Operator Generic Fundamentals Components - Controllers and Positioners

Operator Generic Fundamentals Components - Controllers and Positioners
K1.01 Function and operation of flow controller in manual and automatic modes K1.02 Function and operation of a speed controller K1.03 Operation of valves controllers in manual and automatic mode K1.04 Function and operation of pressure and temperature controllers, including pressure and temperature control valves K1.05 Function and characteristics of valve positioners K1.06 Function and characteristics of governors and other mechanical controllers K1.07 Safety precautions with respect to the operation of controllers and positioners K1.08 Theory of operation of the following types of controllers: electronic, electrical, and pneumatic K1.09 Effects on operation of controllers due to proportional, integral (reset), derivative (rate), as well as their combinations K1.10 Function and characteristics of air-operated valves, including failure modes K1.11 Cautions for placing a valve controller in manual mode Operator Generic Fundamentals Components - Controllers and Positioners

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: Describe the arrangement and operation of typical controllers and positioners within process control systems. TLO’s

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-integral- derivative (PID) control. 1.5 Describe the operation of a controller in the automatic and manual modes. TLO 1

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. ELOs

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 Related KA K1.05 Function and characteristics of valve positioners ELO 1.1

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 ELO 1.1

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. ELO 1.1

Operation of a Simple Controller
Temp of oil leaving heat exchanger is measured by temp element Temp transmitter sends actual signal to temp controller Closed Loop Temp controller compares actual temp to setpoint temp, creates error signal Temp controller output moves the control valve to desired position An example of an OPEN-LOOP control process system would be two separate controllers controlling flow of COLD and HOT waters. In an OPEN –LOOP system the parameter being maintained is NOT sent to the controller to control. In a CLOSED-LOOP system the parameter being maintained IS sent to the controller. Figure: Process Control System Operation ELO 1.1

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 No directly related NRC KAs ELO 1.2

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 ELO 1.2

Two-Position Controller Example 1
Controller switches from OFF to ON when measured variable increases above setpoint Controller switches from ON to OFF when measured variable decreases below setpoint Once above setpoint, magnitude of error signal does not effect output Shown with zero deband, which causes excessive cycling right at setpoint Figure: Input/Output Relationship for a Two-Position Controller ELO 1.2

Two-Position Controller Example 2
Controlled process is volume of water in tank Controlled variable is level in tank Level measured by level detector that sends information to controller Output of controller sent to final control element (solenoid valve) that controls flow of water into tank When water level decreases to setpoint Bistable energizes to open makeup valve When level reaches “re-setpoint” Bistable de-energizes to close makeup valve Figure: Two-Position Control System ELO 1.2

Figure: 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) In most cases, bistables indicated by box or circle Box style used by NRC Lines in or around bistables not only mark them as bistables, also indicate how they function Part (B) of figure shows various conventions used to indicate bistable operation If necessary, type “12, ENTER” to go back to previous slide to show figure. When ready to answer question, type “13, ENTER” to return back to this slide. Answer revealed on last mouse click. Keep in mind that the “default” operation for an NRC Exam Question bistable is to turn ON to do something and to turn OFF to stop doing it. There might be some applications that require the action to be taken when the bistable is turned OFF (fail-safe to take action on loss of power as well). However, all exam questions assume the action is taken when bistable turns ON. Figure: Bistable Symbols ELO 1.2

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 Energize on decreasing RCS pressure Deenergize on different higher RCS pressure Parameter Value Bistable Setting Low High OFF ON Setpoint Reset point Level decreases to Setpoint Bistable turns ON to open makeup valve Level increases to Reset point Bistable turns OFF to close makeup valve Animation clicks used to demonstrate previous tank example. One click required to start entire animation. ELO 1.2

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. NRC Bank Question – P4508 Analysis: Because the warning light actuates when the temperature reaches a high setpoint, symbols 1 and 2 are eliminated. The key word which eliminates distractor ‘C’ is that the warning light extinguishes after the temperature decreases 5ºF below after reaching the setpoint, indicating the deadband shown in symbol 4. Bank questions that refer to symbols 2 & 4 provide different SET and RESET values Bank questions that refer to symbols 1 & 3 usually say something like, “immediately after…” Correct answer is D. ELO 1.2

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 Images on upcoming slides help to reinforce the concepts relating to each of these terms. ELO 1.3

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 An example of an “Open-Loop” controlled process would be the following: If you wanted to control the Letdown Heat Exchanger outlet temperature you could control the Letdown flow as well as the cooling water flow. In this case, the parameter being controlled (Letdown Heat Exchanger outlet temperature) does NOT feed into the controller. ELO 1.3

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 ELO 1.3

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 Output to air-operated valve goes from 15 to 3 psi Exam bank examples of Direct-Acting and Reverse-Acting applications will be provided at the end of the next section. We must discuss Proportional type system control in order to fully understand direct and reverse-acting. ELO 1.3

Process Control Terms - Application
Consider the following controller face images to apply these definitions: Image 1 Image 2 Output 0 % 100 % 50 % Input 70°F 120°F 95oF Controller Face What is the GAIN? 2 Gain = Output/Input 100%/(120°F-70°F) 100/50 = 2 Direct or Reverse Acting? Direct As temperature increases from Output goes from 0-100% Still 2 Reverse As temperature decreases from Recall that Proportional Band is the ratio of Input/Output, or “how much input is required to provide 100% output”. Since Gain is = 2, PB is equal to 0.5, or 50% (or, 50oF)! If you increased the Gain to 4, what would the PB be? or 25%, or 25oF As you increase the Gain, you “tighten” or lower the PB. If this was an AOV for a heat exchanger, the cooling water AOV would move more for a given temperature change. ELO 1.3

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 ________. offset gain deadband feedback Correct answer is A. Correct answer is A. NRC Bank Question – P17 Analysis: A. CORRECT. OFFSET is the steady-state deviation between the measured parameter and the setpoint once the process has stabilized (supply and demand matched). It will be explained in the next section that OFFSET exists in a Proportional controller that does not use the integral feature. B. WRONG. Gain - The ratio of the control signal change to the error signal change. (OUT/IN). C. WRONG. Deadband - Range of values around the setpoint within which no response will occur D. WRONG. Feedback – The portion of a closed loop controller output signal that is compared to the input of the controller in an attempt to provide the proper controller response. ELO 1.3

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... gain bias feedback error Correct answer is C. Correct answer is C. NRC Bank Question - P1615 Analysis: A. WRONG. Gain - The ratio of the control signal change to the error signal change. (OUT/IN) B. WRONG. Bias - the control effort required to maintain the process variable at its setpoint in the absence of a load. C. CORRECT. Feedback is information in a closed-loop control system about the condition of a process variable. A valve position, for example, is fed back to the controller which might change the output signal to the valve as it gets closer to the desired valve position. D. WRONG. Error – difference between the measurement and the setpoint. ELO 1.3

Operation of an Automatic Controller
ELO 1.4 – Describe the operation of an automatic controller, including proportional control, proportional-integral (PI) control, proportional- derivative (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 Related KAs: K1.01 †Function and operation of flow controller in manual and automatic modes K1.09 Effects on operation of controllers due to proportional, integral (reset), derivative (rate), as well as their combinations ELO 1.4

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-plus- rate) [PID] ELO 1.4

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” Proportional Mode: Linear relation between value of controlled variable and position of final control element Amount of valve movement is proportional to amount of signal deviation ELO 1.4

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 Click “Start Flow” to show how makeup valve opens as level decreases. Click “Stop Flow” to show how flow stops when tank at high level. Figure: Proportional System Controller ELO 1.4

Proportional band is input band over which controller provides a proportional output and is defined as follows: 𝑃𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛𝑎𝑙 𝐵𝑎𝑛𝑑= % 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡 % 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 ×100% 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 One additional mouse click to reveal entire slide contents. ELO 1.4

If fulcrum setting changed: 2 inches, or 50% of input, causes full 4 inch stroke, or 100% of output Proportional band would become 50% Gain is 2 Recall, the “smaller” the band, the “larger” the gain Figure: Proportional System Controller ELO 1.4

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 ELO 1.4

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” Integrator provides linear output with magnitude of output directly related to amplitude of step change input and a constant that specifies function of integration ELO 1.4

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 The point is not for the student to do algebra with the time constants, but rather to realize that changing the time constants will alter the response of the circuit. For instance, this is why Tech Spec Instrument surveillances verify the time response of the protection circuits as well as the nominal setpoints. Figure: Integral Controller Output for a Fixed Input ELO 1.4

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 ELO 1.4

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 6- in control valve to position of 3- in open Figure: Integral Flow-Rate Controller ELO 1.4

Measured variable decreases from 50 gpm to 45 gpm ⇒ positive error of 10% applied to input of controller Controller has a constant of seconds-1; controller output magnitude is 1% per second Controller output increases from initial point of 50% at 1% per second Causes control valve to open further at rate of 1% per second ⇒ increasing flow The top line (Actual System Flow) shows a step drop in actual flow from 50 gpm to 45 gpm. The subsequent change to flow is then shown. Since the controller is a Proportional Integral type, the measured variable will return to 50 gpm. Figure: Integral Controller Response ELO 1.4

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 ELO 1.4

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 Figure: Integral Controller Response ELO 1.4

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 ELO 1.4

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 ELO 1.4

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 ELO 1.4

Proportional-Integral Control Example
Heat exchanger system - equipped with proportional-integral controller Figure: Heat Exchanger Process with PI Control ELO 1.4

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 Figure: Effects of Disturbance on a PI Controller ELO 1.4

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 Figure: Effects of Disturbance on a PI Controller ELO 1.4

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 ELO 1.4

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 Higher error signal rate of change ⇒ sooner final control element is positioned to desired value Added derivative action reduces initial overshoot of measured variable ELO 1.4

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 Figure shows the input versus output relationship of a differentiator Figure: Derivative Output for a Constant Rate of Change ELO 1.4

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 Figure shows the input versus output relationship of a differentiator Figure: Derivative-Control Output ELO 1.4

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 ELO 1.4

Proportional-Derivative Control Example
Same heat exchanger system as previously analyzed Temperature controller now uses PD controller Figure: Heat Exchanger Process with PD Control ELO 1.4

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 Does not eliminate offset error Figure: Effect of Disturbance on a PD Controller ELO 1.4

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 ELO 1.4

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 ELO 1.4

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 Figure: PID Control Responses ELO 1.4

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 ELO 1.4

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 Rate action combined with proportional action reduces initial overshoot and cyclic period Figure demonstrates the combined controller response to a demand disturbance. Most PID controllers have the Derivative features “zeroed out” due to potential instability. Figure: PID Controller Response Curves ELO 1.4

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. proportional, integral, and derivative proportional and integral proportional only bistable Correct answer is C. Correct answer in C. NRC Bank Question – P818 Analysis: A. WRONG. No integral features are present as tank level is not returned to original setpoint; also, no rate response or derivative features were discussed. B. WRONG. No integral features are present as tank level is not returned to original setpoint. C. CORRECT. In a proportional controller, the controller produces an output signal that is proportional (a multiple) of the input signal. Without any additional features, proportional-only controllers do not cause the controlled parameter to return to the original setpoint, nor do they have any rate/derivative features. In the above example since a “new steady state level” is established, the controller cannot use the integral feature. D. WRONG. Bistable control will open the drain valve at a set tank level (usually off a level switch) and close the drain valve at a lower tank level. However, bistable control causes a binary change in state (valve either full-open or full-closed). The stem discusses the controller to ‘begin to open’ the valve. This eliminates the possibility of bistable control as a plausible choice. ELO 1.4

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 __________. open; decrease open; increase closed; decrease closed; increase Correct answer is A. Correct answer is A. NRC Bank Question – P617 Analysis: If the temperature transmitter fails high, then the controller will respond to attempt to reduce lube oil temperature. In order to reduce lube oil temperature, the temperature control valve must open to pass more cooling water through the heat exchanger. This will result in actual lube oil outlet temperature decreasing, due to more heat transfer. ELO 1.4

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? 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. The controller will develop an output signal that will remain directly proportional to the difference between the controlled parameter and the controller set point. 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. 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. Correct answer is A. NRC Bank Question – P319 Analysis: For a direct-acting proportional-integral controller, the controller output will begin to rise when the controlled parameter increases above the controller setpoint. Also, when the controller output signal equals the controller setpoint, the controller output will stop increasing, but not become zero. This is because for this discrete amount of time, the controlled parameter has exceeded the setpoint, to return the parameter to the setpoint (fundamental principal of the integral function in the controller). The controller output signal must still exist to force the controlled parameter to return to its setpoint. ELO 1.4

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 Extremely versatile Can be adapted to control various types of industrial equipment and processes Pressure, temperature, valve position, etc. Related KAs K1.01 Function and operation of flow controller in manual and automatic modes K1.03 Operation of valves controllers in manual and automatic mode K1.07 Safety precautions with respect to the operation of controllers and positioners K1.11 †Cautions for placing a valve controller in manual mode ELO 1.5

Controller Operation Operated in either automatic or manual mode
Mode depends on: complexity of process being controlled specific operational requirements Figure: Typical Digital Controller ELO 1.5

Controller Operation Pulser knob Display pushbutton
Alphanumeric display Auto/manual pushbutton Pulser knob Adjusts the setpoint or output of the controller Display pushbutton Toggles parameter for digital display Alphanumeric display Programmable to display error codes in controller Auto/manual pushbutton Places controller in automatic or manual control Figure: Typical Digital Controller ELO 1.5

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 ELO 1.5

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 ELO 1.5

Controller Transfer Operation
When transferring control from automatic to manual: Normally manual tracks automatic Usually no perturbation shifting to Manual During instrument failures Manual removes failed instrument 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) When properly executed, this shift of control from Automatic to Manual or Manual to Automatic is called a "bumpless" transfer. Failure to correctly match the controller outputs would result in a sudden valve repositioning during the transfer. In a lot of cases, controller operation is designed such that Manual tracks Auto so merely placing a controller in Manual wouldn’t cause a system perturbation. ELO 1.5

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 NOTE: The top left most part of this drawing shows the output signal from the controller. The 4 – 20 ma error signal from the controller goes to an I/P converter (converts current into air pressure) to send to the final control element. NOTE: The term “valve positioner” is explained in the last section of this presentation. Figure: Pneumatic Control System - PWR ELO 1.5

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? Only the derivative feature will be lost. Only the integral and derivative features will be lost. All proportional, integral, and derivative features will be lost. All control features will continue to influence the controller output. Correct answer is C. Correct answer is C. NRC Bank Question – P3715 Analysis: When a controller is placed in manual mode, the operator directly controls the output signal of the controller. Any changes in system parameters which previously affected controller output will be lost, the only way to control output is the operator manipulating the controller. Therefore, ALL automatic features are lost. ELO 1.5

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 Only part that might differ is what gets operated as a result Might compare feedback from final control element to sensor input To adjust error signal Related KA: K1.04 Function and operation of pressure and temperature controllers, including pressure and temperature control valves ELO 1.6

Proportional Temperature Control
Steam heats cold water supply Temperature detector monitors hot water outlet 3-15 psi output signal for range of ºF Controller compares measured variable signal with setpoint Sends 3-15 psi output to final control element 3 inch control valve Controller set for proportional band of 50% 50ºF change causes 100% controller output change Controller is reverse-acting Valve throttles closed as hot water temperature increases Valve throttles open as hot water temperature decreases Figure: Proportional Temperature-Control System ELO 1.6

Controller Response to Demand Changes
Purpose of system is to provide hot water at setpoint of 95ºF System must handle demand disturbances that affect outlet temperature Controller set up to function as shown in figure Figure: Proportional-Controller Characteristics ELO 1.6

If measured variable drops below setpoint Positive error is developed Control valve opens further Figure: Proportional-Controller Characteristics ELO 1.6

If measured variable goes above setpoint Negative error developed Control valve throttles down (opening is reduced) Figure: Proportional-Controller Characteristics ELO 1.6

50% proportional band causes full stroke of valve between a +25ºF error and a -25ºF error Figure: Proportional-Controller Characteristics ELO 1.6

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 One additional mouse click to reveal entire slide contents. Figure: Proportional-Controller Characteristics ELO 1.6

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 __________. closed; increase closed; decrease open; increase open; decrease Correct answer is A. Correct answer is A. NRC Bank Question – P2016 Analysis: NOTE: This question tests not only understanding of controllers but heat transfer as well! If cooling water inlet temperature decreases, this will increase the heat transfer rate, causing the lube oil outlet temperature to decrease. The temperature controller will then attempt to compensate by throttling down on the TCV to reduce cooling water flow. This will tend to raise lube oil outlet temperature back to its original value. Because mass flow rate of the cooling water was lowered, in order to achieve the same steady state heat transfer rate, the cooling water differential temperature must increase. ELO 1.6

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 Related KA K1.02 Function and operation of a speed controller K1.06 Function and characteristics of governors and other mechanical controllers NOTE: The concept of DROOP and ISOCHRONOUS will be introduced in this section and reinforced in – Motors and Generators. ELO 1.7

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 ELO 1.7

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 Figure: Mechanical Speed Sensor ELO 1.7

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 ELO 1.7

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 Click “Start Flow” to show how an increase in load will decrease the speed, which will cause the fuel racks to open more to bring speed back up to setpoint. Figure: Mechanical Governor ELO 1.7

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 ELO 1.7

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 ELO 1.7

Droop Mode vs Isochronous Mode
The type of speed control utilized by the diesel generator (D/G) varies Droop Mode 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 Maintains D/G speed at speed changer setting backed up by mechanical governor % Speed Droop = No Load - Full Load Speed / No Load Isochronous Mode (Emergency Mode) Used when D/G is ONLY source to AC Vital Bus Controller returns D/G to speed setpoint for 60 Hz for any change in load Loads sequenced on to minimize impact on D/G NOTE: This is a very rudimentary analogy but you can almost think of Droop Mode as “proportional-only” control and Isochronous Mode as “proportional-integral” control since the D/G ALWAYS return to the rpm associated with 60 Hz in isochronous mode. Another way of looking at Speed Droop mode is as you increase the speed setting to pick up load when the D/G is paralleled to the bus, if this load was removed (and D/G didn’t trip on overspeed), the D/G speed would increase to the setting of speed control device difference between Full Load and No Load. Or, if load was introduced to the D/G while in Speed Droop mode (and not paralleled to the bus), the D/G speed would drop to this same percentage change in speed from No Load to Full Load ELO 1.7

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. speed; air speed; fuel load; air load; fuel Correct answer is B. Correct answer is B. NRC Bank P218 Analysis: A Diesel Generator governor senses engine speed, which is proportional to frequency (F = NP/120). In order to control speed, the governor causes terminal shaft to rotate and position the fuel racks. Fuel rack position determines how much fuel is delivered by the fuel oil pump. When the diesel generator is supplying an emergency bus alone it is said to be operating in “isochronous” mode. This means it runs at 60 Hz and will adjust fuel to the governor as load is started/stopped to maintain a constant frequency. Air is only used to help get the diesel generator up to its initial speed on a start signal. ELO 1.7

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 ___________. counteract; apart aid; together counteract; together aid; apart Correct answer is C. Correct answer is C. NRC Bank P419 ELO 1.7

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... increase, then decrease and stabilize below the initial value. increase, then decrease and stabilize above the initial value. decrease, then increase and stabilize below the initial value. decrease, then increase and stabilize above the initial value. Correct answer is B. Correct answer is B. NRC Bank P2818 Analysis: (House Curve shown on next mouse click) The key word in the stem is speed droop. In speed droop mode, diesel generator frequency will be dependent on loading. The best way to visually demonstrate how diesel loading is affected by a large load trip while in speed droop mode is using ‘house curves’. To use house curves for real load, draw real load on the x-axis, and frequency on the y-axis, and using a rectangle to represent the initial load. The diesel loading must always be represented by a similar rectangle and must remain on the load curve (diagonal line). Therefore, a reduction in load results in a higher frequency. It is easier to visualize this type of question on a START of load. The speed will drop due the load (and high starting current). Once starting current diminishes, speed will increase, but still be less than it was before the load was started. NOTE: The “then decrease” in Choices “A” and “B” might be questionable, but Choice “B” is correct because frequency (speed) will be above the initial value if load is reduced. ELO 1.7

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. Related KA s - K1.08 Theory of operation of the following types of controllers: electronic, electrical, and pneumatic Logic diagrams have many uses Principal diagram for the design of solid state components such as computer chips Used by mathematicians to help solve logical problems (called Boolean algebra) Principle application is to present component and system operational information ELO 1.8

Logic Symbols The states of a gate are also commonly referred to as High or Low, 1 or 0, True or False On = High = 1 = True Off = Low = 0 = False States also referred to as output Determined by status of inputs to gate Each type of gate responds differently to combinations of inputs Common logic symbols shown on next slide NRC uses symbols similar to GE style ELO 1.8

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 On On Off Off Off Off On Off On Off Off On On On On Off Correct answer is B. Correct answer is B. NRC Bank Question – P5009 Analysis: Since the “CLOSE” signal comes from a 2/3 Conditional AND box, Choice “B” is the only one that meets the 2/3 requirements. ELO 1.8

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 Related KAs - K1.05 Function and characteristics of valve positioners K1.10 Function and characteristics of air-operated valves, including failure modes ELO 1.9

Pneumatic Valve Actuator
Recall that Pressure is Force/Area. To overcome the force of the spring, the air pressure has to be such that when applied to the area of the actuator it can cause valve movement (Force = Pressure x Area). There are several bank questions in this chapter on this concept. Pneumatic Valve Actuator: Operates by combination of force created by air and spring tension Actuator transmits its motion through stem Rubber diaphragm separates actuator housing into two air chambers Supply air pressure in upper chamber controls valve position Bottom chamber contains spring Local indicator connected to stem Figure: Pneumatic-Actuated Control Valve ELO 1.9

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 Figure: Pneumatic-Actuated Control Valve ELO 1.9

Actuator Failure Position
Four mouse clicks to reveal entire slide contents. Click “NEXT” arrow button to move to next slide. An actuators failure position is provided by the spring Maintains valve in a safe position if loss of supply air occurs On a loss of supply air, this actuator will fail open Referred to as “air-to-close, spring-to-open“ or "fail-open” Other valves fail in closed position Referred to as "air-to-open, spring-to-close" or "fail-closed" Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

Figure: Pneumatic Actuator with Controller and Positioner
Valve Positioner Purpose Converts the 3-15 psi control air pressure to a higher supply air pressure to move the valve actuator 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) 3-15 psi Supply air pressure available Usually > 100 psi Supply air pressure to actuator Varies Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

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 ELO 1.9

Valve Positioner Knowledge Check – NRC Bank
The purpose of the valve positioner is to convert... a small control air pressure into a proportionally larger air pressure to adjust valve position. a large control air pressure into a proportionally smaller air pressure to adjust valve position. pneumatic force into mechanical force to adjust valve position. mechanical force into pneumatic force to adjust valve position. Correct answer is A. Correct Answer is A. NRC Bank Question – P318 Analysis: A. CORRECT. Valve positioners receive a demand signal from the valve controller, compare valve controller output signal to valve position, and adjust air pressure to the valve actuator to position the valve. In other words, they regulate the Instrument (or Service) air pressure (120 psig) supplied to the valve actuator based on the smaller control air pressure (3-15 psig) supplied by the controller and I/P converter. B. WRONG. Valve positioners convert the small control air pressure from the controller to a larger air pressure to adjust valve position. C. WRONG. This is the purpose of the actuator, not the positioner. D. WRONG. Valve actuators convert pneumatic force into mechanical force to adjust valve position, however this is the wrong component and wrong description for a positioner. ELO 1.9

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 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 ELO 1.9

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 Each component of the hydraulic actuator is animated on with mouse clicks. Figure: Piston-Type Hydraulic Actuated Control Valve ELO 1.9

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 Figure: Piston-Type Hydraulic Actuator ELO 1.9

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 Five mouse clicks to reveal entire slide contents. Figure: Electric Solenoid Actuator ELO 1.9

Solenoid Actuator Advantages & Disadvantages
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 Six mouse clicks to reveal entire slide contents. ELO 1.9

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 Three mouse clicks to reveal entire slide contents. Figure: Motor Actuator ELO 1.9

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 Two additional mouse clicks to reveal entire slide contents. Figure: Motor Actuator ELO 1.9

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 One additional mouse click to reveal entire slide contents. Figure: Motor Actuator ELO 1.9

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? 21 psig 34 psig 43 psig 64 psig Correct answer is A. Correct answer is A. NRC Bank Question – P2517 Analysis: (See info on slide) Solve in class as a review of this type problem P = F/A Where: P = pressure (psi) F = force (lbf) A = area acted upon by system pressure A = r2 A = x 36 A = 113.1 P = 2400/113.1 P = 21.2 psig, so closest answer is A You can also use the following formula for area: A = ¼╥D2 ELO 1.9

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 1.5 K1.04 Function and operation of pressure and temperature controllers, including pressure and temperature control valves 2.8 3.0 1.6 K1.05 Function and characteristics of valve positioners 2.5 1.1, 1.9 K1.06 Function and characteristics of governors and other mechanical controllers 2.3 K1.07 Safety precautions with respect to the operation of controllers and positioners K1.08 Theory of operation of the following types of controllers: electronic, electrical, and pneumatic 2.1 1.8, 1.9 K1.09 Effects on operation of controllers due to proportional, integral (reset), derivative (rate), as well as their combinations 2.4 K1.10 Function and characteristics of air-operated valves, including failure modes 1.9 K1.11 Cautions for placing a valve controller in manual mode 2.9

Operator Generic Fundamentals Components - Controllers and Positioners

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Operator Generic Fundamentals Components - Controllers and Positioners

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