Operator Generic Fundamentals Components - Controllers and Positioners

Operator Generic Fundamentals Components - Controllers and Positioners

Controllers and Positioners
Introduction – Purpose of Study Controllers and positioners are used in power plants to manipulate equipment and control process parameters. For example, a controller monitors and maintains system temperature by Measuring system temperature Comparing that measurement to a setpoint (the desired temperature) Adjusting a control element to change the process temperature Related KAs: Controllers and Positioners (CFR 41.7) 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 Introduction

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

Enabling Learning Objectives for TLO 1
Describe the characteristics of a control system, including process controllers and position controllers. Define the following process control related terms: proportional band, gain, closed loop system, offset, feedback, deviation, deadband, direct acting, and reverse acting. Describe the operation of an automatic controller, including proportional control system, proportional-integral (PI) control, proportional-derivative (DP) control, and proportional-integral- derivative (PID) control. Describe the operation of a controller in the automatic and manual modes. Describe the operation of temperature controllers and pressure controllers. ELOs

Describe the operation of mechanical and electronic speed- control devices. Describe the operation of bistable alarm and control circuits. Interpret logic diagrams and determine controller outputs. Describe the design and operation of the following types of valve actuators: pneumatic, hydraulic, solenoid, and electric motor. ELOs

TLO Introduction TLO 1 – Describe the arrangement and operation of typical controllers and positioners within process control systems. Controllers and positioners manipulate equipment to control process parameters. Controllers and positioners: Monitor and operate many systems simultaneously Maintain fine control of processes Ensure operating limits are maintained TLO 1

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 Controller Sensor
Device that generates output based on input received Sensor Detect actual value of controlled parameter Temperature Pressure Flow Measured parameter must be converted into usable signal for control system ELO 1.1

Process Controllers Transducer Controller
Converts detector output into pneumatic or electrical signal that is sent to controller. Controller Compares actual value of measured parameter to desired value or setpoint Develops error signal, difference between desired and actual readings Error signal is used to regulate control signal sent to final control element such as a valve 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 Temp controller compares actual temp to setpoint temp, creates error signal Temp controller output moves the control valve to desired position Temp of oil leaving HX is measured by temp element Temp transmitter sends actual signal to temp controller Temp Controller compares actual temp to setpoint temp creates error signal Temp controller output to final control element to adjust as necessary Figure: Process Control System Operation ELO 1.1

Two-Position Controller
Simplest type of controller Device that has two operating conditions: Completely on Completely off Termed Bistable ELO 1.1

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 Figure: Input/Output Relationship for a Two-Position Controller ELO 1.1

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 Water Level Decrease Water level decreases, point is reached where measured variable drops below setpoint Creates positive error signal Controller opens final control element fully Water flows into tank, water level rises Water Level Increase Water level rises above setpoint, negative error signal is developed Negative error signal causes controller to shut final control element Stops water flow to tank Opening and closing of final control element results in cycling characteristic of measured variable Figure: Two-Position Control System ELO 1.1

Characteristics of Controllers and Positioners
Knowledge Check A two-position controller will switch to the ______________ state when its measured variable increases above the high-level setpoint. off throttle on open Correct answer in C. Correct answer is C. ELO 1.1

Process Control Terms Proportional Band Gain
ELO 1.2 – Define the following process control related terms: proportional band, gain, closed loop system, offset, feedback, deviation, deadband, direct acting, and reverse acting. Proportional Band Change in value of controlled variable that results in full travel of the final control element Gain Ratio of amount of change in final control element to amount of change in the controlled variable Factor by which magnitude of error signal will be increased Gain is reciprocal to proportional band Also called sensitivity ELO 1.2

Process Control Terms Closed-Loop System Offset
System in which controlled variable is used to adjust any inputs into the process Offset Deviation that remains after a process has stabilized Difference between setpoint and steady-state value of the controlled parameter Also called droop ELO 1.2

Process Control Terms Feedback Deviation Deadband
Information on controlled variable sent back to the controller for finer control Deviation Difference between setpoint and the actual value Deadband Range of values around setpoint of measured variable where no action occurs Prevents oscillation or hunting in proportional control systems ELO 1.2

Process Control Terms Direct Acting Reverse Acting
Direct acting controllers or actuators will respond in the same direction as the control signal, eg: if the controller sends an open signal, the valve will go in the open direction. Reverse Acting Reverse acting controllers or actuators will respond in the opposite or reverse direction as the control signal, eg: if the controller sends an open signal, the valve will go in the closed direction. ELO 1.2

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 ________. feedback deadband gain offset Correct answer is D. offset Correct answer is D. ELO 1.2

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 from the valve, which is proportional to valve position, is returned to the controller. This signal is referred to as... feedback error gain bias Correct answer is A. Correct answer is A. Bank P1615 ELO 1.2

Operation of an Automatic Controller
ELO 1.3 – 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 1.3

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 1.3

Proportional Controller
Proportional Mode Referred to as throttling control Linear relation between value of controlled variable and position of final control element Amount of valve movement is proportional to amount of signal deviation Proportional Control Output Proportional controller provides linear stepless output that positions valve at intermediate positions, as well as "full open" or "full shut” Controller operates within a 0–100% proportional band, where controller output is proportional to the input signal ELO 1.3

Proportional Level Controller Example
Flow of supply water into tank controlled to maintain tank level within narrow band Components Fulcrum and lever assembly used as proportional controller Float chamber is level measuring element 4 inch stroke valve is final control element Figure: Proportional System Controller ELO 1.3

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% One additional mouse click to reveal entire slide contents. Figure: Proportional Controller ELO 1.3

Change fulcrum setting so that level change of 2 inches, or 50% of input, causes full 4 inch stroke, or 100% of output Proportional band would become 50% Proportional band of proportional controller is important because it determines range of outputs for given inputs Figure: Proportional System Controller ELO 1.3

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 is the 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 Rarely used in PI control mode only ELO 1.3

Definition of Integral Control
Device that performs mathematical function of integration is called integrator Mathematical result of integration is called integral 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 1.3

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 Figure: Integral Controller Output for a Fixed Input ELO 1.3

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 1.3

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 1.3

Measured variable decreases from 50 gpm to 45 gpm ⇒ positive error of 10% applied to input of controller Controller has a constant of 0.1 seconds-1; controller output magnitude is 1% per second Controller output increases from initial point of 50% at 1% per second Causes control valve to open further at rate of 1% per second ⇒ increasing flow Figure: Integral Controller Response ELO 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 Proportional-Integral Control ELO 1.3

Proportional-Integral Control Example
Heat exchanger system - equipped with proportional- integral controller Figure: Heat Exchanger Process with PI Control ELO 1.3

Response curves illustrate Demand 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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 1.3

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 7-21 demonstrates the combined controller response to a demand disturbance. Figure: PID Controller Response Curves ELO 1.3

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 only proportional, integral, and derivative proportional and integral proportional and derivative Correct answer in A. Correct answer is A. ELO 1.3

Knowledge Check – NRC Bank If the temperature transmitter fails high (high temperature output signal), the temperature controller will ________ the temperature control valve, causing the actual heat exchanger lube oil outlet temperature to ________. open; decrease open; increase close; decrease close; increase Correct answer is A. Correct answer is A. ELO 1.3

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. ELO 1.3

Knowledge Check – NRC Bank 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. ELO 1.3

Automatic and Manual Controller Operation
ELO 1.4 – 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 1.4

Controllers Digital controller
Popular controller found in industrial applications Extremely versatile Can be adapted to control various types of industrial equipment and processes ELO 1.4

Controller Operation Controllers can be operated in either automatic or manual mode Mode depends on complexity of process being controlled and the specific operational requirements Figure: Typical Digital Controller ELO 1.4

Controller Operation 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 1.4

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 Three mouse clicks to reveal entire slide contents. ELO 1.4

Manual Operation Controller does not attempt to maintain its programmed setpoint Maintains constant output to its control element regardless of changes in controlled parameter For example, used during equipment switching operations Pulser knob must be adjusted by operator in order to change output of controller Requires constant attention by operator When transferring control from automatic to manual, Operator must adjust manual control such that the loss of automatic signal does not cause "bump," or system perturbation Automatic and manual controller outputs must be matched Not matching outputs could cause valve to reposition suddenly When properly executed, this shift of control from Automatic to Manual is called a "bumpless" transfer. Failure to correctly match the controller outputs would result in a sudden valve repositioning during the transfer. ELO 1.4

System Response to Controller Inputs
A decreasing SG water level will: Increase SG level control signal Raise control air pressure Causing feed control valve to open further Figure: Pneumatic Control System - PWR ELO 1.4

System Response Practice Question
If personnel manually decrease the level control signal, how will the pneumatic control system affect SG level Level will decrease because the valve positioner will close more, reducing control air pressure, which causes the feed control valve to close more. Level will decrease because the valve positioner will open more, increasing control air pressure, which causes the feed control valve to close more. Level will increase because the valve positioner will close more, reducing control air pressure, which causes the feed control valve to open more. Level will increase because the valve positioner will open more, increasing control air pressure, which causes the feed control valve to open more. If personnel decrease the level control signal, the signal will cause the valve positioner to close, which will reduce air supply to the feed control valve causing the valve to close down, reducing SG level until reaching a new equilibrium level. The correct answer is A. If personnel decrease the level control signal, the signal will cause the valve positioner to close, which will reduce air supply to the feed control valve causing the valve to close down, reducing SG level until reaching a new equilibrium level. The correct answer is A. Figure: Pneumatic Control System - PWR ELO 1.4

Automatic and Manual Controller Operation
Knowledge Check – NRC Bank If a typical flow controller is in manual control, the output of the flow controller is determined by the ___________. plant computer operator flow error signal system feedback Correct answer is B. Correct answer is B. ELO 1.4

Temperature and Pressure Controller Operation
ELO 1.5 – Describe the operation of temperature controllers and pressure controllers. Essentially, control systems function in the same manner, whether temperature or pressure are controlled. They determine a deviation and adjust the process to return it to the desired setpoint. Related KA K1.04 Function and operation of pressure and temperature controllers, including pressure and temperature control valves ELO 1.5

Proportional Temperature Control
Process system using proportional temperature controller to provide hot water Steam enters heat exchanger to raise temperature of cold water supply Temperature detector monitors hot water outlet Produces 3-15 psi output signal for controlled variable range of º C Controller compares measured variable signal with setpoint Sends 3-15 psi output to final control element, a 3 inch control valve Figure: Proportional Temperature-Control System ELO 1.5

Proportional Temperature Control Example
Controller set for proportional band of 50% Change of 60º C, causes 100% controller output change Controller is reverse-acting Control valve throttles down to reduce steam flow as hot water outlet temperature increases Control valve opens further to increase steam flow as water temperature decreases Figure: Proportional Temperature-Control System ELO 1.5

Controller Response to Demand Changes
Purpose of system is to provide hot water at setpoint of 70º C System must handle demand disturbances that affect outlet temperature Controller set up to function as shown in figure Figure: Proportional-Controller Characteristics ELO 1.5

If measured variable drops below setpoint Positive error is developed Control valve opens further Figure: Proportional-Controller Characteristics ELO 1.5

If measured variable goes above setpoint Negative error developed Control valve throttles down (opening is reduced) Figure: Proportional-Controller Characteristics ELO 1.5

50% proportional band causes full stroke of valve between a + 30 ºC error and a -30 ºC error Figure: Proportional-Controller Characteristics ELO 1.5

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 1.5

Temperature and Pressure Controller Operation
Knowledge Check 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 ________ the temperature control valve, causing the actual heat exchanger lube oil outlet temperature to ________. open; increase close; decrease open; decrease close; increase Correct answer is C. Correct answer is C. ELO 1.5

Operation of a Speed Controller
ELO 1.6 – 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 ELO 1.6

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 1.6

Ballhead force balanced by force of compression of a speeder 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 1.6

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 1.6

Simple Mechanical Governor
For example, load on a diesel engine is increased Speed decreases Flyweights move inward Speeder rod lowers Directs more fuel to the engine Figure: Mechanical Governor ELO 1.6

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 1.6

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 1.6

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; fuel load; air speed; air load; fuel Correct answer is A. Correct answer is A. NRC Bank P218 ELO 1.6

Knowledge Check – NRC Bank If the turbine shaft speed signal received by a typical turbine governor control system fails low during turbine startup, the turbine governor will cause turbine speed to... decrease to a minimum speed setpoint. increase, until the mismatch with demanded turbine speed is nulled. decrease, until the mismatch with demanded turbine speed is nulled. increase, until an upper limit is reached or the turbine trips on overspeed. Correct answer is D. Correct answer is D. NRC Bank P417 ELO 1.6

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 1.6

Bistable Operation ELO 1.7 – 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 to prevent excessive cycling No directly related NRC KAs ELO 1.7

Figure: Bistable Symbols
In most cases, bistables indicated by box or circle 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 Two mouse clicks to reveal entire slide contents. Figure: Bistable Symbols ELO 1.7

Bistable Operation Knowledge Check
Which of the following bistable trips on an increasing signal and resets on different signal decreasing? A. B. C. D. Correct answer is C. Correct answer is C. ELO 1.7

Interpret Logic Diagrams
ELO 1.8 – Interpret logic diagrams and determine controller outputs. 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 Related KA s - K1.08 Theory of operation of the following types of controllers: electronic, electrical, and pneumatic ELO 1.8

Introduction to Logic Diagrams
Logic symbology allows 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 The logic symbols, or gates, depict operation/start/stop circuits of components and systems ELO 1.8

Logic Symbology Three basic types of logic gates: AND OR NOT
Each gate is a very simple device that only has two states, on and off. Five mouse clicks to reveal entire slide contents. ELO 1.8

Logic Symbology 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 Three additional mouse clicks to reveal entire slide contents. ELO 1.8

Logic Symbology AND gate – provides output (on) when all its inputs are on When any one of inputs is off, gate's output is off OR gate – provides output (on) when any one or more of its inputs is on Gate is off only when all inputs are off NOT gate – provides reversal of input If input is on, output off If input is off, output on Two additional mouse clicks to reveal entire slide contents. ELO 1.8

Logic Symbology NOT gate is frequently used with AND and OR gates,
Special symbols represent these combinations Combination of an AND gate and a NOT gate is called a NAND gate Combination of an OR gate with a NOT gate is called a NOR gate Two additional mouse clicks to reveal entire slide contents. ELO 1.8

Logic Symbology NAND gate - opposite (NOT) of AND gate's output
Provides output (on) except when all inputs are on NOR gate - opposite (NOT) of OR gate's output Provides output only when all inputs are off ELO 1.8

Logic Symbology ELO 1.8

Reading Basic Logic Diagrams
Step What Happens 1 Determine all inputs to a logic gate. 2 Determine the normal state of each logic gate. 3 Determine the effect of the input state on the gate. 4 Determine the output state of the logic gate. 5 Carry the output of the logic gate to the next circuit component(s). ELO 1.8

Logic Diagram Example Answer Discussion – Any combination where the 1 or 2/3 logic is met will result in a close signal; an open signal is received when Close logic is NOT met. The only combination where it is not met is in D because 1 is off and 3 is off. Refer to the valve controller logic diagram in the figure. Which one of the following combinations of inputs will result in the valve receiving an open signal? Inputs 1. 2. 3. A. On Off B. C. D. One click reveals answer. ELO 1.8

Logic Diagrams Knowledge Check
In the logic drawing to the right, in order to get a signal out of the "OR gate," how many total signals must be present? One Two Three Four Correct answer is A. Correct answer is A. ELO 1.8

Logic Diagrams Knowledge Check
In the logic drawing below, what signals must be present to open the valve? (Select all that apply.) 1 2, 3, or none 2 or 3 1 and 2 Correct answer is C. Correct answer is C. 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
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
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" Four mouse clicks to reveal entire slide contents. Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

Figure: Pneumatic Actuator with Controller and Positioner
Positioners The output pressure of a pneumatic controller is typically insufficient to drive a valve actuator accurately Valve will not operate as designed Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

Figure: Pneumatic Actuator with Controller and Positioner
Positioners To overcome this problem, a valve operating control loop uses a positioner to provide sufficient pressure for proper operation Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

NRC Exam Example The output pressure of a pneumatic controller is typically insufficient to drive a valve actuator accurately. To overcome this problem, a valve operating control loop would normally employ a... valve actuating lead/lag unit. pressure regulator. valve positioner. pressure modulator. Correct answer is C. ELO 1.9

NRC Exam Example 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. ELO 1.9

Figure: Piston-Type Hydraulic Actuated Control Valve
Hydraulic Actuators Pneumatic actuators are normally used to control processes requiring quick and accurate response, do not require a large amount of motive force If large amount of force is required to operate a valve (for example, large steam system valves), hydraulic actuators are normally used Hydraulic actuators - many designs Piston types most common Figure: Piston-Type Hydraulic Actuated Control Valve 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 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 Initially, with no hydraulic fluid pressure, spring force holds valve in closed position As fluid enters lower chamber, pressure in chamber increases Results in force on bottom of piston opposite to force caused by spring When hydraulic force is greater than spring force, piston begins to move upward Spring compresses, and valve begins to open As hydraulic pressure increases, valve continues to open If hydraulic oil is drained from cylinder, hydraulic force becomes less than spring force Piston moves downward, and valve closes By regulating amount of oil supplied or drained from actuator, valve is positioned between fully open and fully closed Figure: Piston-Type Hydraulic Actuator ELO 1.9

Hydraulic Actuator Operation
Operation of hydraulic actuator like pneumatic actuator Each uses motive force to overcome spring force to move valve Can also be designed to fail-open or fail-closed to provide a fail-safe feature 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

Electric Solenoid Actuator Design
When current flows through coil, magnetic field forms around coil Attracts armature toward center of coil As armature moves upward, spring collapses and valve opens When circuit is opened and current stops flowing to coil, magnetic field collapses Allows spring to expand and shut valve 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
Electric motor actuators vary widely in their design and applications Some electric 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 and 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 Limit switches: de-energize motor when valve reaches specific position Torque limiters: de-energize motor when amount of turning force reaches specified value Turning force greatest when valve reaches fully open/closed position Can prevent damage to actuator/valve if valve binds in an intermediate position Figure: Motor Actuator One additional mouse click to reveal entire slide contents. ELO 1.9

Knowledge Check Select all that apply. A major advantage(s) of solenoid actuators is... they are easier to install. they produce much force. quick operation. they have only two positions. Correct answers are A and C. Correct answers are A and C. ELO 1.9

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 P = F/A A = r2 A = x 36 A = 113.1 P = 2400/113/1 P = 21.2 psig, so closest answer is A Correct answer is A. 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 Correct answer is A. ELO 1.9

Controllers and Positioners Summary
Now that you have completed this lesson, you should be able to: Describe the arrangement and operation of typical controllers and positioners within process control systems.

Describe the characteristics of a control system, including process controllers and position controllers. Define the following process control related terms: proportional band, gain, closed loop system, offset, feedback, deviation, deadband, direct acting, and reverse acting. Describe the operation of an automatic controller, including proportional control system, proportional-integral (PI) control, proportional-derivative (DP) control, and proportional-integral- derivative (PID) control. Describe the operation of a controller in the automatic and manual modes. Describe the operation of temperature controllers and pressure controllers.

Describe the operation of mechanical and electronic speed- control devices. Describe the operation of bistable alarm and control circuits. Interpret logic diagrams and determine controller outputs. Describe the design and operation of the following types of valve actuators: pneumatic, hydraulic, solenoid, and electric motor.

Operator Generic Fundamentals Components - Controllers and Positioners

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Operator Generic Fundamentals Components - Controllers and Positioners

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