Troubleshooting Process Plant Control
Contents
1. And I said Oh Is there a difference So that s how I came to be ejected from the faculty of Northwestern Uni versity after my first day of teaching LEARNING FROM RELATIONSHIPS My ex girlfriend used to tell me Norm the reason we get along so well is that I give you a lot of positive feedback From this I developed the impres sion that positive feedback is good Which is true in a relationship with your girlfriend But when involved in a relationship with a control loop we want negative feedback Control logic fails when in the positive feedback mode of control For example Distillation As process engineers and operators we have the expectation that reflux improves fractionation which is true up to a point That point where more reflux hurts fractionation instead of helps is called the incipi ent flood point Beyond this point the distillation tower is operating in a positive feedback mode of process control That means that tray flooding reduces tray fractionation efficiency More reflux simply makes the flood ing worse Fired Heaters Increasing furnace fuel should increase the heater outlet temperature But if the heat release is limited by combustion air then increasing the fuel gas will reduce the heater outlet temperature But as the heater outlet temperature drops the automatic control calls for more fuel gas which does not burn As the heater outlet temperature continues to fall because c2. I met this lady at a bar Norm I introduced myself and asked her name Hi Ym Linda Can I buy you a drink Linda Actually Jake I m perfectly capable of buying my own drink Norm I hate when girls blow me off like that I couldn t think what else to say xi Xii INTRODUCTION A HISTORY OF POSITIVE FEEDBACK LOOPS Okay Jake Here s the correct line I advised Say Linda I m conducting a survey Which is better negative feedback or positive feedback Well Jake Linda will answer I m a positive thinking person Therefore I m sure that positive feedback is best Not so Linda you ll say As a highly paid Process Control Engineer I ve found negative feedback is best Do you mind if I explain Okay Jake she ll say Maybe I will have a small scotch and soda Let s say Linda you re driving your car uphill The car is in automatic cruise control As the gradient increases the car slows The gas pedal is auto matically depressed to accelerate the car But as the car engine slows the amount of combustion air drawn into the engine is reduced As the air flow drops the incremental gasoline injected into the cylinders does not burn But as the gasoline vaporizes it also cools the engine s cylinders This reduces the cylinder pressure and hence the force acting on the pistons heads This decreases the engine horsepower The engine and c
3. The purchased propylene charge pump had a spillback line as shown in Figure 1 2 I opened the spillback valve The pump discharge pressure dropped and the propylene feed valve opened to 30 The control valve was now operating in its linear range Now when I design a control valve to handle a large reduction in flow I include an automated spillback valve from pump discharge to suction The spillback controls the pump discharge pressure to keep the FRC valve between 20 and 80 open Whenever I sketch this control loop I recall that dark night in El Dorado I also recall the value of learning even the most basic control principles by personal field observations 4 LEARNING FROM EXPERIENCE SPILLBACK C VALVE PROPYLENE gt l l To gt gt REACTOR Figure 1 2 Opening spillback to keep FRC valve in it s linear operating range LEARNING FROM MISTAKES Adolf Hitler did not always learn from his mistakes For example he once ordered a submarine to attack the Esso Lago Refinery in Aruba The sub sur faced in the island s harbor and fired at the refinery But the crew neglected to remove the sea cap on the gun s muzzle The gun exploded and killed the crew I too had my problems in this refinery The refinery flare was often very large and always erratic The gas being burned in the flare was plant fuel The plant fuel was primarily cracked gas from the delayed coker supplemented as shown in Fig 1 3 by vaporized LPG So
4. executed by operators or other engineers as if I had myself originated rather than just participated in such events I have consistently failed throughout this text to assign credit to other individuals for their ideas which I have not stolen but only borrowed Any names of actual individuals or process plants or refineries have been chosen at random and do not correspond to any real events connected with the people or locations mentioned in my text Norman P Lieberman Copyright 2009 by John Wiley amp Sons Inc All rights reserved Published by John Wiley amp Sons Inc Hoboken New Jersey Published simultaneously in Canada No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 978 750 8400 fax 978 750 4470 or on the web at www copyright com Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 201 748 6011 fax 201 748 6008 or online at http www wiley com go permission Limit of Liability Disclai
5. much fuel gas was lost by flaring that 90 of the Aruba s LPG production had to be diverted to fuel via the propane vaporizer I analyzed the problem based on the dynamics of the system I modeled the refinery s fuel consumption vs cracked gas production as a function of time The key problem based on my computer system dynamic analysis was the cyclic production of cracked gas from the delayed coker complex My report to Mr English the General Director of the Aruba Refinery concluded 1 The LPG vaporizer was responding too slowly to changes in cracked gas production from the delayed coker 2 The natural log of the system time constants of the coker and vaporizer were out of synchronization 3 A feed forward advanced computer control based on real time dynam ics would have to be developed to bring the coker vaporizer systems into dynamic real time equilibrium 4 A team of outside consultants experts in this technology should be con tracted to provide this computer technology Six months passed The complex feed forward computer system was inte grated into the LPG makeup and flaring controls shown in Figure 1 3 Adolf LEARNING FROM MISTAKES 5 REFINERY FUEL GAS CRACKED 7 TO REFINERY GAS PROPANE VAPORIZER FLARE 2 Y STM INSTRUMENT AIR SIGNAL MAKEUP LPG Figure 1 3 Unintentional flaring caused by malfunction of LPG makeup control valve is an example of split range pressure control Hi
6. 2 2008032181 Printed in the United States of America 10987654321 Dedication One of life s little pleasures is working with quality and dedicated people such as April Montecino Winn and Phil Negri Persistently and consistently they have worked with my wonderful draftsman Roy Williams to bring order to my scribbled manuscript and illegible drawings to produce this book Inspired by the Creator these three wonderful people have brought Order out of Chaos I too have been inspired by my equally wonderful partner in life Liz She is a light unto my life Contents PREFACE INTRODUCTION A HISTORY OF POSITIVE FEEDBACK LOOPS Chapter 1 Learning from Experience Chapter 2 Process Control Parameter Measurement Chapter 3 Dependent and Independent Variables Chapter 4 Binary Distillation of Pure Components Chapter 5 Distillation Tower Pressure Control Chapter 6 Pressure Control in Multicomponent Systems Chapter 7 Optimizing Fractionation Efficiency by Temperature Profile Chapter 8 Analyzer Process Control Chapter 9 Fired Heater Combustion Air Control Chapter 10 Sizing Process Control Valves Chapter 11 Control Valve Position on Instrument Air Failure Chapter 12 Override and Split Range Process Control Chapter 13 Vacuum System Pressure Control Chapter 14 Reciprocating Compressors Chapter 15 Centrifugal Compressor Surge vs Motor Over Amping 59 67 75 85 93 101 107 115 123 vii viii CONTENTS Chapter 16 Contro
7. ER MEASUREMENT e Liquid levels Temperature e Pressure e Differential pressure Flow My experience is limited to continuous processes but excludes solids and high viscosity fluids So let s limit our study accordingly My experience in the petrochemical and refining industry has taught me that most control problems are a consequence of improper parameter measurement most especially levels HOW ARE LIQUID LEVELS MEASURED Most liquid level measurement is made by a level trol The level trol is served by two pressure transducers A pressure transducer is a mechanical device that converts a pressure in an electronic signal Car engines have a transducer to measure the engine oil pressure Figure 2 1 shows the arrangement of the pressure transducers one at the top and one at the bottom of the level trol The level trol is a pipe a few feet long The difference in the electrical output between the dual pressure trans ducers is proportional to the difference in pressure between the top and bottom of the level trol This delta P is caused by the head of liquid in the level trol The electrical output generated by this pressure difference is called UL jo PRESSURE TRANSDUCER y LEVEL TRANSMITTER MILLIAMP OUTPUT OX Fm lt m k Figure 2 1 Measuring levels by sensing liquid head pressure
8. Jake will get very far in finding a new girlfriend with this grim tale of global warming Especially if Jake tells Linda the end of the story Eventually the small percentage of ocean surface waters that have warmed and become acidic will mix with deeper cooler neutral pH layers of the ocean which contain the vast bulk of the planet s water This will stop the positive feedback loop and global warming So my story does have a happy ending if Linda and Jake can just wait a few thousand years REFERENCE 1 Desonie Dana Climate Our Fragile Planet Chelsea House Publishers 2006 Learning from Experience An old Jewish philosopher once said Ask me any question and if I know the answer I will answer it And if I don t know the answer Pll answer it anyway Me too I think I know the answer to all control questions The only problem is a lot of my answers are wrong I ve learned to differentiate between wrong and right answers by trial and error If the panel board operator persistently prefers to run a new control loop I ve designed in manual if he switches out of auto whenever the flow becomes erratic then I ve designed a control strategy that s wrong So that s how I ve learned to discriminate between a control loop that works and a control strategy best forgotten Here s something else I ve learned Direct from Dr Shinsky the world s expert on process control Lieberman if it won t wor
9. The Process Control Engineer who with his degree in hand must now face up to the real world INTRODUCTION A HISTORY OF POSITIVE FEEDBACK LOOPS xiii The Process Engineer who must design control loops for new or revamped units The text is very basic and very simple Lots of new control concepts are presented But they are all based on old technical concepts Likely you are reading this text because you are one of the 12 000 students who have already attended one of my seminars or read one of my other books or viewed my videotapes on process technology If not let me tell you something about myself My descriptions about control loops process control optimization process instrumentation and control valves in this book are based on my personal experience which encom passes 44 years Except for the story about Linda and Jake I just made that up LATER THAT EVENING My wife and partner Liz has just read the story about Linda and Jake Liz says Can t you give an example of a positive feedback loop without all of the sexist stupidity Okay I will CO accumulates in the atmosphere The rate of CO accumulation between 1968 and 2008 was 0 51 com pounded annually e Global warming has increased by 1 F since the early 1900s including the surface of the oceans For the 1960 2000 period sulfur emissions from oil and coal combustion generated atmospheric sulfates which reflected sunlight and suppressed global w
10. Troubleshooting Process Plant Control Norman P Lieberman WWILEY A John Wiley Sons Inc Publication Troubleshooting Process Plant Control Troubleshooting Process Plant Control Other Books by Norman P Lieberman e Troubleshooting Refinery Operations Penn Well Publications e Troubleshooting Process Operations 4th Edition PennWell Publications e A Working Guide to Process Equipment with E T Lieberman 3 Edition McGraw Hill Publications e Troubleshooting Natural Gas Processing order by e mail at norm lieberman eng com e Process Design for Reliable Operations 3rd Edition order by e mail at norm lieberman eng com Copies of the first three texts are best ordered from the publishers but may be ordered through us E mail norm lieberman eng com Troubleshooting Refinery Operations 1980 has been incorporated into Troubleshooting Process Operations and Troubleshooting Natural Gas Processing Troubleshooting Process Plant Control Norman P Lieberman WWILEY A John Wiley Sons Inc Publication DISCLAIMER Company names and the names of individuals used in this book are entirely fictitious I have selected company names and the names of colleagues entirely at random Technically the stories I have related are correct but are sometimes combinations of actual incidents Any similarity to actually existing events process plants or individuals is purely a coincidence Also I sometimes have related projects
11. ar both slow further This signals the automatic cruise control to inject more gasoline into the cylinders As the engine is already limited by combustion air and not by fuel the extra gasoline just makes the problem worse If you don t switch off the cruise control and return to manual operation the car will stall out and the engine will flood with gasoline The problem Linda is positive feedback The loss of engine speed reduced the air flow to the engine and also automatically caused more gasoline to flow into the engine The problem fed upon itself That s why we call this a positive feedback loop What the Process Control Engineer wants is negative feedback For a negative feedback loop to work the engine cannot be limited by the combustion air flow Linda I run into this problem all the time It happens in distillation towers vacuum jet systems and fired heaters You see control loops for process equip ment only function properly when they are running in the range of a negative feedback response Positive feedback is dangerous in that it leads to process instability Jake has subsequently tried this approach with Gloria Janet and Carol all with the same result When he comes to the part about process instability each lady suddenly remembers an important appointment and rushed out of the bar NOTE TO READERS I ve written this book for three groups of people The experienced plant panel board operator
12. arming Sulfates are scrubbed from the atmosphere by rain every year Sulfur emissions in the past decade have mostly been stopped and the rate of global warming has increased Only 60 of the CO generated from combustion of oil gas coal and cement production has been accumulating in the atmosphere The rest is absorbed in ocean surface waters The ocean surface water is becoming more acidic and hotter Both factors reduce the solubility of CO in water As the land becomes warmer ice and snow melt in Greenland and the Antarctic The Earth becomes less reflective to sunlight As the Earth becomes warmer methane emissions from frozen tundra peat bogs and offshore hydrate deposits increase Methane per mole is 23 times more powerful a greenhouse gas than CO xiv INTRODUCTION A HISTORY OF POSITIVE FEEDBACK LOOPS In the next few decades the ocean surface waters will become an emitter rather than an absorber of CO because of warming and increased acidity Accumulating CO has in the absence of increased sulfate concentration in the atmosphere accelerated global warming Accelerating global warming will reduce the absorption capacity of the ocean surface waters for CO and increase methane emissions Warmer global temperature will increase humidity Water is a bigger greenhouse gas than either CO or methane The humidity effect will build upon itself Liz that s a real positive feedback loop But I don t think
13. ck to automatic control Process Control Parameter Measurement I mentioned in Chapter 1 that I was ejected from the faculty of Northwestern University after teaching a single class This was not the end of my academic career I was also an instructor at Louisiana State University Dr Dillard Smythe had hired me on a trial basis to conduct a process control course for undergraduate chemical engineers My course was excellent but judge for yourself Ladies and gentlemen Welcome to Process Control 101 The course is divided into two segments Segment One Measuring Process Control Parameters Segment Two Designing Control Loops for Process Parameters We must measure the parameter before we can control the parameter That s why we will study measurement first The Nazi army was able to initially defeat the allied armies in World War II because of the superior use of tanks It wasn t that the German tanks were better than the Allied tanks It was that the Germans had excellent FM radios in their tanks The data supplied from forward units enabled senior command ers to coordinate the Panzer attack That is the limiting factor for any control strategy is the quality of the data Garbage in garbage out I plan to discuss measurement techniques and problems for the following process parameters Troubleshooting Process Plant Control by Norman P Lieberman Copyright 2009 John Wiley amp Sons Inc 10 PROCESS CONTROL PARAMET
14. f it does work on manual we can automate it LEARNING FROM FIELD OBSERVATIONS Forty years ago I redesigned the polypropylene plant in El Dorado Arkansas T had never paid much attention to control valves I had never really observed how they operate But I had my opportunity to do so when the polypropylene plant was restarted The problem was that the purchased propylene feed valve was too large for normal service I had designed this flow for a maximum of 1600 BSD but the current flow was only 100 BSD Control valve response is quite nonlinear Nonlinear means that if the valve is open by 5 you might get 20 of the flow If you open the valve from 80 to 100 the flow goes up by an addi tional 2 Nonlinear response also means that you cannot precisely control a flow if the valve is mostly closed With the flow only 20 of the design flow the purchased propylene feed was erratic This resulted in erratic reactor tem perature and erratic viscosity of the polypropylene product The plant start up had proceeded slowly It was past midnight The evening was hot humid and very dark I went out to look at the propylene feed control valves Most of the flow was coming from the refinery s own propylene supply This valve was half open But the purchased propylene feed valve was barely open The valve position indicator as best I could see with my flashlight was bumping up and down against the C closed on the valve stem indicator
15. ge feed surge drum than precise level control of the reflux drum is acceptable In order for the instrument technician to tune the level control valve you have to show him what you want To do this put the level valve on manual Next manipulate the light naphtha flow to permit the level swings in the reflux drum you are willing to tolerate But you will find that there is a problem If you try to get back to the 50 level set point quickly you will badly overshoot your level target CN FUEL GAS TO REFINERY NAPHTHA SPLITTER de ke gt LIGHT NAPHTHA 2 O TO DEBUTANIZER Figure 1 4 Tuning a level control valve depends on what is downstream LEARNING FROM RELATIONSHIPS 7 If you return slowly to the set point it s easy to reestablish the 50 level target However the level will be off the target for a long time In conclusion ladies and gentlemen tuning a control loop is a compromise between the speed at which we wish to return to the set point and our tolerance to overshooting the target To establish the correct tuning criteria the control loop is best run on manual for a few hours by the Process Control Engineer Thank you Class adjourned for today My students unfortunately adjourned to Dean Gold s office Dean Gold lectured me about the student s complaints Mr Lieberman did you think you were teaching a junior high school science class or a postgraduate course in process control
16. k in manual it won t work in auto e Most control problems are really process problems I ve no formal training in process control and instrumentation All I know is what Dr Shinsky told me And 44 years of experience in process plants has taught me that s all Ineed to know Troubleshooting Process Plant Control by Norman P Lieberman Copyright 2009 John Wiley amp Sons Inc 2 LEARNING FROM EXPERIENCE LEARNING FROM PLANT OPERATORS My first assignment as a Process Engineer was on No 12 Pipe Still in Whiting Indiana This was a crude distillation unit My objective was to maximize pro duction of gas oil as shown in Figure 1 1 The gas oil had a product spec of not more than 500ppm asphaltines The lab required half a day to report sample results However every hour or two the outside operator brought in a bottle of gas oil for the panel board operator The panel operator would adjust the wash oil flow based on the color of the gas oil While plant supervision monitored the lab asphaltine sample results plant operators ignored this analysis They adjusted the wash oil rate to obtain a clean looking product The operators consistently produced a gas oil product with 50 200 ppm asphaltines They were using too much wash oil And the more wash oil used the lower the gas oil production I mixed a few drops of crude tower bottoms in the gas oil to obtain a bottle of 500 ppm asphaltine material I then instructed the panel b
17. lling Centrifugal Pumps Chapter 17 Steam Turbine Control Chapter 18 Steam and Condensate Control Chapter 19 Function of the Process Control Engineer Chapter 20 Steam Quality and Moisture Content Chapter 21 Level Pressure Flow and Temperature Indication Methods Chapter 22 Alarm and Trip Design for Safe Plant Operations Chapter 23 Nonlinear Process Responses ABOUT MY SEMINARS FURTHER READINGS ON TROUBLESHOOTING PROCESS CONTROLS THE NORM LIEBERMAN VIDEO LIBRARY OF TROUBLESHOOTING PROCESS OPERATIONS INDEX 131 139 147 155 163 179 195 205 213 217 219 221 PREFACE WARNING The Surgeon General Has Determined That This Book Is a Fraud Dr R K Sudkamp Ph D having reviewed this text reports that this book s claim to be a technical work on the exalted subject of Process Control has no basis Further findings by the Federal Communications Commission have determined that this book cannot claim to be a Process Control text as enu merated below e The entire book is free of complex mathematics Even simple equations are rarely encountered The text is much too easy to read for any respectable technical book e Much of the so called advanced technology described by the author is 40 years old and is already in widespread practice in the process industry While Professor Sudkamp notes the potential usefulness of this book to solve practical plant process problems he also observes this is more
18. mer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at 800 762 2974 outside the United States at 317 572 3993 or fax 317 572 4002 Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at www wiley com Library of Congress Cataloging in Publication Data Lieberman Norman P Troubleshooting process plant control Norman P Lieberman p cm ISBN 978 0 470 42514 5 cloth 1 Petroleum refineries Maintenance and repair I Title TP690 3 L534 2009 665 5028 8 dc2
19. oard operators as follows If the sample from the field is darker than my standard bottle increase the wash oil valve position by 5 If the sample of gas oil from the field is lighter than my standard decrease the wash oil valve position by 3 Repeat the above every 30 minutes The color of gas oil from a crude distillation unit correlates nicely with asphaltine content The gas oil when free of entrained asphaltines is a pale yellow So it seems that my procedure should have worked But it didn t The operators persisted in drawing the sample every one to two hours GAS OIL COLORIMETER SAMPLE OIL LAN 1 Figure 1 1 Adjusting wash oil based on gas oil color LEARNING FROM FIELD OBSERVATIONS 3 So I purchased an online colorimeter The online colorimeter checked whether the gas oil color was above or below my set point With an interval of 10 minutes it would move the wash oil valve position by 1 This never achieved the desired color but the gas oil product was mixed in a tank The main result was that gas oil production was maximized consistent with the 500 ppm asphaltine specification One might say that all I did was automate what the operators were already doing manually that all I accomplished was marginally improving an existing control strategy by automating the strategy But in 1965 I was very proud of my accomplishments I had proved as Dr Shinsky said I
20. ombustion is limited by air the outlet temperature drops further The heater automatic temperature control loop is now in the positive feedback mode of control As long as this control loop is on auto the problem will feed upon itself 8 LEARNING FROM EXPERIENCE MOTIVE STEAM m ol 1 EJECTOR Gp CONDENSER NONCONDENSIBLES WATER Figure 1 5 Too much steam flow causes a loss in vacuum Vacuum Ejector Some refineries control vacuum tower pressure by con trolling the motive steam flow to the steam ejector As the steam pressure and flow to the ejector increases the ejector pulls a better vacuum as shown in Figure 1 5 but as the steam flow increases so does that load on the downstream condenser As the condenser becomes overloaded the ejector discharge pressure rises At some point the increased discharge pressure adversely affects the ejector s suction pressure A further increase in motive steam will make the vacuum worse instead of better As the vacuum gets worse the control loop calls for more steam Having now entered the positive feedback mode of control the problem feeds upon itself Many control loops are subject to slipping into a positive feedback loop The only way out of this trap is to switch the controls to manual and slowly climb back out of the trap Once you guess but there is no way to know for sure that you are in the safe negative feedback mode of control you can then safely switch ba
21. than offset by its total lack of applicability as a postgraduate university text Professor Sudkamp of the University of Stockholm has reported a deep loss of personal dignity as a consequence of his exposure to Mr Lieberman s book which he considers to be an insult to his lofty intellect Introduction A History of Positive Feedback Loops Process Control Engineering is the most important branch of Chemical Engi neering Ask any panel board operator in a petrochemical plant or refinery The P amp IDs Process and Instrumentation Diagrams are the definitive engi neering documents describing how a plant works The Process Control Engi neer has the ultimate responsibility for creating maintaining and interpreting the P amp IDs It is his job and perhaps his most important function to explain to the panel operator how the control valves interact with the process plant to achieve unit stability I don t know why I wrote his job because half of the Process Control Engineers I work with are women Women often make better control engi neers and panel board operators than men because they are more patient Men often are driven to reach some distant goal quickly Women being patient will take a more measured approach to restore stability during a process upset For example one question I am frequently asked by younger male process engineers and console operators is how to meet girls One fellow Jake described his problem
22. tler would have been more sympathetic than Mr English The refinery s flaring continued just as before Now what Distressed discouraged and dismayed I went out to look at the vaporizer I looked at the vaporizer for many hours After a while I noticed that the fuel gas system pressure was dropping This happened every three hours and was caused by the cyclic operation of the delayed coker This was normal The falling fuel gas pressure caused the instrument air signal to the LPG makeup valve to increase This was an Air to Open valve see Chapter 11 and more air pressure was needed to open the propane flow control valve This was normal BUT the valve position itself did not move The valve was stuck in a closed position This was not normal You will understand that the operator in the control room was seeing the LPG propane makeup valve opening as the fuel gas pressure dropped But the panel board operator was not really seeing the valve position he was only seeing the instrument air signal to the valve Suddenly the valve jerked open The propane whistled through the valve The local level indication in the vaporizer surged up as did the fuel gas pres sure The flare valve opened to relieve the excess plant fuel gas pressure and remained open until the vaporizer liquid level sank back down which took well over an hour This all reminded me of the sticky side door to my garage in New Orleans I sprayed the control valve stem
23. with WD 40 stroked the valve up and down with air pressure a dozen times and cleaned the stem until it glistened The next time the delayed coker cycled the flow of LPG slowly increased to catch the falling fuel gas pressure but without overshooting the pressure set point and initiating flaring 6 LEARNING FROM EXPERIENCE My mistake had been that I had assumed that the field instrumentation and control valves were working properly I did not take into account the probabil ity of a control valve malfunction But at least I had learned from my mistake which is more than you could say for Adolf Hitler LEARNING FROM THEORY Northwestern University has an excellent postgraduate chemical engineering program I know this because I was ejected from their faculty I had been hired to present a course to their graduate engineers majoring in process control My lecture began Ladies and gentlemen the thing you need to know about control theory is that if you try to get some place too fast it s hard to stop Let s look at Figure 1 4 In particular let s talk about tuning the reflux drum level control valve Do I want to keep the level in the drum close to 50 or doesn t it matter As long as the level doesn t get high enough to entrain light naphtha into fuel gas that s okay What is not okay is to have an erratic flow feeding the light naphtha debutanizer tower On the other hand if the overhead product was flowing into a lar
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