Surge Control 2

8/3/2019 Surge Control 2

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Dick Livingston, SeniorApplications Engineer, FisherContr ols Int erna tional, Inc.,Marsh alltown, Iowa

Continuing a dvances in surge-relief valve design for ga s-

tra nsm ission cent rifugal com-pressors raise the prospect th at awise valve choice can h elp engineer spay f or t he r e s t o f an advan cedsur ge-cont rol system without exceed-ing budget.

Avoiding and escaping surgeevents wi thout u pse t s or chr onicinefficiency is a major aspect of com-pressor-control system design, pro-curement and operation. However,most discussions of the su bject a relimited to t he contr ol stra tegy and

cont roller ha rdwa re. Too often, otherelement s of th e sur ge-cont rol systemar e taken for gran ted—especially therecycle or su rge-relief valve, whichis th e fina l cont rol element .

I ronical ly, for large gas - t ra ns-mission compressors, the valve is byfar the m ost expensive item in thesur ge-cont rol system , possibly cost-ing well into six figures. Historically,th ese valves ha ve been globe style,

often with n oise-att enua ting trim.Today, th ere a re h igh-performan cerotar y valves available.

Basic surge concepts. First , afresh look at basic concepts is inorder. The sur ge problem is inh erent

in dynamic compressors—centrifugalan d axial, as distinguish ed from pos-itive-displacement types. The u sua lexplan ation is based on a compres-sor’s p erforma nce cur ve of differen-tial pressur e head vs. inlet flow at afixed speed for the given gas com-position and inlet temperature.

Here, imagine a gas-turbine-pow-ered pipe line compr essor run ning ata speed which is contr olled to ma in-tain constan t downstream pressure.There is a differen t cur ve for ea ch

speed, altogether forming th e com-pressor ’s “wheel map.” However, speedchanges of such a hea vy rotat ing massrequire a substa ntial lag time whichma y be as long as several minu tes.Therefore, on a shorter term , speedmay be considered constan t. The headimposed on t he compr essor variesindependently as a result of rapidlychan ging downst ream demand, soth at flow is a function of head.

Suppose there is a steady stat e atPoint A and th ere comes a su ddendecrease in gas demand, experiencedby the compressor as a hea d increa se(Fig. 1). At consta nt speed, th e com-pressor cann ot keep up as mu ch flowagain st a h igher pressu re, so flowgoes down t owar d Po i n t B a sreflected in t he curve.

To visualize the physical mecha-nism a t t his point, imagine th e com-

pressor as a single centrifugal stage.The coupling of th e impeller to th eload, mea ning t he gas be ing com-pressed against t he pressure head,depends on ma intaining flow th roughthe impeller. As long as there is appr e-ciable forwar d flow, the impeller k eepsits grip on the gas, so to speak. That is,energy is transferred from the impellerto the ga s in t he form of increasedvelocity—which is converted to pres-sure head as t he gas slows down againin t he diffuser passa ge. This coupling

depends on maintaining some mini-ma l am ount of forwa rd flow.Now suppose downstr eam dema nd

is progressively reduced. The hea dincreases to the ma ximum a chievableby the compressor at tha t speed, shownas P oint B. This is called the su rgepoint. The locus of th e sur ge point forall compressor speeds, above and belowth e cha ra cteristic cur ve in quest ion, iscalled the surge line. The flow here has

PIPE LINE & GAS INDUSTRY / JULY 2000

Design innovations offered in

compressor surge-relief valvesExamining recycle-valve options may help achieve

a flexible control system without increasing costs

Fig. 1. Centrifugal-compressor performance curve showing key points in surge event and a simplified piping and instrument diagram of acompressor installation.

Reprinted from: July 2000 issue, pg. 35-39. Used with permission.



fallen to the least t hat the impeller canhandle without losing its grip on thegas. A furt her decrease in downstr eamdemand no longer results in increasedhead. Instead, the impeller suddenlyloses its effectiveness a s a motive ele-ment and becomes decoupled from theload. Perh aps in a s quickly as a frac-t ion of a second, gas wi th in theimpeller quits being impelled forward

and begins simply spinning ar oundwith t he impeller. The effect is like anautomobile tire skidding, oran a irplane wing stal l ing.The im peller is no longerfunctioning a s an impeller.As the pipe line gas ra pidlycoasts to a stop, the operat-ing point falls to zero flow atPoint C.

But i t doesn’t pa useth ere. Gas previously com-pressed in the volumebeh i nd t he downs t r eam

check va lve begins leak ingbackward through the inter-stices ar ound th e impeller—even through the impelleritself, becau se th e hea d itcan create by cent r i fugalforce alone is consider ablylower tha n it had bu ilt upby velocity. On t he gr ap h,th e beginn ing of this brief flow reversa l is shown a s acontinuation of the jump past PointC to Point D, lying on a th eoreticalextension of th e norma l cha ra cteris-t ic curve int o negat ive flow. Theextension is sh own a s a br oken line.

Then, within a short time—whichmay be a second or so—the head bleedsdown to the minimum tha t th e decou-pled impeller can ma inta in pur ely bycent rifugal force, at Point E . There,the impeller suddenly picks up t he loadagain, thr owing gas forwar d unt il theoperating point hits Point F on the nor-ma l characterist ic curve. If line flow isstill restricted below the surge pointfor t he given speed, the cycle repeats a t

regular intervals, typically on the orderof one second .

Preventing surge. Unrel ieved,repeated surging of a large compres-sor is a fear some thing which ends incatastrophic damage. Because of surge controls and emergency shut-down systems, few operators todayhave witnessed it. Even one cycle canhave undesirable consequences interms of upsetting operations, alter-ing internal clearances and over-

str essing seals. The cost of replacingseals in a 3,000-hp compressor caneasily exceed $20,000. It is far bet terto take action before the compressorreaches the sur ge point, becau se—bythen—nothing can pr event one cycle.Tha t pr eventive action is pr ovided bya dedicated su rge-control system, sep-ar at e from t he compr essor’s othercontrols because of response-speed

requirements (Fig. 2).Of course , t hose o ther cont ro ls

attempt to stay reasonably clear of th e surge line, but only on a r elativelylong t ime horizon. Sur ge events tendto be tra nsient distu rban ces. It wouldbe both im pra ctical an d inefficient t oslow and t hen a ccelerate a lar ge tur-b ine-dr iven compressor as everypotential sur ge comes and goes. If thecompr essor h as var iable inlet van es,the contr ol system typically man ipu-lat es th em t o opt imize fuel efficiency,not to dodge every threa t of a surge.

Inst ead, the a ction provided by th esur ge-cont rol system is t o open t herecycle valve to an appropriate degree.In th e past, when the most sophisti-

cated systems were based on an alogPI (proportional and integral action)ratio controllers, the dominant strat-egy was to begin cracking the recyclevalve well before t he opera ting p ointreached the sur ge line. This led to pro-longed per iods of operation with appr e-ciable recycling and attendant wasteof compressor fuel as well as capa city.

Today’s ph ilosophy is to avoid open -ing th e recycle valve unt il absolut elynecessary, move away from surge asquickly as possible, and then shut th e

valve again .The dedicated digital controller

which is th e hear t of an a dvancedsurge-control system, receives inputs:

• In le t pressure• Temper a t u re• F low• Head across the compressor• Compressor speed.If gas composition is su bject t o

appr eciable change, an indication of that variable, such as density, maybe included. Characteristiccurves for various temper -atu res—and, if app licable,various inlet pressures a ndcompositions—are a vailableto the control algorithm asequa tions or look-up t ables.

The adva nced cont roller’sreaction t ime ma y be wellun der 50 milliseconds. Thealgorithm att empts to dupli-cate th e action of an experi-

enced human opera torhaving an eye on the in-s t rumen t s , a han d on theman ual valve control and awish to minimize energywaste r esul t ing f rom pr o-longed recycling. Thisrequ ires a combina tion of closed-loop (feedba ck) a ndopen-loop (prear ra nged) con-tr ol. The system keeps the

recycle valve closed as long as the oper-a t ing poin t i s wel l away from th eknown sur ge line. As th e operat ingpoint a pproaches the sur ge line, thecont rol algorith m becomes more sen -sitive to its motion. Getting too closeor moving too suddenly toward t he linecauses the valve to open by an appro-priate am ount to halt progress towardsur ge smooth ly without t riggering con-trol oscillat ions. Should extraneous cir-cumsta nces cont inue t o force th e oper-ating point t oward t he sur ge line, therecycle valve opens a ll the way. If a loadreduction persists, then after a r ela-tively long time (perhaps a few min-

ut es), speed typically is redu ced by th ecompr essor cont rol system.

Consequent ly, un less th e r educedflow is beneat h t he capa bility of th ecompr essor, the operating point a ndth e sur ge point m ove away from eachother, an d th e surge cont roller gradu -ally shu ts t he r ecycle valve. The com-pressor has kept a firm grip on the gas.

Valve requirements. The main re-quirements of a compressor recyclevalve are listed in Table 1. These


Fig. 2. Simplified piping and instrument (P&I) diagram of compres-sor installation with surge-control system.



requirement s are also more than ade-qua te for the valve’s other employ-ment, which is unloading the com-pressor during start up and shu tdown.

Fir st and m ost obviously, the valvemust be able to stroke wide open veryquickly, typically within three secondsor less. The faster i t can open, themore likely tha t it will catch an d pre-vent an impending sur ge in time. For

a valve as big as 24-in. or more, th is isno sma ll feat . One implication is lowfriction in the valve seals and st em orsha ft. Another is plent y of force in th eactuat or—which is a ssumed to beeither a fail-open, sp ring-opposedpneum at ic diaphra gm, or a double-acting pneuma tic piston.

Requi red a c tua t or accessor iescould include a large, solenoid-oper-at ed vent valve to speed th e unload-ing of the “closing” side of the actu-a t o r a nd , i n t he ca se o f a p i s t onactu at or, a volum e-booster r elay on

the “opening” side. In a ddition, m anyuser s favor quick closing as well asqu i ck open i ng . Tha t r equ i r e s abooster on a spr ing-an d-diaph ragmactua tor or a s econd booster on th e“closing” side of a piston actuator.

Second an d equa lly obvious, t hevalve should h ave lar ge capacity. Insome cases, th e valve’s nomin al sizewill be as lar ge as th e pipe line itself.Some sur ge-cont rol expert s size th evalve for about t wice th e minimu mflow capacity required simply for sta-ble operat ion un der full recycle con-dition, as in sta rtup and shutdown.That minimum size typically corre-sponds to the flow coefficient for apoint on th e compr essor’s m aximumperform an ce cur ve which is com-forta bly distan t from th e sur ge line.Th i s i s b i g enough t o p r even t ara pidly impen ding sur ge when, forins tance , l ine f low i s suddenlyblocked ent i rely at the sa me t imeth at compr essor speed is falling. Itnot only can ha lt th e motion of theopera t ing poin t toward th e surge

line, but also move it quickly awa y.On t he other ha nd, if the valve’s

capacity is t oo lar ge, it will be suscep-tible to un sta ble control in th rottlingcondit ions. Fu rt hermore, it may allowth e compr essor t o reach th e choke orstonewall region when wide open, lead-ing to shu tdown and possible dam age.

A third r equirement is n oise reduc-t ion . Because of h igh f lowratesthr ough large pressur e drops—up t o500 psi—noise levels inside compres -sor bypass valves are potentially the

highest encoun tered by a cont rol valve.At its peak, when the valve is th rot-tling par t-way open to sta y away fromth e sur ge line, noise inten si ty canreach dam aging levels above 110 deci-bels unless attenu ation m easures areprovided. These provis ions mayinclude source treat ment, in terms of special valve featu res, and pat h tr eat-ment , such as a coust ic insulat ion an dsilencers. Alth ough insu lation h elpsreduce personnel noise exposure, itdoes not pr event acoustic dama ge to

the valve, pipe and compressor. Anappr opriate silencer (resembling anaut omobile exhaust muffler) is a large,expensive item. Therefore it is far bet-ter t o select a valve having inheren tlow-noise chara cteristics.

Finally, on top of all the foregoingrigorous r equirements, the recyclevalve must be capa ble of accur at ethrot t l ing cont ro l . In most surgeevents , the necessity for full opening isavoided by sophisticated controlleraction. Typically, th e valve is requ iredto str oke open quickly and a ccur at elyto a pos it i on t ha t w i ll a r r e s t t heimpending surge, then thr ottle grad-ua lly back towar d th e closed position.There a re few appl ica t ions wh ereth rottling control-valve accur acy andsta bility a re m ore cru cial. Positionoversh oot or delay in valve movementcan tip t he compressor over int o surge.This crit ical factor is often overlookedin valve selection an d is even more fre-quently not thoroughly understood.

The t hrottling contr ol valve mustbe viewed as an int egral assembly

compr ised of severa l component s.These component s ar e the va lve itself,the actuator, a positioner and (typi-cally) a t ransdu cer. These components,when pr operly matched and applied,should form a h igh-per formanceassem bly capa ble of providing th erequired accurate throttling control.

T h e b e s t m e a s u r e m e n t of t h evalve ass embly’s capa bilities is thechange in f low resul t ing f rom achange in inpu t s igna l . Cons iderth ree r otar y products (Valves A, B

and C) with spring-and-diaphragmactua t ors , pos it ioners a nd t r an s-ducers . Valves B and C do notrespond to input step chan ges of lessth an 2%, while Valve A responds t o0.5% changes in th e input signal.

A sur ge-cont rol system u sing avalve with the responsiveness of ValveA could move its opera ting point closerto th e sur ge line. This could ha ve sig-

nificant economic impact. Improvedsur ge protection, of which t he va lve isa critical component , has been r eportedto result in annual energy savings of more th an $250,000 for a 4,500-hpcompr essor. Of course, a ctua l rea lizedsavings depend on numerous var iablesinvolved in t he compr essor’s opera tion.

Anoth er crit ical factor in th rot-tling cont rol is th e valve cha ra cter-istic. Historically, it h as been gen er-a l l y a g r e e d t h a t l i n e a r i s t h ep r e f e r r ed cha r ac t e r i s t i c ve r susquick-opening or equal-percentage.

Today’s digita l contr ollers can ea s-ily compensa te for varia tions fromth e linear char acteristic.

Another cons idera t ion i s theamount of overall pressur e drop tha tis taken by the piping system r elativeto th e cont rol valve. If the piping sys-tem t akes t oo large a percenta ge of the pressur e drop (typically more than30%), the system characteristic canbecome qu ick open ing. A quick-open -ing cha ra cteristic can resu lt in poorth rottling cont rol. The goal is to havethe valve determine the system char-acter istic. The us e of some form of equa l-percent age cha racteristic, cou-pled with t he piping-system pressu redrop, results in a system t hat can pro-vide accurate throttling control.

Sliding stem, rotary. For years, th etypical r ecycle valve for a lar ge com-pressor wa s a cage-guided type withfabricated a ngle body which provideddesirable long plug t ravel. While well-suited for large flows and noise-at ten-ua ting feat ures, th ese valves in 24-in.

size stand 12 to 14 ft tall, and thean gle bodies can lea d to some convo-luted piping arr angements.

For a pplicat ions wit h sm aller flowrequirem ents , typically up to 16 in.,certain cage-guided globe bodies—cast r ath er tha n fabricated—haveproved effective. These valves can beversat ile since some can be left in th eline while servicing tr im or replacingit to change flow char acterist ic, capac-ity, or noise-reduction capability.

In today’s a pplications, n oise con-


Table 1. Typical requirements ofa surge-relief valve

1. Extremely fast stroking speed whenopening (typically under 3 seconds)

2. High capacity (double the minimum forstartup and shutdown)

3. Extreme noise abatement provisions(up to 30 decibels)

4. Very stable throttling control.



tr ol is the m ost cha llenging techn icalor design concern. Noise generation

in a cont rol valve depends on severa lvariables such a s inlet pressure, pres-sure drop, flowrat e, outlet pressureand pipe-wall attenu ation. It is h ighlysens itive to flow-pa th geomet ry.

Noise can be reduced by us ingnu merous sm all flow restr ict ionsrat her th an one or a few large ones.The prevailing approach in sliding-stem valves is to use a cage-guideddesign with n umer ous small holesdrilled th rough t he wa ll of the cylin-drical cage. At h igher pressu re dr ops,the holes are spaced fart her apa rt t oreduce interaction between jets fromadjacent holes. A baffle resemblingth e cage ma y be moun ted concentr i-cally around it t o weaken jet int erac-tions an d stage th e pressure drop. Inextrem e cases, a second concent ricperforated baffle may be used, pro-viding a t hird st age of pressu re dropthat helps achieve an overall noiseatt enua tion—as m uch as 30 decibels.

Noise-cont rol trim of th is genera ltype also meets th e other thr ee mainrequirements of a compressor recycle

valve. The cylindrical plug slidinginside the cage is pressure-balan cedby one or more passa ges next to thestem, so that relatively little actu a-tor force is requ ired t o overcome t hed i ff er en t i a l p r e s su r e ac r os s t hevalve. Throttling is a chieved by m ov-i ng t h e p l ug t o un cove r m or e o rfewer holes, an inherent ly stable pro-cess which is not susceptible to cha t-tering or buffeting. Fisher Cont rols

can addr ess the capa city challengeby special pat ter ns of hole spacing

and diameter s thr ough th e cage. Theresu l t i s a potent ia l r edu ct ion of requ ired body size.

While the cont rol-valve industry hasachieved stea dy improvement s in slid-ing-stem valves for compressor recycleapplications, these innovations havema inly consisted of incrementa l refine-ment s. Rotar y valves ha ve chan gedmore dramatically. Improvements inball and disk valve technologies overthe pa st 20 years h ave propelled thesedevices int o more applications. Unt ilrecently, difficult ies with noise aba te-ment a nd sta ble throttling have keptrotaries out of large, high-pressu re com-pressor recycle applications. However,a met hod now has been found to applyhigh-performance, full-bore ball va lvesin t his difficult service, represen ted byFisher ’s Design V260, first of its t ype(Fig. 3).

In essence, these new devices applyth e concept of th e specially drillednoise-abatement cage to a compact,heavy-dut y ball valve of proven qua l-ity an d performa nce. A perfora ted

dome, spann ing the ent ire valve pas-sage an d contoured to fit th e ball veryclosely, is mount ed aga inst the down-str eam s ide of th e ball. As th e ball isrotat ed through 90 degrees, from fullyclosed to fully open, its cylindrical pa s-sage un covers m ore a nd m ore of theholes in the dome. For compressorrecycle service, thes e openings ar es ized and spaced to provide thedesired s table flow chara cteristic, cou-

pled with much h igher capacity thansam e-size sliding-stem valves. Noise

redu ctions up to about 25 decibels areachievable by th is meth od. Str okingspeed is enhanced by low-friction sealsand shaft packing. Stable throttlingcont rol is furth er pr omoted by tightmechanical l inkages, includingsplined atta chm ent of the sha ft to theball and crank .

The fina l selection of a r ecyclevalve for a lar ge compressor depen dson man y de ta i l s of the par t i cu larapplication—especially th e line pr es-sure, the pr essure drop across thevalve an d th e capacity required. Asliding-stem type may be favored if the head is h igher, while a rotary typemay be m ore attr active if the capacityrequ iremen t is larger. The lowest bidmay not be the optimum choice dueto un cert ainties su ch as controllabil-ity and noise. Innovations make thedecision more challenging, but theresu lts, in term s of economy an d per-form an ce, ar e definitely improvingyear by year.

F/QTY/9-2000 Copyright © 2000 by Gulf Publishing Company. All rights reserved. Printed in USA.

D350790X012

The authorDick Livingston, senior applications engineer, is a graduate of Iowa State University and has worked for Fisher Controls in Mar- shalltown, Iowa, since 1974.Mr. Livingston has spent 19 years of his career with

Fisher specializing in the oil and gas industry, with particular emphasis and experience in pipe lines and production. He serves on the board of the Gas Processors Suppliers Association.

Fig. 3. A new design in large ball valves for surge relief, the Fisher Design V260, incorporates a noise-attenuating device for up to a 25- deci-bel reduction.

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