3A2 Class Power Engineering - A2 -
Part 3 Final Exam and All Correct
Answers 2026-2027 Updated.
On/off control systems are used where the most basic control is required. The requirements for
successful on/off control are: - Answer 1) Economics of the process do not require precise
control
2) Precise set point is unnecessary
3) Variables such as Heat and Level are generally slow and work best with on/off control
4) Manipulated variable energy or volume flowing into the process is relatively small compared
to the capacity of the process
5) The operation of the FCE produces small changes rather than throwing the process out of
stability
3 common examples of on/off control? - Answer 1) A temperature controlled exhaust fan in a
compressor building
2) A thermostat controlled forced air furnace
3) A compressed air supply to a receiver
It is very common for on/off control to be used in Flow control. True or False? - Answer False,
it is very seldom for Flow to use on/off control
What is Offset? - Answer - The difference between the set point and the actual value of the
process variable is known as offset
- Offset is an inherent characteristic of all proportional only controllers, and is a sustained error
than cannot be eliminated by means of the proportional mode of control
As sensitivity of a controller is increased, offset is decreased. True or False? - Answer True
What principle do Pneumatic controllers work on? - Answer They work on the principle of
Force moments. They contain 3 bellows, containing internal pressure related to the set point,
process variable and controller output.
What is used to stabilize a proportional controller? - Answer A constant spring force, which is
termed the 'manual reset'. When the process variable is at set point, the force in the negative
feedback bellows will equal the spring force. (Clockwise moments = counter-clockwise
moments)
,What is the 'Proportional Band' of a controller? - Answer The percent of the process variable
range that causes 100% change in a controller output is the proportional band
What is the 'Gain' of a controller? - Answer The ratio of Output to change in Input is referred
to as the Gain of a proportional controller.
As the gain is increased, or proportional band is decreased, the Offset of the proportional
controller is decreased. True or False? - Answer True
At what percentage proportional band will a controller start to act in on/off control? - Answer
A proportional band of 2% or less will create on/off control
Some processes cannot tolerate the effect of reset. How is this problem overcome? - Answer
A proportional plus reset controller is used. These controllers have an automatic reset bellows
in place of the constant spring, manual reset adjustment.
Integral is the same as...? - Answer Reset
Operation of a Proportional Integral controller? - Answer - Responds to both the amount and
the duration of the process deviation
- The correction to the controller output will continue as long as the deviation from set point
continues
- The pressure in the negative feedback bellows will equal the pressure in the integral bellows
when set point is reached
What is the purpose of the "capacity tank" in a proportional integral controller? - Answer The
capacity tank causes a delay in the integral action by providing a small "reservoir" for the air to
the positive feedback bellows. This provides more stable control.
In proportional integral controllers the offset due to proportional action is eliminated over a
period of time. True or False? - Answer True
What is 'Reset Time'? - Answer The time it takes integral action to reproduce the
proportional action, expressed in minutes. Integral action can also be expressed in repeats per
minute which is the number of times the initial proportional action is repeated by integral
action.
What happens if the restriction valve in a Proportional Integral controller is wide open? -
Answer Pressures in the positive and negative feedback bellows will increase simultaneously
and create an on/off control action
,Derivative control is popular for which process variable? - Answer Temperature control
What is Derivative control? - Answer Derivative control takes into account the speed at which
the variable is deviating from its set point. If the process variable starts to deviate from the set
point at a faster rate, rate contribution to controller output will increase.
Derivative action is recommended where slow processes undergo large load changes, and not
where abrupt changes occur in a system where there is quick response. True or False? - Answer
True.
Feedforward uses a feedback trim loop to control the input. True or False? - Answer True
What is the objective of any control system? - Answer To maintain a balance between supply
and demand over a period of time.
What is Feedback (closed loop) control? - Answer - A measurement is made of the controlled
variable, which is compared to a set point. A controller will provide any corrective signal to the
final control element to bring the process back to set point.
Feedback control is better suited for the supply side of a process, and feedforward control is
better suited for the demand side of a process. True or False? - Answer True
2 popular process conditions that can make the effectiveness of Feedback control
unsatisfactory? - Answer 1) When large magnitude disturbances of such frequency occur that
the feedback control system cannot get the process under control
2) Processes that produce large amounts of lag cannot be controlled effectively with feedback
control
The "slave" of a cascade controller set up is usually which of 2 controllers? - Answer The
controller that is closest to the supply input.
What is Cascade control? - Answer When one controller works off another controller and
there is only one Final Control Element
For cascade control to be effective, the Secondary loop must have a faster response than the
primary loop. True or False? - Answer True
4 advantages of Cascade control? - Answer - Provides isolation from supply upsets
, - Improves loop dynamics
- Removes the influence of the valve characteristic with respect to the primary loop, since it
defines the amount of supply input
- The secondary loop permits an exact manipulation of the flow of mass or energy into the
process by the primary controller
Explain the operation of Ratio control. - Answer - One process variable is controlled in a
specific ratio to another
- Typically uses one conventional flow controller (primary) and then one Ratio flow controller
(secondary)
- Auto-selector control allows the automatic selection between 2 or more measurement inputs
and provides a single control output to manipulate one final control element
What is Split Range control? - Answer - Has one controlled variable as an input, and two
manipulated variables as outputs
- Used when precise manipulation of 2 variables is required to maintain a controlled variable as
close as possible to set point
- Also referred to as DUPLEX control
What is Auto Select control? - Answer - Allows the automatic selection between 2 or more
measured or controlled variables to produce a single output to a control element
- If 3 variables are selected, the lowest output will be selected as the output to the final control
element
Traditionally a loop alarm is dedicated to a single purpose. These purposes may include: -
Answer 1) A condition in the process may lead to personnel injury
2) Process upset may lead to damage of equipment
3) A control loop is operating outside of set parameters
Purpose of Emergency Shutdown Systems? (ESD's) - Answer When an operator is required to
quickly and safely shut down facilities, or portions of facilities, in a manner that will isolate and
contain the problem.
2 types of ESD modes? - Answer 1) Shut down and isolate rotating equipment (system
remains pressurized)
2) Shut down, isolate and depressurize.
2 ways ESD's may be activated? - Answer Either manually or automatically
Part 3 Final Exam and All Correct
Answers 2026-2027 Updated.
On/off control systems are used where the most basic control is required. The requirements for
successful on/off control are: - Answer 1) Economics of the process do not require precise
control
2) Precise set point is unnecessary
3) Variables such as Heat and Level are generally slow and work best with on/off control
4) Manipulated variable energy or volume flowing into the process is relatively small compared
to the capacity of the process
5) The operation of the FCE produces small changes rather than throwing the process out of
stability
3 common examples of on/off control? - Answer 1) A temperature controlled exhaust fan in a
compressor building
2) A thermostat controlled forced air furnace
3) A compressed air supply to a receiver
It is very common for on/off control to be used in Flow control. True or False? - Answer False,
it is very seldom for Flow to use on/off control
What is Offset? - Answer - The difference between the set point and the actual value of the
process variable is known as offset
- Offset is an inherent characteristic of all proportional only controllers, and is a sustained error
than cannot be eliminated by means of the proportional mode of control
As sensitivity of a controller is increased, offset is decreased. True or False? - Answer True
What principle do Pneumatic controllers work on? - Answer They work on the principle of
Force moments. They contain 3 bellows, containing internal pressure related to the set point,
process variable and controller output.
What is used to stabilize a proportional controller? - Answer A constant spring force, which is
termed the 'manual reset'. When the process variable is at set point, the force in the negative
feedback bellows will equal the spring force. (Clockwise moments = counter-clockwise
moments)
,What is the 'Proportional Band' of a controller? - Answer The percent of the process variable
range that causes 100% change in a controller output is the proportional band
What is the 'Gain' of a controller? - Answer The ratio of Output to change in Input is referred
to as the Gain of a proportional controller.
As the gain is increased, or proportional band is decreased, the Offset of the proportional
controller is decreased. True or False? - Answer True
At what percentage proportional band will a controller start to act in on/off control? - Answer
A proportional band of 2% or less will create on/off control
Some processes cannot tolerate the effect of reset. How is this problem overcome? - Answer
A proportional plus reset controller is used. These controllers have an automatic reset bellows
in place of the constant spring, manual reset adjustment.
Integral is the same as...? - Answer Reset
Operation of a Proportional Integral controller? - Answer - Responds to both the amount and
the duration of the process deviation
- The correction to the controller output will continue as long as the deviation from set point
continues
- The pressure in the negative feedback bellows will equal the pressure in the integral bellows
when set point is reached
What is the purpose of the "capacity tank" in a proportional integral controller? - Answer The
capacity tank causes a delay in the integral action by providing a small "reservoir" for the air to
the positive feedback bellows. This provides more stable control.
In proportional integral controllers the offset due to proportional action is eliminated over a
period of time. True or False? - Answer True
What is 'Reset Time'? - Answer The time it takes integral action to reproduce the
proportional action, expressed in minutes. Integral action can also be expressed in repeats per
minute which is the number of times the initial proportional action is repeated by integral
action.
What happens if the restriction valve in a Proportional Integral controller is wide open? -
Answer Pressures in the positive and negative feedback bellows will increase simultaneously
and create an on/off control action
,Derivative control is popular for which process variable? - Answer Temperature control
What is Derivative control? - Answer Derivative control takes into account the speed at which
the variable is deviating from its set point. If the process variable starts to deviate from the set
point at a faster rate, rate contribution to controller output will increase.
Derivative action is recommended where slow processes undergo large load changes, and not
where abrupt changes occur in a system where there is quick response. True or False? - Answer
True.
Feedforward uses a feedback trim loop to control the input. True or False? - Answer True
What is the objective of any control system? - Answer To maintain a balance between supply
and demand over a period of time.
What is Feedback (closed loop) control? - Answer - A measurement is made of the controlled
variable, which is compared to a set point. A controller will provide any corrective signal to the
final control element to bring the process back to set point.
Feedback control is better suited for the supply side of a process, and feedforward control is
better suited for the demand side of a process. True or False? - Answer True
2 popular process conditions that can make the effectiveness of Feedback control
unsatisfactory? - Answer 1) When large magnitude disturbances of such frequency occur that
the feedback control system cannot get the process under control
2) Processes that produce large amounts of lag cannot be controlled effectively with feedback
control
The "slave" of a cascade controller set up is usually which of 2 controllers? - Answer The
controller that is closest to the supply input.
What is Cascade control? - Answer When one controller works off another controller and
there is only one Final Control Element
For cascade control to be effective, the Secondary loop must have a faster response than the
primary loop. True or False? - Answer True
4 advantages of Cascade control? - Answer - Provides isolation from supply upsets
, - Improves loop dynamics
- Removes the influence of the valve characteristic with respect to the primary loop, since it
defines the amount of supply input
- The secondary loop permits an exact manipulation of the flow of mass or energy into the
process by the primary controller
Explain the operation of Ratio control. - Answer - One process variable is controlled in a
specific ratio to another
- Typically uses one conventional flow controller (primary) and then one Ratio flow controller
(secondary)
- Auto-selector control allows the automatic selection between 2 or more measurement inputs
and provides a single control output to manipulate one final control element
What is Split Range control? - Answer - Has one controlled variable as an input, and two
manipulated variables as outputs
- Used when precise manipulation of 2 variables is required to maintain a controlled variable as
close as possible to set point
- Also referred to as DUPLEX control
What is Auto Select control? - Answer - Allows the automatic selection between 2 or more
measured or controlled variables to produce a single output to a control element
- If 3 variables are selected, the lowest output will be selected as the output to the final control
element
Traditionally a loop alarm is dedicated to a single purpose. These purposes may include: -
Answer 1) A condition in the process may lead to personnel injury
2) Process upset may lead to damage of equipment
3) A control loop is operating outside of set parameters
Purpose of Emergency Shutdown Systems? (ESD's) - Answer When an operator is required to
quickly and safely shut down facilities, or portions of facilities, in a manner that will isolate and
contain the problem.
2 types of ESD modes? - Answer 1) Shut down and isolate rotating equipment (system
remains pressurized)
2) Shut down, isolate and depressurize.
2 ways ESD's may be activated? - Answer Either manually or automatically
