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© Copyright 2014Operator Generic Fundamentals Operator Generic Fundamentals Components - Controllers and Positioners.

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1 © Copyright 2014Operator Generic Fundamentals Operator Generic Fundamentals Components - Controllers and Positioners

2 © Copyright 2014Operator Generic Fundamentals 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 2 Introduction

3 © Copyright 2014Operator Generic Fundamentals 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. 3 TLO’s

4 © Copyright 2014Operator Generic Fundamentals Enabling Learning Objectives for TLO Describe the characteristics of a control system, including process controllers and position controllers. 2.Define the following process control related terms: proportional band, gain, closed loop system, offset, feedback, deviation, deadband, direct acting, and reverse acting. 3.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. 4.Describe the operation of a controller in the automatic and manual modes. 5.Describe the operation of temperature controllers and pressure controllers. ELOs

5 © Copyright 2014Operator Generic Fundamentals Enabling Learning Objectives for TLO 1 6.Describe the operation of mechanical and electronic speed- control devices. 7.Describe the operation of bistable alarm and control circuits. 8.Interpret logic diagrams and determine controller outputs. 9.Describe the design and operation of the following types of valve actuators: pneumatic, hydraulic, solenoid, and electric motor. 5 ELOs

6 © Copyright 2014Operator Generic Fundamentals TLO Introduction 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 6 TLO 1 – Describe the arrangement and operation of typical controllers and positioners within process control systems.

7 © Copyright 2014Operator Generic Fundamentals Characteristics of Controllers and Positioners 7 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 ELO 1.1

8 © Copyright 2014Operator Generic Fundamentals Process Controllers Controller 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 8 ELO 1.1

9 © Copyright 2014Operator Generic Fundamentals Process Controllers Transducer 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 9 ELO 1.1

10 © Copyright 2014Operator Generic Fundamentals Operation of a Simple Controller 10 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 ELO 1.1 Figure: Process Control System Operation

11 © Copyright 2014Operator Generic Fundamentals Two-Position Controller Simplest type of controller Device that has two operating conditions: –Completely on –Completely off Termed Bistable 11 ELO 1.1

12 © Copyright 2014Operator Generic Fundamentals 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 12 ELO 1.1 Figure: Input/Output Relationship for a Two-Position Controller

13 © Copyright 2014Operator Generic Fundamentals 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 13 ELO 1.1 Figure: Two-Position Control System

14 © Copyright 2014Operator Generic Fundamentals Characteristics of Controllers and Positioners 14 ELO 1.1 Knowledge Check A two-position controller will switch to the ______________ state when its measured variable increases above the high-level setpoint. A.off B.throttle C.on D.open Correct answer is C.

15 © Copyright 2014Operator Generic Fundamentals Process Control Terms 15 ELO 1.2 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

16 © Copyright 2014Operator Generic Fundamentals Process Control Terms Closed-Loop System 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 16 ELO 1.2

17 © Copyright 2014Operator Generic Fundamentals Process Control Terms Feedback 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 17 ELO 1.2

18 © Copyright 2014Operator Generic Fundamentals Process Control Terms Direct 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. 18 ELO 1.2

19 © Copyright 2014Operator Generic Fundamentals Process Control Terms 19 ELO 1.2 Correct answer is D. Knowledge Check – NRC Bank The difference between the setpoint in an automatic controller and the steady-state value of the controlled parameter is called ________. A.feedback B.deadband C.gain D.offset

20 © Copyright 2014Operator Generic Fundamentals 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... A.feedback B.error C.gain D.bias Correct answer is A. 20 ELO 1.2

21 © Copyright 2014Operator Generic Fundamentals Operation of an Automatic Controller 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 21 ELO 1.3 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).

22 © Copyright 2014Operator Generic Fundamentals 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] 22 ELO 1.3

23 © Copyright 2014Operator Generic Fundamentals 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 23 ELO 1.3

24 © Copyright 2014Operator Generic Fundamentals 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 24 ELO 1.3 Figure: Proportional System Controller

25 © Copyright 2014Operator Generic Fundamentals Proportional Level Controller Example 25 ELO 1.3 Figure: Proportional Controller

26 © Copyright 2014Operator Generic Fundamentals Proportional Level Controller Example 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 26 ELO 1.3 Figure: Proportional System Controller

27 © Copyright 2014Operator Generic Fundamentals 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 27 ELO 1.3

28 © Copyright 2014Operator Generic Fundamentals 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 28 ELO 1.3

29 © Copyright 2014Operator Generic Fundamentals 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 29 ELO 1.3 Figure: Integral Controller Output for a Fixed Input

30 © Copyright 2014Operator Generic Fundamentals Integral Flow Control System Example 30 ELO 1.3

31 © Copyright 2014Operator Generic Fundamentals 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 31 ELO 1.3 Figure: Integral Flow-Rate Controller

32 © Copyright 2014Operator Generic Fundamentals Integral Flow Control System Example – Controller Operation Measured variable decreases from 50 gpm to 45 gpm ⇒ positive error of 10% applied to input of controller –Controller has a constant of 0.1 seconds -1; controller output magnitude is 1% per second –Controller output increases from initial point of 50% at 1% per second –Causes control valve to open further at rate of 1% per second ⇒ increasing flow 32 ELO 1.3 Figure: Integral Controller Response

33 © Copyright 2014Operator Generic Fundamentals Integral Flow Control System Example – Controller Operation Controller acts to return process to setpoint –Repositions control valve –Measured variable moves closer to setpoint –New error signal is produced –Cycle repeats until no error exists 33 ELO 1.3 Figure: Integral Controller Response

34 © Copyright 2014Operator Generic Fundamentals Integral Flow Control System Example – Controller Operation Controller responds to amplitude and duration of error signal –Can cause final control element to reach "fully open/shut" position before error reaches zero –Final control element could remain at extreme position –Error must be reduced by other means 34 ELO 1.3 Figure: Integral Controller Response

35 © Copyright 2014Operator Generic Fundamentals Combination of proportional and integral modes of control Combining two modes results in gaining advantages and compensating for disadvantages of two individual modes 35 ELO 1.3 Proportional Integral Control

36 © Copyright 2014Operator Generic Fundamentals 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 36 ELO 1.3 Proportional-Integral Control Figure: Response of PI Control

37 © Copyright 2014Operator Generic Fundamentals 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 37 ELO 1.3 Figure: Response of Proportional-Integral Control Proportional-Integral Control

38 © Copyright 2014Operator Generic Fundamentals Proportional-Integral Control Example Heat exchanger system - equipped with proportional- integral controller 38 ELO 1.3 Figure: Heat Exchanger Process with PI Control

39 © Copyright 2014Operator Generic Fundamentals 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 39 Proportional-Integral Control Example ELO 1.3 Figure: Effects of Disturbance on a PI Controller

40 © Copyright 2014Operator Generic Fundamentals 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 40 Proportional-Integral Control Example ELO 1.3 Figure: Effects of Disturbance on a PI Controller

41 © Copyright 2014Operator Generic Fundamentals 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 41 ELO 1.3

42 © Copyright 2014Operator Generic Fundamentals Proportional-Derivative Control Systems Control mode in which derivative section is added to proportional controller Derivative section responds to rate of change of error signal, not amplitude of error –Causes controller output to be initially larger in direct relation with error signal rate of change o Higher error signal rate of change ⇒ sooner final control element is positioned to desired value Added derivative action reduces initial overshoot of measured variable 42 ELO 1.3

43 © Copyright 2014Operator Generic Fundamentals 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 43 ELO 1.3 Figure: Derivative Output for a Constant Rate of Change

44 © Copyright 2014Operator Generic Fundamentals 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 44 ELO 1.3 Figure: Derivative-Control Output

45 © Copyright 2014Operator Generic Fundamentals Definition 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 45 ELO 1.3 Figure: Response of PD Control

46 © Copyright 2014Operator Generic Fundamentals Proportional-Derivative Control Example Same heat exchanger system as previously analyzed Temperature controller now uses PD controller 46 ELO 1.3 Figure: Heat Exchanger Process with PD Control

47 © Copyright 2014Operator Generic Fundamentals 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 47 ELO 1.3 Figure: Effect of Disturbance on a PD Controller

48 © Copyright 2014Operator Generic Fundamentals 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 48 ELO 1.3

49 © Copyright 2014Operator Generic Fundamentals 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 49 ELO 1.3

50 © Copyright 2014Operator Generic Fundamentals 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 50 ELO 1.3 Figure: PID Control Responses

51 © Copyright 2014Operator Generic Fundamentals 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 51 ELO 1.3 Figure: PID Control Responses Proportional-Integral-Derivative Controller Response

52 © Copyright 2014Operator Generic Fundamentals 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 52 ELO 1.3 Figure: PID Controller Response Curves

53 © Copyright 2014Operator Generic Fundamentals Operation of an Automatic Controller Knowledge Check The water level in a tank is being controlled by an automatic level controller and is initially at the controller setpoint. A drain valve is then opened, causing tank level to decrease. The decreasing level causes the controller to begin to open a makeup water supply valve. After a few minutes, a new steady-state tank level below the original level is established, with the supply rate equal to the drain rate. The controller in this system uses __________ control. A.proportional only B.proportional, integral, and derivative C.proportional and integral D.proportional and derivative Correct answer is A. 53 ELO 1.3

54 © Copyright 2014Operator Generic Fundamentals Operation of an Automatic Controller 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 ________. A.open; decrease B.open; increase C.close; decrease D.close; increase Correct answer is A. 54 ELO 1.3

55 © Copyright 2014Operator Generic Fundamentals Operation of an Automatic Controller Knowledge Check – NRC Bank Which one of the following describes the response of a direct acting proportional-integral controller, operating in automatic mode, to an increase in the controlled parameter above the controller set point? A.The controller will develop an output signal that continues to increase until the controlled parameter equals the controller set point, at which time the output signal stops increasing. B.The controller will develop an output signal that will remain directly proportional to the difference between the controlled parameter and the controller set point. C.The controller will develop an output signal that continues to increase until the controlled parameter equals the controller set point, at which time the output signal becomes zero. D.The controller will develop an output signal that will remain directly proportional to the rate of change of the controlled parameter. Correct answer is A. 55 ELO 1.3

56 © Copyright 2014Operator Generic Fundamentals Operation of an Automatic Controller 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. A.proportional integral, and derivative B.proportional and integral C.proportional only D.bistable Correct answer is C. 56 ELO 1.3

57 © Copyright 2014Operator Generic Fundamentals Automatic and Manual Controller Operation 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. 57 ELO 1.4 ELO 1.4 – Describe the operation of a controller in automatic and manual modes.

58 © Copyright 2014Operator Generic Fundamentals Controllers Digital controller –Popular controller found in industrial applications –Extremely versatile –Can be adapted to control various types of industrial equipment and processes 58 ELO 1.4

59 © Copyright 2014Operator Generic Fundamentals 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 59 ELO 1.4 Figure: Typical Digital Controller

60 © Copyright 2014Operator Generic Fundamentals 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 60 ELO 1.4 Figure: Typical Digital Controller

61 © Copyright 2014Operator Generic Fundamentals 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 61 ELO 1.4

62 © Copyright 2014Operator Generic Fundamentals 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 62 ELO 1.4

63 © Copyright 2014Operator Generic Fundamentals 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 63 ELO 1.4 Figure: Pneumatic Control System - PWR

64 © Copyright 2014Operator Generic Fundamentals 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. System Response Practice Question If personnel manually decrease the level control signal, how will the pneumatic control system affect SG level A.Level will decrease because the valve positioner will close more, reducing control air pressure, which causes the feed control valve to close more. B.Level will decrease because the valve positioner will open more, increasing control air pressure, which causes the feed control valve to close more. C.Level will increase because the valve positioner will close more, reducing control air pressure, which causes the feed control valve to open more. D.Level will increase because the valve positioner will open more, increasing control air pressure, which causes the feed control valve to open more. 64 ELO 1.4 Figure: Pneumatic Control System - PWR

65 © Copyright 2014Operator Generic Fundamentals 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 ___________. A.plant computer B.operator C.flow error signal D.system feedback Correct answer is B. 65 ELO 1.4

66 © Copyright 2014Operator Generic Fundamentals 66 ELO 1.5 – Describe the operation of temperature controllers and pressure controllers. Temperature and Pressure Controller Operation ELO 1.5 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.

67 © Copyright 2014Operator Generic Fundamentals 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 67 ELO 1.5 Figure: Proportional Temperature-Control System

68 © Copyright 2014Operator Generic Fundamentals 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 68 ELO 1.5 Figure: Proportional Temperature-Control System

69 © Copyright 2014Operator Generic Fundamentals 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 69 ELO 1.5 Controller Response to Demand Changes Figure: Proportional-Controller Characteristics

70 © Copyright 2014Operator Generic Fundamentals If measured variable drops below setpoint –Positive error is developed –Control valve opens further 70 ELO 1.5 Controller Response to Demand Changes Figure: Proportional-Controller Characteristics

71 © Copyright 2014Operator Generic Fundamentals If measured variable goes above setpoint –Negative error developed –Control valve throttles down (opening is reduced) 71 ELO 1.5 Controller Response to Demand Changes Figure: Proportional-Controller Characteristics

72 © Copyright 2014Operator Generic Fundamentals 50% proportional band causes full stroke of valve between a + 30 ºC error and a -30 ºC error 72 ELO 1.5 Controller Response to Demand Changes Figure: Proportional-Controller Characteristics

73 © Copyright 2014Operator Generic Fundamentals 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 73 ELO 1.5 Controller Response to Demand Changes Figure: Proportional-Controller Characteristics

74 © Copyright 2014Operator Generic Fundamentals Correct answer is C. 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 ________. A.open; increase B.close; decrease C.open; decrease D.close; increase 74 ELO 1.5

75 © Copyright 2014Operator Generic Fundamentals 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 75 ELO 1.6 Operation of a Speed Controller ELO 1.6 – Describe the operation of mechanical and electronic speed control devices.

76 © Copyright 2014Operator Generic Fundamentals 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 76 ELO 1.6 Speed Controllers/Governors

77 © Copyright 2014Operator Generic Fundamentals 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 77 ELO 1.6 Speed Controllers/Governors Figure: Mechanical Speed Sensor

78 © Copyright 2014Operator Generic Fundamentals 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 78 ELO 1.6 Speed Controllers/Governors

79 © Copyright 2014Operator Generic Fundamentals Simple Mechanical Governor 79 ELO 1.6 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

80 © Copyright 2014Operator Generic Fundamentals Speed Controllers/Governors Electronic Speed Teeth attached to rotating shaft rotate through a magnetic field of a permanent magnet –Electrical pulse is induced in a pickup coil –Electrical signal compared to desired speed –Throttles adjust supplied steam accordingly Used to control speed of steam turbine –Turbine may have an additional wheel with 60 teeth on the turbine shaft Overspeed trip mechanism may be similar to the speed sensor –Mechanical arrangement provides a reliable method to protect equipment 80 ELO 1.6

81 © Copyright 2014Operator Generic Fundamentals Speed Controllers/Governors Example: Electrical signal from a steam turbine governor failed low –Speed control governor continues to open –Turbine throttles to raise speed –As the turbine speed increases, Electronic signal feeds the new speed back to the governor and throttle position adjusts as necessary –Electric speed indication is low no matter what the actual turbine speed is so the governor will keep trying to open the throttles –Turbine speed would increase until mechanical overspeed trip point is reached shutting the throttles 81 ELO 1.6

82 © Copyright 2014Operator Generic Fundamentals Operation of a Speed Controller Knowledge Check – NRC Bank An emergency diesel generator (D/G) is operating as the only power source connected to an emergency bus. The governor of the D/G is directly sensing D/G __________ and will directly adjust D/G __________ flow to maintain a relatively constant D/G frequency. A.speed; fuel B.load; air C.speed; air D.load; fuel Correct answer is A. 82 ELO 1.6

83 © Copyright 2014Operator Generic Fundamentals 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... A.decrease to a minimum speed setpoint. B.increase, until the mismatch with demanded turbine speed is nulled. C.decrease, until the mismatch with demanded turbine speed is nulled. D.increase, until an upper limit is reached or the turbine trips on overspeed. Correct answer is D. 83 ELO 1.6 Operation of a Speed Controller

84 © Copyright 2014Operator Generic Fundamentals Knowledge Check – NRC Bank In a flyball-weight mechanical speed governor, the purpose of the spring on the flyball mechanism is to ____________ centrifugal force by driving the flyballs ___________. A.counteract; apart B.aid; together C.counteract; together D.aid; apart Correct answer is C. 84 ELO 1.6 Operation of a Speed Controller

85 © Copyright 2014Operator Generic Fundamentals Bistable Operation 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 85 ELO 1.7 ELO 1.7 – Describe the operation of bistable alarm and control circuits.

86 © Copyright 2014Operator Generic Fundamentals Bistables 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 86 Figure: Bistable Symbols ELO 1.7

87 © Copyright 2014Operator Generic Fundamentals Bistable Operation Knowledge Check Which of the following bistable trips on an increasing signal and resets on different signal decreasing? A. B. C. D. 87 ELO 1.7 Correct answer is C.

88 © Copyright 2014Operator Generic Fundamentals Interpret Logic Diagrams 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 88 ELO 1.8 ELO 1.8 – Interpret logic diagrams and determine controller outputs.

89 © Copyright 2014Operator Generic Fundamentals 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 89 Introduction to Logic Diagrams ELO 1.8

90 © Copyright 2014Operator Generic Fundamentals 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. 90 ELO 1.8

91 © Copyright 2014Operator Generic Fundamentals 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 91 ELO 1.8

92 © Copyright 2014Operator Generic Fundamentals 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 92 ELO 1.8

93 © Copyright 2014Operator Generic Fundamentals 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 93 ELO 1.8

94 © Copyright 2014Operator Generic Fundamentals 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 94 ELO 1.8

95 © Copyright 2014Operator Generic Fundamentals Logic Symbology 95 ELO 1.8

96 © Copyright 2014Operator Generic Fundamentals Reading Basic Logic Diagrams 96 StepWhat Happens 1Determine all inputs to a logic gate. 2Determine the normal state of each logic gate. 3Determine the effect of the input state on the gate. 4Determine the output state of the logic gate. 5 Carry the output of the logic gate to the next circuit component(s). ELO 1.8

97 © Copyright 2014Operator Generic Fundamentals 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? Logic Diagram Example 97 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. Inputs A.OnOff B.OffOn C.OnOffOn D.OffOnOff ELO 1.8

98 © Copyright 2014Operator Generic Fundamentals Logic Diagrams 98 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? A.One B.Two C.Three D.Four Correct answer is A. ELO 1.8

99 © Copyright 2014Operator Generic Fundamentals Logic Diagrams 99 Knowledge Check In the logic drawing below, what signals must be present to open the valve? (Select all that apply.) A.1 B.2, 3, or none C.2 or 3 D.1 and 2 Correct answer is C. ELO 1.8

100 © Copyright 2014Operator Generic Fundamentals Types of Valve Actuators 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 100 ELO 1.9 ELO 1.9 – Describe the design and operation of the following types of valve actuators: pneumatic, hydraulic, solenoid, and electric motor.

101 © Copyright 2014Operator Generic Fundamentals 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 101 ELO 1.9 Figure: Pneumatic-Actuated Control Valve

102 © Copyright 2014Operator Generic Fundamentals Pneumatic Valve Actuator 102 ELO 1.9 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

103 © Copyright 2014Operator Generic Fundamentals 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" 103 Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

104 © Copyright 2014Operator Generic Fundamentals Positioners The output pressure of a pneumatic controller is typically insufficient to drive a valve actuator accurately Valve will not operate as designed 104 Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

105 © Copyright 2014Operator Generic Fundamentals Positioners To overcome this problem, a valve operating control loop uses a positioner to provide sufficient pressure for proper operation 105 Figure: Pneumatic Actuator with Controller and Positioner ELO 1.9

106 © Copyright 2014Operator Generic Fundamentals 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... A.valve actuating lead/lag unit. B.pressure regulator. C.valve positioner. D.pressure modulator. Correct answer is C. 106 ELO 1.9

107 © Copyright 2014Operator Generic Fundamentals NRC Exam Example The purpose of the valve positioner is to convert... A.a small control air pressure into a proportionally larger air pressure to adjust valve position. B.a large control air pressure into a proportionally smaller air pressure to adjust valve position. C.pneumatic force into mechanical force to adjust valve position. D.mechanical force into pneumatic force to adjust valve position. Correct answer is A. 107 ELO 1.9

108 © Copyright 2014Operator Generic Fundamentals 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 108 ELO 1.9 Figure: Piston-Type Hydraulic Actuated Control Valve

109 © Copyright 2014Operator Generic Fundamentals 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 109 ELO 1.9 Figure: Piston-Type Hydraulic Actuated Control Valve

110 © Copyright 2014Operator Generic Fundamentals 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 110 ELO 1.9 Figure: Piston-Type Hydraulic Actuator

111 © Copyright 2014Operator Generic Fundamentals 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 111 ELO 1.9

112 © Copyright 2014Operator Generic Fundamentals 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 112 ELO 1.9 Figure: Electric Solenoid Actuator

113 © Copyright 2014Operator Generic Fundamentals 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 113 ELO 1.9 Figure: Electric Solenoid Actuator

114 © Copyright 2014Operator Generic Fundamentals Solenoid Actuator Advantages & Disadvantages Advantages Quick operation Easier to install than pneumatic or hydraulic actuators Disadvantages Only two positions: fully open and fully closed Don’t produce much force ⇒ usually only operate relatively small valves 114 ELO 1.9

115 © Copyright 2014Operator Generic Fundamentals 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 115 ELO 1.9 Figure: Motor Actuator

116 © Copyright 2014Operator Generic Fundamentals 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 116 ELO 1.9 Electric Motor Actuator Design & Operation Figure: Motor Actuator

117 © Copyright 2014Operator Generic Fundamentals 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 117 ELO 1.9 Electric Motor Actuator Design & Operation Figure: Motor Actuator

118 © Copyright 2014Operator Generic Fundamentals 118 Knowledge Check Select all that apply. A major advantage(s) of solenoid actuators is... A.they are easier to install. B.they produce much force. C.quick operation. D.they have only two positions. Correct answers are A and C. ELO 1.9 Types of Valve Actuators

119 © Copyright 2014Operator Generic Fundamentals Knowledge Check An air-operated isolation valve requires 2,400 pounds-force applied to the top of the actuator diaphragm to open. The actuator diaphragm has a diameter of 12 inches. If control air pressure to the valve actuator begins to increase from 0 psig, which one of the following is the approximate air pressure at which the valve will begin to open? A.21 psig B.34 psig C.43 psig D.64 psig Correct answer is A. 119 ELO 1.9 P = F/A A = r 2 A = x 36 A = P = 2400/113/1 P = 21.2 psig, so closest answer is A Types of Valve Actuators

120 © Copyright 2014Operator Generic Fundamentals 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. 120

121 © Copyright 2014Operator Generic Fundamentals Enabling Learning Objectives for TLO Describe the characteristics of a control system, including process controllers and position controllers. 2.Define the following process control related terms: proportional band, gain, closed loop system, offset, feedback, deviation, deadband, direct acting, and reverse acting. 3.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. 4.Describe the operation of a controller in the automatic and manual modes. 5.Describe the operation of temperature controllers and pressure controllers.

122 © Copyright 2014Operator Generic Fundamentals Enabling Learning Objectives for TLO Describe the operation of mechanical and electronic speed- control devices. 7.Describe the operation of bistable alarm and control circuits. 8.Interpret logic diagrams and determine controller outputs. 9.Describe the design and operation of the following types of valve actuators: pneumatic, hydraulic, solenoid, and electric motor.


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