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Fundamentals of Instrumentation v.1.2

Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved ControlControlControlStation StationStation Innovative Solutions from the Process control Professionals Practical Process control Fundamentals of Instrumentation and Process control Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Practical Process control Fundamentals of Instrumentation and Process control Copyright 2005 by control Station, Inc. All rights reserved. No portion of this book may be reproduced in any form or by any means except with the explicit, prior, written permission of control Station, Inc. Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Table of Contents Table of Contents .. 3 1. Introduction to Process control .

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 the definition of process control is a controllable process must behave in a predictable manner.

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Transcription of Fundamentals of Instrumentation v.1.2

1 Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved ControlControlControlStation StationStation Innovative Solutions from the Process control Professionals Practical Process control Fundamentals of Instrumentation and Process control Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Practical Process control Fundamentals of Instrumentation and Process control Copyright 2005 by control Station, Inc. All rights reserved. No portion of this book may be reproduced in any form or by any means except with the explicit, prior, written permission of control Station, Inc. Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Table of Contents Table of Contents .. 3 1. Introduction to Process control .

2 1 Objectives:.. 1 Introduction .. 2 Why do we need Process control ? .. 2 2 Quality .. 2 Profit .. 2 What is a Process? .. 4 What is Process control ?.. 5 Basics of Process control .. 8 What is Open Loop control ? .. 8 What are the Modes of Closed Loop control ? .. 12 Manual 12 On-Off control .. 13 PID control .. 15 Time Proportion 16 What are the Basic Elements of Process control ? .. 17 The 18 18 Final control Elements .. 18 The Controller .. 18 Process Characteristics .. 19 Objectives:.. 19 Introduction: Process 19 First Order Processes .. 20 LLeessssoonn What is a First Order Process? .. 20 What is Process Dead Time? .. 21 Measuring Dead 21 What is the Process Time Constant? .. 22 Measuring the Time 22 Controllability of a Process .. 23 What is Process Gain? .. 24 Measuring Process Gain .. 24 Making Gains Unitless.

3 25 Values for Process 26 What is Process Action? .. 27 Process Action and Controller Action .. 27 PPrroocceessss 28 Higher Order Processes .. 28 What are Higher Order Processes? .. 28 Over-damped Response .. 29 First Order Fit of Higher Order Over-Damped 30 First Order Fit of Higher Order Under-Damped Response .. 31 Critically Damped Response .. 32 What is a Linear Process?.. 33 What is a Nonlinear Process? .. 34 Dealing with Nonlinearity .. 35 Disturbance 35 Set Point 36 Process 37 What are Self-Regulating Processes? .. 37 What are Integrating Processes? .. 38 Dead time, Time Constants and Gain in an Integrating Process .. 39 Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Dead Time in an Integrating Process .. 39 Time Constants in an Integrating Process .. 39 Gain in an Integrating Process.

4 39 Introduction to Instrumentation .. 42 Objectives:.. 42 Instrumentation Basics .. 43 What are Sensors and Transducers? .. 43 43 44 What are the Standard Instrumentation 45 Pneumatic .. 45 Current Loop .. 46 Loop Scaling .. 46 Output Scaling .. 46 Input Scaling .. 46 0 - 10 46 What are Smart Transmitters? .. 47 Digital Communications .. 47 47 Signal Conditioning .. 47 47 What Instrument Properties Affect a Process? .. 48 Range and Span .. 48 Match Range to Expected 48 Measurement Resolution .. 49 Accuracy and Precision .. 50 % Error Over a 50 Absolute Over a Range .. 50 Accuracy vs. 51 Instrumentation 52 Instrument Gain .. 52 Instrument Time Constants .. 52 Instrument Dead Time .. 52 What is Input Aliasing?.. 53 Correct Sampling Frequency .. 54 Determining the Correct Sampling 55 What is Instrument Noise?.

5 56 Effects of Noise .. 56 Eliminating Noise .. 57 Low Pass Filters .. 57 Selecting a Filter by Cut-off Frequency .. 57 Selecting a Filter by Time 58 Selecting a Filter by Alpha Value .. 59 Process 60 What is Temperature?.. 60 Units of Temperature .. 60 What Temperature Instruments Do We Use? .. 61 Thermocouples .. 61 Junctions .. 61 Junction Misconceptions .. 62 Lead Wires .. 62 Linearization .. 63 Gain .. 63 Thermocouple Types .. 64 Resistive Temperature Devices .. 65 The Importance of the Temperature Coefficient alpha .. 65 Lead Wire Resistance .. 66 2 Wire RTDs .. 66 3 Wire RTDs .. 67 Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Self Heating .. 67 68 Infrared .. 69 Emittance .. 69 Field of View .. 70 Spectral Response .. 70 What is Pressure? .. 71 Units of 71 Absolute, Gauge and Differential 71 What is Level?

6 73 Point and Continuous 73 Common Level Sensing Technologies .. 74 Non-Contact Level Measurement .. 74 Ultrasonic Measurement .. 74 Radar / 74 Nuclear Level 75 Contact Level Measurement .. 76 Pressure Measurement .. 76 RF Capacitance / Resistance .. 77 Guided Wave 78 What is Flow?.. 79 Factors Affecting Flow 79 Viscosity .. 79 Temperature and Pressure Effects on Viscosity .. 80 Units of Viscosity .. 80 Viscosities and Densities of Common Household 81 Conversion 82 Fluid 84 Newtonian Fluids .. 84 Non-Newtonian Fluids .. 84 Reynolds Number .. 85 Laminar 85 Turbulent 86 Transitional Flow .. 86 Flow 87 Common Flow Instruments .. 88 Units of Volumetric Flow .. 90 Positive Displacement Flow 91 Magnetic Flow Meters .. 91 Orifice Plate** .. 91 Orifice Plate** .. 92 Units of Mass 94 Coriolis Flow Meters .. 94 Turndown.

7 95 Installation and 95 Valves .. 97 What is a control Valve? .. 97 Shut-Off Service .. 97 Divert Service .. 97 Throttling Service .. 97 Parts of a control Valve .. 98 What is an Actuator? .. 99 What is a Positioner? .. 100 What is Cv? .. 101 What are Valve Characteristics?.. 102 Inherent Characteristics .. 102 Rangeability .. 103 Gain .. 103 Equal Percentage 104 Linear 105 Quick Opening Valves .. 105 Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved Installed .. 106 What is Valve Deadband .. 108 Testing for Deadband .. 109 Method A .. 109 Method B .. 110 Effects of Deadband .. 110 What is Stiction? .. 111 Testing for Stiction .. 112 Effects of Stiction .. 112 What are the Types of Valves?.. 113 Linear Motion .. 113 Globe Valve .. 113 Gate Valve .. 114 Diaphragm Valve .. 114 Pinch 115 Rotary Motion.

8 116 Ball Valve .. 116 Butterfly Valve .. 116 Plug Valve .. 117 Pumps .. 118 What is a Centrifugal Pump? .. 118 What is Pump Head? .. 119 Why Do We Use Head and Not PSI? .. 120 What is a Pump Curve? .. 121 What is a System Curve?.. 122 What is the System Operating Point? .. 123 Throttling Valves .. 124 Variable Frequency Drives .. 125 Speed - Capacity Relationship .. 125 Speed - Head 125 Speed - Horsepower Relationship .. 126 What is a Positive Displacement Pump? .. 128 How Does a PD Pump Differ From a Centrifugal Pump? .. 128 Pump Head .. 128 Pump Curve .. 129 Changing the System Operating Point .. 129 Variable Frequency Drives .. 130 Speed - Capacity Relationship .. 130 Speed - Horsepower Relationship .. 130 The PID 131 Objectives:.. 131 The Many Faces of What are the PID Equations?.. 132 Series .. 132 133 Independent.

9 134 PID control Modes .. 135 What are the Modes of Operation? .. 135 What is Proportional control ? .. 136 Bias .. 136 Controller Gain, Proportional Gain or Proportion Band .. 136 Controller Action .. 137 Process Nonlinearity .. 138 What is Integral control ? .. 139 Repeats, Integral or Reset? .. 141 Integral Windup .. 142 What is Derivative? .. 143 Derivative 144 Derivative 146 Practical Process control Copyright 2005 by control Station, Inc. All Rights Reserved What is Loop Update Time?.. 147 What Combinations of control Action Can I Use? .. 148 Proportional Only .. 148 Proportional + Derivative .. 148 Integral 148 Proportional + 148 Full PID .. 148 Fundamentals of Loop Tuning ..150 Objectives:.. 150 Introduction .. 151 What is the Goal of Tuning?.. 151 Operate Within Safe Constraints of the Process .. 151 Maximize Operating Profit.

10 151 Eliminate offset from Set 151 Be stable over the normal operating 151 Avoid excessive control action (not overstress the final control element) .. 152 The Approach .. 152 How Do You Tune by Trial and Error?.. 153 Trial and Error, Proportional First .. 153 Trial and Error, Integral 154 Rules of Thumb .. 155 Good Practice and Troubleshooting .. 156 Common control Loops .. 156 Flow 156 Level 156 Pressure control .. 156 Temperature 156 157 Check each subsystem separately .. 157 Final control Elements .. 157 Common Valve Problems .. 157 158 Common sensor Problems .. 158 Smart Transmitters .. 158 Temperature 158 Pressure 158 Flow Sensors .. 158 The Controller .. 159 Common Controller Problems .. 159 The 159 Common Process Problems .. 159 1 Practical Process control Copyright 2005 by control Station, Inc.