Top 50 Process Loop Components (PLC) Objective Questions and Answers
Top 50 Process Loop Components (PLC) Objective Questions and Answers
1. A basic process control loop will consist of ________a) Signal converter, resistor, knob, and control valve
b) Transducer, valve packing, hex-head wrench, and tubing
c) Transmitter, controller, I/P converter, and control valve
d) Resistor, capacitor, terminal block, and battery
Answer: Transmitter, controller, I/P converter, and control valve
2. In a home heating system, room temperature is________
A) Setpoint
B) Manipulated variable
C) Process variable
D) Load Variable
Answer: Process variable
3. Which is/are the component/s are needed for a 4 mA to 20 mA signal?
A) A power supply
B) Wire
C) A transmitter
D) All of the above
Answer: All of the above*
4. Identify the proper sequence of valve actions for a three-valve manifold and bleed when taking a differential pressure transmitter out of service:
A) Close one block valve, open equalizing valve, close other block valve, open bleed
B) Open bleed, close one block valve, open equalizing valve, close other block valve
C) Open equalizing valve, close both block valves (simultaneously), open bleed
D) Open equalizing valve, open bleed, close both block valves (simultaneously)
Answer: Close one block valve, open equalizing valve, close other block valve, open bleed
5. A d/p cell used to measure liquid level has a “suppressed zero.” This means:
A) The transmitter is located above the 0% liquid level mark
B) The transmitter uses filter circuits to suppress noise to a zero level
C) The liquid is less dense than water
D) The transmitter is located below the 0% liquid level mark
Answer: The transmitter is located below the 0% liquid level mark
6. In the four wire transmitter ________
A) Separate power wires are not required in the installation
B) Power wires are not required in the installation
C) Four wires are not required in the installation
D) Separate power wires are required in the installation
Answer: Separate power wires are required in the installation
7. In a direct acting controller, the output increases when the process measurement (variable)________
A) Increases
B) Decreases
C) Slowly decreses
D) Remains unchanged
7. In a direct acting controller, the output increases when the process measurement (variable)________
A) Increases
B) Decreases
C) Slowly decreses
D) Remains unchanged
Answer: Increases
8. Which of the following controllers has maximum offset ?
A) P-controller
B) P-I controller
C) P-I-D controller
D) P-D controller
Answer: P-controller
9. PID controller is used when system requires________
A) System changes are small
B) Offset must be eliminated
C) Fast recovery time
D) All above
Answer: All above
10. If the proportional band of the controller is adjusted to the minimum possible value, the control action is likely to be________
A) On/off
B) With maximum offset
C) Excellent
D) Inoperative
Answer: On/off
11. A two-position controller compares conditions to a set point and:
A) A controller output signal
B) Starts or stops a process
C) Modulates the final control element
D) Automatically readjusts the set point
Answer: Starts or stops a process
12. If one is to use Ziegler-Nichols method for tuning and Ku = 4.5 and Pu=2.8 then the values of PID elements are
A) Kp = 2.7, Ï„i = 1.4, Ï„d = 0.35
B) Kp = 1.4, Ï„i = 1.4, Ï„d = 0.93
C) Kp = 2.1, Ï„i = 1.4, Ï„d = 0.5
D) Kp = 0.9, Ï„i = 1.4, Ï„d = 0.83
Answer: Kp = 2.7, Ï„i = 1.4, Ï„d = 0.35
13. An automatic toaster is a ______ loop control system.
A) Open
B) Closed
C) Partially Closed
D) Partially Opend
Answer: Open
14. In a PID controller, the overshoots has increased. The derivative time constant has to be __ so as to reduce the overshoots:
A) Reduced to zero
B) Increased
C) Reduced
D) None of the above
Answer: Increased
15. What is P to I converter in process control?
A) Analog to Digital Convertor
B) Electronic to Pneumatic Convertor
C) Pneumatic to Current Convertor
D) Pneumatic Transmitter
Answer: Electronic to Pneumatic Convertor*
16. The device which converts 4-20mA signal to 3-15 psi standard process signal is called__
A) Electronic to Pneumatic Convertor
B) Analog to Digital Convertor
C) Pneumatic to Current Convertor
D) Pneumatic Transmitter
Answer: Electronic to Pneumatic Convertor*
17. The Ziegler-Nichols method:
A) A graphical solution method
B) Can be used to determine gain factors of a PI loop
C) Is a method of control
D) Both A and B
Answer: Both A and B
18. A condition where integral control action drives the output of a controller into saturation is called:
A) A wind up
B) Repeat
C) Self-bias
D) Noise
Answer: A wind up
19. The quarter-amplitude decay ratio is basically a design criteria specified by Zeigler-Nichols method implies that the amplitude of an oscillation must be reduced by a factor of __________
A) Four over a whole period
B) Four over an half period
C) Four over a quarter period
D) Four over a quarter and a half period
Answer: Four over a whole period
20. Digital control systems are generally superior to analog control systems, but they are always worse with regard to:
A) Diagnostics
B) Speed
C) Noise immunity
D) User-friendliness
Answer: Speed
21. How is the noise immunity of PLC to electrical noises as compared to that of conventional relay controllers?
A) Poor
B) Excellent
C) As good as noise immunity of conventional relay controllers
D) Unpredicatable
Answer: Poor
22. In a counter instruction if the accumulator value becomes greater than or equal to the preset value then which of the following is true?
A) CU bit goes on
B) DN bit goes on
C) Both CU and DN bits go on
D) None of the above
Answer: DN bit goes on
23. Solenoids, lamps, motors are connected to:
A) Digital output
B) Analog output
C) Digital input
D) Analog input
Answer: Digital output
24. The address T4:6.ACC is used to address what?
A) Accumulator for timer 4 in file 6
B) Preset value of timer 6 in file 4
C) Accumulator for timer 6 in file 4
D) Accumulator for counter 6 in file 4
Answer: Accumulator for timer 6 in file 4
25. The term reset control refers to:
a) Proportional
b) Integral
c) Derivative
d) None of the above
Answer: Integral
26. In a PLC, the scan time refers to the amount of time in which ________
A) The technician enters the program
B) Timers and counters are indexed by
C) One “rung” of ladder logic takes to complete
D) The entire program takes to execute
Answer: The entire program takes to execute
27. Which one of the following is not a PLC manufacturer.
A) Microsoft
B) Mitsubishi
C) Siemens
D) ABB
Answer: Microsoft
28. The OR function implemented in ladder logic uses________
A) Normally closed contacts in series
B) Normally open contacts in series
C) Normally closed contacts in parallel
D) Normally open contacts in parallel
Answer: Normally open contacts in parallel
29. The________instruction is used to turn an output ON or OFF after its time has been on for a preset time interval________
A) Retentive time
B) Timer and Delay
C) Timer On Dealy
D) Timer Off delay
Answer: Timer Off delay*
30. For PLC sca'sn cycle which one is correct?
A) Program Execute-Output write- Input Read
B) Input Read-Output write- Program Execute
C) Output write-Input Read-Program Execute
D) Input Read-Program Execute-Output write
Answer: Input Read-Program Execute-Output write
31. The three most common types of flow characteristics are:
A) Linear, Graphical, Random
B) Equal Percentage, Un-equal percentage, Quick opening
C) Non-linear, Equal percentage, Fast opening
D) Linear, Equal percentage, Quick opening
Answer: Linear, Equal percentage, Quick opening
32. In a PID controller, the overshoots has increased. The derivative time constant has to be __ so as to reduce the overshoots:
a) Increased
b) Reduced
c) Reduced to zero
d) None of the above
Answer: Increased
33. When a valve is installed in a process with where the differential pressure (drop) across the valve decreases with increasing flow, the best trim characteristic to choose for the valve would be:
A) Ported
B) Quick-opening
C) Equal percentage
D) Linear
Answer: Equal percentage
34. Calculate the necessary Cv rating for a liquid service valve, given a pressure drop of 24 PSID, a specific gravity of 1.3, and a maximum flow rate of 140 GPM. Assume there will be no flashing or choked flow through the valve.
A) 0.1319
B) 32.58
C) 7.583
D) 2585
Answer: 32.58
35. A typical check valve is designed to________
A) Permit flow in only one directio
B) Isolate system components
C) Prevent system overpressure
D) Perform automatic pump venting
Answer: Permit flow in only one directio
36. Proportional band of the controller is expressed as:
a) Gain
b) Ratio
c) Percentage
d) Range of control variables
Answer: Percentage
37. Which among the following are not the main selection criteria of the control valves?
A) Type of actuation
B) Environmental conditions
C) Space requirement
D) Software support
Answer: Software support
38. A pneumatically actuated control valve which opens on air failure has a:
A) Direct acting actuator and direct acting valve.
B) Direct acting actuator and reverse acting valve.
C) Reverse acting actuator and direct acting valve.
D) Is typically used for a burner firing rate control valve
Answer: Direct acting actuator and direct acting valve
39. A globe valve is the most suitable for applications, in which________
A) Fluid flow control is required
B) Fluid contains dispersed solid particles
C) Valve is required to be either fully open or fully closed
D) One way flow is required
Answer: Fluid flow control is required
40. A control valve that has one plug that moves upward to close the port is called:
A) Reverse acting single ported.
B) Double seated direct acting.
C) Reverse acting diaphragm actuated.
D) Direct acting single ported.
Answer: Reverse acting single ported.
41. What is the rate of flow of water in Litres per second from a cylindrical valve if the cylindrical valve has a diameter of 10 cm and velocity of water is 12m/s?
a) 100L/s
b) 127L/s
c) 157L/s
d) 94.2L/s
Answer: 94.2L/s
42. Assume that a control valve regulates stream flow to a process and that high temperature makes the reaction hazardous. The usual pneumatically operated control valve utilizes the following action for fail-safe operation
A) Air to open
B) Air to close
C) 3 psi (20 kpa) to fully open
D) 15 psi (100 kpa) to fully close
Answer: Air to open
43. A high- pressure flow process requires a valve with tight packing. This would suggest that :
A) A valve positioner should be employed
B) The actuator must be sized to provide adequate force
C) Oversized pneumatic signals lines are required
D) The controller supplying the signal to the valve must have a very narrow proportional band.
Answer: The actuator must be sized to provide adequate force
44. A fluid flows through a linear valve at a rate of 15 m3/h when the valve is 15 % open. When the valve opens to 30 %, the flow rate increases to 30 m3/h. What is the flow rate in m3/h when the valve opens to 45 %? The pressure drop across the valve remains constant.
A) 15
B) 30
C) 45
D) 60
Answer: 45
45. Due to cavitation, shock waves created are________
A) With zero power
B) Less powerful
C) More powerful
D) None
Answer: More powerful
46. Cavitation causes________
A) Erosion
B) Corrosion
C) Both corrosion and erosion
D) None
Answer: Erosion
47. Diaphragm valves are used for handling ________fluids.
A) Corrosive
B) Viscous
C) Non-Newtonion
D) Solid suspended
Answer: Corrosive
48. The material designation of a valve:
A) Gives where the material is used
B) Tells what material makes up the valve
C) Gives the temperature to which the valve can be subjected
D) Tell what the material is suited for
Answer: Tells what material makes up the valve
49. Rangeability of a control valve is ratio of ________
A) Maximum flow to Maximum flow
B) Minimum flow to Maximum flow
C) Maximum flow to minimum flow
D) Minimum flow to Minimum flow
Answer: Maximum flow to minimum flow*
50. Control valve sizing is required to avoid ________
A) Oversizing
B) Undersizing
C) Maximum cost
D) All of the above
Answer: All of the above*
8. Which of the following controllers has maximum offset ?
A) P-controller
B) P-I controller
C) P-I-D controller
D) P-D controller
Answer: P-controller
9. PID controller is used when system requires________
A) System changes are small
B) Offset must be eliminated
C) Fast recovery time
D) All above
Answer: All above
10. If the proportional band of the controller is adjusted to the minimum possible value, the control action is likely to be________
A) On/off
B) With maximum offset
C) Excellent
D) Inoperative
Answer: On/off
11. A two-position controller compares conditions to a set point and:
A) A controller output signal
B) Starts or stops a process
C) Modulates the final control element
D) Automatically readjusts the set point
Answer: Starts or stops a process
12. If one is to use Ziegler-Nichols method for tuning and Ku = 4.5 and Pu=2.8 then the values of PID elements are
A) Kp = 2.7, Ï„i = 1.4, Ï„d = 0.35
B) Kp = 1.4, Ï„i = 1.4, Ï„d = 0.93
C) Kp = 2.1, Ï„i = 1.4, Ï„d = 0.5
D) Kp = 0.9, Ï„i = 1.4, Ï„d = 0.83
Answer: Kp = 2.7, Ï„i = 1.4, Ï„d = 0.35
13. An automatic toaster is a ______ loop control system.
A) Open
B) Closed
C) Partially Closed
D) Partially Opend
Answer: Open
14. In a PID controller, the overshoots has increased. The derivative time constant has to be __ so as to reduce the overshoots:
A) Reduced to zero
B) Increased
C) Reduced
D) None of the above
Answer: Increased
15. What is P to I converter in process control?
A) Analog to Digital Convertor
B) Electronic to Pneumatic Convertor
C) Pneumatic to Current Convertor
D) Pneumatic Transmitter
Answer: Electronic to Pneumatic Convertor*
16. The device which converts 4-20mA signal to 3-15 psi standard process signal is called__
A) Electronic to Pneumatic Convertor
B) Analog to Digital Convertor
C) Pneumatic to Current Convertor
D) Pneumatic Transmitter
Answer: Electronic to Pneumatic Convertor*
17. The Ziegler-Nichols method:
A) A graphical solution method
B) Can be used to determine gain factors of a PI loop
C) Is a method of control
D) Both A and B
Answer: Both A and B
18. A condition where integral control action drives the output of a controller into saturation is called:
A) A wind up
B) Repeat
C) Self-bias
D) Noise
Answer: A wind up
19. The quarter-amplitude decay ratio is basically a design criteria specified by Zeigler-Nichols method implies that the amplitude of an oscillation must be reduced by a factor of __________
A) Four over a whole period
B) Four over an half period
C) Four over a quarter period
D) Four over a quarter and a half period
Answer: Four over a whole period
20. Digital control systems are generally superior to analog control systems, but they are always worse with regard to:
A) Diagnostics
B) Speed
C) Noise immunity
D) User-friendliness
Answer: Speed
21. How is the noise immunity of PLC to electrical noises as compared to that of conventional relay controllers?
A) Poor
B) Excellent
C) As good as noise immunity of conventional relay controllers
D) Unpredicatable
Answer: Poor
22. In a counter instruction if the accumulator value becomes greater than or equal to the preset value then which of the following is true?
A) CU bit goes on
B) DN bit goes on
C) Both CU and DN bits go on
D) None of the above
Answer: DN bit goes on
23. Solenoids, lamps, motors are connected to:
A) Digital output
B) Analog output
C) Digital input
D) Analog input
Answer: Digital output
24. The address T4:6.ACC is used to address what?
A) Accumulator for timer 4 in file 6
B) Preset value of timer 6 in file 4
C) Accumulator for timer 6 in file 4
D) Accumulator for counter 6 in file 4
Answer: Accumulator for timer 6 in file 4
25. The term reset control refers to:
a) Proportional
b) Integral
c) Derivative
d) None of the above
Answer: Integral
26. In a PLC, the scan time refers to the amount of time in which ________
A) The technician enters the program
B) Timers and counters are indexed by
C) One “rung” of ladder logic takes to complete
D) The entire program takes to execute
Answer: The entire program takes to execute
27. Which one of the following is not a PLC manufacturer.
A) Microsoft
B) Mitsubishi
C) Siemens
D) ABB
Answer: Microsoft
28. The OR function implemented in ladder logic uses________
A) Normally closed contacts in series
B) Normally open contacts in series
C) Normally closed contacts in parallel
D) Normally open contacts in parallel
Answer: Normally open contacts in parallel
29. The________instruction is used to turn an output ON or OFF after its time has been on for a preset time interval________
A) Retentive time
B) Timer and Delay
C) Timer On Dealy
D) Timer Off delay
Answer: Timer Off delay*
30. For PLC sca'sn cycle which one is correct?
A) Program Execute-Output write- Input Read
B) Input Read-Output write- Program Execute
C) Output write-Input Read-Program Execute
D) Input Read-Program Execute-Output write
Answer: Input Read-Program Execute-Output write
31. The three most common types of flow characteristics are:
A) Linear, Graphical, Random
B) Equal Percentage, Un-equal percentage, Quick opening
C) Non-linear, Equal percentage, Fast opening
D) Linear, Equal percentage, Quick opening
Answer: Linear, Equal percentage, Quick opening
32. In a PID controller, the overshoots has increased. The derivative time constant has to be __ so as to reduce the overshoots:
a) Increased
b) Reduced
c) Reduced to zero
d) None of the above
Answer: Increased
33. When a valve is installed in a process with where the differential pressure (drop) across the valve decreases with increasing flow, the best trim characteristic to choose for the valve would be:
A) Ported
B) Quick-opening
C) Equal percentage
D) Linear
Answer: Equal percentage
34. Calculate the necessary Cv rating for a liquid service valve, given a pressure drop of 24 PSID, a specific gravity of 1.3, and a maximum flow rate of 140 GPM. Assume there will be no flashing or choked flow through the valve.
A) 0.1319
B) 32.58
C) 7.583
D) 2585
Answer: 32.58
35. A typical check valve is designed to________
A) Permit flow in only one directio
B) Isolate system components
C) Prevent system overpressure
D) Perform automatic pump venting
Answer: Permit flow in only one directio
36. Proportional band of the controller is expressed as:
a) Gain
b) Ratio
c) Percentage
d) Range of control variables
Answer: Percentage
37. Which among the following are not the main selection criteria of the control valves?
A) Type of actuation
B) Environmental conditions
C) Space requirement
D) Software support
Answer: Software support
38. A pneumatically actuated control valve which opens on air failure has a:
A) Direct acting actuator and direct acting valve.
B) Direct acting actuator and reverse acting valve.
C) Reverse acting actuator and direct acting valve.
D) Is typically used for a burner firing rate control valve
Answer: Direct acting actuator and direct acting valve
39. A globe valve is the most suitable for applications, in which________
A) Fluid flow control is required
B) Fluid contains dispersed solid particles
C) Valve is required to be either fully open or fully closed
D) One way flow is required
Answer: Fluid flow control is required
40. A control valve that has one plug that moves upward to close the port is called:
A) Reverse acting single ported.
B) Double seated direct acting.
C) Reverse acting diaphragm actuated.
D) Direct acting single ported.
Answer: Reverse acting single ported.
41. What is the rate of flow of water in Litres per second from a cylindrical valve if the cylindrical valve has a diameter of 10 cm and velocity of water is 12m/s?
a) 100L/s
b) 127L/s
c) 157L/s
d) 94.2L/s
Answer: 94.2L/s
42. Assume that a control valve regulates stream flow to a process and that high temperature makes the reaction hazardous. The usual pneumatically operated control valve utilizes the following action for fail-safe operation
A) Air to open
B) Air to close
C) 3 psi (20 kpa) to fully open
D) 15 psi (100 kpa) to fully close
Answer: Air to open
43. A high- pressure flow process requires a valve with tight packing. This would suggest that :
A) A valve positioner should be employed
B) The actuator must be sized to provide adequate force
C) Oversized pneumatic signals lines are required
D) The controller supplying the signal to the valve must have a very narrow proportional band.
Answer: The actuator must be sized to provide adequate force
44. A fluid flows through a linear valve at a rate of 15 m3/h when the valve is 15 % open. When the valve opens to 30 %, the flow rate increases to 30 m3/h. What is the flow rate in m3/h when the valve opens to 45 %? The pressure drop across the valve remains constant.
A) 15
B) 30
C) 45
D) 60
Answer: 45
45. Due to cavitation, shock waves created are________
A) With zero power
B) Less powerful
C) More powerful
D) None
Answer: More powerful
46. Cavitation causes________
A) Erosion
B) Corrosion
C) Both corrosion and erosion
D) None
Answer: Erosion
47. Diaphragm valves are used for handling ________fluids.
A) Corrosive
B) Viscous
C) Non-Newtonion
D) Solid suspended
Answer: Corrosive
48. The material designation of a valve:
A) Gives where the material is used
B) Tells what material makes up the valve
C) Gives the temperature to which the valve can be subjected
D) Tell what the material is suited for
Answer: Tells what material makes up the valve
49. Rangeability of a control valve is ratio of ________
A) Maximum flow to Maximum flow
B) Minimum flow to Maximum flow
C) Maximum flow to minimum flow
D) Minimum flow to Minimum flow
Answer: Maximum flow to minimum flow*
50. Control valve sizing is required to avoid ________
A) Oversizing
B) Undersizing
C) Maximum cost
D) All of the above
Answer: All of the above*
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