Advanced Control Unleashed - Plant Performance Management For Optimum Benefit PDF

 

Advanced Control Unleashed

Plant Performance Management for Optimum Benefit Terrence L. Blevins Gregory K. McMillan Willy K. Wojsznis Michael W. Brown

ISA-The Instrumentation Systems and Automation Automation S ocie ociety ty

 

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Dedication This  book  is  dedicated  to  Karen  Blevi Blevins ns Cathy MacDonell Brown arol

 McM ill illan an and ojsznis whoour havecareers. provided encouragem ent and Susan supportW throughout

 

  cknowledgement

Th e au th o r s w is h to ex p r es s th ei eirr ap p r eciatio n to M ar k N ix o n an d R o n Ed d ie f r o m Emer s o n P r o ces s M an ag emen t, f o r th eir en th u s ias tic s u p p o r t and co nm Schleis m itm ens,t John ooff resourc esand ffor or Gil tthis his Par book JimEm Hoff Hoffmaster, master, Bud Ma Keyes, Du nca Schleiss, Berra, Pareja eja, to fr from om erson Process n ag em en t f oorr th eir iinn s p ir atio n an d s u p p o r t in es tab lis h in g th e D elta eltaV V advanced control program, to Karl Astrom from Lund University, Tom Edgar from the University of Texas at Austin, Dale Seborg from the University of California, Santa Barbara and Tom McAvoy from the Univer sity of Maryland for their guidance in the pursuit of new technologies, Mike Gray and Mark Mennen from Solutia Inc. for the initiation and s u s ten an ce o f ad v an ced co n tr o l ap p licatio n s an d in n o v atio n s , K en Schibler from Emerson Process Management for his help in setting the direction of the book, Robert Cameron, Michael Mansy, Glenn Mertz, and Gina U nd erw oo d from Soluti Solutiaa IInc. nc. fo forr their valua ble com m en ts, and finally, Scott Weidemann from Washington University, and Jim Cahill, B r en d a F o r s y th e, an d C o r y Walto n f r o m Emer s o n P r o ces s M an ag emen t for their essential contributions to the videos and demos on the CD. The au th o r s ex ten d th eir th an k s ttoo th e d ev elo p e r s ooff tthh e ad v an ce d co n tr ooll tools that we re the inspiratio n for this boo k. This inclu des V asi asili liki ki Tzovla, Ron Ottenbacher, Dirk Thiele, Ashish Mehta, Yan Zhang, Peter Wojsznis, J o h n G u d az, I an N a d a s an d M ei Y aann g . A ls o , w e w o u ld li likk e ttoo r eco g n ize the valuable contribution of Tom Aneweer, Dennis Stevenson, Jay Colclazier, zie r, Darrin Kuc hle, Dick Seem ann , Joe Joe S S.. Qin, Steve M orrison , Mike Ott, an d S ai G an es amo o r th i. The discipline of Process Control and Advanced Process Control is an exciting, challenging and rewarding field of engineering. Some of us mo v ed in to tthh is d is cip lin e b y ch an ce, w h ile o th er s m ad e a co n s cio u s d eci sion to bec om e Process Con trol En ginee rs. Reg ardless ooff ou r entry po int or xiii

 

motivation, we all appreciate the fact that engineers before us took the time and effort to teach us the tools and techniques that allowed us to ach iev e s u cces s. s. Th is b o o k p a s s es alo n g th e k n o w led g e o f m an y y ear s an d man y p eo p le an d ack n o w led g es th e ef f o r ts o f o u r en g in eer in g men to r s . We ho pe it will allow o thers not only to benefit benefit from all the experien ce we ha ve benefited fr from om so greatly bu t also to take the techno logy to the next level.

 

  bo bout ut t he uthors Terrence L Blevins   has been actively involved in the application and desi gn of proce ss control system s thro ug ho ut his car career. eer. For over fi fift fteen een

y ear s , h e w o r k ed as a s y s tems en g in eer an d g r o u p man ag er in th e d es ig n an d s t ar tu p ooff ad v an c ed co n tr o l ssoo lu tio n s ffoo r tthh e p u lp an d p ap er in d u s tr y . Ter r y w as in s tr u men tal in th e es tab lis h men t o f Emer s o n P r o ces s M an ag e men t' s A d v an ced C o n tr o l P r o g r am. H e is th e F ield b u s F o u n d atio n team leader for the Function Block Specification. In this capacity, Terry is involved in the movement of Fieldbus Foundation function block work into international standards. Terry is the US expert to the IEC TC65 WG6 and SC65C WG7 function block committees. He wrote the fieldbus section included in the   Process/Industrial Instrumentation and Controls  h a n d b o o k . Terry has eight patents and has written over forty papers on process con trol system de sign an d app lication s. H e rreceived eceived a B BEE EE from from the Unive r sity of Louisville in 1971 and a Master of Science degree in Electrical En g in eer in g f ro ro m P u r d u e U n iv er s ity in 1973. Presently, Terry is a principal technologist in DeltaV Product Engineering and the team leader for D e l ta ta V a d v a n c e d c o n t r o l p r o d u c t d e v e l o p m e n t . Ph on e: (512 (512)) 418-46 418-4628 28 E-mail:   [email protected]   E-mail: G r e g o r y K M c M illa n   retired as a Senior Fellow after a 33 year career with M o n s a n to an d S o lu ttia ia IInn c, w h er e h e s p ecial ecialized ized in im p r o v in g lo o p p er f o r  man ce, co n tr o ller tu n in g , v alv e d y n amics , o p p o r tu n ity as s es s men ts , d y n am ic s imu latio n , f er men to r co n tr o l, p H co n tr o l, an d r eacto r co n tr o ll.. G r eg is th e au th o r o f n u m er o u s ar ticl ticles es an d b o o k s , h is mo s t r ecen t b o o k b ein g :  Good Tuning - A Pocket Guide.  H e h as co n tr ib u ted to s ev er al h a n d  boo ks and is the editor of the   Process/Industrial Process/Industrial Instrumentation and Controls h an d b o o k an d th e co au th o r o f a mo n th ly co lu mn titled   Con trol Talk . xv

 

xvi

dvanced Control

Greg is an ISA ISA Fellow Fellow and received the ISA Kerm it Fischer Fischer E nviro nm en tal Aw ard for for pH con trol in 1991, the Co ntrol M aga zin e Eng ineer of of the Year A wa rd for the Process In du stry in 1994 1994,, an d w as one of the fi first rst in d u ctees into in to th e C o n tr o l M ag azin e P r o ces s A u to m atio n H all of F am e in 2001. He rece ived a B.S. B.S. fr from om Ka nsas U nive rsity in 196 19699 in E ng ine erin g Physics and a M.S. from University of Missouri - Rolla in 1976 in Control Theory. Presently, Greg is an affiliate Professor at Washington University in Saint Louis, M issouri and is a con sultan t thro ug h E DP Contra ct Services in Austin, Texas. Cell Phone: (314) 703-9981 E-mail:   g k mcmi@ms n . co m   E-mail: Willy K Wojsznis   h as b een in v o lv ed in d ev elo p in g ad v an ced co n tr o l products over the last twelve years focusing on model predictive control and auto tuning. Over the previous nearly 25-year of his career he was d ev elo p in g co mp u ter co n tr o l s y s tems an d ap p licatio n s in cemen t, s teel,

min in g an d p ap er in d u s tr ies . H is p r o f es s io n al w o r k r es u lted in a n u mb er of successful and innovative advanced control products, fourteen patents, and above thirty technical papers. He received control engineering degree (EE) from from Kiev Kiev Technical Technical Un iversity in l964 , M.S. in A ppl ied M athe m atics from rom W roclaw Univ ersity in 1972, 1972, and Ph.D . from from W arsaw Unive rsity of of Tech nology in 1973. Presently, Willy is a pa rt of DeltaV a dv an ce d control g r o u p . H e co n d u cts ap p lied r es ear ea r ch in in th e ar eas of o p timizatio n , ad a p tiv e control and model predictive control. Phone: (512) 418-7475 E-mail:   [email protected]   E-mail: Michael W Brown   has spent his entire career in the application of

A d v an ced P r o ces s C o n tr o l tech n o lo g ies in in th e co n tin u o u s p r o ces s in g indu stries. Ove r the pr evio us 15 years of his his career career,, he ha s served as an A d v an ced P r o ces s C o n tr o l C o n s u ltan t, p r o v id in g imp lemen tatio n ex p er  tis e an d tech n o lo g y g u id an ce f o r man y o p er atin g co mp an ies . H is ex ten  sive knowledge and experience in the areas of model based predictive co n tr o l an d r eal- time o p timizatio n h av e as s is ted man y co mp an ies in cap  tu r in g th e b en e n ef its its of of th th es e p er f o r man ce imp r o v e m en t tech n o lo g ies . H is work has resulted in several technical papers, published in various control journals. Michael is a Chemical Engineer, received a B.A.Sc. from the Uni versity of of W aterloo in in 1987 1987 and com plete d h is M asters in App lied Science Science in 1989. 1989. Presently, Michael is the A pplica tions B usiness Ma nager, with M a tr ik o n I n c. c . , w h er e h e co n tin u es to w o r k w ith in d u s tr y to b r in g Advanced Control Technology to the next level. Phone: (905) 282-9248 E-mail:   m i k e . b r o w n @ m a t r i k o n . c o m  E-mail: m 

 

Foreword

There has been been a dynam ic developm ent of cont control rol over the past  5 years. Many new m ethods have app eared. The m ethods have tradi traditional tionally ly been presented in highly specia specializ ed ol boottheory. ks written resear researchers chersbeen or en gineers w ith adva nced degrees in lized contr control heory. Theseforbook s have very u se ful ful to adva nce th e sate ooff the art. They are how eve r diffi difficult cult for an ave rage engineer.. The reasons are that it is necessary to read m any book s to get a engineer good coverage ooff advanc ed control techniques and that the level of m athe matics used requ ires a substantial prepa ration. This is is a dilemm a b ecause several of the advanc ed con trol techniques hav e indeed be en very benefi benefi ci cial al in industrial an d m ore engineers sho uld be aw are of them . Even iiff man y details ooff the new me thods are compli complicate catedd the bas basic ic und erlying ideas are oft often en q uite sim sim ple. Man y m etho ds have also been pack aged so that they are relat relatively ively easy to to use.  It is thus highly desirable to presen t the  use. It industriall industr iallyy p roven control methods to ordinary engineers working in industry. This book is a fir first st attem pt to d o this. The book p rovid es a b asis for assessing the benef benefit itss of advan ced control. It covers auto-tun ing, m odel predictive control, control, optimization, estimators, neu ral netwo rks, fuzzy control, simu lators, expert system s, diagnostics, and p erformance assess m ent. The book is written by four seasoned p ractitioners ractitioners of control, hav  ing joint jointly ly m ore tha n   1 years of real indus trial experience in the deve lopm ent an d u se of of advanced control. The book is well positioned to provide the bridge over the inf infamous amous Gap betw een Theory an d Prac Practi tice ce in control. Karl  J Astrom

xvii

 

Contents ACKNOWLEDGEMENT

xiii   xiii

ABOUT THE AUTHORS

xv 

FOREWORD Chapter  

xvii   xvii

INTRODUCTION

1 

Chapter 2 SETTING THE FOUNDATION Practice 5 Overview   5 Opp ortunity Assess Assessment ment   12 Examples 1 Examples  155 Application 20 General Procedure 2 Procedure  200

5 

Ap plicati plication on D etai etaill  26 Rules of Thu m b  7  744 Theory 76 Process Time Constants an d G ains ains 76  76 Process Time Delay 79 Delay  79 Ultimate Gain and Period Period 8  800 Peak and Integrated Error 82 Error  82 Feedforward Feedfor ward Control Control 84  84 Dead T ime ffro rom m Valve Dead Band Band 84  84 Nomenclature 85 References 86

vii

 

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Advanced Control Unleashed

Chapter 3  APC PATH PATHWAYS WAYS 89 89  

Practice 89 Overview 8 Overview  899 Opportunity Assessment 9 Assessment  944 Examples 103 Examples  103 Application 106 General Procedure 106 Procedure  106 Ap plication Detai Detaill  108 Rules of of Thum b  115 References 116

Chapter 4 EVALUATI EVALUATING NG SYSTEM PERFORMANCE

Practice 119 Overview   119 Overview 119   Opportunity Assessment 121 Assessment  121 Examples 125 Examples  125 Application 129

119  

General Procedure Procedure 129 Ap plicati plication on Detail Detailss 129  131 Rules of of Thum b  143 Guided Tour 144 Theory 147 Using Statistics for Control Performance Evaluation 150 Evaluation  150 Extending the Co ncept to tthe he Multi-var Multi-variable iable Env ironme nt nt   153 Addressing Advanced Control 154 Control  154 Diagnostic Tools 156 Tools  156 References 160

Chapter Chapt er 5 ABNORMAL SITUATI SITUATION ON MANAGE MANAGEMENT MENT 163 163   Practice 163

Overview   163 Overview  Opportunity Assessment 165 Assessment  165 Examples 166 Examples  166 Application 168 General Procedure 168 Procedure  168 Ap plication Detail Detailss  169 Rules of of Thum b  171 Guided Tour 173 Theory 177 Introduc tion to Expert Syst Systems ems 177  177 Rules 178 Rules  178 Inference Engine 180 Engine  180 Facts 181 Facts  181 References 182

 

Table of Con tents

Chap ter 6 AUTO MATE D TUNING

83  

Practice 183 O v e r v i e w 18 3   O p p o r t u n i t y A s s e s s m e n t  t  18 1855   E x a m p l e s  s   187 A p p licatio n 197 G en er al P r o ced u r e 197 A p p licatio n D etail  etail   200 R u les of of Th u m b   202 G u i d e d T o u r  r   206 T h e o r y 208 I n t r o d u c t i o n t o A u t o T u n e r s  s   208 Basics of Relay-Oscillation Tuning  Tuning   210 M o d el B as ed Tu n in g  g   218 R o b u s tn es s B as ed Tu n in g  g   221 A d a p t i v e C o n t r o l  l  225 References 237

Chap ter 7 FUZZ FUZZY Y LOGIC CONTRO L

23 9  

Practice 239 O v e r v i e w  w   239 O p p o r t u n i t y A s s e s sm s m e n t  t   240 E x a m p l e s  s   240 A p p licatio n 241 G e n e r a l P r o c e d u r e  e   2 41   R u les o f Th u mb  mb   242 G u i d e d T o u r  r   24 2   T h e o r y 244 I n tr o d u ctio n to F u zzy Lo g ic ic C o n tr o l  l   244 Building a Fuzzy Logic Controller  Controller   247

Fuzzy Logic PID Controller  Controller   251 F u zzy Lo g ic ic C o n tr o l N o n lin ear P I R elatio n s h ip  ip   25 4   FPID and PID Relations  Relations   257 A u to matio n o f F u zzy Lo g ic C o n tr o ller C o mmis s io n in g  g   258 References 259

Chap ter 8 PROPERTIES ESTI ESTIMATION MATION

26

Practice 261 O v e r v i e w  w   261 O p p o r t u n i t y A s s e s s m e n t 263 Ex amp le — D y n amic Lin ear Es timato r  r   265 E x a m p l e s - N e u r a l N e t w o r k s  s   269 A p p licatio n 274 G e n e r a l P r o c e d u r e  e   274 A p p licatio n D etail  etail   279

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Advanced Control Unleashed

Rules of of Thum b  289 Guided Tour 289 Tour  289 Theory 294 Dyn amic Linear Esti Estimator mator 294  294 Neural Networks 296 Networks  296 References 305 Chapter 9 MODEL PREDICTIVE CON TROL Practice 307 Overview 307 Overview  307 Opportunity Assessment 310 Assessment  310 Examples 316 Examples  316 Application 337 General Procedure 337 Procedure  337 Ap plicat plication ion D etai etaill  339 Rules of of Thu m b  353 Guided Tour 355 Tour  355

30 7  

Theory 362 The B asics asics ooff Process M odeling  364 Identifying the Process Model 367 Model  367 Unconstrained Model Predictive Control 369 Control  369 Integrating Con straints H and ling Op timization and M odel Predictive Con trol 373 References 381 Chapter 10 VIRTUA L PLANT 3 8 3   Practice 383 Overview 383 Overview  383 Opportunity Assessment 386 Assessment  386 Examples 387 Examples  387

Application 389 General Procedure 389 Procedure  389 Online Adaptation 393 Adaptation  393 App lication lication D etail etail  395 Rules of of Thum b  399 Guided Tour 400 Tour  400 Theory 403 References 408 Appendixx A ADDITIONAL OPPORTUNITY ASSESSMENT QUESTIONS Appendi QUESTIONS Appendixx B BATCH TO CONTINUOUS TRANSITION Appendi TRANSITION Appendix C DEFINITIONS DEFINITIONS

419 

415 

409 

 

Table of Contents

Appendix D TOP 20 MISTAKES INDEX

431 

425 

xi

 

  Introduction The advent of powerful and friendly integrated software has moved adva nced process control ((APC) APC) ffro rom m the realm of consu ltants into the arena of the ave rage ess controlhave engineer. The ofand infrastruc ture and special skillproc requirements started to obstacles disappearof we are poised for for an accelerated application of APC . It is well kno wn that APC seeks to di discover scover,, incorporate, an d exploit know ledge about raw materials, materials, process process,, produc t, equipment, instrumen tation, and final elements. What is not often recognized is the significant increase in tthe he kno wled ge base ooff both p lant an d fie fielld opera tions th at occurs as the APC system is developed . In ffac act, t, an appreciable po rtion of the benef benefit itss are achiev achieved ed by im provem ents m ade in operati operating ng procedu res, set points, sensors, an d con trol val valves ves as a result ooff the m ethodical analy sis,  testing,  testing, mo deling, and prototy ping that are part of the best practices use d in the iimp mp leme ntation of APC systems. Until recent recently, ly, mo st ooff this know ledge en ded up with co nsultants, and the successs ooff the application oft succes often en deteriorated once they de pa rted. There is no w a n opp ortu nity for the engineers cl closest osest to tthe he process and daily operations to take take a much m ore act active ive rol rolee in in the developm ent an d sup  po rt of APC applications. It is a win-w in situation in th at the cost ooff APC can be reduced by using consultant consultantss primarily in a higher-value-added role of conceptual design a nd o ptimization. Even mo re impo rtantly, greater great er und erstanding , supp ort, and involvement of onsite engineers can increase the success rate, the on-stream time, and the longevity of an A PC app lication. This decreas e in the cost an d inc rease in the benefit benefitss w ill in tur n lead to a lar larger ger num ber of suc succes cessf sful ul A PC installat installations ions and a greater interest in APC as a me thod of im pro vin g process effi effici cienc encyy an d capacit capacity. y.

 

 

dvanced Control Unleashed

However, much of the purpose and use ooff APC has been clouded in tthe he ory.. The theory is scatt ory scattered ered am ong m any bo oks written for g rad ua te school prog ram s in advan ced p rocess control control.. App licati lication on pa pe rs typi callyy conc entrate o n the benefi call benefits ts of sspeci pecifi ficc A PC projects and serve m ore as advertisem ents for particular consulting or sof software tware firms than as implem entation guid es. Lit Littl tlee iiff anythin g ha s been written for for the practic ing engineer on ho w to ssele elect, ct, design, confi configure, gure, commission, and tun e APC system s. The purp ose of this book is to demystif demystifyy APC an d m ake it more accessible.  To that end , the book focuses on practi practice ce and applications backed up by eno ugh theory to insure a deeper u nderstan ding. Each chapter is organized to pro vide concise practi practical cal informati information on that a user can readily explore and reference to start and complete a successful implem entation. Each chap ter has three major sections, entitled  P ractice ractice Application a n d  Theory. T h e  PRACTICE secti  section on starts w ith an Overview that prov ides a concise explanation of for technology technology an dtime its impo rtance. and IItt pro vid uin es the m otiva tion and basis fthe or investing more iinn learning p urs g the tech nology. Next is an   Opportunity ssessment subsection   subsection that offers a simple app roach to determ ine whe ther the technology is applicable to a particular un it ope ration. It consist consistss of a sset et ooff concepts and questions to start the thou gh t process and discussions to ffin indd potential applications. The  Examples subsection rou nd s out the secti section. on. The sam ple ooff applications pre sented here help to insti instill ll a better practical practical unde rstan din g of the use of the technology in the process indu stry. T h e  APPLICATION   secti section on st starts arts with a General Procedure subsection   subsection that presen ts a go-to checkl checklis istt to introduc e the user ttoo the no rm al sequence of even ts ffor or a suc successf cessful ul app lication. This llist ist pro vid es a good refer reference ence to ma ke sure all bases are cover covered ed an d is us useful eful fo forr plan ning , schedu ling, estimating, and m onitoring A PC proj projects ects.. Next there is an   pplication Detail subsect  subsection ion that sum marizes m ost ooff wh at a user need s to know. A building-block appro ach is used whe rever possible, starting with a basic bare-bo nes application and ad din g ssuccessi uccessively vely mo re capabili capabilities ties to en d up with a fful ulll c omp lement of advan ced featur features. es. Next is a series of Rules of Thumb conc  concise isely ly ph rase d to be rreadily eadily re refer ferenced enced and remem bered. A brief bri ef ex planation w ith an y notable exceptions fol follows lows each rule. The sec tion tion end s with a  Guided Tour to   to give the reader a fe feel el ffor or how adva nces in software have made implementation easy enough that the user can focus on the o ppo rtunity offe offere redd by these APC tools to discover discover,, incorporate, and exploi exploitt plant know ledge. T he  THEORY section presen ts the major facets facets of selected app roa che s to the dep loym ent of each APC technology as part of a stat state-ofe-of-thethe-art art tool set set..

 

Chapter  

Introduction

3

For brevit brevity, y, tthe he section section doe s not surv ey all the possible me thodolog ies an d technique s, bu t ffocus ocuses es on those that are iinnov nnov ative and sim ple enou gh to be integrated into a distributed control system. This book c overs a great deal ooff gro un d. Each ooff the technologies dis cussed he re could easily ffil illl a book in  itself. However, users today do n't hav e th e time or inclinati inclination on to read a lot of material. List Lists, s, hints , rules ooff thum b, and conc concis isee explanations explanations are employed to save the reader time time and to provide both a better perspect perspective ive on the who le picture picture and an improved ability abili ty to drill do w n to obtain spec specif ific ic implem entation g uidan ce. The book concentrates on wh at is mo st impo rtant. Users can quickly get ttoo the hea rt of the matter without getting lost in the details associated with a specific tool or suffering from information overload. W hile a user can go directly ttoo a given chapter to learn abou t a particular technology, technol ogy, tthe he auth ors recom men d tha t Chap ters 2 and 3 be read firs first. t. They provide the necessary necessary foundation foundation on which to build an APC appli applica ca tion an d th e logi logicc to se sele lect ct the most ap pro priate tec technology. hnology. Included with the book is a compact disc that contains a set of exam ples ooff the technologies discussed discussed in the book. They dem onstrate, by m ean s of of a step-by-st stepby-step ep procedure and a detai detailed led dynam ic proc process ess model, how to configure, confi gure, test test,, and r un each  APC application. Con figurat figuration ion an d case fi file less use a virtua l plant that ha s a complete scal scalable able Distr Distributed ibuted Con trol Syste System m (DCS) with a suite ooff APC tools and a high-fi high-fideli delity ty plan t simulation. A co mp anion set ooff Pow er Po int sli slides des that illustrates all of the major Fig ures, equation s, tabl tables, es, li lists sts and ru les included in the book is on th e CD. These slides slides and the han ds-on exer exercis cises es mak e the book p ractical as a text boo k fo forr courses on both basic and adv anced process control control.. Ch apters 2 and 6 rece receive ive the most extensi extensive ve treatment becau se introductory courses are most comm on. Also, stud ents an d u sers alike alike need to ffir irst st co ncentrate on getting the basic regulatory con trol system design ed corre correctl ctlyy a nd tuned properly bef before ore mov ing on to more advanced topic topics. s. Most ooff the material has been tested in an introdu ctory co urse on process control ffor or junior a nd senior chemica chemicall engineers at Washington Un iversity in Sai Saint nt Louis.  The  These se students have de mon strated the abil abilit ityy to immediately a pply these APC tools to example pro blem s af afte terr a br brie ieff tutorial, usin g their com puter skill skillss and a powerful integrated Wind Windows ows® ® environme nt. The sound mathematical ffoundation oundation of  AP  APC C makes it easier to learn tha n basic control, wh ich is mo re heuristic.

 

4

Advan ced Control Unleashed

The tutorials and presentations on the CD do n ot require any special soft soft ware or hard wa re beyond a PC with a media player player,, speakers, and a dis play with a screen area of at least  1 24 by  b y 76  768 8 pixels. This book w ith its appendices a nd CDs should en able tthe he average p rocess engineer to deve lop a good un der stan din g of the rrepresen epresen tative principles and techniques of  APC. This knowled ge w ill be helpful in setting obj objec ec tives,  evaluati  evaluating ng potential APC opportunities, and applying the most app ropr iate APC technologies. Readers sh ould fee eell fr free ee to contact the authors at their  e mail addresses if if they have any questions about the use of the book, exercises, exercises, dem os, slides, or APC tools desc ribed. All royalties fr from om this boo k will be give n directly to universities , conso rtia, and ed ucational program s to promote and enhance the developmen t an d use of adva nced process control. A benef benefici iciary ary of each ye ar's royalti royalties es w ill be chosen by the authors.

 

2 S et ettting the Foundation Practice verview

The advan ced co ntrol proj project ectss with the largest benefits benefits usua lly have m ad e signif sign ifica icant nt imp rovem ents in the basic regulatory regulatory control system. While adv anced process control ((APC) APC) ttechniques echniques can partially comp ensate for for such limitations limitations as missing measurem ents, exc excessi essive ve dead time, an d poo r signal-to-noise ratios, a sol solid id found ation will pro vid e the lowest total cost, greatest total benefit, benefit, an d th e longest llif ifecyc ecycle le for for the adva nce d control system. Defici Deficienc encie iess in the measu rem ent and the fi final nal element can increase the tim tim e required fo forr process testing and identificati identification on by a factor of 5 or more an d can signi signifi ficant cantly ly re duce the im prove m ent in proce ss capacity and efficiency provided by APC. The core core ooff a sol solid id foundation for adv anced process control is good m ea surements an d fi final nal el elements. ements. The measuremen t is the wind ow into the process and m ust be able to prov ide an undisto rted v iew ooff small changes in the proces s. The final final elem ent is the m ean s of of aff affect ecting ing the proce ss a nd m ust be able to m ake sm all changes to the process. This overview pro  vides a perspective of how these obj object ectiv ives es are best m et by reduc ing the reproducibility error, error, noise, and int interfe erferences rences in the measu rem ent and decreasing the stick-sl stick-slip ip and dead b an d in the control valve. Measurement

Reproducibility is the closene Reproducibility closeness ss ooff agreem ent of an ou tpu t for an inp ut appro aching fr from om either dir direction ection at the sam e operating con ditions over a period of  time.  time. Repeatabil  Repeatability ity is the closeness closeness ooff agreem ent of an o utp ut fo forr 5

 

6

Advanced Control Unleashed

successive inp uts app roachin g fr successive from om the same direction at the sam e ope rat ing con ditions. Reproducibil Reproducibility ity includes the repeatabilit repeatabilityy as it deteriorates over time plu s dri drift ft,, an d is tthe he better num ber for control. An other imp or tant co nsideration is the iinter nterfer ference ence fr from om changes in process fl fluids uids and ope rating co nditions. Unfortunate Unfortunately, ly, the speci specifi ficat cations ions given by man ufac turers for su ch m easure m ents as accur accuracy, acy, lineari linearity, ty, or rangeabili rangeability ty are extraneou s iiff not misleading because they are either no t as im porta nt as reproducibility, drift, and interference or are generated under fixed labora tory cond iti itions. ons. If the me asurem ent is noisy or not reprod ucible, the controll controlled ed variable will change w he n there is no chan ge. IIff the change in the process v ariable is less tthan han the resolution limit of of the sensor or its digit digital al representation or conversion, there is no cha nge. Consequently, it iiss impo rtant to mak e su re the me asurem ent reproduc ibil ibility ity err error, or, resolution, and noise ba nd total lesss than 1 /5 of the all les allowab owab le control control ba nd , or 1/5 of the permissible vari abilit abil ityy about the set point. Measurem ent resolution withou t any sensor limitations 0. 0.05% 05%except of spaw n he forn alarge tw elve bits digital co , so itocou is nor ma lly no t anis issue span as associat sociated ednversion w ith therm ple and resis resistance tance temp erature d etector (RTD (RTD)) inp ut cards are used in loop s that ne ed to control the tem peratu re w ithin 0. 0.5° 5° Cen tigrade of set point. It is imp ortant to make sure that measurement time time delay and time con stants do no t ex excess cessive ively ly slow do w n or attenua te the view of the actual changes in the process variable. Of les lesser ser concern is an offs offset et a nd non linearityy in a me asurem ent, since these can be com pensated for earit for by a sh shif iftt in the set poin t or a simple zero adjustment in the transm itter and signal characterization, respectively. Ch anges in process or amb ient conditions show u p as dri drift ft or slow noise. For exam ple, changes in a fluid' fluid'ss density will change the li liquid quid head and hence the level readin g from from a diff differe erenti ntial al pressu re mea surem ent; cha nges in a fluid's k inem atic viscosity will change th e m eter coeffi coeffici cient ent a nd hen ce the flow flow read ing from a vortex m eter; and ch ang es in the acti activity vity coe coeff ffii cient of the hydr oge n ion will change the glass electrode electrode poten tial an d hen ce the p H read ing . In In this text, drif driftt w ill be con sidered to be in ef effe fect ct a long-term reproducibilit reproducibilityy error. error. Final Element

The mos t comm on fin final al element is the control valve. Con trol troller ler ou tpu ts also m anipu late the speed of pum ps a nd p ow er to heaters. With ffina inall el ele e m en ts tha t are totall totallyy electr electronically onically set, there are n o issues of sstick tick and slip as there there are ffor or control valves, and any de ad b and that exist existss is is purp osely introduce d and adjust adjustable able to variable reduce the response response of the man ipulated (fl (flow ow for tthe hetopunoise. m p anAlso, d heathe t for for the heater) is linear with controll controller er ou tpu t. Variabl Variable-spe e-speed ed drives ha ve essen-

 

Chapter   - Setting Setting the Foundation Foundation

7

tially no time delay or time con stant and rate limiting is norm ally adjust tially adjust able and not an issue except fo forr su rge control. He aters are inherently slow, bu t most tem peratu re processes are also also sslow. low. Usually, a control valve wil Usually, willl not m ove o n its ow n or w he n th e controller ou tpu t is constant unless the actuator is und ersized or the positioner is unstable. A lso, if the valve w ere to drif drift, t, the po sitioner an d process con troller would correct for  it. Th us, long-term reprodu cibility cibility an d no ise are not no rma lly issues ffor or control valves. While noise is no t genera ted in th e valve stroke, noise in the process variable can be passed on as rap id changes in the valve signa l, wh ich, iiff they exceed exceed the resolution limit limit or dea d ban d of the contr control ol valve, can cause eexcess xcessive ive we ar an d tear an d pr e m ature fai failur luree of the packing. If a control valve st sticks, icks, there is no ch ange in flo flow w until the accum ulated changes exceed the resolution resolution of the the valve.  Iff the va lve slips, the chang e in  valve. I flow flow is mo re di discret scretee than continuous an d is much lar larger ger than dem and ed  [2.3]. by the corresponding inin control cont roller ler outpu  For deaddirec ban d or backlash, there is nochange there cha nge fl flow ow w hen thet  [2.2] controll controller er reverses ti tion on until the change exceeds exceeds the dead ban d as show n in Figure 2-1.

Stiction, Sticti on, a com bination of the w ord s stick an d fri fricti ction, on, ad ds time delay to a loop ffor or any change in the controll controller er outp ut; dead b an d ad ds time d elay only for reversals iinn directi direction; on; and slip causes the process vari able to pas s right by the set point. The dead time fr from om sti sticti ction on an d back lash will increase the error ffro rom m load up sets du e to the additiona l time delay but do esn't slow dow n the set point response as m uch if the changes in set poin t are rather large. Studies that introduce w hite noise or set point changes instead of load u psets w il illl report a negligible iincrease ncrease in inte grated error fro from m dead b and   [2.2]. Oth er studies that concentrate on the eff effect ect of the additional time delay fr from om dea d ban d o n load upse ts show a 50 50% %  increase iinn the pea k error an d a 100% increas  increasee in the integrated error from   10 10% % dead ba nd  [2.4]. Oscill Oscillati ations ons in loops loops with d ead b and tend to dissipate u nless there are interactions or aggressi aggressive ve tu ning . Sli Slip, p, on the other h an d, w ill caus causee a limit cycl cyclee if if there is any reset acti action, on, because the process variable will not come to rest exact exactly ly at set po int an d w hen reset wo rks to eliminate the of offs fset et the e ventua l chang e in fl flow ow is too large. The strokin g tim e ooff the control valve is no t as big an issu e except ffor or very fas fastt loops and v ery large actuators. Th us, iinn the no rm al schem e of of things, sl slip ip is w orse tha n stick, and sti stick ck is wo rse than dead b and , and dead ba nd is wo rse than str stroking oking ti time. me. Fo Forr sliding stem valves, sti stick-sl ck-slip ip and de ad ba nd go hand in ha nd since the com m on cause is excessi excessive ve pack ing frict friction. ion. In fact fact,, if if the slip is equal to the stick, it it is eff effect ective ively ly the sa m e thin g as the res olution limit. The resolu-

 

8

Advanc Advanced ed Cont Contro roll Unleashed tion of of sliding st stem em v alves can be est estimated imated as hhal alff of the dead ba nd   [2.4]. In other words, wh ere you hav e exc excess essiv ivee dead band , you tend al also so to ha ve excessive st stick-sli ick-slip. p. H ow ever, in rotary valv es, there are dif differe ferent nt sources of of st stickick-sli slipp an d dead ban d. A rotary valve could hav e a large dea d b an d b ut li litt ttle le st stick-s ick-slip. lip.

Figure  2-1. Definition of Dead Band and Stick-Slip Un til recent years, w hen you asked a cont control rol valve m anufacturer to es esti ti m ate the dynam ic response of a control valve, you w ere given the stroking time of the actuator. actuator. Even now, iiff you ask fo forr a response time that includes the va lve, it will be ffor or a cha nge of   10% in controll controller er o utp ut at 50%  5 0% posi tion so that the eff effec ectt of sti stick-sli ck-slipp an d d ead ba nd are largel largelyy rem ove d   [2.5]. In actual operation, the chang e in control controller ler ou tpu t per scan is typi typicall callyy less than  0.5% and can occur at positions less than  20 20% % w her e the fricti friction on of the sealing surfaces increases the stick-slip. Test Testss don e at the se con di tions will un earth the real response problem s. IInn valves, ssti tick ck and slip go together an d can be ide identif ntified ied wh ile the lloop oop is opera ting fo forr fas fastt m easure  m ents, aass show n in Figure  2-2. Here, sti stick ck is the am oun t of change in the controller control ler outp ut w here there is no chan ge in the process variable and slip is the rapid change in the process variable divided b y the pro du ct of the valve and process gain. The eff effec ectt of nonline arity in the control valve is oft often en m isu nd ersto od . In order to an alyze the ef efffec ectt on the control loop, yo u m us t look at the p rod  uct of the valve gain an d the process gain. Si Since nce the process gain is inversely propo rtional to fflo low w for tempe rature an d com position control [2.5],  the equ al percentage characteri characteristi stic, c, where the valve gain is pro por tional to flow, helps compensate nonlinearity. trol loops w here the to process gain isfor notprocess-gain inversely propo rtional to For fflo low wcon or w here the installed characterist characteristic ic deviates from from the ideal equal percentag e

 

Chapter 2 - Setting the Founda tion

9

Figure 2-2. Identification Identification of Stick and Slip in in a Closed-loo p Respo nse

characteristic, characterist ic, the nonlinearity of the va lve characteris characteristic tic becomes imp or tant an d there is a range of perm issible issible valve gain, as show n in Figures 23a throu gh 2-3 2-3c. c. Also, the ope rating-point no nlinearity from from the installed valve characterist characteristic ic has a greater de trimental ef efffec ectt tha n the operating poin t nonlinearity associat associated ed w ith a proces processs variable, becau se the op erat ing point of the valve changes with load pe r the dem an ds of tthe he controller wh ereas the ope rating poin t ooff the process variable is drive n back to the set poin t. Correspondingly, proces processs nonlinearity in second ary loops in a cascade casca de or advanced cont control rol sys system tem becomes m ore imp ortant because the set point is dri driven ven to new operating points to meet the d em and s of the master, supervisory, supervisory, or m odel pred ictive cont control roller ler.. The seconda ry loop shou ld be 5 times fa fast ster er tha n the prima ry loop so that the prim ary loop isn't affe affect cted ed by the non linearity inside the seconda ry loop. Signal characterization of the controllerit ou tpu t is relatively fi fixx fo forr un w an ted v alve nonlinearit nonlinearity y because is easy to aimp lem ent quick in Fieldbus functional funct ional blocks. W hat is not so easy to do is to calcul calculate ate the actual shap e of the instal installed led characteristic, characteristic, unless the chan ges in th e inlet and outlet pressu re to the control valve are m easured . Also, the opera tor m us t be m ad e awa re ooff the fa fact ct that the control controller ler outp ut no w represe nts percen t desired flow flow instead of percent valve position. position. For m aintenanc e and trou bleshooting, the characterizer characterizer outp ut tha t represents the cal calculated culated valve position target should b e displayed along with any feedback of actual valve position. In addition , signal characteri characterization zation w ill ill incre increase ase a nd decrease the ef effec ectt of dea d ba nd or suction for for o perating p oints on the steep an d flat portio ns of the installed characteristic, respecti respectively. vely. [2. [2.5] 5] Th us, a greater improv em ent in loop performance is reali realized zed o n the fla flatt tail of the installed characteristic curve associated with rotary valves in Figures 2-3a 2-3a and 2-3b. How ever, the the user mu st remem ber that these curves can becom e so fl flat at above 60 degrees o penin g that there is essent essentiall iallyy no

 

10

Advan ced Control Unleashed

chang e in fflow low ffor or a change in rot rotation ation and the valve gain approa ches zero. To sum m arize, tthe he num bers that tradi traditi tionally onally have been cited by the m an ufacturer fo forr valve perform ance, su ch as lleakag eakag e, strokin g time, li linearity, nearity, and rangeabi rangeabili lity, ty, do not prov ide the information information ne eded to m easu re con trol loop performance. T he user nee ds to know the stickstick-sli slip, p, de ad ban d, and sensit sensitivit ivityy of tthe he installed installed valve assem bly at operating conditions.

Figure 2-3a. Installed Characteristic of a Butterfly Valve

Figure 2-3b. Installed Characteristic of a Ball Valve

 

Chapter   - Setting Setting the Foundation Foundation

•

11

Figure 2-3c. Installed Installed C haracteristic haracteristic of a Sliding Stem Valve Effect on APC

Ad vance d con trol tools such as feedforward feedforward control, online estima tors, and m odel pred ictive control controllers lers can minim ize to a ssigni ignifi ficant cant de gree th e effect of measurement deficiencies. Feedforward control can bypass the irregularities irregulari ties and delay in the contr controlled olled variable but stil stilll m ust w ork throu gh th e ma nipu lated variable. Since Since the exac exactt si size ze ooff st stick ick and de ad ba nd is extremely variable, underco rrection is norm al and the overall imp rovem ent is minimal. Filters Filters can reduce the eff effec ectt of noise; of noise;  and mod el predictive con trol can reduce the adv erse ef efffec ectt of noise, resolution, an d reproducibility by minim izing the error between a process vector created from from a mo del of tthe he process and the set poin t vector  [2.6]. Howe ver, its mo basedpas ont the assum thatroller theercontr control val actuall y mo con ved for for del the is recent changes inption the controll cont outpol ut.valve Thve us,actua advally nced trol algorithms are more v ulnerable to def defic icien iencie ciess in the control valve than m easurem ent. While a kicker algorithm can theoret theoretical ically ly reduc e the effect of dead band and stiction, overcorrection will cause excessive move m ent similar similar to slip  [2.4]. There is no com putational correct correction ion for valve slip.  The effe effect ct of slip is am plified plified by hig h valv e sensitivity (valve gain) and high process sensiti sensitivity vity (process gain). The only solution fo forr slip, an d the best solution for for sticti stiction on and dea d ba nd , is a change in the valve typ e, assembly, asse mbly, and access accessori ories es or the use of a variable-speed drive . To sum m arize, while it is desirable for for the m easurem ent to be linear an d accurate and the control valve's eefffec ectt on th e process to be linear an d pre cise,  it is cise, it is most im po rtant that the signalsignal-to-noi to-noise se ratio, reproducibilit reproducibility, y, time delay, delay, and time lag of the m easurem ent, and the st stickick-sli slipp an d dea d ba nd of the control valve, be scrutinized in case they prev ent the adva nced

 

12

Advanced Control Unleashed

control system system fr from om doing its  job. This scrutiny scrutiny involves a n analysis of the type, location, location, and instal installati lation on of the instrum ent and final final element. Both the degree to which defici deficienci encies es in the measu rem ent can be co mp ensated fo forr by adv anced control techniques, and the permissible am oun t of ssti tickckslipp and dea d ba nd , mu st be part of a cos sli costt-benef benefit it analysis. This chapter of offe fers rs enou gh theo ry to prov ide a deeper un der stan din g ooff the origin origin ooff the relationships and ru les prese nted, including the funda m entals necess necessary ary ffor or gradu ate studies and to develo p new tools ffor or adv anced control control.. The Theory sect section ion covers the setup a nd direct solution of the the diff differe erenti ntial al equa tions fo forr m aterial and en ergy balances to show the fundamental relationships between process parameters and process time constants an d process gains. IItt aalso lso covers covers Frequency Response in en oug h detail to show the source of the equations ffor or the ultimate period and gain and the basis ooff the the fir first-orderst-order-plus-dead-time-approximation plus-dead-time-approximation m etho d for for loop analysis. The theory behind the various ad vanc ed control tools is dis cussed in succeeding chapters.

Opportunity Assessment In this section, som e question s are of offe fere redd tha t could form a n OA to find find imp rovem ents in a basic control ssystem. ystem. Que stion   1) deals w ith the abilit abilityy to overdrive the man ipulated variable on startup or fo forr a major major set point chang e in a batch op eration to reduce th e am oun t of ttime ime it ttakes akes fo forr th e controlled controll ed variable to reach set point. This question is also imp ortan t in performance of an advance d control system to help redu ce the time lag in the m anip ulated variable fo forr th e m odel p redictive cont control roller ler.. There is, ooff course, a trad tradeof eofff betw een rise time and degree of perm issibl issiblee oversho ot, bu t in general, for temp erature a nd co mp ositi osition on control of volu m es w ith mixing an d for the start of a continuous o r batch process, the ou tpu t shou ld led initi initially ally be saapproaches turated high ut backed off from this li limit mit before ore the controlled control variable setb point. Various algorithms a ndbef tunin g m etho ds are avail available able to prov ide overdrive. The ffrac racti tion on that the startu p time or a batch cycle cycle can be redu ced is propo rtional to the ratio ooff the m issi issing ng area ooff overdrive to the ttotal otal ar area ea ooff the man ipulate d v ariable d ur ing the rise time. Qu estions (2 (2)) throu throu gh (4) determ ine w hether a control loop is creat creating ing variability variabil ity instead of transf transferri erring ng the proper am oun t of variabili variability ty from from controlled control led variables to m anip ulated variables. Qu estions (5 (5)) and (7) involve looking at an application and instal installati lation on a nd spotting som e obvi ou s fl flaws. aws. The next tthree hree questions look fo forr op portu nities to app ly cas cade an d ratio control ((al also so kno w n as fflo low w feedforward). These qu estions generally apply to both batch and continuous u nit operati generally operations. ons. Q uestions (11) 11) thro ugh (14 14)) look ffor or op portun ities to red uce batch cycl cyclee time. A ppe n d ix  B expan ds u po n these questions to take a clos closer er look look at the m etho ds to

 

Chapter Chapt er 2 - Setting Setting the Foundation Foundation

13

reduce the time to reach reach a batch set poin t and elim inate operator attention requests. 1.

Co uld the valve position be init initiall iallyy drive n bey ond the resting position w he n the controll controller er settl settles es out at set poin t, to reduce continu ous process startu p tim e or batch process cycle cycle ti time? me?

2 . 

Is the variabil variability ity less in the controll controlled ed variable of the loop w he n the controller control ler is in m anua l?

3 . 

Is the variabil variability ity less less in other loops w hen the control controller ler is in manual?

4 . 

Is the variabil variability ity less less in the process variable for an im portan t constraint if the controller gain or rate setting is decreased or integral time is increased?

5.  

Would better reproducibilit reproducibilityy and less noise in m easu rem ent reduce the variability variability in a process variable for for an im porta nt constraint?

6.

H av e tight shut shutoff off   valves, high valves, high tem perature packing, key lo lock ck shafts, shaf ts, vane actua tors, sscotch cotch yoke actua tors, or valves w ithou t digital positioners positioners been use d in cont control rol loops that af afffec ectt im porta nt constraints?

7. 7.  

Hav e any of the top 20 mistakes been m ade in an impo rtant loop? (See (See A ppen dix D fo forr a lis listt ooff the m istakes ma de ev ery year for the last forty years.)

8.

Are there opp ortunities to linear linearize ize the m anip ulated variable for a prim ary co ntrol ntroller ler by creating a secondary loop tha t encl encloses oses the nonlinearity?

9.

Are there opportunities to attenuate a load upset to a prim ary loop

10. 10 . 

by creating a secondary loop that enclos encloses es the disturban ce? Are there fl flows ows that can be ratioed and use d as a feedforward signal to enfo enforce rce a m aterial balance fo forr sta rtup a nd to co m pensa te for for ch ang es in fflow low rate?

11.  

For batch opera tions, can phase s be eliminated by going ffro rom m sequential to parallel actions, actions, such as sim ultaneo us heating , fil filli ling, ng, pressurization, and venting?

12 12..  

Can batch cycle cycle time be reduce d by a decrease in w ait times, hold perio ds, operator attention requests, m anu al actions, or lab sam ple analysis time?

13 13..  

Can batch end points be autom ated by the use of a property estimator, traje trajector ctory, y, or su staine d ra te ooff chan ge?

 

14

Advanced Control Unleashed

14 14.. 

Can batch cycl cyclee time be reduce d by overd rive or an allall-out out ru n and coast?

If the variabili variability ty in a loop decreases w hen the loop is in man ual, it indi cates cat es that the loop was do ing mo re harm than good, due to poor contr control ol valve performance, inappropriate tuning, and/or interaction. If the valve do es no t resp on d to sm all steps (e.g. (e.g.,, 0.25% 0.25% to 00..5%) in the controller o ut pu t, the oscill oscillati ations ons are proba bly du e to the co ntrol valve. IIff an increase in the controller controller g ain or a decrease in the integral tim e increase increasess the variabil it ity, y, it iiss mo stly d ue to incorrect tun ing . Lastly, Lastly, if the v ariability in othe r loops is less less wh en a loop is pu t in m anu al, the variabili variability ty is the result ooff interaction. If the variabili variability ty in a loop increase increasess wh en the loop is in m anu al, there are load upsets that were being attenuated attenuated b y the loop and it was doing some good . If If the variabil variability ity stays the sam e, the the fluctuat fluctuations ions are mostly d ue to noise or lack of of me asurem ent reproducibi reproducibilit lity. y. There are also also ssom om e obvious fl flaws aws th at will stand out fr from om som e sim ple tests. If there are ssigni ignifi ficant cant non-un iform fluctuations fluctuations in the m easurem ent regard less of the m od e ooff the controller, controller, the n the select selection ion and installation of the the transm itter are suspect. These problem s are m ost oft often en associat associated ed w ith ins insuff uffic icie ient nt run s ooff straight straight pipe up stream or sensing lline ine problem s. If the loop deve lops a saw tooth oscill oscillati ation on in the control controller ler out pu t w he n it is mo ved out of m anu al, it is m ost li likely kely due to ex excess cessive ive pack ing or seat ing fri fricti ction, on, backlash iinn the linkage, sha shaft ft win du p, an d /o r p oor or m issing positioners. The selecti selection on an d instal installati lation on co nsiderations th at lead to these problem s will be discussed in the App licat lication ion Detail sect section ion of tthis his chap  ter. On e ooff the la last st and m ost obvious o ppo rtunities is the use of cascade con trol and ratio control. The mo st com mo n type of cascade contr control ol is a flow flow loop that d eals with the non linearity linearity of the control character characterist istic ic and com pensates for for pressure upse ts so tthat hat the prim ary control lloop oop can m anipu  late flow flow instead of valve po sition an d no t see the eeff ffec ectt of press ure sw ings. The next mo st comm on cascade control system uses a jjacket acket,, or coill inlet or outp ut tem peratu re secondary loop , to insulate a prim ary coi crystalli crysta llizer zer or reactor tem perature control loop loop from changes in coolant temperature and the nonlinearity associated with the manipulation of coolant m ake up fl flow. ow. The largest largest and m ost ffrequent requent op portu nities in basic control are sum m a rized in Tabl Tablee  2  2-1 -1 an d discussed in detail throu gho ut the rest of Chap ter 2. Simple fororthe relationship between stand ardequations deviationfor thefundam peak orental integrated error fo for r up setseither can bethe use d for for each type ooff improv em ent.

 

Chapter 2 - Setting the Foundation

15

Table  2 - 1 .  Largest and Most Frequent Opportunities in in Basic Control •

Decrease stick-slip stick-slip and improve improve the sensiti sensitivity vity of the final element Standard deviation is the the produ ct of stick-slip, stick-slip, valve sensitivity sensitivity valve gain), and pro cess gain): Use properly tuned smart positioners, short shafts with tight connections, and low-friction packing and seating surfaces to decrease valve slip-stick and dea d low-friction band. If Graphoil™ packing must be used, aggressively tune/monitor the smart  posi tioner. Improve valve type/sizing or add signa l characterization characterization to increase valve  sen sitivity. Use variable speed drives where appropriate for the best precision and sensi tivity.

• 

Improve the short- and long-term long-term reproducibili reproducibility ty and reduce the interference interference and noise in the measurement Standard deviation deviation is proportional proportional to repro ducibility ducibili ty and no ise): Use vortex, magnetic, and coriolis mass flow meters to eliminate sensing lines. • Use smart transmitters to reduce process and amb ient temperature/press ure effects. Use RTDs and narrow-span temperature transmitters to decrease noise and drift.

• 

Reduce loop dead time Integrated Integrated error is proportional to the dead time squared): • Decrease valve dead time stick and dead band). Decrease transport plug flow volume) and mixing delay turnover time ). Decrease measurem ent time constants sensor  lag, dam pening, and filter filter time). • Decrease discrete device delays scan or update time).

• 

Tune the controllers controllers Integrated Integrated error is inversely inversely proportional to the contro l ler gain and d irectly irectly prop ortional to the controller integral time). time).

•

Add cascade cascade control Standard deviation deviation is is proportional to the ratio ratio of the period of the secondary loop to the process time time con stant of the primary loop).

•

Add feedforward control Standard deviation deviation is proportional to the root mean square RMS ) of the measurement, feedforward feedforward gain , and timing timing errors).

Exampl es Neutralization Neutraliz ation Process Figure 2-4a 2-4a show s a two-stage neutralization p rocess. The economic vari able is is yi yield. eld. The optim um yield is fo forr pH betw een  6 a  ann d 8. A   A byprodu ct is formed for med that iiss 11% % of the total total produ ct w hen the pH goes above  8.  Of greater concern is the fact fact that the reaction time increases from from 2 m inutes by a factor factor of ten ffor or each p H un it below 6 pH . The fi first rst stage is a static mixer w ith a residence ti tim m e of of  2 seconds an d th e second stage is a well mixed vessel w ith a residence time of 20 m inutes. The ttitr itration ation curve ha s a

 

 6

Advan ced Control Unleashed

  ratio and particularly steep sl particularly slope ope between 6 and 8 pH  (1  Δ  Δ p H p e 0.0001 r w ill greatly amplify a valve stick-sli stick-slipp limit cycle, as sho w n in Figu re 22-4b. 4b.

The fol followi lowing ng improvem ents can be ma de to reduce pH vari variabil abilit ityy and improve the yield. 1.

Ch ange the reagent valves fr from om ball valves w ithou t positioners to sliding stem control valves with d igital positioners; otherwise the stick-sl sti ck-slip ip m ultiplied by the valve gain w ill be a limit limit cycle that causes a dram atic reduction in yield. M ake the secon d-stage reagent valve  10  times smaller than the first stage.

2.  

Close couple the second-stage reagent valve to the recir recirculati culation on li line ne just before before reen tering the vessel to eli elimina mina te the hu ge reagent delivery-time delivery-t ime delay fr from om a small reagent fflo low w dribbling d ow n a dip tube backfilled with process fluid.

3 . 

A dd a secondary reagent flow control loop for the second stage to create pH-to-fl flow ow cascade control system pensates for for ups ets ainpH-toreagent pressu re and lineari linearizes zes thethat mancom ipulate d variable fo forr the p H loop. Do not add a cascade control system to the firs firstt stage because the secondary loop is not 5 ti tim m es fa fast ster er th an the prim ary loop an d the equa l percentage valve characteri characteristi sticc has a valve gain that is prop ortional to ffllow that helps com pensate for for the process gain th at is inversely inversely prop ortional to ffllow. The large well mixed volum e of the second stage has a time co nstant inversely proportional to flow that cancels out the effect of the process gain, which is al also so inversel inverselyy pro portion al to fl flow ow (see (see Equ ation 2-24) 2-24);; so the linearizat linearization ion by the ad dition of  a  flow  f low loo p is desirable for for the second stage .

4.  

A dd inp ut signal characterizat characterization ion to translate the controll controlled ed variable fro from m p H to   reagent dem and to llinearize inearize tthe he fi firs rstt stage sincee it will operate over a wid e portion of the pH titr sinc titration ation cu rve. Signal characterization is no t necessary for the second stage since it should be operating well within the contr control ol band of 7 to 9 pH . Provide a faceplate faceplate for for the operator th at displays the actual p H rather tha n the li linearized nearized control controlled led variable of reagent de m and for the first stage.

5.  

A dd a feedf feedforward orward ssignal ignal to the second stage p H control controller ler ou tpu t to han dle change s iinn fee feedd flow and startu p. Use a feedfor feedforward ward sum m er instead of a feedf feedforward orward m ultiplier to the second -stage p H control controller ler so that a nonlinearity, nonlinearity, which is prop ortional to fl flow, ow, is no t introduc ed by m ultiplying the contr controlle ollerr ou tpu t by fe feed ed flow. Set u p the sum m er so that a cont controll roller er outp ut of  50 is no correction and feedback corrections of  -50 and +50 are

 

Chapter 2 - Setting the Foundation

17

prov ided by th e contr controll oller. er. Displ Display ay th e actual and desired ratio of reagent flow to feed feed fl flow ow for the second stage and pro vide a bum pless trans transfer fer to feedf feedforward orward control. 6.

A dd a head start to the ou tpu t of the first first-st -stage age p H control controller ler to help sp eed based u p theonstartu p of the system. The hea startflow is calculated estimated ratio of reagent to dfeed captured fr from om the last run . The cal calculati culation on m ust includ e the eff effec ectt of the nonlinear flow characteristic of the reagent valve. Since the reagent flow flow is small, the pressure dro p is rrelat elatively ively constant and the inherent charact characteri eristi sticc can be used as the instal installed led characterist character istic ic for for th e control valve. The head start is use d to initial init ialize ize the control controller ler outp ut at startup . The pH control controller ler ultimate period should be less than   1 m inute for the iinline nline system.

7.

Move the reagent fl flow ow m eters ups tream of the control valve for for both stages, both  too greatly improv e the fl flow ow p rofi rofile le for the meter a nd to  stages, t reduce reagent delivery delay. If coriolis flow meters are not used, ma ke sure tthere here are enoug h strai straight ght runs upstream and dow nstream of the fflo low w meter. A dd an isolat isolation ion valve cl close ose coupled to the injection point that will automatically close just beforee a nd reope n just aaft befor fter er the reagent con trol valve clos closes es an d  respectively, vely, to irti irtinimize nimize the backfi backfill ll of pro cess fluid into opens, respecti opens, the reagent piping.

8.

M ove the p H electrodes for the first first stage  20 pipe diameters dow nstream of the stat static ic mixer outlet ttoo provide eno ugh distance fo forr the streams div ided b y the mixer to recombine.

9.

Mo ve the p H electr electrodes odes fo forr the second stage ups tream of the hea t exchanger in the recircul recirculati ation on line to within  20 pip e diam eters ooff the discharge discharge p um p to minimize the tr transportation ansportation del delay. ay.

10. 10 .  

Use a midd le-signal sel select ector or fo forr each stage to choose the m iddle reading from from the thre threee pH measu rements, thus ignoring a singl singlee signal fail failure ure ooff any typ e and reduc ing noise and spikes.

The benefits benefits fr from om the redu ction in variability aff afforded orded by the listed improvem ents will be esti estimated mated in Chapter   3 . The imp rovem ents 1-9 are illustrated in Figure 22-4c. 4c. Distillation Process

Figure 2-5a 2-5a show s a disti distillat llation ion colum n, fe feed ed tan k, and a storage tan k for for the distillate pro du ct. The series ooff plots in Figure 2-5b are indicative of the non linear relat relationship ionship between tray tem peratu re and bo th the dist distill illate-t ate-toofeedd ratio (F d /F f ) and the impurity in the product. The proces fee processs gain seen by th e temp erature loop is the slope ooff the plot versu s F d / F f . The inverse of the slope ooff the plot ooff temp erature v ersus im purity concentration

 

18

Advan ced Control Unleashed

Figure 2-4a. Basic Neutralizer Control System   Before  improvements

Figure 2-4b. Nonlinearity and Sensitivity of pH

expresses the the sensitivit sensitivityy of tthe he imp urity concen tration tration to m easure m ent error. Thermocouple cards with a 400°C 400°C span are used fo forr the tem perature mea surem ents. The distill distillate ate contr control ol valve has Graphoil™ packing an d a pne um atic positioner. positioner. The st storage orage tank residence time is 4 hou rs and the

 

Chapter 2 - Setting the Founda tion

Figure 2-4c. Basic Neutralizer Control System  

fter 

19

Improvements

time delay in the temperature loop is 1 h our. Th e ref refluxlux-toto-feed feed ratio is 10 10.. If concentrati concentration on of impu rities in tthe he prod uct in th e storage tank exceeds the spec by m ore than  0.1%, the produ ct m ust be recycl recycled. ed. For every 0.1% reduction in impu rity the steam flow to the rreboil eboiler er mu st be iincreased ncreased by 0.1%.

 

1.

A dd a digital positioner to the disti distillat llatee control valve and tun e it aggressively aggressi vely to deal w ith the hig h fri frict ction ion fr from om the G raphoil raphoil™ ™ packing and reduce sti stick-sl ck-slip. ip. It iiss impo rtant to realize that settings obtained from the instruction manual or from running a preset autom ated step test are general generally ly too sluggish.

2 . 

Ch ange the pairing of the loops at the top of the colum n so that the tem peratu re contr controll oller er man ipulates the disti distillat llatee fflow low and the distillate receiver receiver level controll controller er m anip ula tes th e ref reflux lux flo flow, w, since the d istillate fl flow ow is ttoo oo sm all ffor or level control.

3 . 

Use tray 6 instead of tray 10 f  for or temp erature control because tray 6 has a larger larger and more symm etr etrical ical ttem em perature response, per Figure 2-5b, fo forr a ch ange in d istillate flow. Also, the slo pe ve rsu s imp urity concentration is higher, wh ich me ans the reproducibility error and resolution of of the the measu rem ent sho ws u p as a ssm m aller concentration error as sh ow n in Figure 2-5b 2-5b.. This is general generally ly m ore imp ortant th an the incr increase ease iinn the ti time me delay associate associatedd w ith the lower tray.

20

Advanced Control Unleashed Unleashed

4 . 

 off the sensor dow n into the tray so it always is Move the locat location ion o immersed  i  in n the  the liquid rather than in the vapor, or even worse, a splashing liquid.

5.  

Replac Replacee the therm ocoup le and i  its ts DCS input with a 3  3-- or 4-w ire ire

6.

RTD with  a smart temperature transmitter. Tune the ove rhead distillate receiver level controller w ith  a high controller gain to insure that the eff effec ectts   of s  sma ma ll changes in  and nd thus distillate flow translate into changes  in reflux flow a changes  in the col column umn temperature.

7.

Tune the feed tan k level controller w ith  a low controller gain to smooth out the change changess  in feed  to the colum n. Consider the use ooff error squa red control control.. For a batch-t  batch-to-continuous o-continuous transition in an und ersized fe feed ed tank, use  an ada pted veloci velocity ty llimited imited feedf feedforward orward  forr optimu m smoothing. per Appendix  B fo

8.  

Add signal characterization to the controlled variable to  forr the nonlinearity in the process variable depicted in compensate  fo Figure 2-5b. Prov ide  a faceplate  fo forr t  the he operator that displays the

actual tray tray temp erature rath er than the linearized controlled  off disti variable o distillat llatee dem and . 9.  

Add a secondary flow controll controller er to each colum n loop to  forr t compensate  fo  the he non linearit linearityy associated w ith the contr control ol valve and to prepare the column loop  f  for or  feedforward control.

10.  

Add a flow feedforward signal to the temperature and level controller control ler outp uts and display the actual and desire d ratio ooff distillate  to fee feed. d. M ake sure the feedfor feedforward ward acti action on is act active ive w he n

the temp erature contr controll oller er is in man ual an d the op erator can easil easilyy  fo or the startup of the g o  to fl  flow ow ratio control and adjust the ratio f column. The benefits from the reduction  in variability aff afforded orded  by the listed improvem ents w il illl be esti estimated mated  in Chapter  3 . The improvem ents are ill illus us trated   in Figure 2-5c.

pplication

General Procedure 1.

Tra Track ck do w n  and cor  correct rect the sou rce o  off sustained oscillations. A  to o find pow er spectrum analyzer may be requi required red  t find the loops w ith the common period  of oscill  oscillation. ation. B eware o  off a slow scan time o off and and controller  a th thaam t will cause  slower than actual period   a/O  orr a ned I  smaller  smaller than actual plitud e fr from om aliasi aliasing. ng. For trend s o data obtained from data historians, make sure the data highway

 

Chapter Chapt er 2 - Setting Setting the Foundation Foundation

21

Figure 2-5a. Basic Column Control System   Before Improvements

Figuree 2-5b. No nlinearity Figur nlinearity and Sensitivity of Tray Tem peratures peratures

reporting an d th e ti time me intervals betw een d ata po ints ffor or histori historical cal data are not too slow slow a nd th e tri trigger gger fo forr exception reporting and com pression is not set too high. Also, the d ata m ust be saved for for a t least a m on th to catch di diff ffere erent nt process cond itions itions an d m ode s ooff operation.

 

22

Advanc ed Control Unleashed

Figure 2-5c. Basic Column Control System  

fter 

Improvements

The correction correction most ofte oftenn involves imp roving the response s of of control valves an d the tun ing of controll controllers. ers. Stick-s Stick-sli lip, p, dea d ba nd , an d time delay fr from om the final final element can be eli eliminated minated by the u se of  a vari  variable-speed able-speed drive . Wave for form m recognition, recognition, step tests, an d an understanding of the relative effects of valve response, controller tunin g, noise, resonance, inter interactions, actions, an d n onlineariti onlinearities es can pin poin t the root cause. a. The most li likely kely source ooff a sustained oscillation oscillation is is a control valve w ith stick-sl stick-slip ip fr from om excessive frict friction ion in eithe r th e pack ing or sealing surf surfaces. aces. This occurs mo st often often in valves with high-tem perature or environmental pack ing, particularl particularlyy if it has bee n tigh tened . The next-greatest so urce of stick-sli stick-slipp is high-performance valves designed to provide tight  shutoff.  It can also occur in almost any control valve so severely oversized tha t it rides the seat where the valve alternately alternately closes clos es and then brea ks fre free. e. In some cases, an aggressively aggressively tune d digital positioner m ay be suf suffi fici cient ent.. It is im porta nt to realize reali ze that these oscil oscillat lations ions are alwa ys pres ent be cause all valve s hav e som e stick-s stick-slip lip (a ffini inite te res olution). It is is just a m atter of deg ree. IIff the stick-sl stick-slip ip is sm all enou gh , it is is smo othed o ut by si signal gnal fi filt lteri ering ng and data com pression or

 

Chapter 2 - Setting the Founda tion

23

w ashe d ou t by backback-mixed mixed v olum es. It is not surp rising that hid de n oscill oscillati ations ons have been found in most of the loops ooff a plant  [2.2]. b .  The second m ost li likely kely source ooff osci oscillati llations ons is a llevel evel

controller control ler that ha s a control valve withaseither stick-sli slipp o r d ead ba nd . For iintegrating ntegrating processes such leve level, l, sticksustained oscillat oscil lations ions occu occurr fo forr valves w ith excess excessive ive dead ba nd beside s stick-slip, w hich is the source of limit cycl cycles es in processes w ith a self-regul self -regulati ating ng response. Dead ba nd , als alsoo kno w n as backlash, can result fr from om linkages and shaft connections that are not tigh t an d is thus mo re llikel ikelyy to occur in rotary valves because they involve either a translation translation of ver vertical tical actuator mo tion to rotary motion or the connection of a rotary actuator shaft to the shaft of the ball or disc. or  disc. The  The Application Application Detail secti section on explains h ow dead ba nd incre increases ases the susceptibi susceptibilit lityy of the level lloop oop to a reset cyc cycle le an d h ow the integral time m ust be incre increased ased   [2.3]. The best fix, fix, outlin ed in the A pplication Detail section, can be relatively expensive in that it requires a new control valve relatively designed to minimize backlash and fric fricti tion on or a variable-speed drive. An alternative that can mitigate but no t eli elim m inate the limit cycle cycle is a level level-t -to-fl o-flow ow cascade lo op . c. Co ntrollers can create perio dic up sets ffrom rom noise if the reaction to the noise causes the cont controll roller er ou tpu t to excee exceedd th e d ead ban d of the control valve fr from om the gain or rate setti setting ng being too large. This most oft often en h app ens in leve levell control controllers, lers, where controller gains can be quite large. d. Controllers can ampli amplify fy periodic upsets w hos e period is clos closee to the natura l period of the control lloop. oop. Resonance occurs fr from om the feedba feedback ck acti action on of the contr controller oller being in ph ase w ith the dis turb anc e oscillat oscillation. ion. IItt m ost oft often en o ccurs for control loops in series series that have similar loop time delays su ch as liquid pressu re and flow, and inli inline ne equip m ent in series ((heat heat exchang ers, stat static ic mixers, and desup erheaters). e. Interacting controllers can cause susta ined oscillati oscillations. ons. H ere, the output of a controller affects another controller and vice versa. A s teady state rel relative ative gain analysis (RGA) can reveal the nature of the int interacti eraction. on. How ever, tthe he dynam ics mu st be con sidered as well si since nce the interaction is particu larly s evere iiff the pe riod s of oscil oscillati lation on of the loop s are similar similar.. The best solution is a change in pairing of the control loops loops p er the RGA . If this is no t feasi feasible, ble, mo del p redictive control (MPC) should be used.

 

24

Advanced Control Unleashed

How ever, bew are of ssttiff or singular processes that ma y no t even be han dled b y MPC . A singular matrix occurs w he n process gains fo forr one control variable are a fact factor or of the proce ss gains fo forr ano ther controlled variable, m aking it iimp mp ossibl ossiblee to achi achieve eve inde pen den t set poin ts ffor or these controlled variables (see Chapter 9 for more details). These processes are becom ing mo re comm on as plan ts push the capaci capacity ty llimits imits of thei theirr pla nts w ithou t mak ing the necessary modifi modificati cations ons to the process eq uipm ent. a. A m easurem ent resolut resolution ion that appro aches the size of the control ban d can cause su stained oscil oscillat lations. ions. With the disappe arance of mechanical sensors, tthis his primarily occu rs for tempe rature cont control rol loops loops that use tthermocou hermocou ple inp ut cards instead of narrow-range smart transmitters. b .   Altho ugh less com mo n, sustained oscillat oscillations ions can also occur fr from om controllers controllers tuned so aggre aggressivel ssivelyy tha t they bang back and forth for th betwe en ou tpu t li limits, mits, and fr from om nonlinear loop s that have a very high central central gain gain region region surrou nde d by exceptionally low gain reg ions. This can occur ffor or co ntrol valves when an insufficient fraction of the system drop has been allocate allocatedd as a valve valve dro p, strong strong acid an d base titr titration ation curves, and the temperature response ooff some mon om er an d wa ter distil distillati lations. ons. For process nonlinearities, the the add ition of signal characterization of the controlled variable and rate action acti on can eliminate or m itigate the limit cycle. cycle. For valve dro p problems, the the si size ze of the piping a n d /o r the pu m p impell impeller er m ay need to be increased.

2 . 

Tra Track ck do w n the source of long settling times . H ere, the oscillat oscillations ions eventually d ie out but take too long or ca cause use too mu ch vari variabil abilit ity. y. The most com m on cause iiss inapp ropria te control controller ler tuning , such as the us e of too much reset aacti ction on (too (too small an integral time) in evaporator, reactor reactor,, or colum n tem peratu re or concentration controllers, controll ers, or other loops dom inated by a large ti tim m e constant; too m uch gain or rate acti action on in llevel evel cont controll rollers ers on surge tan ks; and too m uch gain or rate action in liquid pre ssure, fl flow, ow, inline concentration (blending), or sheet gauge or mo isture controller controllerss or in loops loops dom inated by a lar large ge time time delay (dead time do m inant).

3 . 

Check the sensor selecti selection, on, installation, an d location fo forr opp ortunities, pe r best p racti ractices, ces, to improv e the rel reliabi iabili lity, ty, reproducibility, rangeability, and resolution, and to reduce noise and decrease lloop oop time delay. delay. Ori Orifi fice ce meters an d chrom atograp hs are some of the the least rel reliabl iablee mea surem ents a nd are the biggest sources of of excessi excessively vely ffast ast and slow noise. Chrom atogra phs are alsoo the largest als largest source ooff mea surem ent tim e delay ffro rom m sam ple transpo rtation and analysis cyc cycle le time. Lo Look ok ffor or wa ys to elim inate

 

Chapter 2 - Setting the Foundation

25

sensing li lines nes and sam ple lines lines by the use of of sensors that m ou nt directly direct ly in the pip eline or on the vessel [2.7]. 4 . 

Look fo forr w ays to reduce the time delay in control loops by changes in the design of the equ ipm ent, piping , instrum entation, final final elemen ts, an d the pairing ooff cont controll rolled ed variables w ith m anip ulated variables.

 

5.  

Tune the controll controllers ers for the best com prom ise betw een robu stness (stabil (st abilit ity), y), performance, and smo othness. It iiss imp ortan t to realize that the tun ing rules change w ith the ratio of time delay to time constant an d tha t all all loops will see bo th load up sets an d set po int changes. Me thods that focus focus on set po int chan ges (servo control control)) and noise iintroduced ntroduced into the the measurem ent are appli applicable cable to aerospace, w eb, and p arts man ufacturing bu t not to processes fo forr the chemical, petroleum, food, and drug industries and environmental control. Make sure the tuning method takes into account the relati relative ve degree of of dead tim e and pro vides the p rope r capability for load rejection. rejection. Beware ooff any c ontrol loop analy sis that concentrates solel solelyy on set poin t response, and th e in troduc tion of noise or ups ets dow nstream of the process and direct directly ly into the measurement   [2.8]. The  These se method s were developed fr from om contr control ol prog ram s in system scie science nce or ele electr ctrica icall engineering a nd tend to ignore the ef effec ectt of the process an d eq uipm ent dyna m ics, characteristics, and object objectives. ives.

6.

Find opp ortun ities to em ploy cascade control. W herever there is a reliab rel iable le fl flow ow measurem ent an d a prim ary loop w hose time delay and time constant are mo re than fi five ve times sl slow ow er than a flo flow w loop, a secondary flow flow control controller ler shou ld be created. There are some cases wh ere the dynam ics are not app ropria te ffor or cascade control. Exam ples ooff unde sirable choices choices w ould be inline pH -toreagent flow flow and liquid pressure-t pressure-to-flow o-flow cascade cont control rol because the primary a nd secondary loops have abo ut the same time dela delay. y.

7. 7.  

Look fo forr op po rtun ities to ad d feedforward control, especially flow flow feedforward feedf orward w here m anip ulated flows flows are rati ratioed oed to a fe feed ed flow. flow. Make sure the feedforward feedforward signal does not arrive too soon an d cause inverse response.

8.

For improvem ents that cannot be covered covered by the m aintenance bu dg et, the benef benefit itss from from the reduction in variabili variability ty can be estimated by the calculat calculations ions in C hapter 3 to jjust ustif ifyy the proj project ect..

26

Advanced Control Unleashed

pplication Detail This secti section on w il illl take a cl closer oser llook ook at the m etho ds to im prove the response of valves and m easurem ents, reduce the total lloop oop time delay, tun e con trollers, troll ers, em ploy cascade control, and ad d feedforward control.

Valvee Selection Valv Selection Control valves are often often sel select ected ed based on the low est cost valve th at h as the required materials of construction. Often tight shutoff is sought. No w here in the valve speci specifi ficat cation ion is there a requiremen t that th e control valve m ove or respon d to a chang e in si signal. gnal. Consequently, rotary valves are chosen because they are the least least expensi expensive ve an d of offfer m odels w ith low leakage ra tes. They a re also tho ug ht to of offfer th e highe st rangeability. In reali rea lity, ty, the rotary v alve ha s the least usable rangea bili bility ty bec ause the installed characteristic gets too flat for small and large controller outputs. Figure 2-3a 2-3a show s how the character characterist istic ic is ttoo oo fl flat at below 5 degrees and above  45 degrees fo forr a butterfly butterfly valve. Figure 2-3b 2-3b show s how the charac teristic is tootake flat into belo w  10 degrees andstick-sli  60  6  deg rees for a ball valve. If yo u further accoun t that the stiabove ck-slip p 0signif significantl icantly y increases w he n these valves are lless ess than 15 degrees ope n, the actual usable ran geability of of thes e valves is less less than  20:1, instead of the 200:1  a n d 400:1 stated in the literature. By contrast, the slidi sliding ng stem valve install installed ed charact characteris eristic tic doe sn't get too fl flat at u ntil it gets below  5%  5 % open or above  75% 75 % open as illustrated in Figure 2-3c. 2-3c. Also, its sti stick-sl ck-slip ip is an order of m agn itude or mo re less and usua lly doe sn't increase dramatically un til the valve iiss less less than   5% open. Also, unlike the rotary va lve, the trim m ovem ent of a sliding sliding stem valve closel closelyy ma tches the actuator shaft shaft mo vem ent so that a digital positi positioner, oner, wh ose feedback fee dback is ttypicall ypicallyy actuator-stem position, can, by aggressive tun ing, actually co mp ensate for high p acking fr actually fric icti tion. on. A s a result result,, the real range ability abili ty of slidi sliding ng stem valves w ith d igital posit positioners ioners is  40:1. Of course, valve m anufacturers w ho off offer er o nly rotary control valves will develo p cleve cleverr wa ys of diverting attention fr from om these iss issues ues or even p itch the oppo site by the use of label labelss li like ke high perform ance and high rang eability that ignore ffla latt iinstalled nstalled characteristics an d stick-slip. stick-slip. The user m ust re realiz alizee that high perform ance indicates the abil abilit ityy ooff the valve to prov ide tight sshut hutof offf and to with stand high tem peratu res. Thes Thesee sam e fe fea a tures translate into excessive friction and low performance in terms of con trol. The u se of  a digital position er cann ot correct for for the inhe rent stick slip problem s of rrotary otary valves an d can essentiall essentiallyy deceive the user in to think ing it is do ing a great job by fancy plo ts and statis statistics tics ooff the step resp ons e of the a ctuator stem position. Unfort Unfortunatel unately, y, the ball or dis discc position does not trac trackk th e actuator shaft po sition, sition, because of gap s in linkages, tolertoler-

 

Chapter Chapt er 2 - Setting Setting the Foundation Foundation

27

an ces o f s h af aftt co n n ectio n s , an d s h af aftt w in d u p . A s en s itiv e, lo w - n o is e f lloo w m e a s u r e m e n t i s n e e d e d t o ttee s t t h e r e s p o n s e o n l iinn e   [2.9]. O  O t h e r w i s e , t h e v alv e h a s to b e r em o v e d an d a s en ssitiv itiv e tr av el g au g e m o u n te d o n th e b all o r d is c to d etect actu al mo v em en t f oorr s mall s tep s o r a s lo w r am p in th e valve signal. N ew d es ig n s ooff s lid in g s tem ( g lo bbe) e) v alv es , s u c h as th a t s h o w n in F ig u r e 2-6 2- 6 , r ed u c e th e am o u n t ooff me tal u s ed in th e b o d y an d p o c k ets an d cr ev ices w h e r e p r o ces s ma ter ial can s tag n ate an d acc u m u late. Th is m ak es th e v alv e m o r e co m p etitiv e w i th th e r o tar y v alv e f oorr ex o ti ticc mater ials , lar g e lin e sizes, an d fouling o r slu rry service. Abo ve 6 inche s in line size, the cost ooff s lid in g s tem tem ( g lo b e) v alv es can b eco me lar g e en o u g h to w ar r an t f u r th er investig ation. IIff the redu ctio n in sstick-sli tick-slipp and loop variability offe offered red by a s lid in g s tem tem v alv e d o es n ' t p r o v id e an accep tab le r ate o f r etu r n o n th e ad d itio n al in v es tm en t, th e u s er s h o u ld tak e a cl cloo s er lo o k at r o tar y v a lv es , but avoid any valves originally designed for isolation or interlocks. Sepa r a t e a u t o m a t e d h i g h p e r f o r m a n c e o r   on-off v alv es s h o u ld b e u s ed f oorr isolation or interlocks, an d lo w fri friction ction valve s fo forr throttlin g service. Since m an y of the rota ry valv es are fl flangeless angeless (wafer bo die s), the lifecyc lifecycle le cost should include not only the cost of increased variability, but the increased difficulty of proper installation and alignment and the increased risk of a s afety afety in cid en t an d r ep o r tab le r eleas e o f h a zar d o u s m ater ials .

Figuree 2-6. Sliding Stem Valve with Figur with Streamlined Passages Passages and Less Metal

R o t a r y v a l v e s m u s t a l s o p a s s t h e c hhee c k s o n t h e m a x i m u m p r e s s u r e d r o p . Th e r o tar y v alv e mu s t meet th e max imu m p r es s u r e d r o p r atin g at s h u to f f an d th e m ax im u m allo w ab le p r es s u r e d r o p to av o id ch o k e d fflo low w , f las h in g , cav itatio n , an d ex ceed in g th e n o is e limit. I n g en er al, s lid in g s tem v alv es o ffff er h ig h er p r es s u r e d r o p r atin g s , h ig h er allo w ab le p r es s u r e d r o p s to p r e vent cavitation, and noise reduction trim, and are thus the first choice for h ig h p r es s u r e , b o il iler er - ffeed eed w ater , s team a n d co n d en s ate s y s tem s .

 

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Advanced Control Unleashed Unleashed

If a rotary v alve is st stil illl the best choice, m ake s ure the con nection of the actuator shaft to the bal balll or disc st stem em is a splined connection, as sho w n in Figure  2-7, to  to minim ize the toler tolerance ance and associat associated ed p lay in the connection so that the backlash is les lesss than  0.5%. Key lock lock connections can cause a backlash of  8%  8%.. Also, the sha shaft ft diam eter sho uld b e large and the sha shaft ft length shou ld be short so that sshaf haftt w ind up does not cause a st stickick-sli slipp  0.5% [2.10].  0.5% greater than  [2.10].

Figure 2-7. Splined Shaft Connection

To sum m arize the performance dif differ ferences ences,, sliding stem valves with low fric fricti tion on packing a nd digital posit positioners ioners hav e a st stickick-sli slipp that appro aches the analog-to-digital converter resolution of 0.05% of  0.05% and  and a rangeability rangeability that approaches  50:1. Rotary valves exhibit a stick-sli stick-slipp an d b acklash fr from om 0.5% to  1  10% 0% an d a rangeabilit rangeabilityy of 10:1  to 20:1, even with a digital positioner a nd low fric fricti tion on packin g, with the great greatest est deteriorati deterioration on found in designs that em ploy keyed connections, long slender shafts, shafts, an d hig h fri frict ction ion sealing  To help avoid the man y trap s ooff creat of surfaces for tight  shutoff. To creative ive advertising, adverti sing, tthe he user should keep in m ind the pop ular m yths li liste stedd in Table 2-2. Table 2-2. Popular Myths About Control Valves Valves 1.  R otary valves have the greatest rangeability. rangeability. 2 .  High Perform ance valves offer offer high control-loop control-loop performance .

3. A digital positioner will make any control valve perform  well. 4 . A fast a ctuator will make any valve fast.

5. A piston is the fastest a nd m ost reliable actuator. 6. Tight shutoff is desirable for a control valve. 7. The m ost cost-effective cost-effective va lve is a rotary valve. 8. Fast loops should use b oosters instead of positioners.

In the old day s ooff analog controllers, controllers, theoret theoretical ical studie s based o n N yqu ist plots show ed that, fo forr fast fast loops such as flow flow and liquid pres sure loops, a

 

Chapter 2 - Setting the Founda tion

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booster w ould prov ide better control than a positioner. IInn real llif ife, e, un kn ow n bench settings, excessi excessive ve booster dea d ban d, positive feedback from from high boo ster outlet port sensit sensitivi ivity, ty, an d th e high break aw ay torque of tight shuto shutoff ff valves mak es the om issi ission on of a positioner risky bu siness [2.10]. Com press pressor or surge valves have slamm ed shut d ue to booster posi tive fee feedback dback w he n a booster w as used w ithou t a positioner   [2.1]. In fact, w here large actuators ne ed the high relay capacity ooff a booster, booster, iitt sho uld be used on the outlet ooff a positioner and a byp ass aro un d th e booster adjusted for stability  [2.1] [2.11]. In the chemical industry, positioners hav e either impro ved, or at w orst had a benign ef efffec ectt on, the performance of all types of control loops  [2.12]. The scan tim e and op tional fil filter ter of a d igital process controller, and the tuning flexibility of a modern positioner, elimi nate th e concern about the violation ooff the cascade rule wh ere the second ary valve pos ition loop is no t ssuff uffici icientl entlyy fast faster er tha n the m aster p rocess loop. Actuall Actually, y, there there never w as m uch of a concern concern because flow flow loops w ere tuned w ith mostly integral, rather tha n gain, ac action. tion. Pneu m atic and s, electro-pneumatic positioners oners entust, outand of cal calibrat ibration w ithin 6 m onth the llinkages inkages werepositi dif diffi ficul cult t towadj adjust, remo te ion indica tion of of positi position on requ ired the m oun ting of a separate valve position position tran s m itter. Digital positioners h old their calibration, of offfer a n ind ication ooff actual valve position (AV (AVP) P),, and perform a wid e spectrum of diagnostics and tests that can be used fo forr predictive rather than reactive mainten ance. T h e  A  AV VP can be monitored and com pared to the cont controll roller er outp ut to deter m ine dead ba nd , sti stick-sli ck-slip, p, and dea d tim e ffor or sliding sliding stem va lves. For rotary valves, a low-noiselow-noise-fl flow ow m easurem ent is need ed sinc sincee the actuator stem po sition doe s not re refl flect ect the actual ball or di disc sc position . A piston a ctuator can reduc e the stroking time of large large valves once a valve starts to m ove. How ever, the design of mo st pistons exhibit exhibitss poo r resolu tion and d ead b and that will cause an excepti exceptionally onally slow slow respo nse to small changes in valve position position th at can get worse if the cylinders are not p rop  erly lubricated. lubricated. For small changes in valve position, a diap hrag m actuator is generall generallyy fas faster ter and mo re pr precise. ecise. Also, a diap hra gm actuator d oes n ot require lubrication lubrication or as m uch m aintenance un less iits ts tem pera ture rating is exceeded. For concentrated concentrated slurries, particularly those with clum ps of material or rocks, such as w aste treatme nt lime slurr slurry, y, rotar rotaryy valves m ay be the only choice. choic e. In tthese hese cases cases,, pulse interval m odu lation h as been use d to prov ide an extended rangeabili rangeability ty when the dow nstream back-mixed back-mixed volum e wa s sufficient to attenuate the pulses  [2.1]. Finally Final ly,, pharmace utical and fo food od applications ma y require sanitary co ntrol valves. Angle plug, ball ball,, and diap hragm valves are used. The slidi sliding ng O -

 

30

Advan ced Control Unleashed

ring stem seal ooff an ang le valve creates a dif diffi ficult cult-t -to-cle o-clean an void betw een bonnet an d stem, and does not insure produc t media w ill ill not lleak eak past the O-ring and into the environm ent. The rotary ball valve also has a vo id betw een seals wh ere produ ct can accumulate, and h as excessi excessive ve operating fric fricti tion. on. By fa farr the m ost pop ular choi choice ce iiss tthe he Sau nders d iaph ragm valve, bu t both it and the pinch va lve have a very limited rangeability ooff fl flow ow and a quick-opening type of flo flow w charact characteris eristic tic that is und esirable for con trol.. A crevi trol crevicece-fr free ee roll rolling ing m etal diaph ragm control valve like like that sho wn in Figure 2-8 ha s an almost fr frict ictionion-fr free ee m echa nism an d slidin g stem ac tu ation that affords exceptional rangeability and sensitivity  [2.13].

Figure 2-8. Crevice-free Sanitary Valve with High Rangeability and Sensitivity

In all all valves, there is a valve pres troke time delay : the time it takes for eno ug h air to mo ve into or out of of the act actuator uator to change the air pressu re eno ug h to start to m ove the actuator stem . The stroking time is the time required to complete its its transit transition ion to the new stem position aft after er the actua tor starts to move. The ttests ests to docum ent the prestroke time delay an d the stroking time typically consisted of   10% or larger steps, don e with the valve disconnected. The results depe nde d solely solely on the type an d size of actuator an d the type an d fl flow ow capacit capacityy of the actuator connecti connections ons a nd accessories; they d id n ot includ e the ef effe fect ct of valve de ad b an d or stick stick slip. Until recent recently, ly, valve manu facturers rep orted the actuator prestroke dead time time and stroking stroking ttime ime wh en you asked how fa fast st a valve valve would respond. These days, a valve manufacturer will perform actual step tests tests ooff the com plete valve assembly b ut w ill ill chos chosee a step size ooff   10% and an initial initial valve position of about  50%, i  inn order to show the fas fastes testt response .

 

Chapter   - Setting Setting the Foundation Foundation

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To get a realist realistic ic picture of valve respon se u nd er actual loop c onditions, the smallest and largest step size size at the min im um expected throttle posi tion should b e used. For llarge arge valves used for for com pressor surge or pres sure control, step sizes of 10 to 50  50% % ar  aree appropriate and the prest prestroke roke time time delay and stroking ti tim m e dictat dictatee the speed of response. For m ost other con trol valve s, step sizes of 0.5% or lless ess shou ld be used and the valve de ad ban d an d stic stick-sl k-slip ip will dom inate the response. A ram p (a ser series ies of small steps held fo forr the loop scan ti time) me) w ould better duplicate the actual valve respon se for clos closed-loop ed-loop control. The use of a ramp is particularly important for pneumatic positioners and boosters because they exhibit a drastic drastic incr increase ease iinn response time as you appro ach the resolution li limit mit of the the linkages an d flapper flapper nozzle assembly. T Too quan  ti tify fy d ea d b an d, stick and slip, a series of of step s, ffir irst st for a reversal of posi tion and then in the same directi direction on are used . Each step is held for for the prestroke de ad time and stroking ti time me identi identifi fied ed fr from om a   10% step. The steps are are conti continued nued until movem ent ent occurs.  The actual actual val valve ve m ovem ent in  occurs. The excess of of the size of the last step is iindicative ndicative of the a m ou nt of slip. Figure 2-9 2-9 show s how the response tim e changes as a ffuncti unction on of the ty pe of control control valve, sshaf haftt connections, actuator, actuator, and po sit sitioner. ioner. D iaphrag m actuators, sliding sliding stem valves, and digital positioners hav e the fas fastes testt response b y far to a ran ge of small step sizes, wh ich is the goal of  99% 99 % of all control control valves. The combination of an ele electr ctrica icall or h ydrau lic actuator and a sl sliding iding stem valve can have an even better resoluti resolution. on. The only de ad ba nd is w hat is iintroduce ntroduce d in the setup of the the positi positioner oner to eliminate dither. The prestroke de ad time is essent essential ially ly zero but the stroking time increases incre ases proportionally to the step size and can becom e quite large ffor or the electrical actuator. Hydraulic actuators provide the fastest response for small and large st step ep sizes bu t are complex complex an d expensive.

Valve Installation The installati installation on and location location requirem ents for a control valve are gen erall erallyy less tha n fo forr a sensor sensor.. Ideall Ideally, y, control valves shou ld h av e the sam e stra ight ru n of of pip e upstream and dow nstream as a di diff ffere erenti ntial al h ead flow flow m eter eter,, since bo th c onstitute a v ariable orif orifice ice  [2.9]. Ad herence to the straight-run requirem ents rarely occurs in iindu ndu stry bu t is com mo n in the fflo low w test llabs abs use d to establish flo flow w characteristics and flow flow coef coeffi ficie cients nts  [2.9]. The repro ducib ility error fr from om an erratic fl flow ow profi profile le is no t as imp orta nt for a final final elemen t as it is fo forr a m easurem ent because th e control loop w ill drive the m anip ulated variable as necessary to reach sset et point. Ho wev er, iiff the flow flow is going to be com puted throu gh th e contr control ol valve based on valve position and pressure d rop , the reproducibil reproducibility ity ooff the resulting fflo low w m easure m ent, and hence the str straight-r aight-run un requirements, requirements, become more important.

 

32

Advanced Control Unleashed Unleashed

Figure 2-9. Response Times Times for Different Types Types of Final Elements Elements Valves, Actuators and Positioners)

If the control valve is up stream of equipm ent w ith lar large ge pressure d rop s, the m inimu m pressure in the valve iiss more li likel kelyy to stay above the v apor pressu re that starts fl flashing. ashing. A llocati ocation on that offer offerss a lower tem pera ture, besides a higher p ressure, can preven t fl flashi ashing. ng. Cavitation can be pre  ven ted by a staged p ressure d rop o r a low-recovery control valve, such as a sliding stem va lve, so that the bubbles do n ot im plod e fr from om the rise of the discharge discharge pressure above the vapo r pressure. The st staged aged pressure dro ps can be d one internally internally by speci special al trim, trim, tw o control valves in ser series, ies, or a restriction orifice   [2.9]. If cavit cavitation ation is unav oidab le, the dam age to pip  ing can be avoided if the valve is m ou nted on a vessel nozzle so that the bubbles im plode insi inside de the vess vessel el vapo r space  [2.9]. A hug e source ooff time delay occurs iinn concentration control loops w hen  ever the volum e between a cont control rol valve and the iinj nject ection ion poin t into equ ipm ent or piping is either parti partially ally fill filled ed or large com pared to the fl flow ow rate. For partially partially fi fill lled ed lines, there is an excess excessive ive time delay even w he n the valve stays ope n. A chang e in valve positi position on cau ses a cre crest st or val valley ley in the w ave to travel dow n the pipe or channel. channel. The transportation delay is the distance d ivided by th e veloc velocit ityy of the w ave b ut th e veloci velocity ty is ddif iffi ficul cultt to estimate. For a very low fl flow ow do w n a vertical pipe, the velocit velocityy of a fal falli ling ng fi fillm can be com puted   [2.14]. W henever th e contr control ol valve cl closes, oses, m anip u lated fluid fluid in the dow nstream pipin g and inj inject ectors ors or dip tubes slowly m igrates into the equ ipm ent o r destination and the process fflui luidd backf backfil ills ls the sam e volum e. The rresult esult iiss a llong ong delay betw een th e cl closure osure ooff the valve and the end of m anipu lated fl fluid uid flow flow and a ssimilarl imilarlyy long delay

 

Chapter  - Setti Setting ng the Foundat Foundation ion

33

betw een the ope ning of the cont control rol valve and th e start of the m anip ulated fluid fluid fl flow ow into tthe he equip m ent or destination. For a pressurized, comp let letely ely ful fulll pipe line, dip tu be, and in inje ject ctor or,, the time delay can be estim estim ated as the volum e between the valve and the inject injection ion po int divide d by fl flow ow of the the m anipu lated fluid. fluid. For pH control systems w here the m anip ulated flui fluidd is a reagent, tthe he fl flow ow can be as low as on e gallon gall on per ho ur a nd just one gal gallon lon ooff volume dow nstream of the valve can result in one ho ur of ti time me d elay every time the reage nt control valve closes  [2.14]. The control valve shou ld h ave block and d rain v alves so that it can be  easily. F rem oved safe safely ly and and easily.  F or large continuous processes, iitt is desirable to have a m anual by pass valve to keep the unit online while the valve iiss tested or repa ired. Also, the ou tlet iisolat solation ion valve can be closed an d the byp ass valve, sho w n in Figure 22-10, 10, open ed for inl inline ine test testing ing and tunin g of the digital positioner at process pressures a nd tem peratu res. For slurry service, rotary valves can be m oun ted in a ve vert rtica icall-fl flow ow u p p ipe to pro  m ote self self-drai -draining ning a nd preve nt solids bu ildu p. For sliding stem valves, a vertical verti cal mou nting w ill cause additional wea r ooff the packing ffro rom m the weight of the actuator.

Figure 2-10. Install Installation ation Requir Requirements ements for a Flow Meter and Control Valve

The packing shou ld be consis consistentl tentlyy tightened w ith a torque wren ch to a specification of less than  90 footfoot-lbs. lbs. A pac king tha t is tighte ned exces sivel sively, y, to supp osed ly p reven t leakage, will cause a li limit mit cycl cyclee and prem a ture failure failure of the packing th at can cause a rel release. ease. For high tem pera tures and highly hazardo us m ateri aterials, als, exten extension sion bonnets and bellows seals seals shou ld be considered instead of highhigh-fr frict iction ion packing .

 

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Advanced Control Unleashed

The Variable-speed Drive Alternative

The cost of the variable-speed driv e (VSD) (VSD),, al also so kno w n as the variable-f variable-fre re quency drive (VFD) (VFD),, has dropp ed to tthe he point where it may now be an attractive attracti ve fi final nal el elem em ent wherev er there is a m otor-driven p u m p or fa fan. n. There is no st is stickick-sli slip andt, there the only dead aneasu d is rable that introduced in the electronics. Inp fac In fact, is al also so nobm tim e delay,by sothe tha ut ser forr fo pressure and fl flow ow loops, the loop dea d time is lar largely gely set by the control controller ler scan tim tim e  [2.1]. Fo  Forr furna furnace ce pressure control control,, the use of a VSD on the induc ed draf draftt fan an d a scan rrate ate ffas astt enou gh to be equivalen t to an analog controller control ler can llead ead to muc h tighter pressure control and fewer pre ssure trips  [2.15]. There are comm on misconcepti misconceptions, ons, inadvertent inadvertently ly prom oted by a m anu factur fac turer, er, that the m ain ad van tage of a V VSD SD is energy sav ings an d th at a VSD is is slow an d lacks rangeability. The time ffro rom m m inim um to m axim um speed is adjusta adjustable ble w ithin the li limits mits impo sed by the impeller iner inertia tia and the motor ho rsepower. The factor factoryy setting setti ng is conservative. The speed response is a veloci velocity-l ty-limited imited ram p rate with n o time delay or lag. Consequently, the response for small speed chan ges is fas fast. t. For exam ple, iiff it takes a drive 9 sec ond s to go from   10% to 100%  speed, it will take only 0  0.1 .1 second to change the speed by 1%. The m inim um speed is also adj adjustable. ustable. The fac factor toryy setti setting ng is es especial pecially ly conservative. Cogg ing is not a problem for tod ay 's inverters, an d over heating is not a problem for vari variable-torque able-torque loads typical ffor or a pu m p o r a fan. fan. Test Testss hav e sh ow n th at the rangeab ility ility of a VSD is great greater er th an the range ability of a m agn etic fl flow ow m eter (25:1 (25:1), ), instead of tthe he  5  5::1 suggested by the manufactur manufacturer. er. How ever, the minim um speed m ust provide a total discharge pressu re that is greater than the stati staticc pressure to p revent reverse fl flow. ow. To sum m arize, the VS VSD D provides a consist consistent ent and rapid respon se. IInn fa fact ct,, the response can be so ffas astt th at it scares scares opera tors because the flow flow can reach a set poin t in less less than a second. The ccontrol ontroller ler ou tpu t an d set p oint limits m ust be set to prevent excessi excessivel velyy high or low fl flow ow rates and the process variable filt filter er m ust be set higher to prev ent reaction to noise since there is no d ead ban d. Set poin t velocit velocityy limits m ay also be required. A n isolation valve may n eed to be add ed fo forr shut shutoff off a nd fai faill-saf safee action. The inverter invert er m ust be m ounted in a climat climate-c e-contr ontroll olled ed room and be m aintained by technicians with special skills for troubleshooting complex electronics. In addition, it may be necessary to increase increase the mo tor enclosure size to meet the area classification because of the reduced cooling capacity of the m oto r's fan fanss at low speed s. Most mo tor vendo rs will only rate the m otor for the area classi classific ficati ation on if they sup ply the m otor a nd V SD as a packag e.

 

Previous Page Chapter 2 - Setting the Founda tion

35

Finally, Final ly, isolat isolation ion transformers and best practices practices m ust be use d to preve nt the introduction of noise ffro rom m the inverter into the elect electri rical cal grou nd sys tem . Co nsequ ently, the installed cost an d th e li lifefe-cycl cyclee cost ooff a VSD ma y be m ore than a contr control ol valve if is isolati olation on valves m ust be ad de d, the pla nt is not set up for for VSD m aintenance , and the benefit benefitss fro from m faster faster or mo re pre cise manipulation of flow are relatively small. For strong acid acid an d base pH systems, tthe he requirem ent for pre precise cise adjust adjust m ent w ou ld best be me t by a VSD, bu t the flow flow rates are of ofte tenn too small for for a centr centrif ifugal ugal p um p an d the locati location on of of the the pu m p on the grou nd creates a hu ge reag ent de livery transpo rtation del delay. ay. Instead, an electr electronica onicall llyy set metering pu m p is used with the piping designed to st stay ay ful full.

Measurement Selection The num ber of measu rem ent devices iiss too llarge arge ffor or a com prehensive treatment. This chapter off offer erss an im portant pe rspective, highlighting the most desirable alternatives and the relative performance advantages and limitations limitat ions ooff the most com mo n types fo forr the mo st general types of appli cations. There are excepti exceptions ons to every rule. The user sh ould consult han d books, manuals, and the manufactur manufacturer er befor beforee p roceeding with an implementation. The objective objective is to sel select ect a m easu rem ent w ith th e best relia reliabili bility, ty, repro du c ibili ibi lity, ty, rangeability rangeability and resoluti resolution, on, an d the smallest am ou nt of noise by m eans of digital device devicess and the elimination of process lines, process and am bient ef effe fect cts, s, and m echanical com pone nts. Smart transm itters sho uld alwa ys be used because they pro vide the best accur accuracy acy and rangeabi rangeabilit lity, y, compensate for extraneous effects, and offer online diagnostics. However, the device with the inherently best principle principle of of operation sh ould be favored over the one with the fanciest diagnostics that will constantly rem ind y ou of a bad choice. For flo flow w m easure m ents, inli inline ne m eters such as corioli corioliss mass flo flow w m eters, magnetic flow meters, vortex shedding meters, and thermal mass flow m eters should be considered because these m eters elim elim inate sensing li lines, nes, external connections, and small holes that are the largest source of errors, failures, leaks, leaks, and m ainten anc e. To To redu ce the ef effe fect ct of fflow low profiles an d chan ges in process flui fluid, d, preferred ord er of select selection ion is the ord er listed. affec ecte tedd by R ey Coriolis  mass flow  meters  require no straight run s, are not aff nold s Nu m ber or fluid fluid pro perties, and hav e by fa farr the best rel reliabi iabili lity, ty, reproducibilit reproducibi lity, y, rangeabi rangeabilit lity, y, and resolution fo forr the m easurem ent of both ma ss fl flow ow and densit density. y. The coriol coriolis is m eter is the only true m ass fl flow ow meter. Mag netic and vortex fflo low w m eters are veloci velocity ty volume tric device devicess and can be used to com pute m ass ffllow only ffor or a fixe fixedd and kno w n com positi position on by

 

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the me asurem ent of tem pera ture an d pressu re. This iiss also true fo forr pitot tubes with differ different ential ial pressu re, pressu re, and te m pera ture transm itters that are advertised as mass fl flow ow m eters. The therm al mas s fflo low w m eter is not a volumetric meter but its reading will depend up on the heat capaci capacity ty and thu s the composition ooff the fluid. Coriolis fl Coriolis flow ow m eters above 2 iinches nches get expensive, bu t stil stilll ma y be justi fied fied w here the abilit abilityy to measu re and control a m ass balance is im portan t. Oftenn o verlooked are the benefi Ofte benefits ts ooff an accurate density m easure m ent and an approximate tem perature measurem ent (the tempe rature senso sensorr is on the outsid e surf surface ace of the tube an d is not in contact with th e fl fluid) uid) to create online estima tors ooff fflui luidd com position. Unfortunately, the the ffluid luid m ay be too corrosive for for the mate rials ooff construction offer offered, ed, th e fflui luidd tem pera  ture or concen trat tration ion of parti particles cles may b e too high, or the pipe size too large. Coriolis fl Coriolis flow ow m eters are so accurate that they c an be used to replace load cellss or w eigh ta nks for batch charg es. Also, ffor cell or fl flat at titration cu rves an d constant com positi position on feeds and rea gents, simple ma ss rati ratioo control with corioli cori oliss fflo low w m eters has prove n to be more accurate and mo re rel reliable iable than a p H control control.. The har dw are cost ooff a cori corioli oliss fflo low w m eter is high com pared to a diffe different rential ial he ad m eter, bu t the installation cost m ay be less since there are no straight-r straight-run un requirements or additional measu rements to com pen sate for for pressure and tem peratu re. The projec projectt cost may b e sti still ll higher, bu t the life-cycle cost is often significantly better for the coriolis meter, as show n in Figur Figuree  2-11, because the coriolis coriolis meter requires less ma intenance and accum ulates bene benefit fitss from from tighter control.

Figure  2-11. Life-cycle Cost Comparison of a Coriolis and Orifice Meter

 

Chapter 2 - Setting the Foundation

37

Next to the coriol coriolis is mass flow m eter, eter, the  magnetic flow  meter  has the fewest interferences interfer ences since it is no t af affe fect cted ed by either R eyno lds Nu m be r or viscos ity and is relati relatively vely insensitive to fl flow ow profil profile. e. The ma in limitation is th at the fluid fluid con ductivity m ust be greater tthan han   1  micromho/cm  0.1 0.1 for spe cial un its). For erosive service, cer ceramic amic lining s are off offere ered. d. The flow profile is a big factor for the  vortex  meter particularly near the low end of the me ter's ra nge. IIff the vel velocit ocityy d rop s below  1 fps, or Rey no lds N um ber is lless ess than 20, 20,000 000,, or the viscosi viscosity ty is greater than 30 centi 2 % , the vortices are not poises, or the co ncentration of particl particles es is above  2% shed uniformly and the vortex ffreque requency ncy d eviates ffro rom m a prop ortional relationship to flow. flow. At low velocities velocities the readin g can becom e very errati erratic. c. A n  orifice meter  may ha ve a good short-term repeatabilit repeatabilityy speci specifi ficat cation ion un de r laboratory test conditions. How ever, tthe he ef effe fect ctss of Reynolds N um  ber, flow flow profil profile, e, ori orifi fice ce w ear, and sensing lines ssignifi ignificantly cantly dete riorate s the long-term reproducibilit reproducibility. y. Measurem ent noise and the sq uare-root relationsh ip also lead to a range ability of just  4:1. Sm art ttransm ransm itters can do ub le this iiff the straigh t ru n of pipe u pstre am is suf suffi ficie cient. nt. If a large li line ne size requires the u se of a dif differe ferenti ntial al he ad m eter, a large-bore direct m ou nted sm art ttransm ransm itter should be used a s sho wn in Fi Figure gure 2-12a. 2-12a. For lesss sensit les sensitivit ivityy to Reynolds N um ber, fl flow ow profi profile, le, and process co nditions, use a purge d averaging pitot tube or a llargearge-slot slot A nnu bar w ith a dire direct ct integral-mounted integra l-mounted smart tra transmitter nsmitter with press pressure ure and tem perature com pensa tion. The pu rge is used to preven t the plugging of the small sensing holes of a pitot tube. Table 2-3 sum m arize s the relative performanc e capab ilit Table ilityy of each ooff the most common types of flow measurement. The rangeability and accuracy of the meas urem ent can be improv ed generall generallyy by a fa fact ctor or of two by th e use of a sm art transmitter. ori orifi fice, ce, in vort vortex, ex,ndand maho gnetic flow ow m eters are volumetric meters, so ffllThe ow ranges pou s per ur arefl based on an assum ed velocit velocityy prof profil ilee a nd density or concentrati concentration on iiff temperature-cor rected. The n um be rs in Table 22-33 are llargely argely fro from m Refer Reference ence 2.16, except fo forr the reproducibilit reproducibilityy percentage s. They are based on the au tho r's exp eri ence an d ref refle lect ct the ef efffec ectt of we ar an d th e chang e in meter coeff coeffic icie ient nt w ith ope rating conditions such as Reynolds Num ber. The man ufacturer shou ld be consulted fo forr an estimated reproducibilit reproducibilityy for the actual range of oper ating conditions for the application. The drift from wear will be greatest fo forr slu rries or corrosive service and h igh v elociti elocities. es. The hardware and setup costs of the  radar level measurement  has decreased and m any of the ca calibr librati ation on com plexi plexiti ties es have been autom ated to the po int w her e the li lifec fecycl yclee cost is is ooft ften en less than the diff differenti erential al pre ssu re (d /p ) m ethod of level level me asurem ent. IItt is the most accurate of the com mo n

 

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Table 2-3. Compariso Comparison n of Common of Common Flow  Flow Measurem Measurements ents [2.16] [2.16]

Type

Sizes

Range

Piping A/B**

Interferences

Reproducibility

Coriolis

¼ -6 n

80:1

1/1

solids, alignment, vibration  

Magmeter Vortex

¼-78 ½-12

25:1 9:1*

5/1 10/5

conductivity, electrical noise 0.5 conductivity, profile,  viscosity, hydra ulics 1.0

of rate of span

Orifice

¼-78

4:1

10/5

profile,   Reynolds Number

of span

0.1

5.0

of rate

* Assumes  a minimum and maximum velocity velocity of  of  an d  9 fps, respectively. respectively. **  A is the number of upstream pipe diameters and B is the number of  of downstream pipe diameters  of straight run required (see Figure 2-10).

types  o  off leve levell measu rem ent. The m ain limitation o  off rad ar is that the fluid fluid mu st hav havee  a dielectric constant greater than  2 . For tall tall narrow vessels, the minimum  8 degree angle o  off divergence o  off the beam m ay result in the gaug e not being able di discer scernn the bottom  o  off vess vessel. el. The gauge m ust be p ro gram me d to iignore gnore any obstructi obstruction, on, including including d ip tubes and agitat agitator or blad es, typical typically requires an em ptyare tank  some point d uring cal cal  ibration ibrat ionand proced ure.ly  Pulsed lasers  that not at adversely affecte affec tedd bythe d ust or vapo r can potenti potentiall allyy become an even m ore accurat accuratee m ethod  o  off level  level m ea surement   [2.17]. The angle o  off divergence is less than a degree and there is no dielectric dielectric rrequirem equirem ent. How ever, a relat relatively ively cl clean ean sight glass w ind ow is required. T h e  differential pressure level measurement  depe nds u po n density ooff the flu fluid id and the condit condition ion  o  off the sensing an d equalization lines. A second d /p with bo th conne connecti ctions ons below below the m inimum level level can be ad ded to provide a representative representative measurem ent  o  off the density if  if the vessel is we ll mixed , althou gh th e accuracy is usually good to only two significa significant nt digits. The sensing and equalization lines can be eliminated by th e use  o  off capillary  orr transm itters mo unte d dire systems  o directl ctlyy on vessel fl flanges anges fo forr both the bottom total pressure and top equalization pressure. However, the error introduc ed by capill capillary ary system s can be si signif gnifica icant nt   if there are bubbles in the fill,  or differences in the temperature or length o  off the capill capillary. ary. Th e communication  o  off signals ffor or the co m putation o  off level fr from om du al transm it ters is is best do ne digitall digitallyy to eliminate eliminate digital-to-anal digital-to-analog og (D /A ) an d analogto-digital to-digit al (A /D ) converter errors. Even  so  s o , the error increases as the vessel pressu re increas increases es and can become unacceptable because the bottom  d /p measures both liquid head and vessel pressure relative to atmospheric pressure.

T h e  admittance-probe type  of  llevel evel measurement  dep end s upo n the diele dielectr ctric ic constant  o  off the flui fluid. d. A second probe can can be added to provide a represen  off the dielectric consta nt if  if the vessel is well m ixed. tative tat ive m easuremen t  o Since the dielectric constant is rarely kn ow n accurately, the vessel level

 

Chapter  2 - Setting Setting the Foundation Foundation

39

m ust be vari varied ed a know n am ount d uring the cal calibr ibrati ation. on. Fo Forr haza rdou s m aterials, there is usua lly no w ay to visuall visuallyy check the level and anoth er metho d of le level vel m easuremen t is needed. T h e  ultrasonic level measurement device beam  is scatt scattered ered by foam o r d us t and is affe affect cted ed by an ything that af affe fect ctss the speed of sou nd . Thus, read ings are aff affec ecte tedd by ch anges in the pressure, tem peratu re, and com position of the vapor space. isolated from from the process, the process, but  but are Nuclear level measurements are completely isolated affe affect cted ed by density un less a second device is ad de d. Strip sources are rec om m end ed to eliminate the need for for com pensation of the dif differ ference ence in radiation pa th len gth fr from om a point source to the strip detec detector. tor. The lice license nse proce dure is consider considered ed a hassle and an ything nuc lear tend s to scare peo  ple even tho ugh the exposure is less than w hat they recei receive ve ffro rom m the su n. F or  temperature  measurement a  3- or  o r 4 wire resis resistance tance tempe rature detector (RT RTD) D) is preferr preferred ed over a therm ocou ple because it has mu ch less dr drif iftt and a m uch b etter repeatabili repeatability ty a nd resolution, per Tab Tablle 22-4. 4. For very h igh temperatures, however, a thermoco thermoco uple m ay be the only option if an o pti cal pyrom eter is not feasi feasible ble.. A sm art transmitter sh ould be used for for con trol loops instead of ttherm herm ocou ple or RTD inp ut cards in the  DCS. The speed of response of an RTD iiss only slower th an a therm ocou ple for a bare-element installation installation of bo th because the therm ow ell and its air gap is the largest source ooff mea surem ent lag. ent  lag. Bare-element  Bare-element installations are rare and even then the dif diffe fere rence nce is only a ffew ew seconds, which is negligible negligible com pared to the other thermal lags in m ost processes. processes. Table Tabl e 2-4. Comparison Comparison of  T y p e 

ommon   Temperatu Temperature re

Range (°F) 

Measurement Measurementss [2.19 ]

Drift

Repeatability Repeatability (o F )

*F/yr) 

RTD

-200 -200 to16 00

0.02 0.02 to 0.2 0.2

0. 0.05 05 to  

Thermocou Ther mocoupl ple e

-300 -300 to 3100 3100

2 to 40

2 to 15

Resolution (oF) 0 002 

0.1

Power (watts) -2

4x10 2x10

-7

For analyti analytical cal measu rem ents, iinline nline m eters and pro bes that do no t require sam ple systems will pa y of off by the elimination ooff samp le transp ortation dela ys an d th e reduc tion in the li lifefe-cycl cyclee cost ooff a sam ple system . If a den  sity mea sure m ent is suff suffici icient, ent, th e coriol coriolis is m eter off offer erss the fastes fastestt an d m ost accurate and relia reliable ble response. For simple wa ter mixture an d com  plex general mixtures, inl inline ine meters such as microwave and nuclear m ag netic resonance (NMR) should be investigated respecti respectively. vely. [M icr icrowave owave can 2 or 3 compo nent w ater m ixtures, ixtures , wh ile NMR handbe le used aqueofor us simple an d n on-aqueou s complex mixtures mixtures]. ]. F For or coa coal, l, oil andcan mineral slurries, slurries, the prom pt gam ma neutron acti activati vation on analyzer can

 

40

Advanced Control Unlea Unleashed shed

deliver rap id, sample-f sample-free, ree, elemental analysis on large top-size, bulk m ate rials m oving a t meters per second   [2.18]. For covalent chemical bon ds, Ram an scatter scattering ing emission spectropho tom eters can provide fast analysis of mu ltiple com pone nts in multiple stream s by the use of fiberfiber-opti opticc probes  [2.23]. The hardw are and setup costs of these analyzer analyzerss h ave decreased w ith advan ces in com putational capabilit capabilityy for for complex spec trum analysis. Once the system is su succes ccessfu sfull llyy calibr calibrated ated a nd com mis sioned, the m aintainabili aintainability ty a nd rel reliab iabil ilit ity, y, not to m ention the spee d, are m uch better than wha t can be achi achieved eved with chrom atographs. How ever, the extensive ex perience base of analyzer sp ecialist ecialistss (lar (largely gely chem ists) w ith chro m atograp hs, and a llack ack ooff appreciation fo forr the value of a tradeoff of som e accurac accuracyy for better avail availabili ability ty an d elimination of dead time, hav e slowed d ow n the introduct introduction ion of new inl inline ine meter an d prob e technol technolo o gies. Unfortunately, noth ing has proven to be rel Unfortunately, reliabl iablee and accu rate enoug h to replace the glass m easurem ent electrode. The ac accuracy curacy ooff iridium oxide sensors is too adversely aff affec ecte tedd by oxidation red uction po tentials, and the Field Eff Effec ectt T ransist ransistor or (F (FET) ET) ha s not yet been m ad e rug ge d e no ug h to be a viable general-purpose replacement for the glass electrode. Significant imp rovem ents hav e been m ade in the ref refere erence nce electr electrode ode by the use of a solidd po lyme r w ith a large ar soli area ea acti active ve and imb edd ed elect electrolyt rolyte, e, iinstead nstead of a tiny por ous junction that can plu g an d internal liquid or gel ffiill that can be contam inated w ith process m aterial. How ever, tthe he greatest accurac accuracyy and speed of acclimation requires the use of a pressurized flowing refer ence junction  [2.18]. A three-probe installation installation with the m iddle rea ding select selected ed as the con trolled variab le of offe fers rs the grea test accuracy an d availabili availability. ty. This ha s been the best practi practice ce ffor or imp ortant p H loops in M onsanto a nd Soluti Solutiaa since 1986.. M iddle 1986 will iinhe nhe igndiagnostics, ore a si single ngle and fail failure ure e rron eou reading of anyselection type , will prov iderently online willo rreduce th e s noise band withou t the ad diti dition on of a lag. Two pr probes obes are quite com mon bu t introduce m ore questions than an swe rs si since nce the elect electrodes rodes rarely agree . Ho we ver, iiff the prob e llif ifee is sho rt du e to chemical attack of the glass, the prefer preferred red soluti solution on is then a piston ac tuated retract retractable able assembly w ith an autom ated imm ersion, w ash, cali calibrati bration, on, and soak cycl cyclee  [2.18]. The probes should be immersed only long enoug h  (e.g., 2 minu tes) to get a valid read ing to reduce expo sure time and any drif driftt as the ref refere erence nce seeks to reach reach equilibrium w ith the process. The auto m ated calibr calibration ation check can be don e on a shi shift ft or batch basis ttoo reduce th e time between successi successive ve insertions to less less than 5 minu tes.

 

Chapter 2 - Setting the Foundation

41

Measurement Installati Installation on Sensing lines should b e el eliminated iminated w herever possible by direct mo un ting a diff differe erenti ntial al p ressure (d /p ) transm itter ffor or flow flow m easure m ent or pressu re m easurem ent, as show n in Figure 22-12a 12a an d 2-12b, to the pipe connection. Isolat Isolation, ion, fflus lush, h, che d rain, and during equalization valves to inimize the exposure to chemicals micals the remov al oofare f thenecessary tr transm ansm itt itter. er.mAn eq ual ization valve is is used w hen bo th the high and low sides are connected to the process to of offfer the o ppo rtunity for a zero adjustment and to protect the d / p from over-range fr from om just seei seeing ng the upstream pressure.

Figure 2-12a. A Direct-mounted d/p Transmitter for a Flow Measurement

Figure 2-12b. A Direct-mounted d/p Transmitter for a Pressure Measurement

The flo flow w m eter should be m oun ted upstrea m of the control valve as sho w n in Figure 22-10 10 ttoo m inimize the distortion of the fl flow ow prof profil ilee and to pro vide a more con stant pressure. For liquid liquid flow, flow, the up stream locat location ion helps p reven t a partially fi fill lled ed meter, besides redu cing e xposu re to flash flash ing an d c avitation. Bubbles adversely aff affec ectt the accuracy of all fl flow ow m eters and the implosion of bubb les can cause serious dam age in additio n to

 

42

Advanced Control Unleashed

erratic readings. Isol Isolation, ation, flush, and drain v alves are again recom m ended for saf safee rem oval of an inli inline ne meter, althou gh some practices ffor or high haz  ard ou s materials sseek eek to minimize the num ber of connections connections and leak points. A bypa ss valve al allows lows the plant to run on m anua l while the instru m ent is repaired. The is isolati olation on valves upstream of the flow flow m eter sh ow n m ust be wid e ope n w he n the flow flow meter is in ser service vice for for the sam e reasons that a con trol valve iiss undesirab le up stream of a fflo low w meter. IIff there are solids, the m eter can be instal installed led in vertical pip e with flow flow up to help drain the pipin g. For cor coriol iolis is m eters, a si single ngle straight tub e is desir desirable able to eliminate erosion erosion at the be nd s ooff a U-tube an d u neq ual d istribution of sol ids in a du al tube. The meter size size should be chosen to provid e the opti m um velocity to m inim ize the effe effect ct of solids concen tration on accuracy. Figure 2-10 2-10 and Tabl Tablee 2-3 2-3 show the relative relative straight r un requirem ents fo forr different diff erent ty pes of fl flow ow m eters (the A and   B va lues are in Table 22-3). 3). A n or i fi fice ce with a large beta ratio (high ori orifi fice ce bore to inside p ipe diam eter ratio) has the greatest upstream straight-run requirement, followed by the vor tex meter o perating at a llow ow fl fluid uid velocit velocity. y. The upstrea m requirem ents also increase for multiple fittings in different planes or valves upstream that are not completely open or are partially plug ged . The upstre am straight-run requirem ent can be dram aticall aticallyy redu ced by th e use of straightening van es. The m anufacturer shou ld be consulted ffor or actual requirements based on your piping details and process conditions. The coriolis flow meter has no upstream or downstream straight-run require ments. Sam ple li lines nes should be eliminated wh erever possible ffor or all types of ele elec c trod es by th e use of iinsertio nsertio n or inje injector ctor assem blies. Inj Inject ector or electrode holde rs with ma nua l or autom atic retract retraction ion are no w off offer ered. ed. Even tho ug h these assemblies have built-in isolation, flush, and drain capability, the user ay choose to have thprotection e piping set show Figure For 2-13a 2-13a and 213b tompro vide a dditional for for up hazasardo us nmin aterials. three electrodes, electr odes, a seri series es arrangem ent, as sh ow n in Figure 22-13b, 13b, is ffavored avored to help k eep the velocit velocityy and concentration the sam e ffor or all three me ters. The electr ele ctrodes odes must be pointed do w n at a  30 - to 60-degr 60-degree ee angle, as show n in Figure 2-13c 2-13c,, to preven t a bubble fr from om becom ing lodge d in the tip or at an internal electrode. The first electrode should be at least  20 pipe diameters from from the discharge ooff a pu m p or stat static ic mixer to rreduce educe pre ssure p ulsation and fac facil ilit itat atee some m ixing. The elect electrodes rodes shou ld also be sepa rated b y 10 pip e diam eters to help establ establish ish a m ore unif uniform orm veloci velocity ty and they sho uld be inserted fa farr en oug h into the line or vessel to to get a rrepresentative epresentative read ing. The m oun ting of elect electrodes rodes in a pipeline w ith a 5 to 9 fps fps velocit velocityy is preferred preferr ed to a vessel because the high er veloci velocity ty in the pipe m akes the electrodes respo nd faste electrodes fasterr and keeps the m cleaner cleaner.. The bulk fluid fluid veloci velocity ty in even h ighly ag itat itated ed vessels rarely rarely exceeds 1 fps except near the agitator

 

Chapter 2 - Setting Setting the Foundation Foundation

43

blad e tip. For ssolids olids and high causti causticc or tem peratu re service, there is a com prom ise to keep the veloc velocit ityy low to decrease erosi erosion on an d chemical attack and yet high eno ugh to reduce coatings. The slot in the protective shro ud of the elect electrode rode tip should be oriented to shield the elect electrode rode from from abrasion an d chemical at attack tack bu t provid e a sweeping acti action on to decrease fouling. fouli ng. Final Finally ly,, the transpo rtation delay betw een the vessel or the po int of reagent add ition an d the elect electrodes rodes should n ot exceed exceed 5 seconds.

Figure 2-13a. Parallel Installation of pH Electrodes

Figure 2-13b. Series Installation of pH Electrodes

Figure 2-13c. Orientation and Insertion of pH Electrodes

For tem tem peratu re sensors, insert insertion ion length should b e maximized in tthe he cen ter of of the pipe so that the tip tem peratu re is cl close ose ttoo the actual process tem  perature d espite the conduction ooff heat between the tip and the s enso r's external connection. This This is particularly im portan t for an RTD since the

 

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Advanced Control Unleashed Unleashed

resistance ooff the entire element respo nds to tem peratu re, and for polym ers resistance because of a greater tem peratu re diffe differe rence nce be twee n the wa ll an d th e centerline of of the pip e an d a llow ow er convective heat transfer coeffi coeffici cient ent.. H ow  ever, an exces excessi sivel velyy long sensor may vibrate an d fai fail. l. Co m puter pro gram s can calcul calculat atee the conducti conduction on error and m aximu m length to prevent vibra tion for for a given thermow ell and sen sor construction construction an d process condi  The use of an elbow connection with the sensor or therm ow ell tions. The tions. facin fac ingg in to the fl flow ow as show n in Figure 22-14 14 prov ides th e greatest accu racy. rac y. The least desirable choice is op tion  4 , particularly fo forr sm all diam eter pipes.

Figure 2-14. Orientation and Insertion of Thermowells

At the o utlet ooff a stat static ic mixe mixer, r, desup erheater, or m ultiple-pass h eat exchangerr there should be abou t  20 pipe diameters between the equip exchange m ent ou tlet and the sensor locati location, on, as show n in Figure 2-15, for th e streams to recomb recomb ine to prov ide a more representative tem pera ture. Also, it is desirable ffor or the velocity to be between 5 and   11 fps to to maximize th e convective hea t tr transfer ansfer coeff coeffic icie ient nt at the tip ffro rom m a higher velocity an d to m ainta in a cl cleaner eaner surface. The higher coef coeffi fici cient ent n ot only im pro ves th e speed of response bu t also reduces the he at cond uction error. For the stat static ic mixer, the therm ow ell lag is is the lar largest gest lag in in the loop . For no n-h azard ous and non-corrosive fl fluids uids at ssaf afee tem peratures and pressu res, a bare ele m ent with a protective shro ud cou ld be considered for for tighter tem pera ture control of blend ing ope rations by elimination of the the therm ow ell lag. For d / p m easure m ents ooff llevel, evel, either the sensing or equalization line should be purged as shown in Figure Figure  2 16a. N ormally a li liquid quid such as w ater is used for the sensing li line ne and a gas such as nit nitrogen rogen for for the eq ual ization  line. I  Iff nitrogen is used fo forr the bottom connection, a sl slotted otted bu bbler tube is used.  T  To o preven t dryin g ou t the tip of bubbler, which can lead to solidi sol idifi ficati cation on of ma terial and p lugg ing, water is som etimes purg ed along w ith nitr nitrogen. ogen. The pu rge flow flow rates are norm ally regulated an d indicated

 

Chapter 2 - Setti Setting ng the Foundat Foundation ion

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Figure 2-15. Location of Temperature Sensor

the water pu rge an d the nitr nitrogen ogen pu rge lines m ust each have a ch check eck valve beforee being co mb ined, and each lline befor ine mus t hav e a che check ck valve. The sens ing an d equ alizati alization on lines for low to mo dera te vessel pressure s can be eliminated by the use of sepa rate d/ p s for the total pressu re and equaliza tion, tion, di direct rect m ounted on the bottom an d top nozzles as show n in Figure 216b. third d / py,can direct ect m ounted at ancan intermediate to co com m pu te A fl fluid uid densit density, an dbea dir tem peratu re sensor be used to nozzle c om pensate fo forr the ch anges in the dim ensions of the vessel vessel,, ttoo provid e a m ore ac curate level level measurem ent. Flus Flushh connect connections ions or ext extended ended diaph ragm s are used fo forr th e lower nozzles to help keep the d iaph ragm clean. The signals are com mu nicated digitall digitallyy to a com puter or a prog ram m able elec electr tronic onic con trol system.

Figure 2-16a. Purged Sensing and Equalization Lines for Level Measurement

Open Loop Response If a contr controlle ollerr is pu t in ma nu al, and a step change is ma de in the controll controller er outpu t, the trend recording wil willl show the open loop response. In indu s trial processes, tthe he o pen loop respo nse is rarely osci oscill llator atoryy b ut fol follows lows a

 

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Advanc ed Control Unleashed

Figure 2 16b. Direct mounted Transmitters for Level Measurement

path of a sel self-r f-regulat egulating, ing, integrating, or a runaw ay p rocess as show n in Figure  2-17. I  Iff oscil oscillat lations ions occ occur, ur, the root cause is hu nte d do w n and cor rected. Sometimes Sometimes oscillati oscillations ons occur in the op en loop respo nse of a pri m ary controller of a cascade con trol system, but this is easily ffixe ixedd by retuning the secondary controller to provide a smoother response. Indus trial practice now is to eliminate oscillations even in the closed-loop response (the response w hen the control controller ler iiss in in automa tic). The q uarter am plitud e decay for cl closed-l osed-loop oop con trol in the lit literature erature is a be nchm ark for minim um peak error an d is only seen as pa rt ooff a closedclosed-loop loop tunin g proce dure that then seeks to sup press th e osci oscill llati ation on as described in the next section on controller tuning. Thus, fo  forr p urpo ses of this chapter chapter,, the op en loop respo nse w ill be a fi firs rsttorder resp onse th at can be characteri characterized zed b y a total time dela delay, y, a neg ative or positive feedba feedback ck time constant, and a steady state gain. The time it takes the process variable to get out of of the noise band afte afterr a step change in the controlle controllerr ou tpu t  (CO), is the observed ti time me d elay  (dτ ) , or dead time. It excludes any time delay d ue to valve dead b an d o r sti sticti ction. on. The Theory section sect ion show s how to estimate the additional time delay fr from om valve d ead ban d. The time it takes af afte terr the time delay for the respon se of the process variab le (PV) ttoo reach  6 3 of the fina finall cha nge for a self-regulati self-regulating ng response is the negative feedb feedback ack open loop time constant   (oτ ),  or time lag.

Alternatively, a tangent to the inflection point can be visualized on the trend. The intersections of the tangent with the initial and final values of the PV ma rk the end of the tot total al ti time me delay an d the open loop time con stant, respectively. The final change in the process variable in percent divided by the step change in the cont controll roller er ou tpu t in percent iiss the o pen loop steady state (s (sta tati tic) c) gain, mo re comm only kn ow n as the process gain even tho ug h it iiss the the prod uct of the  valve, process, and m easurem ent static  2-1 1 d Figure gains as shown by Equation 2 an  2-18. Included in the process  2-18.

 

Chapter 2 - Setting the Foundation

47

gain for for tem peratu re an d co mp ositi osition on control loops is a fl flow-rat ow-ratio io gain tha t is the inv erse of the fe feed ed flow as sh ow n in Figure 2-18 for pH . All fl flow, ow, liquid pressure, most gas pressure, temperature, and composition loops hav e a self self-regul -regulati ating ng response. For an integrating or a runaw ay response, the process variable does no t line out at a new steady state, bu t continues to ram p o r accel accelerat erate, e, respec tively, tively, un til a physical limit is hit. For an integra ting re spo nse, there is an integrator gain that is the ram p rate iinn percent per m inute d ivided b y the step change in controll controller er ou tpu t in percent, per Equation  2-2. I  Itt can b e approx imated as a large open loop time constant and gain. In ffac act, t, pro  cesses cess es with a large time constant look li like ke a ram p in the control region a nd are called called pseudo -integrators. For a runa w ay process, there is a positive feedback fee dback tim e constant that is the time, aaft fter er the tim e delay and any nega tive feedba feedback ck time constant, ffor or the op en loop resp onse to reach 172% of the self-regulat self-regulating ing response as show n in Figure 22-17 -17.. The ope n loop respon se ooff each maj major or co m ponen t of the plan t (control valve, process, and m easurem ent) in Figure 22--19 has a fir first-or st-order-plusder-plusdead -time ap proxim ation. The contr controller oller al also so contributes a ttime ime delay from the scan time and time constants from the signal filters. Material and energy balances are used in the Theory secti section on to show origin ooff the pro  cesss time ces time constants and gains and how they change with operating cond i tions. Equations in the Theory section al also so estimate the dea d time from from mixing, transportation delay, and valve dead band.

Figure 2-17. First Order-plus-Dead Time Approximation for an Open Loop Response

 

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Figure 2-18. Steady-State (Static) Gains for a pH Loop

F i g u r e 2 - 1 9 .   Block Diagram for a First Order-plus-Dead Time Approximation quations

 

Chapter 2

Setting the Foundation

49

where: K o  K i  K m v  IC pv   K cv   τ o 

= = = = = =

open loop gain ( / ) integrator gain  ( /minute/ ) steady-state gain of the ma nipu lated variable (val (valve ve gain) steady -state gain of the proce ss variab le (process gain) steady-state gain of controll controlled ed variable (measu remen t gain) open loop time constant (minutes)

The total loop loop time delay (dead time) is the m ost imp ortan t of the th ree key variables that describe the open loop respon se of a contr control ol loop. It delay s the ability ooff a controller to see or react to a disturb anc e. The minimum peak error is the maximum excursion of the process variable du ring this ti time me del delay. ay. The oscil oscillat lation ion period is also proportion al to the time delay. delay. Thu s the integrated error is propo rtional to the time d elay squared for self-regulating processes with a large time constant that limits the excursion within the time delay to less than the fu full ll change of the process variable. The equations to app roxim ate these relationships are develop ed in the Theory sect section. ion. Perfect con trol is theoreticall Perfect theoreticallyy p ossible iiff the total tim e delay is zero an d there is just a single proces processs time constant. How ever, in indus trial pro cesses cess es the total loop time delay is never negligible because even if the pro  cess time delay is negligibl negligible, e, the ad dition of a me asurem ent, valve, an d digital controller ad ds tim e del delay. ay. For fflo low, w, pressu re, and level control, mo st ooff the ti time me delay in a contr control ol lloop oop comes fr from om the autom ation system [2.1]. W hile the tim e delay c anno t be zero, the objecti objective, ve, partic ularly for for loops w ith operating poin t nonlinearities, like like the pH a nd co lumn temp er ature control examples, iiss to reduce the total time delay. This   s because the extent of of the excursi excursion on on the tit titration ration curve or tray tem pera ture c urve, an d hen ce the ef efffec ectt of the nonlinearity, is decreas ed by a red uctio n in loo p dead time. Pure de ad times ccome ome fr from om transpo rtation delay s (pipes, sam ple lines, lines, static mixers,  coils, jackets, conv eyors, sh eet lines, and texti textile le fi fiber ber lines), valves (prestroke (prestroke dead time, dea d ban d an d stict stiction), ion), and any thing digital or with a cycle cycle ttime ime (micropr (microprocessor ocessorss and analyzers). Equivalent dea d time comes from from time constants in ser series ies ffro rom m instrum entation (sens (sensor or time lags, thermow ell ti time me lags, and transm itter ffil ilte terr times and dam pen  ing adjustments), ana log in pu t cards, (analog fi filt lters ers), ), and process v ariable fi filt lter er tim es (digital fil filter ters). s). The exact value s are not im po rtan t, just the rel ative sizes. sizes. The engineer or technici technician an sh ould wo rk on the largest largest,, mo st cost-ef cost -effec tive ve the sources of deengineer ad time who [i [itt isisnores t just th e jjob obfo of the ipm con ent trol engi neer bufecti t also process responsible ponsible for r equ design a nd the sel selecti ection on of instrum ents for small autom ation project projectss and

 

50

Advanced Control Unle Unleashed ashed

the technician technician wh o oft often en d etermine s the sensor llocation ocation and loop scan times.  Some plants do n't h ave a control engineer]. A tim e cons tant is benefici beneficial, al, iiff it is located in the pro cess afte afterr the en try poin t ooff load up sets, because it sl slows ows d ow n the excursion of the process variable from from the disturba nce. IIff a time constan t is betw een the control controller ler ou tpu t and the entry point of the load upset, iitt slows dow n the correcti correction on for the disturbance from the controller. If a time constant is between the process output and the controller input, it affects the recognition of the disturbanc e. In the bl block ock diagram in Figure 2-19, the o nly benefi beneficial cial tim e constant is the second process time con stant   (τp2 p2 ). The largest time time constant does no t have to be in the process. For processes no t dom inated b y time dela delay, y, an increas increasee in tthe he time constant, no ma tter where it appears in the  loop, will allow allow an increase in controll controller er g ain, even tho ug h th e fi final nal ef effe fect ct is no t benef beneficia icial. l. For exam ple, a large tim e co nstan t in the me asurem ent, such as a lar large ge thermo well lag or process variable fi fil l ter time setting, will allow allow a higher control controller ler gain and give the illusion ooff better control because the controlled variable is an a ttenua ted version of the real process variab le. Equation 3-2 can be u sed to estima te the effe effect ct.. All time constants mu ch sm all aller er than the largest time constant are con verted to equivalent dead time. W hil hilee tthe he fract fraction ion con verted to dea d time de pen ds u po n the relati relative ve size ooff the small to the large time constant, very small time time constants can be sum m ed u p as ttotall otallyy converted to dea d time bec aus e it is dif diffi ficul cultt to ffind ind a nd e stima te all the sm all ttime ime co nstan ts an d sources of of de ad time. Thus, the total ti time me de lay for a control loop is the sum of all the pu re time delays an d the sm all ti time me con stants. Dead time com pensators and mo del predictive controll controllers ers can account ffor or the eff effect ect of time delay on the respon se to changes in the control controller ler outp ut, bu t th e min imum peak error or unmea sured upsets and initi tial all time delay before the sta rt oof f the ffor set poin t resp onsload e is stil still l fixe fixed d b ythe theini tota delay. The op en loop time time co nstant is approxim ately the largest ooff the time con stants plu s the portion of all of the small time constants not conv erted to time delay. If If each of the sma ll time consta nts is less tha n   10 of the larg est time time con stant, so that each is ess essenti entiall allyy co nverted to equivalent ti time me delay, the largest ttime ime constant can be considered to be the open loop time constant. The purp ose here is to show the relati relative ve sizes and sources of time delay. There are too man y un kn ow ns to cal calculate culate an exact value. In industry, the open loop time constant, ttotal otal time time delay, and ope n loop gain can not be accuratel accuratelyy calculated, except possibly for fflo low w an d level level,, and m ust obtained by p lant tests. tests.

 

Chapter  2 - Setti Setting ng the Foundat Foundation ion

51

The ope n loop gain (sens (sensit itivi ivity) ty) can be too low or too h igh. IIff the v alve gain is ttoo oo low (valve sensitivit sensitivityy is too low ), the con troller has little effe effect ct on the process and the controlled controlled variable will w an de r an d be at the m ercy of upse ts  [2.5]. If  If the process o r m easurem ent gain (sens (sensit itivi ivity) ty) is too lo low, w, the con trol trolled led v ariable is not representative of the p rocess performan ce. IIff the v alve gain (sensit (sensitivit ivity) y) is too hig h, th e ef effe fect ct of stick-slip is excessi excessive ve and just the act of of puttin g a controll controller er in autom atic can cause unac ceptable oscillations. oscillati ons. IIff the process or me asu rem ent gain (sensitivity) iiss too h igh , the nea rly ful fulll-scal scalee oscillat oscillations ions will scare m ost pe op le even if the a ctua l performance of th e process is accepta acceptable. ble. The cl class assic ic examp le of this pro b lem is the the p H control ooff a strong acid and base system w ith a stati staticc mixer. Try Try explai explaining ning to the operator tha t the actual change in hydro gen ion con centration is tiny for for a system that oscill oscillates ates betw een 2 and   12 pH . The total loop time dela delay, y, open loop time constant, and ope n loop ga in are rarely constant bu t rather a fu functi nction on of ope rating cond itions. For examp le, the process gain for for comp ositi osition on a nd tem peratu re control iiss inversely pro portional to feed feed ra te, and the process time constant is iinversely nversely p ropo r tional to fee feedd fl flow ow fo forr back-mixed v olum es, wherea s the time de lay is inversely proportional to feed flow for plug flow. The theory section show s the source of these process nonlinearit nonlinearities. ies. PID Controller Tuning

Prop ortional-Integral-D erivative (PI (PID) D) controller controllerss in the basic proc esscontrol system m ust be tune d for bo th servo response (set poin t changes) and regulation (l (load oad rej reject ection ion). ). W hil hilee eit either her of these may be m ore imp or tant than the other to a given applicati application, on, loop tunin g me tho ds that foc focus us on o ne to the exclus exclusion ion of tthe he other are not seeing the w hole p icture. Increasi Incr easingly, ngly, the control loop set point is changed b y either a m aster loop for cascade control or a model predictive controller for advanced control, or by a unit operation for an automated startup sequence, product transi tion, and batch o peration. Even ffor or those loops who se set poin t is not changed fr from om a remo te source, the loc local al set point is a han dle u sed for for star t u p , sweet spots, and to rreli elieve eve bored om . Opera tors are noto rious for for mo ving set points d espite clai claims ms to the contrar contrary. y. Plus, the op erator h as to start up the unit, wh ich m ay consist ooff a seri series es ooff set poin t changes a s he walks the unit u p to operati operating ng con dit ditions. ions. On the other han d, iiff ther theree were no proces processs upsets, you w ould n't need a controller control ler:: Yo Youu could fi find nd and m anually set an ou tpu t to a fin final al elem ent that w ou ld be goo d inde indefi finit nitel ely. y. The traditional performance index has been integrated a bsolute error  that i  iss , the integral integral ooff the absolute error between the set point a nd (IAE), that (IAE),

 

52

Advance d Control Unleashed

co n tr o lled v ar iab le. Th e in teg r a ted er r o r ( Ei) can b e es timated f r o m th e tu n in g s ettin g s an d is eq u iv alen t to th e IA IA E if if th e r es p o n s e is n o t o s ci cill lla a tory. Of increasing importance is the settling time (T s ), w hi ch is the tim e it takes for for a loop to stay w ithi n a specifi specified ed b an d ar ou nd the set po int aft after er a s et p o in t ch a n g e o r lo ad u p s et to d etect s u s tain ed o s cill cillatio atio n s . S in ce p r o  ces s es an d e q u i p m en t h av e limits th at can tr ig g er in in ter lo ck s , v io late en v i ronmental constraints, or initiate side reactions, the overshoot (E 1 ) for set p o i n t ch an g es an d th e p e ak er r o r fo fo r lo ad u p s ets ( E x ) ar e als o im p o r ta n t. Th e d ecay r atio is th e am p litu d e o f th e fir f irss t p ea k   ( E 1   o r Ex ) d iv id ed b y th e s eco n d p eak ( E 2 ). Finally, th e rise ti m e (T r) (time it takes th e co ntrolled variable to first reach a specified band around the set point) is important f or or r ed u cin g b atch cy cle, cle, s tar t u p , an d tr an s itio n time an d d ecr eas in g th e o p en lo o p r es p o n s e time ( T 98 ) (time (time to reach 98 % of the fi final nal value ) for m as ter a n d m o d e l p r ed ictiv e co n tr o ller s. s . F ig u r es 2 -2 -2 00aa an d 2 -2 - 2 0 b s h o w th e clo s ed - lo o p p er f o r man c e in d ices fo fo r a s et p o in t ch an g e an d a lo ad u p s et , r es p ectiv e ctively ely . I n th e Th eo r y s ectio n , eq u a tio n s ar e d e v elo p ed to es tim ate E i a n d E x   for load upsets.

Figure 2-20a. Closed Loop Performance Indices for a 10% Setpoint Change

T h e  proportional mode  p r o v id es a co n tr ib u tio n to th th e co n tr o lller ler o u t p u t th at is the pr od uc t of the controller ga in (K c ) an d th e control error, w hic h is the d iff iff eren eren ce b etw een th e s et p o i n t an d th e co n tr o lled v ar iab le. I n F ig u r e 2 2 1 ,  the contro ller is is no t actually con nected to a proce ss so the controller r es p o n s e can b e is o lated f ro ro m th e p r o ces s r es p o n s e. F ig u r e 2 -2 -2 1 s h o w s th at th e r es p o n s e o f th e p r o p o r tio n al mo d e to a s tep ch an g e in th e meas u r e-

 

Chapter 2 - Setting the Foundation

53

Figure 2-20b. Closed Loop Performance Indices for a 40% Load Upset

m en t iiss a step s tep ch an g e in th e co n tr o ll ller er o u tp u t. C o n s eq u en tly , an y ab r u p t ch an g e in th e er r o r w ill b e p as s ed o n to th e o u tp u t. F o r a p er s is ten t co n tr o l error, the proportional mode does nothing more after the initial reaction. The proportional mo de prom otes an imm ediate response, but will not elim inate a n offs offset et.. Th e size of the offs offset et is inverse ly prop ort ion al to con troller gain , ssoo for for high g ains the offs offset et is sm all. Ho we ver, dig ital disp lay s p r o mo te u n r ealis tic ex p ectatio n s an d s o me o p er ato r s w ill g et co n cer n ed ove r insignificant persi sten t errors and ask fo forr the contro ller to to be tu ne d to get rid of of an offs offset et.. Th e direction of the cha nge in controller ou tp ut d e p e n d s u p o n t h e s i g n o f t h e c h a n g e i n er e r ro ro r , s o t h e p r o p o r t i o n a l m o d e has some sense of direction of approach to set point. An increase in the controller gain will greatly reduce the peak error, the r ise ise time, an d th e r etu r n time, b u t ma y in cr eas e tthh e o v er s h o o t an d th e s et tling time. High controller gains will amplify noise, increase interaction, an d p as s o n mo r e v ar iab ility ility fr fr o m th e co n tr o lled v ar iab ility ility to th e m an ip u  lated variab le. Figure s 2-22 2-22aa an d 2-22b show the effe effect ct of the c ontroller gain setting on the response of an effectively proportional-only controller to a lo ad u p s et an d a s et p o in t ch an g e, fo fo r a p r o ces s w ith a time co n s tan t tha t is 5 tim es larger th an the tim e delay. A n increase in contro ller g ain d r am atically s p e ed s u p th e in itial itial r ate o f ap p r o ach to s et p o in t b y o v er  driv ing the controller ou tpu t. It will also start to ba ck of offf th e co ntroller o u tp u t f ro ro m th e o u tp u t limit as so s o o n as th e co n tr o lled lled v ar iab le c o m es w ith in th e p r o p o r tio n al b an d , w h ic h is is th e p er cen t ch an g e in th e co n tr o l er r o r ( d if if fer fer en en ce b etw een th e meas u r em en t a n d s et p o in t) n eces s ar y to

 

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Advanced Control Unlea Unleashed shed

Figure   2-21.  Contribution of the Proportional,  Integral, and Derivative Modes

cause a full-scale change (100%) in the controller output. For a propor tio n al- o n ly co n tr o ller ller w ith zer o b ias an d a 5 0 % s et p o in t, th e p r o p o r tio n al b a n d is cen ter ed o n th e s et p o in t. H ig h co n tr o ller g ain s b y th em s elv es d o not cause a classic overshoot but instead an oscillatory approach to set p o in t. I t is h ig h co n tr o ller ller g ain co m b in ed w ith in teg r al actio n th at cau s es o v er s h o o t.

Figure 2-22a. Response of a Proportional-only Controller to a 10%  Se t point Change

 

Chapter  Chapter   2 - Setting the Foundation

55

Figure 2-2 2b. Response of a Proportional-only Controller to a 40% Load Upset

T h e  integral m ode s  contrib ution to controller ou tpu t is the produ ct of the controller gain (K e ), the in vers e ooff the integra l tim e (T i), an d an integra l of the control error. error. Figure 2-2 2-211 show s that the respon se of the integral m od e to a step change in the measurement is a ramp. The time it takes to repeat the contribution of the proportional mode is the integral time in minutes or seconds per repeat. The integral mode is not satisfied with any offset. It provides a gradual but driving action. The direction of the change in con troller output depends solely upon the sign of the error, so the integral mode has no sense of direction or rate of approach to set point. A decrease in integral (reset) time, which is an increase in reset action, has a negligible effect on the peak error for step load upset and the initial approach to set point, but will decrease the rise or return time. Unfortu nately, reset action increases the overshoot, the period of oscillation, and settling time, unless th e controller controller gain is reduc ed or deriva tive (rate) action is a dd ed . Figures 2-23a 2-23a and 2-23b show the ef effe fect ct of the integra l tim e setti ng on the resp on se ooff an effect effectively ively int egra l-on ly co ntrolle r to a load up set and a set set poin t chan ge, ffor or a process with a time constan t that is 5 times larger than the time delay. The controller output gradually approaches the output limit and does not reverse direction until well after the controlled variable has crossed the set point despite anti-reset windup protection. The integral mode does not amplify noise or increase interac tion but will increase the response tim e ooff a master or m odel predic tive controller.

 

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Advanced Control Unleashed Unleashed

Figure 2-23a. Response of an Integral-only Controller to a 10% Set point

Change

Figure 2-23b. Respo nse of an Integral-only Controller to to a 40% Load Upset

T h e   derivative mode derivative  mode s  co n tr ib u tio n to co n ttrr o ller o u tp u t is th e p r o d u c t ooff th e controller gain (K c ), the de riva tive (ra (rate) te) time (T d ) , an d th e r ate of of ch a n g e of the controlled variable. Figure 2-21 shows that the response of the deriv ativ e m o d e to a s tep ch an g e iinn th e m eas u r em en t is a b u m p in s tead o f a

 

Chapter 2 - Setting the Foundation

57

spike becau se ooff a bu ilt-in ffil ilter ter wh ose tim e constan t is typically 1 /8 of the derivative time. The contribution w ill decay to zero since the offs offset et h as a slope of zero. of  zero. The  The directi direction on of the change in cont controll roller er ou tpu t de pe nd s up on the sign of of the chang e or the accelerati acceleration on of the error error,, so tthe he prop or tional mod e has a defini definite te sense of dir direction ection and rate ooff appro ach to set point. The PID algorithm algorithm can have th e derivative action on eit either her the co ntrol error or on the the control controlled led variable. The lat latter ter metho d w as deve loped to reduce the bumps to the controller output from rapid set point changes m ad e by the operator. Unfortunately, Unfortunately, fo forr set poin t changes m ad e by sequences, cascade, and advan ced control system s, the derivative m ode wo rks against tthe he change requested because iitt onl onlyy know s that the m ea surement is starting starting to move a nd that any m ovem ent is undesirable. Fo Forr this reason, derivative action action on control error is is pref preferred erred so that deriva tive action action is benef benefic icial ial iiff the loop is dom inated by a time constan t an d set po int velocit velocityy limits are readily availabl available. e. An increase in derivative time will decrease decrease the peak error and re duce overshoot an d the period of os oscil cil Too o mu ch de rivative action lation.  To action can incre increase ase the ri rise, se, or return, time and the settling settling time. The derivative mo de can respect respectively ively speed u p or slow do w n the ini initi tial al approach for a set poin t change by acting on th e control error or the control controlled led variable. Figures  2 24a an  a n d 2 24b sho w th e eeff ffec ectt of the der ivativ e time se tting on the response of a propo rtional-plus-derivative control controller ler to a load u pse t and a set poin t change, ffor or a process with a time constant tha t is 4 tim es larger than the tim e del delay. ay. The derivative m ode is even m ore like likely ly tha n the gain m od e to amplif amplifyy noise, increase increase interaction, interaction, and tra transfer nsfer variabil ity to the m anipu lated variable. IItt ccan an decrease or incr increase ease the resp onse time of of master or mo del predictive predictive controll controllers ers depe nd ing o n how it is used. It rup It should be on dea d orks tim e-dom inant systemswith or any system w ith ab t or not errat erratic ic used changes. IItt w best on processes large tim e constants, low noise, and g ood m easurem ent repeatabili repeatability ty and resolution so that the respon se of the contr controlled olled variable is smooth. Flow and liquid pre ssure loops norm ally use proportional-integral (PI) (PI) controllers controll ers because the loops response is too fa fast st and abru pt for derivative action. acti on. Leve Levell and gas pressure loops can use a high control controller ler gain fo forr tight control.. In mo st control st cases,  the up per li limit mit on contr controll oller er gain depe nds u po n the  cases, the degree of of measurem ent noise and on h ow m uch variabi variabili lity ty should be passed on to the m anipu lated flow. Derivative act action ion is und esirable except for for those un usu al level loops with large time lags because the com bination of a high gain and derivative action greatly amplifies noise. For level con trol of surge volum deri derivative vative action ou ld to betthe couma nterprod uctive as iitt w ould pass on m orees,vari variabili ability ty fr from om thewllevel evel he nipu lated flow. flow.

 

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Advanced Control Control Unleashed

Figure 2-24a. Response of a Proportional-plus-Derivative Controller to a 10% Set point Change

Figure 2-24b. Response of a Proportional-plus-Derivative Controller to a 40% Load Upset

T e m p e r a t u r e a n d c o m p o s i t iioo n l o o p s a r e t h e p r i m e c a n d i d a t e s f oorr P I D c o n  trollers.

 

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Chapter 2 - Setting the Foundation

59

Nea rly all tem pera ture controllers shou ld use PID controllers controllers because the derivative derivat ive m ode prov ides a phase lead that com pensates for for the phase lag of the therma l lags in the thermow ell and proce ss. W herever the p rocess variable can accelerate, accelerate, wh ether du e to positive fee feedback dback or non lineari linearity, ty, derivative is helpful helpful sinc sincee it rreacts eacts to a change in the ra te change . In In th e distillation-column distillat ion-column exam ple, the control point is on th e kn ee of a plot ooff tray temperature versus distillate-to-feed ratio. An increase in feed can cause the dro p in tem peratu re to accelerate accelerate on the steep slope. How ever, if therm ocou ple or RT RTD D inpu t cards are are used instead of a sm art transm itter, itter, the steps from from hitting the resolution li limit mit seriously seriously redu ce the am oun t ooff derivative action that can be used . A tem porary fix is to add a fi filt lter er that sm ooths o ut the steps. IIff there is an inverse respon se, where th e initi initial al response is opposite to the fin final al response, the derivative mo de cann ot be used . This can occur iinn fur furnace nace tem peratu re control where the controller controller ou tpu t is the fi firi ring ng d em and that wo rks within a cross limit limit to m ake air lead fuel on a load increase. Composition control loops on back-mixed volumes should use PID con trollers troll ers because the derivative mo de prov ides anticipatory action imp or tant for for the slow resp onse. IInn the pH contr control ol exam ple, derivative acti action on shou ld be used in the pH loop on the neutrali neutralizer zer bu t not on the stat static ic mixer. How ever, m any analyze rs have an erratic erratic response an d analyz ers w ith an an alysis ccycl yclee ttime, ime, such as chrom atograp hs, have stai staircase rcase responses tha t preclude th e use ooff derivative action. For tight tight control ooff processes w ith a time constant m uch larger than the dead time, tthere here is benef benefit it in aggressive aggressive preem ptive an d anticipatory action. Th us, tthese hese process processes es should maxim ize the gain and derivative set ti ting ng and overdrive the output to reduce the ri rise se and retu rn time. rn  time. The  The inte gral tim e is increased (reset (reset action decreased ) since it ha s no sen se ooff direction and increases overshoot. If the controller gain is larger than 5, there is enough muscle from the proportional mode, and the derivative setting can be sm all or zero. Derivative m od e is a necessi necessity ty regardless ooff gain se tting fo forr a process wh ose con trol variab le can si significant gnificantly ly acceler acceler ate,  wh ether d ue to a nonlinearity (pseu do-runa wa y) or positive feedbac feedbackk (real (real run aw ay), such as a polymerization reactor or a fermentor in the expon ential grow th p hase . For For these processes processes,, the the integral time setting is increased to abo ut  10 times  times the ultim ate period so that gain an d rate action dom inate the response. Conversely, ffor Conversely, or tight control ooff processes with a time delay m uch larger than the larges largestt ti time me constant, gain and rate action action m ust be m inimized and integral (re (reset set)) action action m aximized, w hich m eans th e integral (rese (reset) t) time must be minimized. The integra integrall mod e ad ds smo othness not inherent

 

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Advanced Control Unleashed

in the process. The integral time factor is is decre ased an d can be as sm all as 1 /8 o f th e u ltim ate p er io d f o r a p u r e d ea d tim e p r o ces s . Figu res 2-2 2-25a 5a thro ug h 22-25 25ff sh ow the ef effe fect ct ooff the three m od e setting s on a PID controller for for a proc ess with a tim e lag 4 tim es larger than the lo op time d elay elay . N o te th at a g ain s ettin g to o lar g e in cr eas es o v er s h o o t d u e to the pre sence of reset acti action. on. A n integra l (rese (reset) t) time setting th at is too small causes a greater overshoot and increases the period of oscillation. To o m u ch d er iv ativ e acti actioo n cau s es an o s cill cillato ato r y r es p o n s e an d a s h o r ter period but no real overshoot. Figures 2-26a through 2-26d show the effect of two mode settings on a PID controller for a process with a time lag 4 times s m aller th an th e lo o p tim e d eelay lay . Th e d o u b l in g o f th e g ain s ettin g is much more disruptive.

Figure 2-25a. Effect of Gain on Set point Response of PID for a Large Time Lag-to-Time Delay Ratio

A s tar tin g p o in t is n ee d ed o n tu n in g s ettin g s , p ar ticu lar ly f o r n e w p lan ts o r ex is tin g p lan ts w ith u p g r ad ed in s tr u men tatio n an d co n tr o l v alv es . I n Tab le le 22-- 5 , th e f iirr st st n u m b e r o u ts id e th e p ar en th es es p r o v i d es a tu n in g , s can time, an d f ilt ilter er time s ettin g s u itab le ffoo r a d o w n lo ad . W ith iinn th e p ar en th e ses is a rang e of typical se ttings   [2.5].  A n au to tu n er s h o u ld b e r u n as s o o n as th e p lan t is u p to n o r m al o p e r atin g co n d itio n s . IIff th e cal calcu cu lated s ettin g s ar e o u ts id e th e r an g e n o ted in Tab llee 2 - 55,, th e au t o tu n e r tes t s h o u ld b e r u n aga the still ou atsid thewrang e,eitmeas m ea unsr emen eithetr othr at u n u in. s u alIIff lo o presults o r th atare th sti er ell is p r oe bof lem ith th coitn triso an l

 

Chapter  - Setting the Foundation Foundation

Figure 2-25b. Effect Figure Effect of Gain on on 40 to-Time Delay Ratio

61

Load Upset to PID for for a Large Large Time La g-

Figure  2-25c. Effect of Reset on Set point Response of PID for a Large Time Lag-to-Time Delay Ratio

v alv e. Wh e n ev er a lo o p is co m mis s io n e d o r tu n ed , iitt s h o u ld b e clo s el elyy w a tch ed f o r s ev er al d a y s f oorr a v ar iety ooff o p er atin g co n d itio n s to m ak e s u r e th e lo o p is s tab le an d th e p er f o r ma n ce is accep tab le.

 

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Advance d Control Unleashed

Fig u r e 2 - 2 5 d . E f f e c t o f R e s e t o n 4 0 to-Time Delay Ratio

L o a d U p s e t t o P IID D f o r a L a r g e Tim e L a g -

Fig u r e 2 - 2 5 e . E f f e c t o f R a t e o n S e t p o in t   R e s p o n s e  o f P I D f o r a L a r g e Tim e Lag-to-Time Delay Ratio

Some loops will fai aill du ring a n auto tuner pretest because the loo p response is too small or too llarge arge within the allowable tim e fr frame ame of th e pretest, or because the valve ha s too mu ch stick-sl stick-slip. ip. Also, ffas astt run aw ay

 

Chapter 2 - Setting the Foundation

Figure  2 25f.  Effect Effect of Rate on 40 Tim e D e la y R a t io

Load Upset to PID for a Large Time  L a g - t o -

Fig u r e 2 - 2 6 a . E f f e c t o f G a in o n S e t p o in t R e s p o n s e o f P I D   f o  r a S m a ll Tim e Lag-to-Time Delay Ratio

and integrating loops cannot be saf safel elyy taken ou t ooff the auto m od e. For these loops, a closed-loop tuning method is best because it is the fastest method for a large time constant, it keeps the controller in service with

 

63

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Advanced Control Unleashed

Figure 2-26b. Effect Effect of Gain on a 20 to-Time Delay Ratio

Load Upset to PID PID for a Sm all Ti Time me L a g -

Figure 2-26c. Effect of Reset on Set point Response of PID for a Small Time Lag-to-Time Delay Ratio

maximum gain, (safest for processes that can get into trouble quickly or ha ve a non-self-regulating respon se), and it includ es the ef efffec ectt of poo r valve response in the the tunin g   [2.29].

 

Chapter 2 Chapter  2  - Setting Setting the Foundation Foundation   

Figure 2-26d . Effect of Res et on a 20 L a g - t o - T i m e D e l a y Ra t i o

Load Upset to to PID for a Sm all Time

Table 2-5. Typical Tuning Settings Settings [2.5] A pplic a tion tion Type

Re s e t seconds) 6 1-12)

Ra te seconds) 0 0-2)  

Me th od

Li qu id Fl ow /Pr ess

Sc a n G a in seconds) 1 0.2-2) 0.2-2) 0.3 0. 0.2-0 2-0.8) .8)

Ti gh t Li qu id Lev el

5 1. 1.00-30) 30)

600 120120-6000) 6000)

0 0-60)

CLM

G as Pr es su re psig )

0.2 0.02-1) 0.02-1) 5.0 0.5-20) 0.5-20)

300 60-6 60-600) 00)

3 0-30)

CLM

Re acto r pH

2 1.0-5 1.0-5))

  120 6060-600) 600)

30 6-30 )

SC SCM M

N eu tra liz er pH

2 1.0-5)

0.1 0.1 0.0010.001-10) 10) 300 60-6 60-600) 00)

70 6-120) 6-120)

SCM

Inline pH  

1  0.2-2)

0. 0.22 0.1-0.3) 0.1-0.3)

30 15-60)

0 0-3) 

λ

R eac to r T em pe ra tu re

5 2.0-1 2.0-15) 5)

5.0 5.0 1.0-1 1.0-15) 5)

300 300 300-3 -3000 000))

70 3030-300) 300)

CLM

Inl ine T em pe ra tu re

2 1.0-5) 1.0-5)

0.5 0.5 0.20.2-2. 2.0) 0)

60 1212-120) 120)

12 12-40) 12-40) 

λ

0.5 0.1 0.1-10 -10))

300 300300-3000) 3000) 70 30-600) 30-600)

 

C ol um n T em p er at u re 10 2.0-30) 2.0-30)

5.0 0.5-25)* 0.5-25)*

1.0 0.001-50)

λ

SCM

* An error square algorithm or gain scheduling should be used for gains < 5 Methods:   - Lambda, CLM - Closed-loop Closed-loop Method, SCM - Shortcut Shortcut Method

 

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Advanced Control Unleashed

Table 2-6. Table  2-6.   Closed-loop Tuning Method 1.  

Put the controller controller in automa tic at its its normal set point. IfIf it is important important not to make big changes in the manipulated variable (analog output), narrow the controller out put limits.

2.  

Decrease reset action (increase (increase reset time) by a factor of 10 10 ifif possible and trend record the process variable (PV) and controller output (CO).

3.

Add a PV filter to keep output fluctuations from noise within the dead band of the control valve. Set the scan time per Table 2-5 based on the loop type.

4 . 

Bump the set point in both directions and increase controller gain if necessary to get a slight oscillation. Note the time delay between the bump and first excursion out of the noise band.

5. 5.    Stop when loop has about  a  qu rter mplitude oscill tion or the gain has reached your com fort limit limit,, and note the the period. For gain settings greater than one, the oscill oscillation ation will be mo re recognizable in the controller output valve's good throttle rang e. (CO).  Make sure C O stays on scale within the valve's 6.

Reduc e the gain until the oscillation just disappears so that recovery is is as smoo th as desired and the overshoo t and settling time are acceptable.

7.

For a temperature loop with a smooth response (no chatter chatter,, inverse inverse response, square wa ve, or intera ction ), use rate action. If the gain is larger than 10, reset and rate action are not needed. If the m anipulated flow will upset other loops, decreas e the gain or use error squared control. If a high gain is use d, add set point ve locity limits, and configure the set point to track the PV in manual and remote output (ROUT) so the loop can restart.

8.

If rate is is used, add set point velocity limits, limits, set the rate rate equal to 1/10 1/10 of  period,   and make sure noise in the output does not cause the valve to dither. dither. Make a nother set point change and adjust the gain to get a smooth response.

9.

Use Equations 2-3 2-3 and 2-4 to to calculate calculate the integral (reset) time setting from the noted time delay and period of oscillation oscillation for quarter amplitude decay. After e ntering the integral time, make another set point change and make sure the rise time, over shoot, and settling time are acceptable.

10 .   If gains less than 5 are used for  m ake sure the integral (reset) time is greater  level, than 10 minutes, and add error squared control and/or a velocity limited feedfor ward as defined in Appendix B.

mits to their proper values values if narrowed for testing. 11.   Return the output lilimits

Set poin t change s are used because they are m ore li likely kely to cause an osci oscill lla a tion than a c hange in the controll controller er ou tput: A step ch ange in a set point is a step chang e in the error seen by the control controller ler,, whe reas a step ch ange in the controller controller outp ut is smoothed out by the time constants in the loop. After Aft er the n ew tunin g settings are entered, the loop sho uld b e checked fo forr a step change in the controller output and the load rejection capability of the new setti settings ngs m onitored a nd c om pared to hist historical orical data for the ol oldd set tings.