NOTICE: The information presented in this publication is for the general education of the reader. Because neither the author nor editor nor the publisher has any control over the use of the information by the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application. Additionally, neither the author nor editor nor the publisher have investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented. Any references to commercial products in the work are cited as examples only. Neither the author nor the publisher endorses any referenced commercial product. Any trademarks or trade names referenced belong to the respective owner of the mark or name. Neither the author nor editor nor the publisher makes any representation regarding the availability of any referenced commercial product at any time. The manufacturer's instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.
Copyright ©2014 by ISA 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709 All Rights Reserved ISBN: 978-1-934394-22-9 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher.
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NOTE: This is the second release of the second edition. It is free of any errors known as of July 21, 2014.
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Tips on How to Use This Study Guide To make the most of this study guide, it may be of interest to use the features built into Adobe Reader. The image below shows where to click, for the display of Page Thumbnails and Bookmarks in this guide. The Bookmarks are a dynamic Table of Contents. See the following images below for illustrations of how thumbnails and bookmarks work. (There is a formula sheet for the exam in the attachments) attachments)
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Using Page Thumbnails to Navigate The Page Thumbnail shows a preview of the pages in this guide. Just click on any thumbnail image to instantly jump to the page in the preview. The default viewing mode in Adobe Reader is one column. If you want to view two columns at the same time as shown below, move your mouse over the divider between the thumbnails and the viewing page and drag the column splitter till you show as many columns as you would like to view at once. I recommend viewing only two columns.
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Using Bookmarks to Navigate
The Bookmarks in this guide are the same as the Table of Contents collapsed. Quickly navigate to the subject of interest and click on the “+” to expand the contents of the subject matter under the subject heading. Click on the “-“ to collapse the addition subject topics. The default viewing mode in Adobe Reader shows wrap around text in the bookmark column. If you would like to read your bookmarks as shown below, move your mouse over the divider between the bookmarks and the viewing page and drag the column splitter till you show as much text width as you desire to view.
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Table of Contents Table of Contents ..................................................................................................................................... vii Preface ...................................................................................................................................................... 1 About The Author ..................................................................................................................................... 1 People who have Contributed to this Manual .......................................................................................... 2 General Information .................................................................................................................................... 3 State Licensing Requirements ................................................................................................................... 3 Eligibility .................................................................................................................................................... 3 Exam Schedule .......................................................................................................................................... 3 Exam Format ............................................................................................................................................. 4 Exam Content............................................................................................................................................4 Reference Materials for the Exam ........................................................................................................... 7 Recommended Books and Materials for Testing ...................................................................................... 7 Books and Courses for Additional Study ................................................................................................... 8 Review of Process Control Subjects ....................................................................................................... 9 Overview of Process Measurement, Control and Calibration .................................................................. 9 Process Signal and Calibration Terminology ........................................................................................... 10 Definition of the Range of an Instrument ............................................................................................... 10 Definition of the Span of an Instrument ................................................................................................. 11 Definition of the use of Zero in Instrumentation .................................................................................... 12 Live-Zero ............................................................................................................................................. 12 Elevated-Zero ...................................................................................................................................... 12 Suppressed-Zero ................................................................................................................................. 12 Illustrations of range and span terminology ........................................................................................... 13 Illustrations of measured variable, measured signal, range and span ................................................... 14 Temperature Measurement and Calibration ......................................................................................... 15 Temperature Measurement Devices and Calibration ............................................................................ 15 Thermocouple Worked Examples (how to read the thermocouple tables) ........................................... 17 RTD (Resistance Temperature Detector) ................................................................................................ 18 RTD Worked Examples ............................................................................................................................ 18 Pressure Measurement and Calibration ............................................................................................... 21 Pressure Measurement and Head Pressure ........................................................................................... 21 Applying Pressure Measurement and Signals Worked Examples ..........................................................22 Differential Pressure and Meter Calibration ........................................................................................... 22 Pressure Change in a Pipe for a given Flow Rate .................................................................................... 23 Pressure Change across the Flow Element for a given Flow Rate .......................................................... 23 Pressure Calibration of Transmitter ........................................................................................................ 24 Level Measurement and Calibration ...................................................................................................... 25 Applying Level Measurement and Calibration Worked Examples .......................................................... 25 Level Displacer (Buoyancy) ..................................................................................................................... 27 Bubbler Level Measurement ................................................................................................................... 29 Density Measurement ............................................................................................................................ 30 Calculating the Volume in Tanks ............................................................................................................. 30 vii
Flow Measurement and Calibration ....................................................................................................... 31 Applying Flow Measurement Devices ..................................................................................................... 31 Turndown Ratio in a Flow Meter ............................................................................................................ 31 ISA Standard Flow Meter Symbols .......................................................................................................... 31 Flow Meter Applications Chart ............................................................................................................... 32 Orifice Tap Dimensions and Impulse Line Connections .......................................................................... 33 Applying the Bernoulli Principal for Flow Control .................................................................................. 34 Orifice Type Meters ................................................................................................................................ 35 Orifice Sizing Factors (The Spink Factor) ................................................................................................. 38 Sizing Orifice Type Devices for Flow Measurement Worked Examples.................................................. 39 Mass Flow Measurement and Control .................................................................................................... 41 Applying Mass Flow Measurement with an Orifice Worked Example ................................................... 44 Turbine Flow Meter Worked Example .................................................................................................... 46 Weight Measurement and Calibration ................................................................................................... 49 Weight Measurement Devices and Calibration ...................................................................................... 49 Sizing Process Control Valves ............................................................................................................... 51 Process Control Valves ............................................................................................................................ 51 Turndown Ratio in Valves ....................................................................................................................... 51 ISA Standard Valve Symbols .................................................................................................................... 52 ISA Standard Pressure Regulating Valve Symbols ................................................................................... 52 Valve Actuators ....................................................................................................................................... 53 ISA Standard Actuator Symbols .......................................................................................................... 53 ISA Standard Symbol for Limit Switches on Valve Actuator ............................................................... 54 Calculating the size of the actuator .................................................................................................... 54 Example Actuator Sizing ..................................................................................................................... 55 Split Ranging Control Valves ................................................................................................................... 57 Valve Positioner Applications ................................................................................................................. 58 ISA Standard Valve Positioner Symbol ............................................................................................... 58 Summary of Positioners ...................................................................................................................... 59 When should a positioner be used? ................................................................................................... 59 Control Valve Application Comparison Chart ......................................................................................... 60 Sizing Control Valves ............................................................................................................................... 61 Sizing Valves for Liquid ............................................................................................................................ 63 Sizing Valves for Gas ............................................................................................................................... 65 The basic equation for gas flow through a control valve is: ................................................................... 65 Sizing Valves for Vapor and Steam.......................................................................................................... 68 Sizing Valves for Two Phase Flow ....................................................................................................... 71 Sizing Pressure Relief Valves and Rupture Disks ............................................................................... 75 ASME VIII Code for Sizing Relief Valves and Rupture Disks .................................................................... 75 Pressure Limits in Sizing .......................................................................................................................... 75 ISA Pressure Relief Valve and Rupture Disc Symbols .............................................................................. 76 Sizing Pressure Relief Valves and Rupture Disks ..................................................................................... 77 Sizing Rupture Disks Worked Examples .................................................................................................. 80 Sizing Pressure Relief Valves Worked Examples ..................................................................................... 83 Table 5 - ASME Standard Nozzle Orifice Data ......................................................................................... 88 Table 6 - Typical Properties of Gases ....................................................................................................... 89
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Process Control Theory and Calculations ............................................................................................ 91 Degrees Of Freedom in Process Control Systems ................................................................................... 91 Controllers and control strategies (models-modes) ............................................................................... 93 Process Characteristics from the transfer function ................................................................................ 95 Controller Tuning Closed Loop ................................................................................................................ 98 Controller Tuning Open Loop ................................................................................................................ 100 A Typical Process Reaction curve for tuning a controller ..................................................................... 101 Block Diagram Algebra .......................................................................................................................... 103 Block Diagram Algebra Reduction (Example) ....................................................................................... 104 Nyquist Stability Criterion ..................................................................................................................... 105 Routh Stability Criterion........................................................................................................................ 107 Check for Stability using Routh (Example) ........................................................................................ 110 A First Analysis of Feedback Control .................................................................................................. 113 Compare Open Loop Control to Closed Loop Control .......................................................................... 113 Open Loop Example – A Mathematical Analysis ................................................................................... 113 Closed Loop Example – A Mathematical Analysis ................................................................................. 115 The Transfer Function for the Automobile ........................................................................................... 117 A First Analysis of Frequency Response ............................................................................................ 119 Electrical Application – A First Order System ....................................................................................... 119 Bode Plot of First Order System ............................................................................................................ 120 Calculate data for the Bode Plot ........................................................................................................... 121 Creating a Bode Plot – First Order System using Frequency ................................................................ 124 Hydraulic Application – A First Order System ....................................................................................... 125 Overview of Discrete Control Subjects ................................................................................................ 127 Overview of Digital Logic ...................................................................................................................... 127 Digital Logic Gate Symbols .................................................................................................................... 127 Digital Logic Gate Truth Tables ............................................................................................................. 128 ISA Binary Logic ..................................................................................................................................... 129 Relay Ladder Logic ................................................................................................................................ 130 Sealing Circuits ...................................................................................................................................... 131 PLC Programming.................................................................................................................................. 132 PLC Programming (RLL) relay ladder logic ........................................................................................ 132 PLC Programming (ST) structured text ............................................................................................. 132 PLC Programming (FBD) functional block diagram ........................................................................... 133 PLC Programming (SFC) sequential function chart ........................................................................... 133 Analog Control Signals .......................................................................................................................... 135 Overview of Analog Signals ................................................................................................................... 135 Typical Analog Loop Wiring Diagram .................................................................................................... 135 Signal Filtering in Process Control ......................................................................................................... 136 Appling Signal Filters......................................................................................................................... 136 Filter Time Constant and Sample Time ............................................................................................. 137 Example of Filter Time Selection ...................................................................................................... 138
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ISA Standards for Documentation ....................................................................................................... 141 ISA Identification Letters ....................................................................................................................... 141 ISA Letter Combinations ....................................................................................................................... 142 ISA Instrument or Function Symbol ...................................................................................................... 145 ISA Line Type Symbols ........................................................................................................................... 146 ISA Standard P&ID................................................................................................................................. 147 ISA Standard PFD .................................................................................................................................. 150 ISA Standard Loop Diagram .................................................................................................................. 151 ISA Standard (HMI) Graphical Display Symbols & Designations ........................................................... 153 NFPA 79 Colors for Graphical Displays (Industrial Machinery) ............................................................. 154 Overview of Safety Instrumented Systems ......................................................................................... 155 Overview of Process Safety and Shutdown .......................................................................................... 155 SIS (Safety Instrumented Systems) ....................................................................................................... 155 SIF (Safety Instrumented Function) ...................................................................................................... 156 SIL (Safety Integrity Level) ..................................................................................................................... 157 SIS Calculations ..................................................................................................................................... 160 Overview of Industrial Control Networks ............................................................................................. 163 Overview of Networks and Communications ....................................................................................... 163 Layers That Make Up the OSI Layers .................................................................................................... 165 Intelligent and Smart Devices ............................................................................................................... 165 Overview of NEC and NFPA Codes .................................................................................................... 167 List of NFPA Codes ................................................................................................................................ 167 NFPA 70 – NEC (National Electrical Code) ............................................................................................ 167 Voltage Drop Calculations ................................................................................................................ 168 Substitute Specific Resistance (k) for Resistance (R) of wire ....................................................... 168 Wire and Cable Sizing formulas for Voltage Drop ........................................................................ 168 Example: Voltage Drop Calculation 1 ..........................................................................................169 Example: Voltage Drop Calculation 2 ..........................................................................................169 Explosion Proof Installations NEC Article 500 (Hazardous Locations) ............................................. 170 Class I Hazardous Location NEC Article 501 ................................................................................. 170 Class I Location Definition ........................................................................................................ 170 Class I Division Definitions ....................................................................................................... 170 Class I Group Definitions .......................................................................................................... 171 Class I Temperature Definition ................................................................................................ 171 Class II Hazardous Location NEC Article 502 ................................................................................ 172 Class II Location Definition ....................................................................................................... 172 Class II Division Definitions ...................................................................................................... 172 Class II Group Definitions ......................................................................................................... 173 Class II Temperature Class ....................................................................................................... 173 Class III Hazardous Location NEC Article 503 ............................................................................... 174 Class III Location Definition ...................................................................................................... 174 Class III Division Definitions ..................................................................................................... 174 Class III Group Definitions ........................................................................................................ 174 Use of Zone Classifications ........................................................................................................... 175 Classification Comparison (Zone/Division) for a Class I Location................................................. 175 Group Comparison (Zone/ Division) for a Class I Location ...................................................... 175 Protection Methods Comparison Class I ...................................................................................... 176 x
Example: Designation of NEC/CEC Classification ......................................................................... 177 Example: Hazardous Location Classification ................................................................................ 178 Purged and Pressurized Systems .................................................................................................. 179 Intrinsically Safe Systems ............................................................................................................. 179 Zener diode barrier (configurations) ............................................................................................ 179 Conventional Passive IS Zener Barriers .................................................................................... 179 Active (Powered) IS Isolation Barriers ..................................................................................... 179 Electrical Enclosures Types and Uses ............................................................................................... 180 Non-hazardous location NEMA enclosure types .......................................................................... 180 Table 10 – Indoor Nonhazardous Locations ................................................................................. 181 Table 11 - Outdoor Nonhazardous Locations .............................................................................. 182 Table 12 - Hazardous Locations .................................................................................................... 183 Determining Temperature Rise .................................................................................................... 183 NFPA 77 Static Electricity ...................................................................................................................... 184 1.2 Purpose ....................................................................................................................................... 184 8.1 General Overview ....................................................................................................................... 184 8.3.1 Charge Generation................................................................................................................... 185 G.1 Grounding Diagrams .................................................................................................................. 186 NFPA 780 Lightning Protection (formerly NFPA 78) ............................................................................. 187 Air Terminal Height ........................................................................................................................... 187 Conductor Bends .............................................................................................................................. 187 Conductor Size and Material ............................................................................................................ 188 NFPA 79 Industrial Machinery .............................................................................................................. 190 Conductor sizing ............................................................................................................................... 190 Conductor colors............................................................................................................................... 190 Pushbutton functions for color ......................................................................................................... 190 Colors for Machine Indicator Lights and Icons Table 10.3.2 ............................................................ 190 NFPA 496 Purged and Pressurized Systems .......................................................................................... 191 Overview of the NFPA 496 articles ................................................................................................... 191 Factors to consider (NFPA 496, Sec. 5-3) .......................................................................................... 191 Location of the control room (NFPA 496, Secs. 5-3.1(c) and 5-3.2) ................................................. 192 Positive pressure air systems (NFPA 496, Sec. 5-4.1) ....................................................................... 192 Type X equipment (NFPA 496, Sec. 5-4.4) ........................................................................................ 192 Type Y equipment (NFPA 496, Sec. 5-4.5) ........................................................................................ 192 Type Z equipment (NFPA 496, Sec. 5-4.5) ........................................................................................ 192 Basic Design of Purged Enclosures ................................................................................................... 193 Basic Design of Purged Buildings ...................................................................................................... 194
The Fisher Control Valve Handbook ................................................................................................... 195 Guide to Using the Control Valve Handbook ........................................................................................ 195 Examination Sample Questions ........................................................................................................... 197 Sample Questions ................................................................................................................................. 197 Answers to Examination Sample Questions ......................................................................................... 204 Explanations and Proofs of Examination Sample Questions ................................................................ 205
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Useful Equations for Pumping and Piping .......................................................................................... 217 Find pipe diameter with velocity of flow known .................................................................................. 217 Find flow velocity with pipe diameter known ...................................................................................... 217 Find pipe diameter with temperature and pressure correction ........................................................... 217 Find flow velocity with temperature and pressure correction ............................................................. 217 Find the Reynolds Number for the flow ............................................................................................... 218 Find the pressure loss in piping system ................................................................................................ 218 Find the pump motor size (break horsepower) .................................................................................... 218 Calculating the Volume of Tanks ......................................................................................................... 219 Cylindrical Tanks Upright ...................................................................................................................... 219 Cylindrical Tanks on Side ....................................................................................................................... 219 Spherical Tanks ..................................................................................................................................... 220 Bullet Tanks ...........................................................................................................................................220 Appendix .................................................................................................................................................. 221 Table A1 – Thermocouple Table (Type J) .............................................................................................. 221 Table A2 - Thermocouple Table (Type K) .............................................................................................. 223 Table A3 - Thermocouple Table (Type E) .............................................................................................. 226 Table A4 - Thermocouple Table (Type T) .............................................................................................. 228 Table A5 - Platinum 100 Ohm RTD Table in ohms................................................................................. 229 Table A6 - Properties of Water Specific Gravity and LBs/HR to GPM ................................................... 230 Table A7 - Properties of Water Specific Volume and Density ............................................................... 231 Table A8 – Properties of Water Kinematic Viscosity centistokes ......................................................... 232 Table A9 - Properties of Saturated Steam ............................................................................................ 233 Table A9 - Properties of Saturated Steam (continued) ......................................................................... 234 Table A9 - Properties of Saturated Steam (continued) ......................................................................... 235 Table A9 - Properties of Saturated Steam (continued) ......................................................................... 236 Table A9 - Properties of Saturated Steam (continued) ......................................................................... 237 Table A10 - Specific Gravity and Gas Constants for Some Common Gases .......................................... 238 Table A11 – Properties and Sizing Coefficients for Globe Valves ......................................................... 240 Table A12 – Properties and Sizing Coefficients for Rotary Valves ........................................................ 241 Table A13 - Numerical Constants for Control Valve Sizing Formulas ................................................... 242 Table A14 – Service Temperature Limits for Non-Metallic Materials .................................................. 243 Table A15 – Standard Pipe Dimensions and Data ................................................................................. 244 Table A16 – NEC Wire Ampacity Table 310.16 ..................................................................................... 245 Table A17 – NEC Table 8 Conductor Properties .................................................................................... 246 Table A18 – NEC Full Load Motor Currents .......................................................................................... 247 Table A19 – Valve Seating Shutoff Pressure and Stem Friction Values ................................................ 248 Applications of Basic Fluid Mechanics in Piping Systems ............................................................... 249 Relationship of Pressure and Flow ........................................................................................................ 249 Applications of the formulas ................................................................................................................. 251 In Summary of Fluid Mechanics for Process Control ............................................................................ 254 References .............................................................................................................................................. 256
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Preface Most state licensing boards in the United States recognize the Control System Engineering (CSE) and offer the NCEES exam in this branch of engineering. There are, however, three states that do not offer the CSE exam—Alaska, Hawaii, and Rhode Island. If you live in one of these states, you may choose to pursue licensing in another discipline (such as electrical, mechanical, or chemical engineering). Or you can try to arrange to take the CSE exam in a neighboring state. The Control Systems Engineering (CSE) exam covers a broad range of subjects, from the electrical, mechanical and chemical engineering disciplines. This exam is not on systems theory, but on process control and basic control systems. Experience in engineering or designing process control systems is almost a necessity to pass this exam. Study of this reference manual should adequately prepare the experienced engineer or designer to take the CSE exam. However, passing the exam depends on an individual applicant’s demonstrated ability and cannot be guaranteed. I have included a list of recommended books and material. The recommended books contain information, invaluable to passing the exam. Even if you could take as many books as you want into the exam site, it is better not to overwhelm yourself —too much information can become distracting. Remember you will be under pressure to beat the clock. Study your reference books and tab the tables and information you need. This will ensure you do not waste time. Study of the Fisher Control Valve Handbook or another manufactory’s book is strongly recommended, to obtain the full benefits of this study review guide. The pages in the handbook are referenced later in this guide. The Fisher Control Valve Handbook can be obtained free or for minimal cost from your local Fisher Valve representative. The book is also available from Brown’s Technical Book Shop, 1517 San Jacinto, Houston, Texas, 77002. The book can be downloaded in PDF format from the Emerson- Fisher web site as well.
About The Author Bryon Lewis is a Professional Engineer (PE), licensed in Control Systems Engineering (CSE). He is also a Senior Member of ISA, a SME Certified Manufacturing Engineer (CMfgE), a Certified Journeyman Electronics Technician in Industrial electronics (CET), an ISA Level III Certified Control System Technician (CCST) and a licensed Master Electrician. Mr. Lewis has over 30 years of experience in electrical, mechanical, instrumentation, and control systems. He holds letters of recommendation from Belcan Engineering, S & B Engineers and Constructors, Enron Corporation and Lee College. His design experience is in electrical and lighting systems design; pharmaceutical and petrochemical plant design and installation, instrumentation and electrical systems design for compressor stations and food manufacturing plants and maintenance. If there are any questions please contact me at my email address
[email protected].
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People who have Contributed to this Manual
Chad Findlay Chad graciously reviewed this manual for errors and made numerous suggestions to improve its content. Chad Findlay is a Lead Controls Engineer for General Electric Company where he has worked for 7 years. He develops gas turbine control systems applied to simple and combined cycle power plants. Chad holds a Masters degree in Mechanical Engineering from the University of California, Davis.
Daniel Masso Daniel also contributed to the review of this manual for errors and made suggestions to improve its content. Daniel Masso has worked as a DCS engineer for Westinghouse and Emerson Electric for 20 years in sales, project and field/start-up engineering capacities in system, control logic and graphic design and programming capacities. He earned a B.Ch.E from Cleveland State University and continued on a M.S. Ch.E at Case Western Reserve University and is employed by Emerson Process Management Power and Water Solutions.
Neil Frihart I would like to thank Neil for his encouragement in writing this manual and his friendship and help over the years. Neil Frihart is Vice President of Engineering for Power & Control Engineering Solutions. He was employed a as a Senior Engineer for Callidus Technologies and was Manager of Systems Engineering at Power Flame, Inc. He earned a BSEE from Kansas State University and MBA from Pittsburg State University
Susan Colwell I would like to thank Susan for her patience and help in the publication of this manual. She was extremely helpful in the publication of the first edition. Susan Colwell is the Publishing Manager for ISA (International Society of Automation).
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General Information State Licensing Requirements Licensing of engineers is intended to protect the public health, safety, and welfare. State licensing boards have established requirements to be met by applicants for licenses which will, in their judgment, achieve this objective. Licensing requirements vary somewhat from state to state but have some common features. In all states, candidates with a 4-year engineering degree from an ABET/EAC-accredited program and four years of acceptable experience can be licensed if they pass the Fundamentals of Engineering (FE) exam and the Principles and Practice of Engineering (PE) exam in a specific discipline. References must be supplied to document the duration and nature of the applicant’s work experience.
Eligibility Some state licensing boards will accept candidates with engineering technology degrees, related-science (such as physics or chemistry) degrees, or no degree, with indication of an increasing amount of work experience. Some states will allow waivers of one or both of the exams for applicants with many years (6 – 20) of experience. Additional procedures are available for special cases, such as applicants with degrees or licenses from other countries. Note: Recipients of waivers may encounter difficulty in becoming licensed by “reciprocity” or “comity” in
another state where waivers are not available. Therefore, applicants are advised to complete an ABET accredited degree and to take and pass the FE/EIT exam. Some states require a minimum of four year experiences after passing the FE/EIT exam, before allowing one to sit for the PE (principals and practices) exam. Some states will not allow experience incurred before the passing of the FE/EIT exam. It is necessary to contact your licensing board for the up-to-date requirements of your state. Phone numbers and addresses can be obtained by calling the information operator in your state capital, or by checking the Internet at www.ncees.org or nspe.org.
Exam Schedule The CSE exam is offered once per year, on the last weekend in October, (typically on Friday). Application deadlines vary from state to state, but typically are about three or four months ahead of the exam date. Requirements and fees vary among state jurisdictions. Sufficient time must be allotted to complete the application process and assemble required data. PE references may take a month or more to be returned. The state board needs time to verify professional work history, references, and academic transcripts or other verifications of the applicant's engineering education. After accepting an applicant to take one of the exams, the state licensing board will notify him or her where and when to appear for the exam. They will also describe any unique state requirements such as allowed calculator models or limits on the number of reference books taken into the exam site.
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Description of Examination Exam Format The NCEES Principles-and-Practice of Engineering examination (commonly called the PE examination) in Control Systems Engineering (CSE) is an eight-hour examination. The examination is administered in a four hour morning session and a four hour afternoon session. Each session contains forty (40) questions in a multiple-choice format. Each question has a correct or “best” answer. Questions are independent, so an answer to one question has no bearing on the following questions.
All of the questions are compulsory; applicants should try to answer all of the questions. Each correct answer receives one point. If a question is omitted or the answer is incorrect, a score of zero will be given for that question. There is no penalty for guessing.
Exam Content The subject areas of the CSE exam are described by the exam specification and are given in six areas. ISA supports Control Systems Engineer (CSE) licensing and the examination for Professional Engineering. ISA is responsible for the content and questions in the NCEES examination. Refer to the ISA web site (http://www.isa.org) for the latest information concerning the CSE examination. The following details what to expect on the examination and breaks down the examination into the six parts. The percentage and number of questions are given for each part of the examination at the time this guide was written.
I.
MEASUREMENT 24% of Examination 19 Questions 1.
Sensor technologies applicable to the desired type of measurement (e.g., flow, pressure, level, temperature, analytical, counters, motion, vision, etc.) 2. Sensor characteristics (e.g., rangeability, accuracy and precision, temperature effects, response times, reliability, repeatability, etc.) 3. Material compatibility 4. Calculations involved in: pressure drop 5. Calculations involved in: flow element sizing 6. Calculations involved in: level, differential pressure 7. Calculations involved in: unit conversions 8. Calculations involved in: velocity 9. Calculations involved in: linearization 10. Installation details (e.g., process, pneumatic, electrical, etc.)
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II.
SIGNAL AND TRANSMISSION 12.5% of Examination 10 Questions A. Signals - 11.5%, 9 questions 1. Pneumatic, electronic, optical, hydraulic, digital, analog 2. Transducers (e.g., analog/digital [A/D], digital/analog [D/A], current/pneumatic [I/P] conversion, etc.) 3. Intrinsically Safe (IS) barriers 4. Grounding, shielding, segregation, AC coupling 5. Basic signal circuit design (e.g., two-wire, four-wire, isolated outputs, loop powering, etc.) 6. Calculations: circuit (voltage, current, impedance) 7. Calculations: unit conversions B. Transmission - 1.25%, 1 question 1. Different communications systems architecture and protocols (e.g., fiber optics, coaxial cable, wireless, paired conductors, fieldbus, Transmission Control Protocol/Internet Protocol [TCP/IP], OLE Process Control [OPC]) 2. Distance considerations versus transmission medium
III.
FINAL CONTROL ELEMENTS 20% of Examination 16 Questions A. Valves - 12.5%, 10 questions 1. Types (e.g., globe, ball, butterfly, etc.) 2. Characteristics (e.g., linear, low noise, equal percentage, shutoff class, etc.) 3. Calculation (e.g., sizing, split range, noise, actuator, speed, pressure drop, air/gas consumption, etc.) 4. Applications of fluid dynamics (e.g., cavitation, flashing, choked flow, Joule-Thompson effects, two-phase, etc.) 5. Material selection based on process characteristics (e.g., erosion, corrosion, plugged, extreme pressure, temperature, etc. 6. Accessories (e.g., limit switches, solenoid valves, positioners, transducers, air regulators, etc.) 7. Environmental constraints (e.g., fugitive emissions, packing, special sealing, etc.) 8. Installation practices (e.g., vertical, horizontal, bypasses, troubleshooting, etc.) B. Pressure Relieving Devices - 5%, 4 questions 1. Pressure Relieving Valves: Types (e.g., conventional spring, balanced bellows, pilot operated, etc.) 2. Pressure Relieving Valves: Characteristics (e.g., modulating, pop action, etc.) 3. Pressure Relieving Valves: Calculations (e.g., sizing considering inlet pressure drop, back pressure, multiple valves, etc.) 4. Pressure Relieving Devices: Material selection based on process characteristics 5. Pressure Relieving Valves: Installation practices (e.g., linking valves, sparing the valves, accessibility for testing, car sealing inlet valves, piping installation, etc.) 6. Rupture discs (types, characteristics, application, calculations, etc.) C. Other Final Control Elements - 2.5%, 2 questions 1. Motor controls 2. Solenoid valves 3. On-off devices/relays 4. Self-regulating devices 5
IV.
CONTROL SYSTEMS ANALYSIS 16% of Examination 13 Questions A. Documentation - 7.5%, 6 questions 1. Drawings (e.g., PFD, P&ID, Loop Diagrams, Ladder Diagrams, Logic Drawings, Cause and Effects Drawings, SAFE Charts, etc.) B. Theory - 6%, 5 questions 1. Basic processes (e.g., compression, combustion, distillation, hydraulics, etc.) 2. Process dynamics (e.g., loop response, P-V-T relationships, simulations, etc.) 3. Basic control (e.g., regulatory control, feedback, feed forward, cascade, ratio, PID, splitrange, etc.) 4. Discrete control (e.g., relay logic, Boolean algebra) 5. Sequential control (e.g., batch) C. Safety - 2.5%, 2 questions 1. Safety system design (e.g., Safety Instrumented System [SIS], Safety Requirements Specification [SRS], application of OSHA 1910, etc.)
V.
CONTROL SYSTEMS IMPLEMENTATION 16% of Examination 13 Questions 1. HMI (e.g., graphics, alarm management, trending, historical data) 2. Ergonomics (e.g., human factors engineering, physical control room arrangement, panel layout) 3. Configuration and programming (e.g., PLC, DCS, Hybrid systems, SQL, Ladder logic, sequential function chart, structured text, function block programming, data base management, specialized controllers, etc.) 4. System comparisons and compatibilities (e.g., advantages and disadvantages of system architecture) 5. Installation requirements (e.g., shielding, constructability, input/output termination, environmental, heat load calculations, power load requirements, purging, lighting, etc.) 6. Commissioning (e.g., performance tuning, loop checkout, etc.) 7. Safety Instrumented System [SIS] model validation calculations (e.g., Safety Integrity Level [SIL], reliability, availability, etc.) 8. Troubleshooting (e.g., root cause failure analysis and correction)
VI.
CODES, STANDARDS, REGULATIONS 7.5% of Examination 6 Questions 1. Working knowledge of applicable Codes, Standards, and Regulations: American National Standards Institute (ANSI) 2. Working knowledge of applicable Codes, Standards, and Regulations: Institute of Electrical & Electronics Engineers (IEEE) 3. Working knowledge of applicable Codes, Standards, and Regulations: ISA 4. Working knowledge of applicable Codes, Standards, and Regulations: National Electrical Code (NEC) 5. Working knowledge of applicable Codes, Standards, and Regulations: National Electrical Manufacturers Association (NEMA) 6. Working knowledge of applicable Codes, Standards, and Regulations: National Fire Protection Association (NFPA) 7. Working knowledge of applicable Codes, Standards, and Regulations: Occupational Safety and Health Administration (OSHA)
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Exam Scoring NCEES exams are scored independently. There are no pre-specified percentages of candidates that must pass or fail. Assisted by a testing consultant, a panel of licensed CSEs uses recognized psychometric procedures to determine a passing score corresponding to the knowledge level needed for minimally-competent practice in the discipline. The passing score is expressed as the number of questions out of 80 that must be answered correctly. The method used for pass-point determination assures that the passing score is adjusted for variations in the level of exam difficulty and that the standard is consistent from year to year. Starting in October 2005, candidates have received results expressed either as “Pass” or “Fail”; failing candidates no longer receive a numerical score. Published passing rates are based on first-time takers only, omitting the results for repeat takers.
Reference Materials for the Exam Recommended Books and Materials for Testing The list of recommended books and materials for testing will be necessary to help you pass the CSE examination. Use a book you are comfortable with. A substitution with the same material and information may be used. The list of recommended books and materials for additional study can be helpful in the review of subjects and preparation for the examination. Remember to keep the review simple. The test is not on control systems theory studies, but rather on simple general functional design. Again keep your studies simple and practical; control systems theory will only encompass about 3% of the examination. Books and Materials for Testing
NCEES APPROVED CALCULATOR (Have a spare with new batteries installed). I recommend the TI-36X Solar (any light). Practice with the calculator you will be using. (See http://www.ncees.org for a current list of approved calculators.)
ISA-5.1-1984 (R1992) - INSTRUMENTATION SYMBOLS AND IDENTIFICATION
ISA-5.2-1976 (R1992) - BINARY LOGIC DIAGRAMS FOR PROCESS OPERATIONS
ISA-5.3-1983 - GRAPHIC SYMBOLS FOR DISTRIBUTED CONTROL/ SHARED DISPLAY INSTRUMENTATION, LOGIC, AND COMPUTER SYSTEMS
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ISA-5.4-1991 - STANDARD INSTRUMENT LOOP DIAGRAMS
Books and Courses for Additional Study
ISA offers a 3-1/2 day instructor led Control Systems Engineer (CSE) PE exam review course at different locations across the nation. The cost of the course is approximately $1,299.
ISA offers an Automation and Control Curriculum - 44 Courses. The cost for all 44 courses is approximately $750.
Norman A. Anderson, INSTRUMENTATION FOR PROCESS MEASUREMENT AND CONTROL (3rd Ed.), CRC Press LLC, Boca Raton, FL, 1997. [Measurement; instrument calibration; orifice sizing; valve sizing; process characteristics; charts; thermocouple tables; RTD tables; general flow and pipe data tables; nomographs; formulas; typical installation details; typical calculations.]
The CSE Study Guide from ISA
The Fisher Control Valve Handbook
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The Fisher Control Valve Catalog
Review of Process Control Subjects Overview of Process Measurement, Control and Calibration The process control industry covers a wide variety of applications: petrochemical; pharmaceutical; pulp and paper; food processing; material handling; even commercial applications. Process control in a plant can include discrete logic, such as relay logic or a PLC; analog control, such as single loop control or a DCS (distributed control system) as well as pneumatic; hydraulic and electrical systems. The Control Systems Engineer must be versatile and have a broad range of understanding of the engineering sciences. The Control Systems Engineer (CSE) examination encompasses a broad range of subjects to ensure minimum competency. This book will review the foundations of process control and demonstrate the breadth and width of the CSE examination. We will review many aspects of process control systems, first the theory, then application and then calibration and installation of process control equipment. First we will start with basic terminology and definitions of process measurement and control signals. We will then review the basic process control elements, their theory of operation and then apply the elements to real world application. We will then review the calculations for sizing of the elements, as well as applicable laws, standards and codes governing the installation of a process control system.
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Process Signal and Calibration Terminology The most important terms in process measurement and calibration are range, span, zero, accuracy and repeatability. Let us start by first defining Span; Range; Lower Range Value (LRV); Upper Range Value (URV); Zero; Elevated Zero; Suppressed Zero.
Definition of the Range of an Instrument Range: The region in which a quantity can be measured, received, or transmitted, by an element, controller or final control device. The range can usually be adjusted and is expressed by stating the lower and upper range values. NOTE 1: For example: Full Range Adjusted Range LRV URV a) 0 to 150°F None 0°F 150°F b) –20 to +200°F –10 to +180°F –10°F +180°F c) 20 to 150°C 50 to 100°C 50°C 100°C NOTE 2: Unless otherwise modified, input range is implied. NOTE 3: The following compound terms are used with suitable modifications in the units: measured variable range, measured signal range, indicating scale range, chart scale range, etc. See Tables 1 and 2. NOTE 4: For multi-range devices, this definition applies to the particular range that the device is set to measure. Range-limit, lower: LRV (Lower Range Value) The lowest value of the measured variable that a device is adjusted to measure. Range-limit, upper: URV (Upper Range Value) The highest value of the measured variable that a device is adjusted to measure. NOTE: The following compound terms are used with suitable modifications to the units: measured variable lower range-limit, measured signal lower range-limit, etc. See Tables 1 and 2. Range-limit, upper: URV (Upper Range Value) The highest value of the measured variable that a device is adjusted to measure. NOTE: The following compound terms are used with suitable modifications to the units: measured variable upper range-limit, measured signal upper range-limit, etc. See Tables 1 and 2, Span: The algebraic difference between the upper and lower range-values. NOTE 1: For example: Range: 0 to 150°F, Span 150°F Range: –10 to 180°F, Span 190°F Range: 50 to 100°C, Span 50°C
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NOTE 2: The following compound terms are used with suitable modifications to the units: measured variable range, measured signal range, etc. NOTE 3: For multi-range devices, this definition applies to the particular range that the device is set to measure. See Tables 1 and 2. Range-limit, lower: LRV (Lower Range Value) The lowest value of the measured variable that a device is adjusted to measure. Range-limit, upper: URV (Upper Range Value) The highest value of the measured variable that a device is adjusted to measure. NOTE: The following compound terms are used with suitable modifications to the units: measured variable lower range-limit, measured signal lower range-limit, etc. See Tables 1 and 2. Range-limit, upper: URV (Upper Range Value) The highest value of the measured variable that a device is adjusted to measure. NOTE: The following compound terms are used with suitable modifications to the units: measured variable upper range-limit, measured signal upper range-limit, etc. See Tables 1 and 2.
Definition of the Span of an Instrument Span: The algebraic difference between the upper and lower range-values. NOTE 1: For example: Range: 0 to 150°F, Span 150°F Range: –10 to 180°F, Span 190°F Range: 50 to 100°C, Span 50°C NOTE 2: The following compound terms are used with suitable modifications to the units: measured variable range, measured signal range, etc. NOTE 3: For multi-range devices, this definition applies to the particular range that the device is set to measure. See Tables 1 and 2.
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Definition of the use of Zero in Instrumentation Live-Zero
The lower range value (LRV) is said to be set to zero, as a reference point, whether it is at zero or not. This LRV can be 0%; -40°F; 4mA; 1V or 3 PSI. All LRVs are an example of the ZERO (Live Zero), in process control signals or elements.
Elevated-Zero
The lower range-value of the range is below the value of zero. The LRV of the range must be raised to Live Zero, for the instrument to function properly. The output signal of the measured value will always be 0 to 100%. If the LRV of the range is too low, the instrument may not be able to reach 100% output . NOTE: For example:
input signal = (-100 in H2O to 25 in H 2O) output signal = (4mA to 20mA)
The output signal may only reach 12mA for 25 in H 2O (100%) input, due to limitation in the electronics or pneumatics. Therefore the Elevate jumper must be set in the transmitter or an elevation kit must be installed in a pneumatic transmitter. See Table 1.
Suppressed-Zero The lower range-value of the span is above the value of zero. The LRV of the range must be lowered to Live Zero, for the instrument to function properly. The output signal of the measured value will always be 0 to 100%. If the LRV of the range is too high, the instrument may not be able to reach 0% output. NOTE : For example:
input signal = (50 in H 2O to 200 in H 2O) output signal = (4mA to 20mA)
The output signal may only reach 6mA for 50 in H2O (0%) input, due to limitation in the electronics or pneumatics. Therefore the Suppress jumper must be set in the transmitter or a suppression kit must be installed in a pneumatic transmitter. See Tab1e 1.
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Illustrations of range and span terminology
Table 1 – Examples of range and span terminology TYPICAL RANGES
NAME
RANGE
LOWER RANGE VALUE
UPPER RANGE VALUE
SPAN
SUPPLEMENTARY DATA
0
+100
—
0 to 100
0
+100
100
—
20
+100
SUPPRESSED ZERO RANGE
20 to +100
20
+100
80
SUPPRESSION RATIO = 0.25
-25
+100
ELEVATED –25 to +100 ZERO RANGE
–25
+100
125
—
ELEVATED ZERO RANGE
-100
0
100
—
-100
-20
80
—
–100
0
–100
–20
–100 to 0
ELEVATED –100 to –20 ZERO RANGE
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Illustrations of measured variable, measured signal, range and span
Table 2 – Examples of measured variable, measured signal, range and span TYPICAL RANGES
THERMOCOUPLE 0
2000°F TYPE K T/C
– 0.68
+ 44.91 mV
FLOWMETER 0
10,000 lb/h
0
100 in H2O
0
10 x1000=lb/h
4
20 mA
1
5 Volts
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TYPE OF RANGE
MEASURED VARIABLE
RANGE
0 to 2000°F
LOWER RANGE VALUE
UPPER RANGE VALUE
SPAN
0°F
2000°F
2000°F
MEASURED SIGNAL
–0.68 to +44.91 –0.68 mV mV
+44.91 mV
45.59 mV
MEASURED VARIABLE
0 to 10 000 lb/h
10,000 lb/h
10,000 lb/h
MEASURED SIGNAL
0 to 100 in H2O 0 in H2O
100 in H2O
100 in H2O
0 lb/h
10,000 lb/h
10,000 lb/h
SCALE AND/OR 0 to 10,000 lb/h CHART
0 lb/h
MEASURED SIGNAL
4 to 20 mA
4 mA
20 mA
16 mA
MEASURED SIGNAL
1 to 5V
1V
5V
4V
Temperature Measurement and Calibration Temperature Measurement Devices and Calibration In the process industry, temperature measurements are typically made with thermocouples, RTDs (Resistance Temperature Detector) and industrial thermometers. Industrial thermometers are typically of the liquid (class I), vapor (class II), and gas (class III) type. The five major types of thermocouple configurations are shown to the left. The first two thermocouples are welded or grounded, as shown, to the outside metal protective sheathing. The bottom three thermocouples are ungrounded and should never touch the metal protective sheathing; otherwise they are shorted to ground.
Thermocouples should be extended with thermocouple extension wire and thermocouple termination blocks, but can be extended with standard copper wire and standard terminal blocks. This is due to the fact that the voltages generated at the extension junctions almost cancel each other out with very little error. One side is positive and the other side is negative. The four major thermocouples used in the process industry are Type J, Type E, Type K, Type T. The red wire is always the negative wire with thermocouples. Thermocouple terminal junction blocks should be made of the same material as the thermocouple wire that is being connected to terminal. This will prevent additional thermocouple (TC) junction points from being introduced in the temperature signal. Some companies use standard terminal strips, this can cause an error in the signal.
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Thermocouple millivolt tables for the examination can be found in the Table A1 – Thermocouple Table (Type J) through Table A4 – Thermocouple Table (Type T) in the Appendix section of this guide. Thermocouple Linearity Chart
Thermocouple Makeup Material and Color Code TC Type
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THEMOCOUPLE MATERIAL
RANGE FOR CALIB. DEG F -300 to 1830
USEFUL RANGE DEF F 200 to 1650
E
Chromel (+) Constantan (-)
J
Iron (+) Constantan (-)
-320 to 1400
200 to 1400 (300 to 800)
K
Chromel (+) Alumel (-)
-310 to 250
200 to 2300
R
Platinum 13% Rodium (+) Platinum (-)
0 to 3100
1600 to 2640
S
Platinum 10% Rodium (+) Platinum (-)
0 to 3200
1800 to 2640
T
Copper (+) Constantan (-)
-300 to 750
-310 to 660
TC COLORS
Purple Wire Jacket Purple (+) Red (-) Black Wire Jacket Black (+) Red (-) Yellow Wire Jacket Yellow (+) Red (-) Green Wire Jacket Black (+) Red (-) Green Wire Jacket Black(+) Red (-) Blue Wire Jacket Blue (+) Red (-)
Thermocouple Worked Examples (how to read the thermocouple tables) Sample problem: What is the Millivolt (mV) output of a Type “J” thermocouple at 218 °F and referenced to a 32°F electronic ice bath? Find the nearest temperature in Table A1 - Thermocouple Table (Type J) in the appendix of this guide. The nearest temperature in the first column is 210. Look at the column headers at the bottom of the chart. Find the column header labeled 8. Follow the column up to the row with the 210 value. Where they meet is a total of 210°F + 8ºF = (218°F). Read the value of mV. The answer is: 5.45 mV
Sample problem: What is the Millivolt (mV) output of a Type “K” thermocouple at 672°F from the data given? Assume the thermocouple is linear. Given: 670°F = 14.479mV 672°F = mV 680°F = 14.713mV We will have to interpolate the mV value for the desired temperature as follows: interpolation:
deg desired - deg lower value
mV upper value mV lower value mV lower value deg upper value - deg lower value
mV
Therefore the new mV for 672°F:
672 - 670 14.713 - 14.479 14.479 680 - 670
14.526
The mV at 672°F is 14.526 mV This can be verified in Table A2 -Thermocouple Table (Type K) in the appendix.
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RTD (Resistance Temperature Detector) The process control industry also uses RTDs (Resistance Temperature Detectors) for many applications, for example, when precise temperature measurement is needed, such as mass flow measurements or critical temperature measurements of motor bearings. RTDs typically come in 10 ohm copper and 100 ohm platinum elements. Their resistance is typically very linear over the scale. Resistance values for the examination can be found in the Table A5 - Platinum 100 Ohm RTD Table in ohms , in the Appendix section of this guide.
2-wire RTD
3-wire RTD
4-wire RTD
Good for close applications, at the transmitter.
Good for further distance applications. Remote from the transmitter.
Best application and usually uses 20 mA driving current and voltage measurement.
RTD Worked Examples Sample problem: A RTD is platinum and has a resistance of 100 omhs at a temperature of 32°F and an alpha 0.2178 ohms per °F. What is the resistance of the RTD at a temperature of 240°F? Find the difference in the temperature first. 240°F – 32°F = 208°F Now find the resistance for the differential temperature: 208°F * 0.2178 ohms/deg F = 45.3 ohms Now we add the change in resistance to the resistance at 32°F: 100 ohms + 45.3 = 145.3 ohms Referring to Table-A5. Platinum 100 Ohm RTD Table in ohms, in the appendix. The resistance value for the RTD can be interpolated and found for a given temperature.
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Sample problem: In the bridge circuit above, if R1 and R2 are 200 ohms and the RTD is at 60°F. What resistance should R3 measure, to balance the circuit and give the meter a reading of 0 volts? The RTD is platinum and measures 100 ohms at 32°F with an alpha of 0.2178 ohms per °F.
Find the difference in the temperature first. 60°F – 32°F = 28°F Now find the resistance for the differential temperature: 28°F * 0.2178 ohms/°F = 6.0984 ohms Now we add the change in resistance to the resistance at 32°F: 100 ohms + 6.0984 = 106.0984 ohms The resistor R3 needs to be 106 ohms to balance the bridge and give 0 volts at the meter.
Sample problem: In the bridge circuit above, R1 and R2 are 200 ohms. R3 is 150 ohms. The excite voltage to the bridge is 10 volts. If the meter is reading 0.4 volts (the positive is on the right side and the negative on the left side) what is the temperature at the RTD? Find the voltage on the left side of the bridge. This is the voltage we will add to the meter voltage on the right side. We will use the voltage divider theorem to find the voltage across R1.
V R1
R1 R1 R2
(10V )
200 200 200
(10V ) 5V
This means the voltage across the RTD is 5.0V + 0.4V = 5.4 volts. We will now use the voltage divider theorem to find the resistance of RTD.
V RTD
R RTD R RTD RR 3
(10V ) ; 5.4V
R RTD R RTD 150
(10V )
Solving for RRTD:
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R RTD 5.4 10 R RTD 150 R RTD 10 10 R RTD 150 10 R RTD 0.54 R RTD 150 RRTD 150 R 150 RTD 0.54( R RTD 150) RRTD 5.4
0.54 R RTD 0.54(150) RRTD 0.54 R RTD 81 RRTD 0.54 R RTD 0.54 RRTD 81 RRTD 0.54RRTD 81 R RTD 0.54 RRTD 81 1 0.54 R RTD 81 0.46 R RTD 81 0.46
0.46 R RTD 0.46
176.087 R RTD We can prove that the 176.087 ohms for the RTD is correct by plugging the value into the voltage divider formula to find the 5.4 volts at the meter.
V RTD
176.087 176.087 150
(10V ) 5.4V
We have the ohms of the RTD, now we can find the temperature. 100 ohms = 32°F, So subtract the difference in ohms 176.087 – 100 = 76.087 ohms. Divide the 76.087 ohms by the alpha 0.2178 ohms per °F.
F
76.087 ohms
0.2178 ohms deg F
349.34 F
Add the 32°F bias for 100 ohms to the 349.34°F for 76.087 ohms and we get: 349.34°F + 32.00°F = 381.34°F.
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Pressure Measurement and Calibration Pressure Measurement and Head Pressure Pressure is measured in typically two different forms. Pounds per square inch (psi) or head pressure. Head pressure is measured in inches or feet of water column (H2O). Head pressure is independent of the tank’s height or area. The transmitter measures head pressure. Head pressure is the measure of the potential energy in the system. The transmitter measurement is from how high is the fluid falling. The distance the fluid falls indicates the force generated (F=ma). This is why the density of the fluid must be known to calibrate a pressure transmitter for a process, to obtain the fluid mass. The calibration process uses specific gravity (S.G.), the ratio of a known density of a fluid divided by the density of water (H2O). To illustrate these facts we will start with one gallon of water. The gallon of water equals 231 cubic inches and weighs approximately 8.324 pounds at 60°F. Pressure is measured in PSI (pounds per square inch). Only one (1) square inch of area is needed to calculate the height of the water and the force it is excerpting. Remember force divided by area = pressure. Stack 231 cubic inches of water on top of each other, to form a tall column of water, with a base of one (1) square inch. The column of water will be 231 inches tall. Divide the height of the column of water, 231 inches, by the weight of one (1) gallon of water, 8.324 pounds. The result will be 27.691 or 27.7 inches of water per pound of water, over a one square inch of area. Therefore 27.7 inches H2O, of head pressure, equals one (1) PSI. By knowing the specific gravity of the fluid to be measured, multiplied by the height of the tank in inches, an equivalent value in inches of water can be found. The transmitter can now be calibrated in inches of water, regardless of the fluid. If the tank’s fluid has a S.G . equal to 0.8 and a height of 100 inches tall, then the height in inches of H 2O will be (100” of fluid x 0.8 s.g.= 80” of H2O). Pressure transmitters are purchased in different sizes of measurement. They are in ranges of inches H 2O, psig (the “g” stands for gauge pressure) or psia (the “a” stands for absolute pressure). When the symbol psid (the “d” stands for differential pressure) is called for, a standard psig transmitter is used. Most industrial pressure transmitters are differential pressure transmitters. They act on differential forces applied to each side of the transmitter. The force is produced by the pressure in the system multiplied by the area of the diaphragm.
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Applying Pressure Measurement and Signals Worked Examples Differential Pressure and Meter Calibration Differential pressure or differential head pressure is used to calibrate transmitters for pressure, level, flow and density measurements. The transmitter has a high side, marked with an H, and a low side, marked with a L. The low side will typically go to atmospheric pressure or to a fixed height wet leg measurement. The high side will typically go to the tank, where the varying height of fluid is to be measured. When calibrating an instrument remember: The low side is the negative scale, below zero, and the high side is the positive scale, above zero. The transmitter’s sensor element is static in position or elevation and therefore the transmitter itself is always equal to zero elevation. This will be discussed in detail in the section on Level Measurement. Transmitters can be purchased in ranges of 25in H 2O, 250in H2O, 1000in H 2O, 300 psi and 2000 psi. The formula for calibration is: (high side inches x S.G.) – (low side inches x S.G.) = lower or upper range value. Note: Gives LRV when empty or minimum and URV when full or maximum
Sample problem: A pressure gauge is reading 25 pisg. It is attached to a tank filled with a fluid. The bottom of tank is 65 feet above the ground. The pressure gauge is 5 feet above the ground. The fluid has a specific gravity of (0.7 s.g.). What is the level of the fluid in the tank? First convert the psi gauge measurement to feet of head measurement. 25 psi * 2.31 feet per psi = 57.75 feet of H 2O. Next find the elevation of the bottom of tank in relation to the elevation of the pressure gauge. Tank bottom in feet – pressure gauge elevation in feet, equals the height in feet to the bottom of tank. 65 feet – 5 feet = 60 feet of head to bottom of the tank. Note: Head is always measured in the standard of inches or feet of water Column.
Multiply the head between the bottom of the tank and the pressure gauge times the s.g. to get the head equal to H2O. 60 feet of fluid * 0.7 s.g. = 42 feet H 2O to bottom of tank from the pressure gauge. Next subtract (the height from the pressure gauge to the bottom of the tank in feet of H 2O), from (the total height of fluid in feet of in H 2O above the pressure gauge) , to find (the height of the fluid in the tank in H2O). (57.75 feet of H2O total head) – (42 feet of H2O below the tank) = (feet of fluid in H2O in the tank). (57.75 feet total) – (42 feet to bottom tank from the pressure gauge) = 15.75 feet in H 2O in the tank Next convert height in feet of H 2O to height of fluid with a specific gravity (s.g.) of 0.7: 15.75 feet of H 2O / 0.7 s.g. = 22.5 feet of total height of the fluid column in the tank
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On the CSE examination you will be asked to correlate signals and measurements using Flow, Pressure and the Output in (4mA to 20mA) signals. A change in flow in a pipe will cause a change in the head pressure across the pipe and measurement element. If the flow decreases in the pipe the pressure in the pipe will increase at any point along the pipe.
Pressure Change in a Pipe for a given Flow Rate If the flow rate increases, the pressure in the piping system decreases. If the flow rate decreases, the pressure in the piping system increases. This is because the total head of the system remains constant due to the head pressure developed by of the pump. The total energy head being endowed into the pump and piping system, remains constant. 2
F h1 1 h2 F 2
h1F12 h2 F22
Sample problem: There is a flow rate of 300 gpm in a piping system. There is a pressure gauge reading 100 psi somewhere in the piping system. If the flow rate is decreased to 240 gpm. What is the new pressure gauge reading in psi in the piping system? a) Find the new pressure at the point of the gauge in the piping system for a flow rate of 240 gpm. 2
F 300 156.25 psi h2 h1 1 100 240 F 2 2
Pressure Change across the Flow Element for a given Flow Rate If the flow in the pipe increases, the head pressure on the outlet of the measurement element will decrease. This correlation can be demonstrated by the following equations for differential head pressure (DP) across the element or section of pipe. See the appendix for applications of basic fluid mechanics in piping systems. 2
h1F22 h2 F12
F h1 2 h2 F 1
Sample problem: a. A flow of 250 gpm has a head pressure measurement of 309 inches of H 2O. If the flow is decreased to 150 gpm, what is the new head pressure in H 2O for the measurement element? b. What would be the new output to the PLC or DCS, in a mA signal, if the transmitter was calibrated in 0 to 400 inches of H 2O? The signal is calibrated for 4mA to 20mA. a. Find the new head pressure for 150 gpm.
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