effect on the control
features when the controller is switched from the automatic mode to the
manual mode?
A. Only the derivative feature will be lost.
B. Only the integral and derivative features will be lost.
C. All proportional, integral, and derivative features will be lost.
D. All control features will continue to influence the controller
output.
Correct answer is C.
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Temperature and Pressure Controller
Operation
ELO 1.6 – Describe the operation of temperature controllers and
pressure controllers.
• Both types function in the same manner:
– Takes the parameter variable from the sensor
– Compares it to a setpoint
– Develops an error signal
– Sends error signal to final control element
o Only part that might differ is what gets operated as a result
– Might compare feedback from final control element to sensor
input
o To adjust error signal
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Proportional Temperature Control
Figure: Proportional Temperature-Control System
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Controller Response to Demand Changes
• Purpose of system is to provide hot water at setpoint of 75ºF
• System must handle demand disturbances that affect outlet
temperature
• Controller set up to function as shown in figure
Figure: Proportional-Controller Characteristics
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Controller Response to Demand Changes
• If measured variable drops below setpoint
– Positive error is developed
– Control valve opens further
Figure: Proportional-Controller Characteristics
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Controller Response to Demand Changes
• If measured variable goes above setpoint
– Negative error developed
– Control valve throttles down (opening is reduced)
Figure: Proportional-Controller Characteristics
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Controller Response to Demand Changes
• 50% proportional band causes full stroke of valve between a +25ºF
error and a -25ºF error
Figure: Proportional-Controller Characteristics
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Controller Response to Demand Changes
• When error is zero, controller provides 50% (9 psi) signal to control
valve
• As error changes, controller produces an output proportional to
magnitude of error
• Control valve compensates for demand disturbances that cause
process to deviate from setpoint in either direction
Figure: Proportional-Controller Characteristics
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Temperature and Pressure Controller
Operation
Knowledge Check
Refer to the drawing of a lube oil temperature control system (see
figure below). The temperature control valve is currently 50 percent
open.
If the cooling water inlet temperature decreases, the temperature
controller will position the temperature control valve more __________,
causing cooling water differential temperature through the heat
exchanger to __________.
A. closed; increase
B. closed; decrease
C. open; increase
D. open; decrease
Correct answer is A.
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Operation of a Speed Controller
ELO 1.7 – Describe the operation of mechanical and electronic speed
control devices.
• Senses speed of component and governs speed
• Speed could be controlled by a throttle such as in a diesel governor
• Servomotor may be used to operate throttles
• Speed can be sensed mechanically, electrically, or a combination of
both
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Speed Controllers/Governors
Mechanical Speed
• Senses speed on rotating element such as diesel or turbine shaft
– Attach flyweights to the shaft
– As shaft rotates, rotational force causes the weights to extend
radially outward
– Force is proportional to the square of rotational speed
• Provides trouble free speed sensing
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Speed Controllers/Governors
• Force balanced by compression
of the spring
• Ballhead rotates with the shaft
• Flyweights move out radially
away from the shaft due to the
rotation
• Flyweight arms in contact with a
non-rotating speeder rod
• Speeder rod is free to move
axially along the shaft
• Transmits radial movement of
flyweights into axial movement
of speeder rod
© Copyright 2016 – Rev 2
Figure: Mechanical Speed Sensor
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Speed Controllers/Governors
• Governors can be used to directly sense speed and adjust the
supplied fuel
– In a diesel generator the speed controls the generator output
frequency
• Speed used to generate an electronic signal to a hydraulic actuator
• Hydraulic actuator generates a corresponding hydraulic signal to
move the fuel racks
– Hydraulics are generally shaft driven by the engine
• Movement of speeder rod can be used to control a fuel mechanism
• Governors can be extremely complex with several modes of control
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Simple Mechanical Governor
For example, load on a diesel
engine is increased
• Speed decreases
• Flyweights move inward
• Speeder rod lowers
• Directs more fuel to the
engine
Figure: Mechanical Governor
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Speed Controllers/Governors
Electronic Speed
• Teeth attached to rotating shaft rotate through a magnetic field of a
permanent magnet
– Electrical pulse is induced in a pickup coil
– Electrical signal compared to desired speed
– Throttles adjust supplied steam accordingly
• Used to control speed of steam turbine
– Turbine may have an additional wheel with 60 teeth on the turbine
shaft
• Overspeed trip mechanism may be similar to the speed sensor
– Mechanical arrangement provides a reliable method to protect
equipment
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Speed Controllers/Governors
Example: Electrical signal from a steam turbine governor failed low
• Speed control governor continues to open
• Turbine throttles to raise speed
– As the turbine speed increases,
• Electronic signal feeds the new speed back to the governor and
throttle position adjusts as necessary
• Electric speed indication is low no matter what the actual turbine
speed is so the governor will keep trying to open the throttles
• Turbine speed would increase until mechanical overspeed trip point
is reached shutting the throttles
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Droop Mode vs Isochronous Mode
• The type of speed control utilized by the diesel generator (D/G) varies
– Droop Mode
o Used when D/G is started and paralleled to bus for testing
– If in Droop Mode on an isolated bus, load changes would effect
speed
o Maintains D/G speed at speed changer setting
– backed up by mechanical governor
o % Speed Droop = No Load - Full Load Speed / No Load
– Isochronous Mode (Emergency Mode)
o Used when D/G is ONLY source to AC Vital Bus
o Controller returns D/G to speed setpoint for 60 Hz for any change
in load
– Loads sequenced on to minimize impact on D/G
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Operation of a Speed Controller
Knowledge Check – NRC Bank
An emergency diesel generator (D/G) is operating as the only power
source connected to an emergency bus. The governor of the D/G is
directly sensing D/G __________ and will directly adjust D/G
__________ flow to maintain a relatively constant D/G frequency.
A. speed; air
B. speed; fuel
C. load; air
D. load; fuel
Correct answer is B.
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Operation of a Speed Controller
Knowledge Check – NRC Bank
In a flyball-weight mechanical speed governor, the purpose of the
spring on the flyball mechanism is to ____________ centrifugal force
by driving the flyballs ___________.
A. counteract; apart
B. aid; together
C. counteract; together
D. aid; apart
Correct answer is C.
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Operation of a Speed Controller
Knowledge Check – NRC Bank
A diesel generator (DG) is supplying an isolated electrical bus with the
DG governor operating in the speed droop mode. Assuming the DG
does not trip, if a large electrical bus load trips, bus frequency will
initially...
A. increase, then decrease and stabilize below the initial value.
B. increase, then decrease and stabilize above the initial value.
C. decrease, then increase and stabilize below the initial value.
D. decrease, then increase and stabilize above the initial value.
Correct answer is B.
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Interpret Logic Diagrams
ELO 1.8 – Interpret logic diagrams and determine controller outputs.
• Logic symbols allow user to determine the operation of a component
or system as the input signals change
• Reader must understand each of the specialized symbols
• Commonly see logic symbols on equipment diagrams
• Three basic types of logic gates:
– AND
– OR
– NOT
• Each gate is a very simple device that only has two states, on and
off.
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Logic Symbols
Symbol/
ANSI
Symbol/
GE
Name
Function
AND
gate
Provides an output (on), when all of its
inputs are on. If any of the inputs is
off, the gate's output will be off.
OR gate
Provides an output (on), when any of
its inputs is on. If all of the inputs are
off, the output will be off.
NOT gate
Provides a reversal of the input. If the
input is on, the output will be off; if
the input is off, the output will be on.
NAND
gate
Provides an output (on), except when
all of the inputs are on. The opposite
of an AND gate's output.
NOR gate Provides an output (on), except when
all of its inputs are off. The opposite
of an OR gate's output.
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Logic Diagram Example
• Refer to the valve controller logic diagram in the figure. Which one of
the following combinations of inputs will result in the valve receiving
an open signal?
• Answer Discussion – Place “1” or “0” working backwards to the
inputs.
• The only combination where it is not met is in D because 1 is off and
3 is off.
One or both of
Inputs
1.
2.
A. On
Off Off
B. Off
On On
C. On
Off On
D. Off
On Off
© Copyright 2016 – Rev 2
Inputs 2 and 3 must
be 0.
3.
0
0
0
1
1
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Operator Generic Fundamentals
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Logic Diagrams
Knowledge Check
Refer to the valve controller logic diagram (see figure below).
Which one of the following combinations of inputs will result in the valve
receiving a CLOSE signal?
INPUTS
1.
2.
3.
4.
A.
On
On
Off
Off
B.
Off
Off
On
Off
C.
On
Off
Off
On
D.
On
On
On
Off
Correct answer is B.
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Types of Valve Actuators
ELO 1.9 – Describe the design and operation of the following types of
valve actuators: pneumatic, hydraulic, solenoid, and electric motor.
• Valves can require remote operation when they
– Are large in size
– Require quick operation
– Located in hazardous areas
• Four types of actuators used for remote operation are:
– Pneumatic
– Hydraulic
– Solenoid
– Electric motor
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Pneumatic Valve Actuator
Figure: Pneumatic-Actuated Control Valve
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Pneumatic Valve Actuator
• Initially, with no supply air,
– Spring forces diaphragm
upward
– Holds valve fully open
• Supply air pressure
increases
– Air pressure forces
diaphragm downward
– Closes control valve
• Supply air pressure
decreases
– Force of spring forces
diaphragm upwards
– Opens control valve
• Valve can be held at
intermediate position
© Copyright 2016 – Rev 2
Figure: Pneumatic-Actuated Control Valve
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Operator Generic Fundamentals
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Actuator Failure Position
Figure: Pneumatic Actuator with Controller and Positioner
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Valve Positioner
• Purpose
– Converts the 3-15 psi control air pressure to a higher supply air
pressure to move the valve actuator
o Supply air is usually from Service Air or Instrument Air
• Valve Positioner for AOV usually consists of three (3) gages
– Control air pressure (from I/P)
o 3-15 psi
– Supply air pressure available
o Usually > 100 psi
– Supply air pressure to actuator
o Varies
© Copyright 2016 – Rev 2
Figure: Pneumatic Actuator with Controller and Positioner
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Operator Generic Fundamentals
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Valve Positioner System Example
• Previous SG Water Level Control drawing
– Control air signal (3-15 psi) operates a pilot valve
– Regulates more or less Supply Air to valve actuator to FRV
Figure: SGWLC Valve Positioner Example
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Valve Positioner
Knowledge Check – NRC Bank
The purpose of the valve positioner is to convert...
A. a small control air pressure into a proportionally larger air
pressure to adjust valve position.
B. a large control air pressure into a proportionally smaller air
pressure to adjust valve position.
C. pneumatic force into mechanical force to adjust valve position.
D. mechanical force into pneumatic force to adjust valve position.
Correct answer is A.
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Hydraulic Actuators
• Operation of hydraulic actuator like pneumatic actuator
• Each uses motive force to overcome spring force to move valve
• Normally used if:
– Large amount of force is required to operate a valve
o for example, large steam system valves
• Piston type most common
• Can also be designed to fail-open or fail-closed to provide a fail-safe
feature
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Hydraulic Actuator Design
• Typical piston-type hydraulic
actuator consists of:
– Cylinder
– Piston: slides vertically
inside separates cylinder
into two chambers
– Spring: contained in upper
chamber of cylinder
– Hydraulic fluid, supply and
return line: contained in
lower chamber
– Stem: transmits motion from
piston to valve
© Copyright 2016 – Rev 2
Figure: Piston-Type Hydraulic Actuated Control Valve
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Hydraulic Actuator Design
• Initially, with no supply air,
– Spring forces piston
upward
– Holds valve fully open
• Hydraulic fluid pressure
increases
– Fluid pressure forces
piston downward
– Closes control valve
• Hydraulic fluid pressure
decreases
– Force of spring forces
piston upwards
– Opens control valve
• Valve can be held at
intermediate position
© Copyright 2016 – Rev 2
Figure: Piston-Type Hydraulic Actuator
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Electric Solenoid Actuators
• A typical electric solenoid
actuator consists of:
– Coil: Provides upward force
– Armature: Transmits force
from coil to vertical motion
– Spring: Applies downward
force
– Stem: Transmits force
motion from armature to
valve
© Copyright 2016 – Rev 2
Figure: Electric Solenoid Actuator
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Solenoid Actuator Advantages &
Disadvantages
Advantages
• Quick operation
• Easier to install than pneumatic or hydraulic actuators
Disadvantages
• Only two positions: fully open and fully closed
• Don’t produce much force ⇒ usually only operate relatively small
valves
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Electric Motor Actuators
• Some motor actuators are designed to operate in only two positions
– fully open or fully closed
• Other electric motor actuators can be positioned in intermediate
positions
Figure: Motor Actuator
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Electric Motor Actuator Design &
Operation
• Motor moves stem through gear assembly
• Motor reverses its rotation to either open or close valve
• Clutch and clutch lever disconnects electric motor from gear
assembly
– allows valve to be operated manually with handwheel
Figure: Motor Actuator
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Electric Motor Actuator Design &
Operation
• Most are equipped with limit switches and/or torque limiters
– Usually limit switch stops motor when opening
– Usually torque limiter stops motor when closing
Figure: Motor Actuator
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Types of Valve Actuators
Knowledge Check
An air-operated isolation valve requires 2,400 pounds-force applied to
the top of the actuator diaphragm to open. The actuator diaphragm has
a diameter of 12 inches.
If control air pressure to the valve actuator begins to increase from 0
psig, which one of the following is the approximate air pressure at
which the valve will begin to open?
A. 21 psig
B. 34 psig
C. 43 psig
D. 64 psig
Correct answer is A.
© Copyright 2016 – Rev 2
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Operator Generic Fundamentals
NRC KA to ELO Tie
KA #
KA Statement
RO SRO
ELO
K1.01 Function and operation of flow controller in manual and automatic modes
3.1
3.2
1.4
K1.02 Function and operation of a speed controller
2.6
2.7
1.7
K1.03 Operation of valves controllers in manual and automatic mode
Function and operation of pressure and temperature controllers, including pressure and
K1.04 temperature control valves
3.1
3.1
1.5
2.8
3.0
1.6
K1.05 Function and characteristics of valve positioners
2.5
2.8 1.1, 1.9
K1.06 Function and characteristics of governors and other mechanical controllers
2.3
2.6
1.7
K1.07 Safety precautions with respect to the operation of controllers and positioners
2.3
2.6
1.5
K1.08 Theory of operation of the following types of controllers: electronic, electrical, and pneumatic 2.1
Effects on operation of controllers due to proportional, integral (reset), derivative (rate), as
K1.09 well as their combinations
2.4
2.6 1.8, 1.9
2.5
1.4
K1.10 Function and characteristics of air-operated valves, including failure modes
2.4
2.8
1.9
K1.11 Cautions for placing a valve controller in manual mode
2.8
2.9
1.5
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Operator Generic Fundamentals