Fundamentals of Instrumentation& Process ControlInteractive Training Workshop
Introduction to Process Control       A common misconception in process control is that it is all       about the controll...
Outline of the CourseCopyright © 2007 Control Station, Inc. All Rights Reserved   3
Introduction to Process Control       Objectives:            Why do we need process control?            What is a process?...
Motivation for Automatic Process Control                        Safety First:                             people, environm...
“Loose” Control Costs Money                                       65                          operating constraint        ...
“Tight” Control is More Profitable                                       65                              operating constra...
Terminology for Home Heating Control         Control Objective         Measured Process Variable (PV)         Set Point (S...
What is Process Control       Terminology:            The manipulated variable (MV) is a measure of resource            be...
What is a Process       A process is broadly defined as an operation       that uses resources to transform inputs into   ...
What is a ProcessCopyright © 2007 Control Station, Inc. All Rights Reserved   11
What is a Process       Each process exhibits a particular dynamic (time       varying) behavior that governs the       tr...
What is a Process       Can you identify some of the elements that will       determine the dynamic properties of this    ...
What is Process Control       Process control is the act of controlling a final       control element to change the manipu...
What is Process ControlCopyright © 2007 Control Station, Inc. All Rights Reserved   15
What is Process ControlCopyright © 2007 Control Station, Inc. All Rights Reserved   16
Section Assessment:                   Basic Terminology AssessmentCopyright © 2007 Control Station, Inc. All Rights Reserv...
What is Open Loop Control       In open loop control the controller output is not       a function of the process variable...
What is Open Loop ControlCopyright © 2007 Control Station, Inc. All Rights Reserved   19
What is Open Loop Control       Can you think of processes in which open loop       control is sufficient?Copyright © 2007...
What is Closed Loop Control       In closed loop control the controller output is       determined by difference between t...
What is Closed Loop ControlCopyright © 2007 Control Station, Inc. All Rights Reserved   22
What is Closed Loop Control       From the controllers       perspective the       process encompasses       the RTD, the ...
What are the Modes of Closed Loop Control       Closed loop control can be, depending on the       algorithm that determin...
What are the Modes of Closed Loop Control   Manual Control       In manual control an operator directly       manipulates ...
What are the Modes of Closed Loop Control   On-Off Control       On-Off control provides a controller output of       eith...
What are the Modes of Closed Loop Control   On-Off Control Deadband       Upon changing the direction of the controller   ...
What are the Modes of Closed Loop Control   PID Control       PID control provides a controller output that       modulate...
What are the Modes of Closed Loop Control   Time Proportion Control       Time proportion control is a variant of PID cont...
What are the Modes of Closed Loop Control   Cascade Control       Cascade control uses the output of a primary (master or ...
What are the Modes of Closed Loop Control   Cascade ControlCopyright © 2007 Control Station, Inc. All Rights Reserved   31
Basic Elements of Process Control       Controlling a process requires knowledge of four       basic elements, the process...
Section Assessment:                   Basic Process Control AssessmentCopyright © 2007 Control Station, Inc. All Rights Re...
Understanding Dynamic Process Behavior            What We Will Learn in This Section                Dynamic Process Behavi...
Dynamic Process Behavior and Controller Tuning              Consider cruise control for a car vs a truck                 h...
Graphical Modeling of Dynamic Process Data            To learn about the dynamic behavior of a process, we            anal...
Modeling Dynamic Process Behavior          The best way to understand process data is through modeling          Modeling m...
Modeling Dynamic Process Behavior                   When a first order plus dead time (FOPDT) model                   is f...
The FOPDT Model is All Important          FOPDT model parameters (Kp, τ p and Өp) are used in          correlations to com...
Step Test Data and Dynamic Process Modeling                              Manual Mode Step Test on Gravity Drained Tanks   ...
Process Gain (Kp) from Step Test Data             Kp describes how far the measured PV travels in             response to ...
Process Gain (Kp) for Gravity-Drained Tanks                                                                        ∆PV = (...
Process Gain (Kp) for Gravity-Drained Tanks                                                                               ...
Additional Notes on Process Gain   Measuring the Process Gain       Process gain as seen by a controller is the       prod...
Additional Notes on Process Gain   Converting Units of Process Gain       There is one important caveat in this process; t...
Additional Notes on Process Gain   Converting Units of Process Gain      Assuming the process gain of the gravity      dra...
Additional Notes on Process Gain   Values for Process Gain       Process gain that the controller sees is influenced      ...
Additional Notes on Process Gain   Values for Process Gain       As a rule of thumb:            Process gains that are gre...
Additional Notes on Process Gain   Values for Process Gain       The result of a final control element being too large (hi...
Additional Notes on Process Gain   Values for Process Gain             The general rule of thumb is the             proces...
Process Time Constant τ p ) from Step Test Data                         (                                                 ...
Process Time Constant τ p ) from Step Test Data                         (                                                 ...
Process Time Constant τ p ) from Step Test Data                         (                                  PV63.2         ...
Process Time Constant τ p ) from Step Test Data                         (                                  PV63.2         ...
Time Constant (τ p) for Gravity-Drained Tanks                                                             Copyright © 2007...
Time Constant (τ p ) for Gravity-Drained Tanks                              PV63.2 = 2.5                                  ...
Time Constant (τ p ) for Gravity-Drained Tanks                              PV63.2 = 2.5                                  ...
Process Dead-Time (Өp) from Step Test Data           Dead time, Өp, is how much delay occurs from the time when           ...
Dead-Time (Өp) is the Killer of Control          Tight control grows increasingly difficult as Өp becomes large          T...
Process Dead-Time (Өp) from Step Test Data                                                       Copyright © 2007 by Contr...
Dead-Time (Өp) for Gravity Drained Tanks                                                Өp = 4.1 – 3.8 = 0.3 min          ...
The FOPDT Model Parameters                                               In Summary        For a change in CO:            ...
Hands-On Workshop           Workshop #1           Exploring Dynamics of Gravity-Drained TanksCopyright © 2007 Control Stat...
Processes have Time Varying Behavior         The CO to PV behavior described by an ideal FOPDT model is         constant, ...
Processes have Nonlinear Behavior                                                                  …cause changing (nonlin...
Processes have Nonlinear Behavior                              3) but behavior of real processes                          ...
What is a Nonlinear Process       While linear processes may be the design goal of process       engineers, the reality is...
What is a Nonlinear Process   Dealing with Nonlinearity – Set Point ResponseCopyright © 2007 Control Station, Inc. All Rig...
What is a Nonlinear Process   Dealing with Nonlinearity       The robustness of a controller is a measure the       range ...
What is Process Action       Process action is how the process variable changes with       respect to a change in the cont...
What is Process Action   Process and Controller Action       The action of a process is important because it will       de...
Fundamentals of Instrumentation& Process ControlInteractive Training Workshop
Introduction to Instrumentation       The intent of this chapter is neither to teach you how to       select a particular ...
Introduction to Instrumentation       Objectives:         What are sensors and transducers?         What are the standard ...
Introduction to Instrumentation       You cannot control what you             cannot measure                            “W...
What are Sensors and Transducers       A sensor is a device that has a characteristic that changes       in a predictable ...
What are Standard Instrumentation Signals       Standard instrument signals for controllers to       accept as inputs from...
What are Standard Instrumentation Signals   Pneumatic       3 to 15 psig            Before 1960, pneumatic signals were us...
What are Standard Instrumentation Signals   Pneumatic Scaling       What would our pneumatic signal be if our       contro...
What are Standard Instrumentation Signals   Current Loop       4-20 milliamp            Current loops are the signal workh...
What are Standard Instrumentation Signals   Current Loop Scaling       Output Scaling            Scale outputs for a one-t...
What are Standard Instrumentation Signals   Current Loop Scaling       Input Scaling            Scale inputs for a one-to-...
What are Standard Instrumentation Signals   0 to 10 Volt       0 to 10 volt is not commonly used in control       systems ...
What are Smart Transmitters       A smart transmitter is a digital device that       converts the analog information from ...
What are Smart Transmitters   Digital Communications       Smart transmitters are capable of digital communications       ...
What are Smart Transmitters   Digital Communications       Most smart instruments wired to       multi-channel input cards...
What are Smart Transmitters   Configuration, Signal Conditioning, Self-Diagnosis       Configuration            Smart tran...
What Instrument Properties Affect a Process       The     instrument’s               range and span.       The     resolut...
What Instrument Properties Affect a Process   Range and Span       The range of a sensor is the lowest and highest       v...
What Instrument Properties Affect a Process   Measurement Resolution       Resolution is the smallest amount of input     ...
What Instrument Properties Affect a Process   Accuracy       Accuracy of a measurement describes how close       the measu...
What Instrument Properties Affect a Process   Precision       Precision is the reproducibility with which       repeated m...
What Instrument Properties Affect a Process   Accuracy vs. Precision       Why is precision preferred over accuracy?Copyri...
What Instrument Properties Affect a Process   Instrumentation Dynamics: Gain and Dead Time       The gain of an instrument...
What Instrument Properties Affect a Process   Instrumentation Dynamics: Time Constant       As for processes, one time con...
What is Input Aliasing       Input aliasing is a phenomenon that occurs from       digital processing of a signal.       W...
What is Input Aliasing                 2 Hz waveform sampled every 0.4 secondsCopyright © 2007 Control Station, Inc. All R...
What is Input Aliasing   Correct Sampling Frequency       Nyquist Frequency Theorem:            To correctly sample a wave...
What is Input Aliasing   Correct Sampling Frequency              2 Hz waveform sampled every 0.025 secondsCopyright © 2007...
Workshop Lab:                   Input Aliasing 1Copyright © 2007 Control Station, Inc. All Rights Reserved   100
What is Input Aliasing   Determining the Correct Sampling Interval       Rules of Thumb            Set the sample interval...
What is Input Aliasing   Determining the Correct Sampling IntervalCopyright © 2007 Control Station, Inc. All Rights Reserv...
Workshop Lab:                   Input Aliasing 2Copyright © 2007 Control Station, Inc. All Rights Reserved   103
What is Instrument Noise       Noise is a variation in a measurement of a       process variable that does not reflect rea...
What is Instrument Noise   Sources       Noise is generally a result of the technology used       to sense the process var...
What is Instrument Noise   Effects of Noise       Noise reduces the accuracy and precision of       process measurements. ...
What is Instrument Noise   Eliminating Noise       The most effective means of eliminating noise is       to remove the so...
What is Instrument Noise   Low Pass Filter       A low-pass filter allows the low frequency       components of a signal t...
What is Instrument Noise   Low Pass Filter       A low pass filter introduces a first order lag into       the measurement...
What is Instrument Noise   Selecting a Low Pass Filter       By Cut-Off Frequency            Cut-off frequency is defined ...
What is Instrument Noise   Selecting a Low Pass Filter       By Time Constant            Some filters are configured by se...
What is Instrument Noise   Selecting a Low Pass Filter       By α Value            Some filters are selected by a value ca...
What is Instrument Noise   Selecting a Low Pass Filter       When a filter is specified by cut-off frequency,       the lo...
Workshop Lab:                   Noise FilteringCopyright © 2007 Control Station, Inc. All Rights Reserved   114
What is Temperature       Temperature is a measure of degree of the       hotness or coldness of an object.       Units of...
What is Temperature   Unit Conversion               9               °F =  x °C  + 32               5                ...
What Temperature Instruments Do We Use       Temperature is one of the most common process       variables.       Temperat...
What Temperature Instruments Do We Use   Thermocouples       Thermocouples are fabricated from two electrical       conduc...
What Temperature Instruments Do We Use   Thermocouple Junctions       There is a misconception of how thermocouples operat...
What Temperature Instruments Do We Use   Thermocouple Junctions       Hot junction voltage proportional to 210°F.       Ex...
What Temperature Instruments Do We Use   Thermocouple Junctions       Use of the correct extension wire is critical in    ...
What Temperature Instruments Do We Use   Thermocouple LinearizationCopyright © 2007 Control Station, Inc. All Rights Reser...
What Temperature Instruments Do We Use   Thermocouple GainCopyright © 2007 Control Station, Inc. All Rights Reserved   123
What Temperature Instruments Do We Use   Thermocouple TypesCopyright © 2007 Control Station, Inc. All Rights Reserved   124
What Temperature Instruments Do We Use   RTDs       A resistance-temperature detector (RTD) is a temperature       sensing...
What Temperature Instruments Do We Use   RTDs: Importance of α       α is, roughly, the slope of the RTDs       resistance...
What Temperature Instruments Do We Use   RTD Lead Wire Resistance       The RTD is a resistive device, you must drive a   ...
What Temperature Instruments Do We Use   RTD Lead Wire Resistance       Error is eliminated through the use of three wire ...
What Temperature Instruments Do We Use   RTD Self Heating Effect       The current used to excite an RTD causes the RTD to...
What Temperature Instruments Do We Use   Thermistors       A thermistor is similar to an RTD in that it is a       passive...
What Temperature Instruments Do We Use   Infrared       Objects radiate electromagnetic energy.            The higher the ...
What Temperature Instruments Do We Use   Infrared Emittance       The emittance of a real surface is the ratio of the     ...
What Temperature Instruments Do We Use   Infrared Emittance       To function properly an infrared temperature       instr...
What Temperature Instruments Do We Use   Infrared Field of View       Infrared temperature instruments are like an optical...
What Temperature Instruments Do We Use   Infrared Spectral Response       The range of frequencies that an infrared sensor...
Section Assessment:                   TemperatureCopyright © 2007 Control Station, Inc. All Rights Reserved   136
What is Pressure       Pressure is the ratio between a force acting on a       surface and the area of that surface.      ...
What is Pressure   Absolute, Gauge and Differential       Gauge pressure is defined relative to atmospheric       conditio...
What is Pressure   Absolute, Gauge and DifferentialCopyright © 2007 Control Station, Inc. All Rights Reserved   139
Section Assessment:                   PressureCopyright © 2007 Control Station, Inc. All Rights Reserved   140
Common Level Sensing Technologies       Point sensing level probes only sense tank level at a       discrete level.       ...
Common Level Sensing Technologies   Non-Contact: Ultrasonic       Ultrasonic makes use of sound waves.       A transducer ...
Common Level Sensing Technologies   Non-Contact: UltrasonicCopyright © 2007 Control Station, Inc. All Rights Reserved   143
Common Level Sensing Technologies   Non-Contact: Radar/Microwave       Radar, or microwave level measurement,       operat...
Common Level Sensing Technologies   Non-Contact: Radar/MicrowaveCopyright © 2007 Control Station, Inc. All Rights Reserved...
Common Level Sensing Technologies   Non-Contact: Nuclear Level Measurement       Radiation from the source is detected on ...
Common Level Sensing Technologies   Non-Contact: Nuclear Level MeasurementCopyright © 2007 Control Station, Inc. All Right...
Common Level Sensing Technologies   Contact: Hydrostatic Pressure       Measurement of level by pressure relies on hydrost...
Common Level Sensing Technologies   Contact: Hydrostatic Pressure       How would you handle measuring other fluids using ...
Common Level Sensing Technologies   Contact: Hydrostatic PressureCopyright © 2007 Control Station, Inc. All Rights Reserve...
Common Level Sensing Technologies   Contact: RF/Capacitance       RF (radio frequency) Capacitance       level sensors mak...
Common Level Sensing Technologies   Contact: RF/Capacitance       Requires the material being measured to have a high     ...
Common Level Sensing Technologies   Contact: RF/CapacitanceCopyright © 2007 Control Station, Inc. All Rights Reserved   153
Common Level Sensing Technologies   Contact: Guided Wave Radar       Guided wave radar is similar to the non-contact radar...
Common Level Sensing Technologies   Contact: Guided Wave RadarCopyright © 2007 Control Station, Inc. All Rights Reserved  ...
Section Assessment:                   LevelCopyright © 2007 Control Station, Inc. All Rights Reserved   156
What is Flow       Flow is the motion characteristics of constrained       fluids (liquids or gases).       Fluid velocity...
Factors Affecting Flow Measurement       The critical factors affecting flow measurement       are:            Viscosity  ...
Factors Affecting Flow Measurement   Viscosity       Dynamic or absolute viscosity (η) is a measure of       the resistanc...
Factors Affecting Flow Measurement   Viscosity       Visualize a flat sheet of glass on a film of oil on top of a flat    ...
Factors Affecting Flow Measurement   Viscosity       Effect of Temperature            The dynamic viscosity of a fluid var...
Factors Affecting Flow Measurement   Viscosity       Viscosity is measured in units of poise or       Pascal·seconds or st...
Factors Affecting Flow Measurement   Viscosity Conversion ChartCopyright © 2007 Control Station, Inc. All Rights Reserved ...
Factors Affecting Flow Measurement   Viscosity of Common FluidsCopyright © 2007 Control Station, Inc. All Rights Reserved ...
Factors Affecting Flow Measurement   Viscosity       The fluid streams in most processes can include       both high and l...
Factors Affecting Flow Measurement   Fluid Type       Newtonian Fluids            When held at a constant temperature, the...
Factors Affecting Flow Measurement   Fluid Type       Non-Newtonian Fluids            When held at a constant temperature,...
Factors Affecting Flow Measurement   Reynolds Number       The Reynolds number is the ratio of inertial       forces to vi...
Factors Affecting Flow Measurement   Laminar Flow       Laminar flow occurs at low Reynolds numbers,       typically Re < ...
Factors Affecting Flow Measurement   Turbulent Flow       Turbulent flow occurs at high Reynolds numbers,       typically ...
Factors Affecting Flow Measurement   Transitional Flow       Transitional flow typically occurs at Reynolds       numbers ...
Factors Affecting Flow Measurement   Reynolds Number                      Flow instruments must be                        ...
Factors Affecting Flow Measurement   Flow Irregularities               Flow irregularities are               changes in th...
Common Flowmeter TechnologiesCopyright © 2007 Control Station, Inc. All Rights Reserved   174
Common Flowmeter Technologies       Volumetric Flow            Magnetic flowmeters            Positive displacement for vi...
Common Flowmeter Technologies   Volumetric Flow       Volumetric flow is usually inferred.            Volumetric Flow = Fl...
Common Flowmeter Technologies   Volumetric Flow: Positive Displacement       Operate by capturing the fluid to be measured...
Common Flowmeter Technologies   Volumetric Flow: Magnetic      Magnetic flowmeters infer the      velocity of the moving f...
Common Flowmeter Technologies   Volumetric Flow: Orifice Plates                                                           ...
Common Flowmeter Technologies   Volumetric Flow: Orifice Plates       Orifice Plate type meters only work well when       ...
Common Flowmeter Technologies   Mass Flow       Measured directly by measuring the inertial       effects of the fluids mo...
Common Flowmeter Technologies   Mass Flow: Coriolis       Coriolis flowmeters are based on the affects of the       Coriol...
Common Flowmeter Technologies   Mass Flow: CoriolisCopyright © 2007 Control Station, Inc. All Rights Reserved   183
Common Flowmeter Technologies   Mass Flow: CoriolisCopyright © 2007 Control Station, Inc. All Rights Reserved   184
Common Flowmeter Technologies   Mass Flow: Coriolis    Coriolis mass flowmeters    work by applying a    vibrating force t...
Common Flowmeter Technologies   Flowmeter Turndown       A specification commonly found with flow sensors       is turndow...
Common Flowmeter Technologies   Installation and Calibration       As with any instrument installation, optimal flow      ...
instrumentation & contoby prashant sakharkar)
instrumentation & contoby prashant sakharkar)
instrumentation & contoby prashant sakharkar)
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instrumentation & contoby prashant sakharkar)

  1. 1. Fundamentals of Instrumentation& Process ControlInteractive Training Workshop
  2. 2. Introduction to Process Control A common misconception in process control is that it is all about the controller – that you can force a particular process response just by getting the right tuning parameters. In reality, the controller is just a partner. A process will respond to a controller’s commands only in the manner which it can. To understand process control you must understand the other partners as well: sensors, final control elements and the process itself. All of these determine what type of response the controller is capable of extracting out of the process. It is not the other way around.Copyright © 2007 Control Station, Inc. All Rights Reserved 2
  3. 3. Outline of the CourseCopyright © 2007 Control Station, Inc. All Rights Reserved 3
  4. 4. Introduction to Process Control Objectives: Why do we need process control? What is a process? What is process control? What is open loop control? What is closed loop control? What are the modes of closed loop control? What are the basic elements of process control?Copyright © 2007 Control Station, Inc. All Rights Reserved 4
  5. 5. Motivation for Automatic Process Control Safety First: people, environment, equipment The Profit Motive: meeting final product specs minimizing waste production minimizing environmental impact minimizing energy use maximizing overall production rateCopyright © 2007 Control Station, Inc. All Rights Reserved 5
  6. 6. “Loose” Control Costs Money 65 operating constraint PV & SP (%) 60 set point far from profit constraint 55 SP more profit 45 Copyright © 2007 40 poor control = large variation in PV by Control Station, Inc. All Rights Reserved Time It takes more processing to remove impurities, so greatest profit is to operate as close to the maximum impurities constraint as possible without going over It takes more material to make a product thicker, so greatest profit is to operate as close to the minimum thickness constraint as possible without going underCopyright © 2007 Control Station, Inc. All Rights Reserved 6
  7. 7. “Tight” Control is More Profitable 65 operating constraint set point near 60 profit constraint PV & SP (%) SP 50 more profit tight control = small variation in PV 45 40 Copyright © 2007 by Control Station, Inc. All Rights Reserved Time A well controlled process has less variability in the measured process variable (PV), so the process can be operated close to the maximum profit constraint.Copyright © 2007 Control Station, Inc. All Rights Reserved 7
  8. 8. Terminology for Home Heating Control Control Objective Measured Process Variable (PV) Set Point (SP) Controller Output (CO) Manipulated Variable Disturbances (D) thermostat controller set point TC TT temperature heat loss sensor/transmitter (disturbance) control signal fuel flow furnace valve Copyright © 2007 by Control Station, Inc. All Rights ReservedCopyright © 2007 Control Station, Inc. All Rights Reserved 8
  9. 9. What is Process Control Terminology: The manipulated variable (MV) is a measure of resource being fed into the process, for instance how much thermal energy. A final control element (FCE) is the device that changes the value of the manipulated variable. The controller output (CO) is the signal from the controller to the final control element. The process variable (PV) is a measure of the process output that changes in response to changes in the manipulated variable. The set point (SP) is the value at which we wish to maintain the process variable at.Copyright © 2007 Control Station, Inc. All Rights Reserved 9
  10. 10. What is a Process A process is broadly defined as an operation that uses resources to transform inputs into outputs. It is the resource that provides the energy into the process for the transformation to occur.Copyright © 2007 Control Station, Inc. All Rights Reserved 10
  11. 11. What is a ProcessCopyright © 2007 Control Station, Inc. All Rights Reserved 11
  12. 12. What is a Process Each process exhibits a particular dynamic (time varying) behavior that governs the transformation. That is, how do changes in the resource or inputs over time affect the transformation. This dynamic behavior is determined by the physical properties of the inputs, the resource, and the process itself.Copyright © 2007 Control Station, Inc. All Rights Reserved 12
  13. 13. What is a Process Can you identify some of the elements that will determine the dynamic properties of this process?Copyright © 2007 Control Station, Inc. All Rights Reserved 13
  14. 14. What is Process Control Process control is the act of controlling a final control element to change the manipulated variable to maintain the process variable at a desired set point. A corollary to our definition of process control is a controllable process must behave in a predictable manner. For a given change in the manipulated variable, the process variable must respond in a predictable and consistent manner.Copyright © 2007 Control Station, Inc. All Rights Reserved 14
  15. 15. What is Process ControlCopyright © 2007 Control Station, Inc. All Rights Reserved 15
  16. 16. What is Process ControlCopyright © 2007 Control Station, Inc. All Rights Reserved 16
  17. 17. Section Assessment: Basic Terminology AssessmentCopyright © 2007 Control Station, Inc. All Rights Reserved 17
  18. 18. What is Open Loop Control In open loop control the controller output is not a function of the process variable. In open loop control we are not concerned that a particular set point be maintained, the controller output is fixed at a value until it is changed by an operator. Many processes are stable in an open loop control mode and will maintain the process variable at a value in the absence of a disturbances. Disturbances are uncontrolled changes in the process inputs or resources.Copyright © 2007 Control Station, Inc. All Rights Reserved 18
  19. 19. What is Open Loop ControlCopyright © 2007 Control Station, Inc. All Rights Reserved 19
  20. 20. What is Open Loop Control Can you think of processes in which open loop control is sufficient?Copyright © 2007 Control Station, Inc. All Rights Reserved 20
  21. 21. What is Closed Loop Control In closed loop control the controller output is determined by difference between the process variable and the set point. Closed loop control is also called feedback or regulatory control. The output of a closed loop controller is a function of the error. Error is the deviation of the process variable from the set point and is defined as E = SP - PV.Copyright © 2007 Control Station, Inc. All Rights Reserved 21
  22. 22. What is Closed Loop ControlCopyright © 2007 Control Station, Inc. All Rights Reserved 22
  23. 23. What is Closed Loop Control From the controllers perspective the process encompasses the RTD, the steam control valve, and signal processing of the PV and CO values.Copyright © 2007 Control Station, Inc. All Rights Reserved 23
  24. 24. What are the Modes of Closed Loop Control Closed loop control can be, depending on the algorithm that determines the controller output: Manual On-Off PID Advanced PID (ratio, cascade, feedforward) or Model BasedCopyright © 2007 Control Station, Inc. All Rights Reserved 24
  25. 25. What are the Modes of Closed Loop Control Manual Control In manual control an operator directly manipulates the controller output to the final control element to maintain a set point.Copyright © 2007 Control Station, Inc. All Rights Reserved 25
  26. 26. What are the Modes of Closed Loop Control On-Off Control On-Off control provides a controller output of either on or off in response to error.Copyright © 2007 Control Station, Inc. All Rights Reserved 26
  27. 27. What are the Modes of Closed Loop Control On-Off Control Deadband Upon changing the direction of the controller output, deadband is the value that must be traversed before the controller output will change its direction again.Copyright © 2007 Control Station, Inc. All Rights Reserved 27
  28. 28. What are the Modes of Closed Loop Control PID Control PID control provides a controller output that modulates from 0 to 100% in response to error.Copyright © 2007 Control Station, Inc. All Rights Reserved 28
  29. 29. What are the Modes of Closed Loop Control Time Proportion Control Time proportion control is a variant of PID control that modulates the on-off time of a final control element that only has two command positions. To achieve the effect of PID control the switching frequency of the device is modulated in response to error. This is achieved by introducing the concept of cycle time. Cycle Time is the time base of the signal the final control element will receive from the controller. The PID controller determines the final signal to the controller by multiplying the cycle time by the output of the PID algorithm.Copyright © 2007 Control Station, Inc. All Rights Reserved 29
  30. 30. What are the Modes of Closed Loop Control Cascade Control Cascade control uses the output of a primary (master or outer) controller to manipulate the set point of a secondary (slave or inner) controller as if the slave controller were the final control element.Copyright © 2007 Control Station, Inc. All Rights Reserved 30
  31. 31. What are the Modes of Closed Loop Control Cascade ControlCopyright © 2007 Control Station, Inc. All Rights Reserved 31
  32. 32. Basic Elements of Process Control Controlling a process requires knowledge of four basic elements, the process itself, the sensor that measures the process value, the final control element that changes the manipulated variable, and the controller.Copyright © 2007 Control Station, Inc. All Rights Reserved 32
  33. 33. Section Assessment: Basic Process Control AssessmentCopyright © 2007 Control Station, Inc. All Rights Reserved 33
  34. 34. Understanding Dynamic Process Behavior What We Will Learn in This Section Dynamic Process Behavior – What It Is & Why We Care What a FOPDT Dynamic Model Represents Analyzing Step Test Plot Data to Determine FOPDT Dynamic Model Parameters Process Gain, Time Constant & Dead Time How To Compute Them From Plot Data How to Use Them For Controller Design and Tuning How to Recognize Nonlinear ProcessesCopyright © 2007 Control Station, Inc. All Rights Reserved 34
  35. 35. Dynamic Process Behavior and Controller Tuning Consider cruise control for a car vs a truck how quickly can each accelerate or decelerate what is the effect of disturbances (wind, hills, etc.) Controller (gas flow) manipulations required to maintain set point velocity in spite of disturbances (wind, hills) are different for a car and truck because the dynamic behavior of each "process" is different Dynamic behavior → how the measured process variable (PV) responds over time to changes in the controller output (CO) and disturbances (D)Copyright © 2007 Control Station, Inc. All Rights Reserved 35
  36. 36. Graphical Modeling of Dynamic Process Data To learn about the dynamic behavior of a process, we analyze measured process variable (PV) test data PV test data can be generated by suddenly changing the controller output (CO) signal The CO should be moved far and fast enough so that the dynamic behavior is clearly revealed as the PV responds The dynamic behavior of a process is different as operating level changes (nonlinear behavior), so collect data at normal operating conditions (design level of operation)Copyright © 2007 Control Station, Inc. All Rights Reserved 36
  37. 37. Modeling Dynamic Process Behavior The best way to understand process data is through modeling Modeling means fitting a first order plus dead time (FOPDT) dynamic model to the process data: dPV τp + PV = Kp ×CO(tθp) − dt where: PV is the measured process variable CO is the controller output signal The FOPDT model is simple (low order and linear) so it only approximates the behavior of real processesCopyright © 2007 Control Station, Inc. All Rights Reserved 37
  38. 38. Modeling Dynamic Process Behavior When a first order plus dead time (FOPDT) model is fit to dynamic process data dPV τp + PV = Kp ×CO(tθp) − dt The important parameters that result are: Steady State Process Gain, Kp Overall Process Time Constant, τ p Apparent Dead Time, ӨpCopyright © 2007 Control Station, Inc. All Rights Reserved 38
  39. 39. The FOPDT Model is All Important FOPDT model parameters (Kp, τ p and Өp) are used in correlations to compute controller tuning values Sign of Kp indicates the action of the controller (+ Kp ⇒ reverse acting; — Kp ⇒ direct acting) Size of τ p indicates the maximum desirable loop sample time (be sure sample time T ≤ 0.1τ p ) Ratio of Өp/τ p indicates whether model predictive control such as a Smith predictor would show benefit (useful if Өp >τ p ) Model becomes part of the feed forward, Smith Predictor, decoupling and other model-based controllersCopyright © 2007 Control Station, Inc. All Rights Reserved 39
  40. 40. Step Test Data and Dynamic Process Modeling Manual Mode Step Test on Gravity Drained Tanks Copyright © 2007 by Control Station, Inc. All Rights Reserved Process starts at steady state in manual mode Controller output (CO) signal is stepped to new value Process variable (PV) signal must complete the responseCopyright © 2007 Control Station, Inc. All Rights Reserved 40
  41. 41. Process Gain (Kp) from Step Test Data Kp describes how far the measured PV travels in response to a change in the CO A step test starts and ends at steady state, so Kp can be computed directly from the plot Steady State Change in the Process Variable,ΔPV Kp = Steady State Change in the Controller Output, ΔCO where ∆PV and ∆CO are the total change from initial to final steady state A large process gain, Kp, means that each CO action will produce a large PV responseCopyright © 2007 Control Station, Inc. All Rights Reserved 41
  42. 42. Process Gain (Kp) for Gravity-Drained Tanks ∆PV = (2.9 – 1.9) = 1.0 m ∆CO = (60 – 50) = 10 % Copyright © 2007 by Control Station, Inc. All Rights Reserved Compute ∆PV and ∆CO as “final minus initial” steady state valuesCopyright © 2007 Control Station, Inc. All Rights Reserved 42
  43. 43. Process Gain (Kp) for Gravity-Drained Tanks ∆PV = 1.0 m ∆PV 1.0 m m Kp = –––– = ––––– = 0.1 ––– ∆CO 10% % ∆CO = 10 % Copyright © 2007 by Control Station, Inc. All Rights Reserved Kp has a size (0.1); a sign (+0.1), and units (m/%)Copyright © 2007 Control Station, Inc. All Rights Reserved 43
  44. 44. Additional Notes on Process Gain Measuring the Process Gain Process gain as seen by a controller is the product of the gains of the sensor, the final control element and the process itself. Process Gain = Gain Process x Gain Sensor x Gain Final Control Element The gain of a controller will be inversely proportional to the process gain that it sees 1 Controller Gain ∝ Process GainCopyright © 2007 Control Station, Inc. All Rights Reserved 44
  45. 45. Additional Notes on Process Gain Converting Units of Process Gain There is one important caveat in this process; the gain we have calculated has units of m/%. Real world controllers, unlike most software simulations, have gain units specified as %/%. When calculating the gain for a real controller the change in PV needs to be expressed in percent of span of the PV as this is how the controller calculates error. ∆PV COSpan Gain = x ∆CO PVSpanCopyright © 2007 Control Station, Inc. All Rights Reserved 45
  46. 46. Additional Notes on Process Gain Converting Units of Process Gain Assuming the process gain of the gravity drained tanks is 0.1 m/% then, to convert to %/% m span CO m 0 − 100% % 0 .1 ⋅ = 0 .1 ⋅ = 1 .0 % span PV % 0 − 10% %Copyright © 2007 Control Station, Inc. All Rights Reserved 46
  47. 47. Additional Notes on Process Gain Values for Process Gain Process gain that the controller sees is influenced by two factors other than the process itself, the size of the final control element and the span of the sensor. In the ideal world you would use the full span of both final control element and the sensor which would give a process gain of 1.0.Copyright © 2007 Control Station, Inc. All Rights Reserved 47
  48. 48. Additional Notes on Process Gain Values for Process Gain As a rule of thumb: Process gains that are greater than 1 are a result of oversized final control elements. Process gains less than 1 are a result of sensor spans that are too wide.Copyright © 2007 Control Station, Inc. All Rights Reserved 48
  49. 49. Additional Notes on Process Gain Values for Process Gain The result of a final control element being too large (high gain) is: The controller gain will have to be made correspondingly smaller, smaller than the controller may accept. High gains in the final control element amplify imperfections (deadband, stiction), control errors become proportionately larger. If a sensor has too wide of a span: You may experience problems with the quality of the measurement. The controller gain will have to be made correspondingly larger making the controller more jumpy and amplifying signal noise. An over spanned sensor can hide an oversized final element.Copyright © 2007 Control Station, Inc. All Rights Reserved 49
  50. 50. Additional Notes on Process Gain Values for Process Gain The general rule of thumb is the process gain for a self regulating process should be between 0.5 and 2.0.Copyright © 2007 Control Station, Inc. All Rights Reserved 50
  51. 51. Process Time Constant τ p ) from Step Test Data ( Copyright © 2007 by Control Station, Inc. All Rights Reserved Time Constant, τ p , describes how fast the measured PV responds to changes in the CO More specifically, how long it takes the PV to reach 63.2% of its total final change (∆PV), starting from when it first begins to respondCopyright © 2007 Control Station, Inc. All Rights Reserved 51
  52. 52. Process Time Constant τ p ) from Step Test Data ( Copyright © 2007 by Control Station, Inc. All Rights Reserved t PVstart 1) Locate t PVstart , the time where the PV starts a first clear response to the step change in COCopyright © 2007 Control Station, Inc. All Rights Reserved 52
  53. 53. Process Time Constant τ p ) from Step Test Data ( PV63.2 ∆PV total 63.2% of ∆PV total Copyright © 2007 by Control Station, Inc. All Rights Reserved t PVstart 2) Compute 63.2% of the total change in PV as: PV63.2 = PVinital + 0.632 ∆PVCopyright © 2007 Control Station, Inc. All Rights Reserved 53
  54. 54. Process Time Constant τ p ) from Step Test Data ( PV63.2 τP Copyright © 2007 by Control Station, Inc. All Rights Reserved t PVstart t 63.2 3) t 63.2 is the time when the PV reaches PV63.2 4) Time Constant, τ p , is then: t 63.2 — tPVstartCopyright © 2007 Control Station, Inc. All Rights Reserved 54
  55. 55. Time Constant (τ p) for Gravity-Drained Tanks Copyright © 2007 by Control Station, Inc. All Rights Reserved tPVstart = 4.1 Here, tPVstart = 4.1 minCopyright © 2007 Control Station, Inc. All Rights Reserved 55
  56. 56. Time Constant (τ p ) for Gravity-Drained Tanks PV63.2 = 2.5 ∆PV = 1.0 m 63.2% of ∆PV total Copyright © 2007 by Control Station, Inc. All Rights Reserved 4.1 PV63.2 = PVinital + 0.632 ∆PV = 1.9 + 0.632(1.0) = 2.5 mCopyright © 2007 Control Station, Inc. All Rights Reserved 56
  57. 57. Time Constant (τ p ) for Gravity-Drained Tanks PV63.2 = 2.5 τP τP = 5.7 – 4.1 = 1.6 min Copyright © 2007 by Control Station, Inc. All Rights Reserved 4.1 5.7 Time Constant, τ p = t 63.2 — tPVstart = 1.6 min τ p must be positive and have units of timeCopyright © 2007 Control Station, Inc. All Rights Reserved 57
  58. 58. Process Dead-Time (Өp) from Step Test Data Dead time, Өp, is how much delay occurs from the time when the CO step is made until when the measured PV shows a first clear response. Өp is the sum of these effects: transportation lag, or the time it takes for material to travel from one point to another sample or instrument lag, or the time it takes to collect, analyze or process a measured PV sample higher order processes naturally appear slow to respond and this is treated as dead time Dead time, Өp, must be positive and have units of timeCopyright © 2007 Control Station, Inc. All Rights Reserved 58
  59. 59. Dead-Time (Өp) is the Killer of Control Tight control grows increasingly difficult as Өp becomes large The process time constant is the clock of the process. Dead time is large or small relative only to τ p When dead time grows such that Өp > τ p , model predictive control strategies such as a Smith predictor may show benefit For important PVs, work to select, locate and maintain instrumentation so as to avoid unnecessary dead time in a loopCopyright © 2007 Control Station, Inc. All Rights Reserved 59
  60. 60. Process Dead-Time (Өp) from Step Test Data Copyright © 2007 by Control Station, Inc. All Rights Reserved Өp tCOstep t PVstart Өp = t PVstart — tCOstepCopyright © 2007 Control Station, Inc. All Rights Reserved 60
  61. 61. Dead-Time (Өp) for Gravity Drained Tanks Өp = 4.1 – 3.8 = 0.3 min Copyright © 2007 by Control Station, Inc. All Rights Reserved tCOstep = 3.8 t PVstart= 4.1 Өp = t PVstart — tCOstep = 0.3 minCopyright © 2007 Control Station, Inc. All Rights Reserved 61
  62. 62. The FOPDT Model Parameters In Summary For a change in CO: Process Gain, Kp → How Far PV travels Time Constant, τ p → How Fast PV responds Dead Time, Өp → How Much Delay Before PV RespondsCopyright © 2007 Control Station, Inc. All Rights Reserved 62
  63. 63. Hands-On Workshop Workshop #1 Exploring Dynamics of Gravity-Drained TanksCopyright © 2007 Control Station, Inc. All Rights Reserved 63
  64. 64. Processes have Time Varying Behavior The CO to PV behavior described by an ideal FOPDT model is constant, but real processes change every day because: surfaces foul or corrode mechanical elements like seals or bearings wear feedstock quality varies and catalyst activity decays environmental conditions like heat and humidity change So the values of Kp, τ p and Өp that best describe the dynamic behavior of a process today may not be best tomorrow As a result, controller performance can degrade with time and periodic retuning may be requiredCopyright © 2007 Control Station, Inc. All Rights Reserved 64
  65. 65. Processes have Nonlinear Behavior …cause changing (nonlinear) response in real processes equal CO steps… Copyright © 2007 by Control Station, Inc. All Rights Reserved The dynamic behavior of most real processes changes as operating level changesCopyright © 2007 Control Station, Inc. All Rights Reserved 65
  66. 66. Processes have Nonlinear Behavior 3) but behavior of real processes changes with operating level 2) FOPDT model response is constant as operating level changes 1) with equal CO steps Copyright © 2007 by Control Station, Inc. All Rights Reserved A FOPDT model response is constant as operating level changes Since the FOPDT model is used for controller design and tuning, a process should be modeled at a specific design level of operation!Copyright © 2007 Control Station, Inc. All Rights Reserved 66
  67. 67. What is a Nonlinear Process While linear processes may be the design goal of process engineers, the reality is that most processes are nonlinear in nature due to nonlinearity in the final control element or the process itself. Example: A heating process is nonlinear because the rate at which heat is transferred between two objects depends on the difference in temperature between the objects. Example: A valve that is linear in the middle of its operating range may become very nonlinear towards its limits.Copyright © 2007 Control Station, Inc. All Rights Reserved 67
  68. 68. What is a Nonlinear Process Dealing with Nonlinearity – Set Point ResponseCopyright © 2007 Control Station, Inc. All Rights Reserved 68
  69. 69. What is a Nonlinear Process Dealing with Nonlinearity The robustness of a controller is a measure the range of process values over which the controller provides stable operation. The more nonlinear a process is, the less aggressive you must be in your tuning approach to maintain robustness.Copyright © 2007 Control Station, Inc. All Rights Reserved 69
  70. 70. What is Process Action Process action is how the process variable changes with respect to a change in the controller output. Process action is either direct acting or reverse acting. The action of a process is defined by the sign of the process gain. A process with a positive gain is said to be direct acting. A process with a negative gain is said to be reverse acting.Copyright © 2007 Control Station, Inc. All Rights Reserved 70
  71. 71. What is Process Action Process and Controller Action The action of a process is important because it will determine the action of the controller. A direct acting process requires a reverse acting controller. Conversely, a reverse acting process requires a direct acting controller.Copyright © 2007 Control Station, Inc. All Rights Reserved 71
  72. 72. Fundamentals of Instrumentation& Process ControlInteractive Training Workshop
  73. 73. Introduction to Instrumentation The intent of this chapter is neither to teach you how to select a particular instrument nor to familiarize you with all of the available types of instruments. The intent of this material is to provide an introduction to commonly measured process variables, including the basic terminology and characteristics relevant to each variable’s role in a control loop. Detailed information and assistance on device selection is typically available directly from the instrumentation supplier.Copyright © 2007 Control Station, Inc. All Rights Reserved 73
  74. 74. Introduction to Instrumentation Objectives: What are sensors and transducers? What are the standard instrument signals? What are smart transmitters? What is a low pass filter? What instrument properties affect a process? What is input aliasing? What is instrument noise? How do we measure temperature? How do we measure level? How do we measure level? How do we measure flow?Copyright © 2007 Control Station, Inc. All Rights Reserved 74
  75. 75. Introduction to Instrumentation You cannot control what you cannot measure “When you can measure what you are speaking about, and express it in numbers, you know something about it. When you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.” - Lord KelvinCopyright © 2007 Control Station, Inc. All Rights Reserved 75
  76. 76. What are Sensors and Transducers A sensor is a device that has a characteristic that changes in a predictable way when exposed to the stimulus it was designed to detect. A transducer is a device that converts one form of energy into another.Copyright © 2007 Control Station, Inc. All Rights Reserved 76
  77. 77. What are Standard Instrumentation Signals Standard instrument signals for controllers to accept as inputs from instrumentation and outputs to final control elements are: pneumatic current loop 0 to 10 voltCopyright © 2007 Control Station, Inc. All Rights Reserved 77
  78. 78. What are Standard Instrumentation Signals Pneumatic 3 to 15 psig Before 1960, pneumatic signals were used almost exclusively to transmit measurement and control information. Today, it is still common to find 3 to 15 psig used as the final signal to a modulating valve. Most often an I/P (I to P) transducer is used. This converts a 4-20 mA signal (I) into a pressure signal (P).Copyright © 2007 Control Station, Inc. All Rights Reserved 78
  79. 79. What are Standard Instrumentation Signals Pneumatic Scaling What would our pneumatic signal be if our controller output is 40%? ( Signal psig = % Controller Output x 12 psig + 3psig )Copyright © 2007 Control Station, Inc. All Rights Reserved 79
  80. 80. What are Standard Instrumentation Signals Current Loop 4-20 milliamp Current loops are the signal workhorses in our processes. A DC milliamp current is transmitted through a pair of wires from a sensor to a controller or from a controller to its final control element. Current loops are used because of their immunity to noise and the distances that the signal can be transmitted.Copyright © 2007 Control Station, Inc. All Rights Reserved 80
  81. 81. What are Standard Instrumentation Signals Current Loop Scaling Output Scaling Scale outputs for a one-to-one correspondence. Controller output is configured for 0% to correspond to a 4mA signal and 100% to correspond to a 20mA signal. The final control element is calibrated so that 4mA corresponds to its 0% position or speed and 20mA corresponds to its 100% position or speed.Copyright © 2007 Control Station, Inc. All Rights Reserved 81
  82. 82. What are Standard Instrumentation Signals Current Loop Scaling Input Scaling Scale inputs for a one-to-one correspondence as well. Example: If we were using a pressure transducer with a required operating range of 0 psig to 100 psig we would calibrate the instrument such that 0 psig would correspond to 4mA output and 100 psig would correspond to a 20mA output. At the controller we would configure the input such that 4mA would correspond to an internal value of 0 psig and 10mA would correspond to an internal value of 100 psig.Copyright © 2007 Control Station, Inc. All Rights Reserved 82
  83. 83. What are Standard Instrumentation Signals 0 to 10 Volt 0 to 10 volt is not commonly used in control systems because this signal is susceptible to induced noise and the distance of the instrument or final control element is limited due to voltage drop. You may find 0-10 volt signals used in control systems providing the speed reference to variable speed drives.Copyright © 2007 Control Station, Inc. All Rights Reserved 83
  84. 84. What are Smart Transmitters A smart transmitter is a digital device that converts the analog information from a sensor into digital information, allowing the device to simultaneously send and receive information and transmit more than a single value. Smart transmitters, in general, have the following common features: Digital Communications Configuration Re-Ranging Signal Conditioning Self-DiagnosisCopyright © 2007 Control Station, Inc. All Rights Reserved 84
  85. 85. What are Smart Transmitters Digital Communications Smart transmitters are capable of digital communications with both a configuration device and a process controller. Digital communications have the advantage of being free of bit errors, the ability to multiple process values and diagnostic information, and the ability to receive commands. Some smart transmitters use a shared channel for analog and digital data (Hart, Honeywell or Modbus over 4-20mA). Others use a dedicated communication bus (Profibus, Foundation Fieldbus, DeviceNet, Ethernet).Copyright © 2007 Control Station, Inc. All Rights Reserved 85
  86. 86. What are Smart Transmitters Digital Communications Most smart instruments wired to multi-channel input cards require isolated inputs for the digital communications to work.Copyright © 2007 Control Station, Inc. All Rights Reserved 86
  87. 87. What are Smart Transmitters Configuration, Signal Conditioning, Self-Diagnosis Configuration Smart transmitters can be configured with a handheld terminal and store the configuration settings in non-volatile memory. Signal Conditioning Smart transmitters can perform noise filtering and can provide different signal characterizations. Self-Diagnosis Smart transmitters also have self-diagnostic capability and can report malfunctions that may indicate erroneous process values.Copyright © 2007 Control Station, Inc. All Rights Reserved 87
  88. 88. What Instrument Properties Affect a Process The instrument’s range and span. The resolution of the measurement. The instrument’s accuracy and precision. The instrument’s dynamicsCopyright © 2007 Control Station, Inc. All Rights Reserved 88
  89. 89. What Instrument Properties Affect a Process Range and Span The range of a sensor is the lowest and highest values it can measure within its specification. The span of a sensor is the high end of the Range minus the low end of the Range. Match Range to Expected Conditions Instruments should be selected with a range that includes all values a process will normally encounter, including expected disturbances and possible failures.Copyright © 2007 Control Station, Inc. All Rights Reserved 89
  90. 90. What Instrument Properties Affect a Process Measurement Resolution Resolution is the smallest amount of input signal change that the instrument can detect reliably. Resolution is really a function of the instrument span and the controller’s input capability. The resolution of a 16 bit conversion is: Input Span The resolution of a 12 bit conversion is: 65,535 The bit error. Input Span 4,095Copyright © 2007 Control Station, Inc. All Rights Reserved 90
  91. 91. What Instrument Properties Affect a Process Accuracy Accuracy of a measurement describes how close the measurement approaches the true value of the process variable. % error over a range ± x% over  % of full scale  % of span Absolute over a range ± x units over  full scale  spanCopyright © 2007 Control Station, Inc. All Rights Reserved 91
  92. 92. What Instrument Properties Affect a Process Precision Precision is the reproducibility with which repeated measurements can be made under identical conditions. This may be referred to as drift.Copyright © 2007 Control Station, Inc. All Rights Reserved 92
  93. 93. What Instrument Properties Affect a Process Accuracy vs. Precision Why is precision preferred over accuracy?Copyright © 2007 Control Station, Inc. All Rights Reserved 93
  94. 94. What Instrument Properties Affect a Process Instrumentation Dynamics: Gain and Dead Time The gain of an instrument is often called sensitivity. The sensitivity of a sensor is the ratio of the output signal to the change in process variable. The dead time of an instrument is the time it takes for an instrument to start reacting to process change.Copyright © 2007 Control Station, Inc. All Rights Reserved 94
  95. 95. What Instrument Properties Affect a Process Instrumentation Dynamics: Time Constant As for processes, one time constant for an instrument is the time it takes to provide a signal that represents 63.2% of the value of variable it is measuring after a step change in the variable. Instrument manufacturers may sometimes specify the rise time instead of the time constant. Rise time is the time it takes for an instrument to provide a signal that represents 100% of the value of the variable it is measuring after a step change in the variable. The rise time of an instrument is equal to 5 time constants.Copyright © 2007 Control Station, Inc. All Rights Reserved 95
  96. 96. What is Input Aliasing Input aliasing is a phenomenon that occurs from digital processing of a signal. When a signal is processed digitally it is sampled at discrete intervals of time. If the frequency at which a signal is sampled is not fast enough the digital representation of that signal will not be correct. Input aliasing is an important consideration in digital process control. Processor inputs that have configurable sample rates and PID loop update times must be set correctly.Copyright © 2007 Control Station, Inc. All Rights Reserved 96
  97. 97. What is Input Aliasing 2 Hz waveform sampled every 0.4 secondsCopyright © 2007 Control Station, Inc. All Rights Reserved 97
  98. 98. What is Input Aliasing Correct Sampling Frequency Nyquist Frequency Theorem: To correctly sample a waveform it is necessary to sample at least twice as fast has the highest frequency in the waveform. In the digital world this means sampling at 1/20th of the period of the waveform. Period = 1/frequencyCopyright © 2007 Control Station, Inc. All Rights Reserved 98
  99. 99. What is Input Aliasing Correct Sampling Frequency 2 Hz waveform sampled every 0.025 secondsCopyright © 2007 Control Station, Inc. All Rights Reserved 99
  100. 100. Workshop Lab: Input Aliasing 1Copyright © 2007 Control Station, Inc. All Rights Reserved 100
  101. 101. What is Input Aliasing Determining the Correct Sampling Interval Rules of Thumb Set the sample interval for an instrument at 1/10th to 1/20th of the rise time (1/2 to 1/4th of the time constant). Set the sample interval to 1/10th to 1/20th of the process time constant. Temperature instrumentation (RTDs and thermocouples in thermowells) typically have time constants of several seconds or more. For these processes sampling intervals of 1 second are usually sufficient. Pressure and flow instrumentation typically have time constants of ½ to 1 second. For these processes sampling intervals of 0.1 second are usually sufficient.Copyright © 2007 Control Station, Inc. All Rights Reserved 101
  102. 102. What is Input Aliasing Determining the Correct Sampling IntervalCopyright © 2007 Control Station, Inc. All Rights Reserved 102
  103. 103. Workshop Lab: Input Aliasing 2Copyright © 2007 Control Station, Inc. All Rights Reserved 103
  104. 104. What is Instrument Noise Noise is a variation in a measurement of a process variable that does not reflect real changes in the process variable.Copyright © 2007 Control Station, Inc. All Rights Reserved 104
  105. 105. What is Instrument Noise Sources Noise is generally a result of the technology used to sense the process variable. Electrical signals used to transmit instrument measurements are susceptible to having noise induced from other electrical devices. Noise can also be caused by wear and tear on the mechanical elements of a sensor. Noise may be uncontrolled, random variations in the process itself. Whatever the source, noise distorts the measurement signal.Copyright © 2007 Control Station, Inc. All Rights Reserved 105
  106. 106. What is Instrument Noise Effects of Noise Noise reduces the accuracy and precision of process measurements. Somewhere in the noise is the true measurement, but where? Noise introduces more uncertainty into the measurement. Noise also introduces errors in control systems. To a controller, fluctuations in the process variable due to noise are indistinguishable from fluctuations caused by real disturbances. Noise in a process variable will be reflected in the output of the controllerCopyright © 2007 Control Station, Inc. All Rights Reserved 106
  107. 107. What is Instrument Noise Eliminating Noise The most effective means of eliminating noise is to remove the source. Reduce electrically induced noise by following proper grounding techniques, using shielded cabling, and physically separating the signal cabling form other electrical wiring. If worn mechanical elements in the sensor are causing noise, then repair or replace the sensor. When these steps have been taken and excessive noise is still a problem in the process variable a low pass filter may be appropriate.Copyright © 2007 Control Station, Inc. All Rights Reserved 107
  108. 108. What is Instrument Noise Low Pass Filter A low-pass filter allows the low frequency components of a signal to pass while attenuating the higher frequency components. Raw (unfiltered) Signal Filtered SignalCopyright © 2007 Control Station, Inc. All Rights Reserved 108
  109. 109. What is Instrument Noise Low Pass Filter A low pass filter introduces a first order lag into the measurement. Raw (unfiltered) Signal Filtered SignalCopyright © 2007 Control Station, Inc. All Rights Reserved 109
  110. 110. What is Instrument Noise Selecting a Low Pass Filter By Cut-Off Frequency Cut-off frequency is defined as frequency above which the filter provides -3dB of signal attenuation.  Amplitude Out  dB of attenuation = 20 log10    Amplitude In  An attenuation of 0 dB would mean the signal will pass with no reduction in amplitude while a large negative dB would indicate a very small amplitude ratio. Select a cut-off frequency that is above the frequency of your process.Copyright © 2007 Control Station, Inc. All Rights Reserved 110
  111. 111. What is Instrument Noise Selecting a Low Pass Filter By Time Constant Some filters are configured by selecting a time constant for the lag response of the filter. The relationship between the cut-off frequency and the time constant of a low pass is approximately given by: 1 Cut - Off Frequency ≈ 5 Time ConstantsCopyright © 2007 Control Station, Inc. All Rights Reserved 111
  112. 112. What is Instrument Noise Selecting a Low Pass Filter By α Value Some filters are selected by a value called alpha (α), notably the derivative filter in a PID controller. α is an averaging weighting term used in controller calculations to impart a first order lag on the measured variable. α generally has values between 0 and 1. A filter with a α = 0 would pass the signal through unfiltered. A filter with a α = 1 would filter everything allowing nothing to pass through the filter.Copyright © 2007 Control Station, Inc. All Rights Reserved 112
  113. 113. What is Instrument Noise Selecting a Low Pass Filter When a filter is specified by cut-off frequency, the lower the frequency the greater the filtering effect. When a filter is specified by time constant, the greater the time constant the greater the filtering effect. When a filter is specified by α, when α=0 no filtering is done, when α = 1 no signal passes through the fitter.Copyright © 2007 Control Station, Inc. All Rights Reserved 113
  114. 114. Workshop Lab: Noise FilteringCopyright © 2007 Control Station, Inc. All Rights Reserved 114
  115. 115. What is Temperature Temperature is a measure of degree of the hotness or coldness of an object. Units of Temperature Two most common temperature scales are Fahrenheit (°F) and Celsius (°C). The reference points are the freezing point and the boiling point of water. Water freezes at 32°F and boils at 212°F. The Celsius scale uses the same reference points only it defines the freezing point of water as 0°C and the boiling point as 100°C.Copyright © 2007 Control Station, Inc. All Rights Reserved 115
  116. 116. What is Temperature Unit Conversion 9  °F =  x °C  + 32 5  5 °C = ( x °F - 32 ) 9Copyright © 2007 Control Station, Inc. All Rights Reserved 116
  117. 117. What Temperature Instruments Do We Use Temperature is one of the most common process variables. Temperature is most commonly measured by Resistance Temperature Devices (RTD) Thermocouples Infrared (IR) (to a lesser degree) Thermistors (may also be found embedded in some control equipment)Copyright © 2007 Control Station, Inc. All Rights Reserved 117
  118. 118. What Temperature Instruments Do We Use Thermocouples Thermocouples are fabricated from two electrical conductors made of two different metal alloys. Hot or sensing junction Cold or reference junction Generate an open-circuit voltage, called the Seebeck voltage that is proportional to the temperature difference between the sensing (hot) and reference (cold) junctions.Copyright © 2007 Control Station, Inc. All Rights Reserved 118
  119. 119. What Temperature Instruments Do We Use Thermocouple Junctions There is a misconception of how thermocouples operate. The misconception is that the hot junction is the source of the output voltage. This is wrong. The voltage is generated across the length of the wire. Another misconception is that junction voltages are generated at the cold end between the special thermocouple wire and the copper circuit. Hence, a cold junction temperature measurement is required. This concept is wrong. The cold end temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.Copyright © 2007 Control Station, Inc. All Rights Reserved 119
  120. 120. What Temperature Instruments Do We Use Thermocouple Junctions Hot junction voltage proportional to 210°F. Extension wire voltage proportional to 18°F.Copyright © 2007 Control Station, Inc. All Rights Reserved 120
  121. 121. What Temperature Instruments Do We Use Thermocouple Junctions Use of the correct extension wire is critical in thermocouple applications. An incorrect extension will cause the temperature differential across the extension leads to be introduced as measurement error. Does this mean you cannot use copper terminal blocks for thermocouples? No, it is highly unlikely there will be a temperature differential across the terminal block; no error will be introduced.Copyright © 2007 Control Station, Inc. All Rights Reserved 121
  122. 122. What Temperature Instruments Do We Use Thermocouple LinearizationCopyright © 2007 Control Station, Inc. All Rights Reserved 122
  123. 123. What Temperature Instruments Do We Use Thermocouple GainCopyright © 2007 Control Station, Inc. All Rights Reserved 123
  124. 124. What Temperature Instruments Do We Use Thermocouple TypesCopyright © 2007 Control Station, Inc. All Rights Reserved 124
  125. 125. What Temperature Instruments Do We Use RTDs A resistance-temperature detector (RTD) is a temperature sensing device whose resistance increases with temperature. An RTD consists of a wire coil or deposited film of pure metal whose resistance at various temperatures has been documented. RTDs are used when applications require accuracy, long- term stability, linearity and repeatability. 100 Ω Platinum is most common.Copyright © 2007 Control Station, Inc. All Rights Reserved 125
  126. 126. What Temperature Instruments Do We Use RTDs: Importance of α α is, roughly, the slope of the RTDs resistance curve with respect to temperature. R100 − R0 α = 100 x R0 A 100Ω platinum RTD with a =0.003911 will have a resistance of 139.11 ohms at 100°C. A 100 Ω platinum RTD with a =0.003850 will have a resistance of 138.50 ohms at 100°C. Common values for the temperature coefficient are: 0.003850, DIN 43760 Standard (also referred to as European curve) 0.003911, American Standard 0.003926, ITS-90 StandardCopyright © 2007 Control Station, Inc. All Rights Reserved 126
  127. 127. What Temperature Instruments Do We Use RTD Lead Wire Resistance The RTD is a resistive device, you must drive a current through the device and monitor the resulting voltage. Any resistance in the lead wires that connect your measurement system to the RTD will add error to your readings. 40 feet of 18 gauge 2 conductor cable has 6Ω resistance. For a platinum RTD with α = 0.00385, the resistance equals 0.6 Ω /(0.385 Ω /°C) = 1.6°C of error.Copyright © 2007 Control Station, Inc. All Rights Reserved 127
  128. 128. What Temperature Instruments Do We Use RTD Lead Wire Resistance Error is eliminated through the use of three wire RTDs. With three leads, two voltage measurements can be made. When the voltages are subtracted, the result is the voltage drop that would have occurred through RT alone.Copyright © 2007 Control Station, Inc. All Rights Reserved 128
  129. 129. What Temperature Instruments Do We Use RTD Self Heating Effect The current used to excite an RTD causes the RTD to internally heat, which appears as an error. Self-heating is typically specified as the amount of power that will raise the RTD temperature by 1° C, or 1 mW/ °C. Self-heating can be minimized by using the smallest possible excitation current, but this occurs at the expense of lowering the measurable voltages and making the signal more susceptible to noise from induced voltages. The amount of self-heating also depends heavily on the medium in which the RTD is immersed. An RTD can self- heat up to 100 times higher in still air than in moving waterCopyright © 2007 Control Station, Inc. All Rights Reserved 129
  130. 130. What Temperature Instruments Do We Use Thermistors A thermistor is similar to an RTD in that it is a passive resistance device. Thermistors are generally made of semiconductor materials giving them much different characteristics. Thermistors do not have standardized electrical properties like thermocouples or RTDs.Copyright © 2007 Control Station, Inc. All Rights Reserved 130
  131. 131. What Temperature Instruments Do We Use Infrared Objects radiate electromagnetic energy. The higher the temperature of an object the more electromagnetic radiation it emits. This radiation occurs within the infrared portion of the electromagnetic spectrum.Copyright © 2007 Control Station, Inc. All Rights Reserved 131
  132. 132. What Temperature Instruments Do We Use Infrared Emittance The emittance of a real surface is the ratio of the thermal radiation emitted by the surface at a given temperature to that of the ideal black body at the same temperature. By definition, a black body has an emittance of 1. Another way to think of emissivity is the efficiency at which an object radiates thermal energy. Emittance is a decimal number that ranges between 0 and 1 or a percentage between 0% and 100%.Copyright © 2007 Control Station, Inc. All Rights Reserved 132
  133. 133. What Temperature Instruments Do We Use Infrared Emittance To function properly an infrared temperature instrument must take into account the emittance of the surface being measured. Emittance values can often be found in reference tables, but such tables will not take into account local conditions of the surface. A more practical way to set the emittance is to measure the temperature with an RTD or thermocouple and set the instrument emissivity control so that both readings are the same.Copyright © 2007 Control Station, Inc. All Rights Reserved 133
  134. 134. What Temperature Instruments Do We Use Infrared Field of View Infrared temperature instruments are like an optical system in that they have a field of view. The field of view basically defines the target size at a given distance. Field of view may be specified as: Angle and focal range (2.3° from 8" to 14) Distance to spot size ratio and focal range (25:1 from 8" to 14) Spot size at a distance (0.32" diameter spot at 8") All of these specifications are equivalent, at 8 inches our field of view will be a 0.32" diameter spot (8"/25), at 14 feet our field of view will be 6.72" (14 / 25). An infrared temperature sensor measures the average temperature of everything in its field of view. If the surface whose temperature we are measuring does not completely fill the field of view we will get inaccurate results.Copyright © 2007 Control Station, Inc. All Rights Reserved 134
  135. 135. What Temperature Instruments Do We Use Infrared Spectral Response The range of frequencies that an infrared sensor can measure is called its spectral response. Infrared temperature sensors do not measure the entire infrared region. Different frequencies (wavelengths) are selected for detection to provide certain advantages. If you wanted to use an infrared temperature sensor to measure the temperature of an object behind glass you would need to select an instrument with a spectral response in the infrared region in which glass is "transparent”.Copyright © 2007 Control Station, Inc. All Rights Reserved 135
  136. 136. Section Assessment: TemperatureCopyright © 2007 Control Station, Inc. All Rights Reserved 136
  137. 137. What is Pressure Pressure is the ratio between a force acting on a surface and the area of that surface. Pressure is measured in units of force divided by area. psi (pounds per square inch) bar, 1 bar = 14.5 PSI, common for pump ratings inH20 (inches of water), 27.680 inH20 = 1 psi, common for vacuum systems and tank levels mmHg (millimeters of mercury), 760 mmHg = 14.7 psi, common for vacuum systemsCopyright © 2007 Control Station, Inc. All Rights Reserved 137
  138. 138. What is Pressure Absolute, Gauge and Differential Gauge pressure is defined relative to atmospheric conditions. The units of gauge pressure are psig, however gauge pressure is often denoted by psi as well. Absolute pressure is defined as the pressure relative to an absolute vacuum. The units of absolute pressure are psia. Differential pressure uses a reference point other than full vacuum or atmospheric pressure.Copyright © 2007 Control Station, Inc. All Rights Reserved 138
  139. 139. What is Pressure Absolute, Gauge and DifferentialCopyright © 2007 Control Station, Inc. All Rights Reserved 139
  140. 140. Section Assessment: PressureCopyright © 2007 Control Station, Inc. All Rights Reserved 140
  141. 141. Common Level Sensing Technologies Point sensing level probes only sense tank level at a discrete level. Typically used for high-high or low-low level sensing to prevent plant personnel and/or process equipment from being exposed to harmful conditions. Also used in pairs in processes in which we do not particularly care what the exact level in a tank is, only that it is between two points. Continuous level probes sense the tank level as a percent of span of the probes capabilities. Continuous level probes are typically used where we need some type of inventory control, where we need to know with some degree of confidence what the particular level in a tank is. Contact and Non-Contact TypesCopyright © 2007 Control Station, Inc. All Rights Reserved 141
  142. 142. Common Level Sensing Technologies Non-Contact: Ultrasonic Ultrasonic makes use of sound waves. A transducer mounted in the top of a tank transmits sound waves in bursts onto the surface of the material to be measured. Echoes are reflected back from the surface of the material to the transducer and the distance to the surface is calculated from the burst-echo timing. The key points in applying an ultrasonic transducer are: The speed of sound varies with temperature. Heavy foam on the surface of the material interferes with the echo. An irregular material surface can cause false echoes resulting in irregular readings. Heavy vapor in the air space can distort the sound waves resulting in false reading.Copyright © 2007 Control Station, Inc. All Rights Reserved 142
  143. 143. Common Level Sensing Technologies Non-Contact: UltrasonicCopyright © 2007 Control Station, Inc. All Rights Reserved 143
  144. 144. Common Level Sensing Technologies Non-Contact: Radar/Microwave Radar, or microwave level measurement, operates on similar principles to ultrasonic level probes but, instead of sound waves, electromagnetic waves in the 10GHz range are used. When properly selected, radar can overcome many of the limitations of ultrasonic level probes. Be unaffected by temperature changes in the tank air space. See through heavy foam to detect the true material level. See through heavy vapor in the tank air space to detect true material level.Copyright © 2007 Control Station, Inc. All Rights Reserved 144
  145. 145. Common Level Sensing Technologies Non-Contact: Radar/MicrowaveCopyright © 2007 Control Station, Inc. All Rights Reserved 145
  146. 146. Common Level Sensing Technologies Non-Contact: Nuclear Level Measurement Radiation from the source is detected on the other side of the tank. Its strength indicates the level of the fluid. Point, continuous, and interface measurements can be made. As no penetration of the vessel is needed there are a number of situations that cause nucleonic transmitters to be considered over other technologies. Nuclear level detection has some drawbacks. One is high cost, up to four times that of other technologies. Others are the probable requirement for licenses, approvals, and periodic inspections; and the difficulty and expense of disposing of spent radiation materials.Copyright © 2007 Control Station, Inc. All Rights Reserved 146
  147. 147. Common Level Sensing Technologies Non-Contact: Nuclear Level MeasurementCopyright © 2007 Control Station, Inc. All Rights Reserved 147
  148. 148. Common Level Sensing Technologies Contact: Hydrostatic Pressure Measurement of level by pressure relies on hydrostatic principles. Pressure is a unit force over a unit area. A cubic foot (12"L x 12"W x 12"H) of water weighs 62.4796 pounds. The area that our cubic foot of water occupies is 144 square inches (12"L x 12"H), therefore our cubic foot of water exerts a force of 62.4796 pounds over 144 square inches, or 0.4339 psig for a 12" water column. 1 inch H20 = 0.0316 psig It would not matter how many cubic feet of water were placed side by side, our pressure would still be 0.4399 psig. Hydrostatic pressure is only dependent on the height of the fluid, not the area that it covers.Copyright © 2007 Control Station, Inc. All Rights Reserved 148
  149. 149. Common Level Sensing Technologies Contact: Hydrostatic Pressure How would you handle measuring other fluids using pressure? Compare the densities. For instance, chocolate weighs approximately 80 pounds per cubic foot. 80 divided 62.5 times 0.0316 = 0.0404 psig. For a change in level of one inch in a liquid chocolate tank the pressure measurement will change by 0.0404 psig. Level measurement by pressure requires a constant density for accurate measurements. If the head pressure in the tank can be other than atmospheric, we must use a differential pressure sensor.Copyright © 2007 Control Station, Inc. All Rights Reserved 149
  150. 150. Common Level Sensing Technologies Contact: Hydrostatic PressureCopyright © 2007 Control Station, Inc. All Rights Reserved 150
  151. 151. Common Level Sensing Technologies Contact: RF/Capacitance RF (radio frequency) Capacitance level sensors make use of electrical characteristics of a capacitor to infer the level in a vessel. As the material rises in the vessel, the capacitance changes. The level transducer measures this change, linearizes it and transmits the signal to the process control system. A point level probe will look for a specific change in capacitance to determine whether it is on or off.Copyright © 2007 Control Station, Inc. All Rights Reserved 151
  152. 152. Common Level Sensing Technologies Contact: RF/Capacitance Requires the material being measured to have a high dielectric constant. Level measurements are affected by changes in the dielectric of the material (moisture content). Proper selection requires informing the probe vendor of the material to be measure, especially in applications where you are measuring conductive materials or have a nonmetallic tank. Point probe sensitivity can be increased by welding a plate on the sensor tip to increase the capacitance, and therefore the sensitivity (gain).Copyright © 2007 Control Station, Inc. All Rights Reserved 152
  153. 153. Common Level Sensing Technologies Contact: RF/CapacitanceCopyright © 2007 Control Station, Inc. All Rights Reserved 153
  154. 154. Common Level Sensing Technologies Contact: Guided Wave Radar Guided wave radar is similar to the non-contact radar probes, only a rod or cable is immersed into the material like an RF capacitance probe. The rod or cable is used to guide the microwave along its length, where the rod or cable meets the material to be measured a wave reflection is generated. The transit time of the wave is used to calculate level very precisely. Unlike an RF capacitance probe, a guided wave radar probe can measure extremely low dielectric material.Copyright © 2007 Control Station, Inc. All Rights Reserved 154
  155. 155. Common Level Sensing Technologies Contact: Guided Wave RadarCopyright © 2007 Control Station, Inc. All Rights Reserved 155
  156. 156. Section Assessment: LevelCopyright © 2007 Control Station, Inc. All Rights Reserved 156
  157. 157. What is Flow Flow is the motion characteristics of constrained fluids (liquids or gases). Fluid velocity or mass are typically not measured directly. Measurements are affected by the properties of the fluid, the flow stream, and the physical installation.Copyright © 2007 Control Station, Inc. All Rights Reserved 157
  158. 158. Factors Affecting Flow Measurement The critical factors affecting flow measurement are: Viscosity Fluid Type Reynolds Number Flow IrregularitiesCopyright © 2007 Control Station, Inc. All Rights Reserved 158
  159. 159. Factors Affecting Flow Measurement Viscosity Dynamic or absolute viscosity (η) is a measure of the resistance to a fluid to deformation under shear stress, or an internal property of a fluid that offers resistance to flow. Commonly perceived as “thickness” or resistance to pouring. Water is very “thin” having a relatively low viscosity, while molasses is very thick having a relatively high viscosity.Copyright © 2007 Control Station, Inc. All Rights Reserved 159
  160. 160. Factors Affecting Flow Measurement Viscosity Visualize a flat sheet of glass on a film of oil on top of a flat surface. A parallel force applied to the sheet of glass will accelerate it to a final velocity dependent only on the amount of force applied. The oil that is next to the sheet of glass will have a velocity close to that of the glass; while the oil that is next to the stationary surface will have a velocity near zero. This internal distribution of velocities is due to the internal resistance of the fluid to shear stress forces, its viscosity. The viscosity of a fluid is the ratio between the per unit force to accelerate the plate and the distribution of the velocities within the fluid filmCopyright © 2007 Control Station, Inc. All Rights Reserved 160
  161. 161. Factors Affecting Flow Measurement Viscosity Effect of Temperature The dynamic viscosity of a fluid varies with its temperature. In general, the viscosity of a liquid will decrease with increasing temperature while the viscosity of a gas will increase with increasing temperature. Viscosity measurements are therefore associated with a particular temperatureCopyright © 2007 Control Station, Inc. All Rights Reserved 161
  162. 162. Factors Affecting Flow Measurement Viscosity Viscosity is measured in units of poise or Pascal·seconds or stokes. Poise or Pascal·seconds are the units of dynamic viscosity. Stokes are the units of kinematic viscosity. Kinematic viscosity is the dynamic viscosity of a fluid divided by the density of the fluid.Copyright © 2007 Control Station, Inc. All Rights Reserved 162
  163. 163. Factors Affecting Flow Measurement Viscosity Conversion ChartCopyright © 2007 Control Station, Inc. All Rights Reserved 163
  164. 164. Factors Affecting Flow Measurement Viscosity of Common FluidsCopyright © 2007 Control Station, Inc. All Rights Reserved 164
  165. 165. Factors Affecting Flow Measurement Viscosity The fluid streams in most processes can include both high and low viscous fluids. The viscosity of the fluid under process conditions must be taken into account when selecting a flow instrument for optimum performance.Copyright © 2007 Control Station, Inc. All Rights Reserved 165
  166. 166. Factors Affecting Flow Measurement Fluid Type Newtonian Fluids When held at a constant temperature, the viscosity of a Non-Newtonian fluid will change with relation to the size of the shear force, or it will change over time under a constant shear force. Water, glycerin, liquid sugar, and corn syrup are examples.Copyright © 2007 Control Station, Inc. All Rights Reserved 166
  167. 167. Factors Affecting Flow Measurement Fluid Type Non-Newtonian Fluids When held at a constant temperature, the viscosity of a Non-Newtonian fluid will change with relation to the size of the shear force, or it will change over time under a constant shear force. Shear Thickening: Peanut Butter, Starch & Water Time Thickening: Milk, MolassesCopyright © 2007 Control Station, Inc. All Rights Reserved 167
  168. 168. Factors Affecting Flow Measurement Reynolds Number The Reynolds number is the ratio of inertial forces to viscous forces of fluid flow within a pipe and is used to determine whether a flow will be laminar or turbulent. Dvρ Where : Re = Reynolds number Re=124 η D = Pipe diameter in inches v = Fluid velocity in ft/sec ρ = Fluid density in lb/ft3 η = Fluid viscosity in cPCopyright © 2007 Control Station, Inc. All Rights Reserved 168
  169. 169. Factors Affecting Flow Measurement Laminar Flow Laminar flow occurs at low Reynolds numbers, typically Re < 2000, where viscous forces are dominant. Laminar flow is characterized by layers of flow traveling at different speeds with virtually no mixing between layers. The velocity of the flow is highest in the center of the pipe and lowest at the walls of the pipe.Copyright © 2007 Control Station, Inc. All Rights Reserved 169
  170. 170. Factors Affecting Flow Measurement Turbulent Flow Turbulent flow occurs at high Reynolds numbers, typically Re > 4000, where inertial forces are dominant. Turbulent flow is characterized by irregular movement of the fluid in the pipe. There are no definite layers. The velocity of the flow is nearly uniform through the cross-section of the pipe.Copyright © 2007 Control Station, Inc. All Rights Reserved 170
  171. 171. Factors Affecting Flow Measurement Transitional Flow Transitional flow typically occurs at Reynolds numbers between 2000 and 4000. Flow in this region may be laminar, it may be turbulent, or it may exhibit characteristics of both.Copyright © 2007 Control Station, Inc. All Rights Reserved 171
  172. 172. Factors Affecting Flow Measurement Reynolds Number Flow instruments must be selected to accurately measure laminar flow or turbulent flowCopyright © 2007 Control Station, Inc. All Rights Reserved 172
  173. 173. Factors Affecting Flow Measurement Flow Irregularities Flow irregularities are changes in the velocity profile of the fluid flow caused by installation of the flow instrument. Normal velocity profiles are established by installing a sufficient run of straight piping.Copyright © 2007 Control Station, Inc. All Rights Reserved 173
  174. 174. Common Flowmeter TechnologiesCopyright © 2007 Control Station, Inc. All Rights Reserved 174
  175. 175. Common Flowmeter Technologies Volumetric Flow Magnetic flowmeters Positive displacement for viscous liquids Orifice Plates Mass Flow Coriolis flowmetersCopyright © 2007 Control Station, Inc. All Rights Reserved 175
  176. 176. Common Flowmeter Technologies Volumetric Flow Volumetric flow is usually inferred. Volumetric Flow = Fluid Velocity x Area Measured directly with positive displacement flow meters. Measured indirectly by differential pressure Volumetric flow meters provide a signal that is in units of volume per unit time. gpm (gallons per minute) cfm (cubic feet per minute) l/hr (liters per hour)Copyright © 2007 Control Station, Inc. All Rights Reserved 176
  177. 177. Common Flowmeter Technologies Volumetric Flow: Positive Displacement Operate by capturing the fluid to be measured in rotating cavities of a known volume. The volume of the cavity and the rate of rotation will give the volumetric flow value. Unaffected by Reynolds number and work with laminar, turbulent, and transitional flow. Low viscosity fluids will slip past the gears decreasing the accuracy of the flowmeter.Copyright © 2007 Control Station, Inc. All Rights Reserved 177
  178. 178. Common Flowmeter Technologies Volumetric Flow: Magnetic Magnetic flowmeters infer the velocity of the moving fluid by measuring a generated voltage. Magnetic flowmeters are based on Faraday’s law of electromagnetic induction - a wire moving though a magnetic field will generate a voltage. Requires a conductive fluid. Flow = Pipe Area ⋅ velocity πD 2 E = kBDv 2 = πr v = v k = Constant 4 B = Magnetic Flux Density πD 2 E πDE D = Pipe Diameter = = 4 kBD 4kB v = Fluid VelocityCopyright © 2007 Control Station, Inc. All Rights Reserved 178
  179. 179. Common Flowmeter Technologies Volumetric Flow: Orifice Plates An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. In practice, the orifice plate is installed in the pipe between two flanges. The orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference.Copyright © 2007 Control Station, Inc. All Rights Reserved 179
  180. 180. Common Flowmeter Technologies Volumetric Flow: Orifice Plates Orifice Plate type meters only work well when supplied with a fully developed flow profile. This is achieved by a long upstream length (20 to 40 diameters, depending on Reynolds number) Orifice plates are small and cheap to install, but impose a significant energy loss on the fluid due to friction.Copyright © 2007 Control Station, Inc. All Rights Reserved 180
  181. 181. Common Flowmeter Technologies Mass Flow Measured directly by measuring the inertial effects of the fluids moving mass. Mass flow meters provide a signal that is in units of mass per unit time. Typical units of mass flow are: lb/hr (pounds per hour) kg/s (kilograms per second)Copyright © 2007 Control Station, Inc. All Rights Reserved 181
  182. 182. Common Flowmeter Technologies Mass Flow: Coriolis Coriolis flowmeters are based on the affects of the Coriolis force. When observing motion from a rotating frame the trajectory of motion will appear to be altered by a force arising from the rotation.Copyright © 2007 Control Station, Inc. All Rights Reserved 182
  183. 183. Common Flowmeter Technologies Mass Flow: CoriolisCopyright © 2007 Control Station, Inc. All Rights Reserved 183
  184. 184. Common Flowmeter Technologies Mass Flow: CoriolisCopyright © 2007 Control Station, Inc. All Rights Reserved 184
  185. 185. Common Flowmeter Technologies Mass Flow: Coriolis Coriolis mass flowmeters work by applying a vibrating force to a pair of curved tubes through which fluid passes, in effect creating a rotating frame of reference. The Coriolis Effect of the mass passing through the tubes creates a force on the tubes perpendicular to both the direction of vibration and the direction of flow, causing a twist in the tubes.Copyright © 2007 Control Station, Inc. All Rights Reserved 185
  186. 186. Common Flowmeter Technologies Flowmeter Turndown A specification commonly found with flow sensors is turndown or rangeability. Turndown is the ratio of maximum to minimum flow that can be measured with the specified accuracy. For example, a flow meter with a full span of 0 to 100 gallons per minute with a 10:1 turndown could measure a flow as small as 10 gallons per minute. Trying to measure a smaller flow would result in values outside of the manufacturer’s stated accuracy claims.Copyright © 2007 Control Station, Inc. All Rights Reserved 186
  187. 187. Common Flowmeter Technologies Installation and Calibration As with any instrument installation, optimal flow meter performance and accuracy can only be achieved through proper installation and calibration. Requirements will vary between type and manufacturer by some general considerations are: The length of straight piping required upstream and downstream of the instrument. This is usually specified in pipe diameters. The properties of the fluid/gas to be measured. Mounting position of the flowmeter. Flowmeters must be kept full during operation.Copyright © 2007 Control Station, Inc. All Rights Reserved 187

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