Description of Surge

7/30/2019 Description of Surge

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21

SECTION 3

descrition o urge

3-1 V all

urge is a ynami instability tat ours in ynami omressors. urge an alsoour in axial an entrifugal ums an blowers, but te ourrene is less fre-quent an te amage less severe. pum surge selom ours exet infrequentlyuring avitation an two-ase ow (ef. 15). Blower surge oes our but oes

not amage te blower internals unless te ressure rise exees 2 si an te sizeexees 150 Bhp (brake orseower) (ef. 22). urge oes not our in ositiveislaement omressors an ums.

tall is anoter instability tat ours in ynami omressors an is some-times onfuse wit surge. e onsequenes of stall are usually less severe. pumstall as ause extensive vibration in te iing of boiler feewater systems (ef.15). tall will be esribe in enoug etail to istinguis it from surge an tounerstan ow it an evelo into surge.

s te ow troug a omressor is reue, a oint is reae were te

ow attern beomes unstable. f te ow osillates in loalize regions arounte rotor, te instability is alle stall. For axial omressors tese regions ofunstable forwar ow an exten over just a few blaes or u to 180 egreesaroun te annulus in te omressor. n rotating stall, te region of unstableforwar ow rotates aroun te annulus. e average ow aross te annulus isstill ositive. e frequeny of te loalize ow osillations ranges from 50 to


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22 Centrifugal and Axial Compressor Control

100 ertz. e stall frequeny is some fration of te omressor see. e stallfrequeny oes not een on te esign of te iing system in wi te om-ressor is installe. tall an evelo into a more global tye of instability allesurge. n surge te average ow aross te annulus goes troug large amlitueosillations. e frequeny of tese osillations ranges from 0.5 to 10 ertz. esurge frequeny eens on te esigns of bot te omressor an iing sys-tem. e surge frequeny for most inustrial omressor installations is sligtlyless tan 1 ertz.

Figure 3-1 sows a compressormap for a variable-see entrifugal omres-sor. omressor ma is te single most imortant iee of information foresribing surge. omressor ma sows te omressor arateristi urvefor ifferent oerating onitions. a urve traes te rise in isarge ressureeveloe by te omressor as te sution ow is varie for a given oeratingonition (su as see). e oerating onition is not limite to tat sownby te few urves lotte but is ontinuously ajustable to intermeiate values.e Xaxis is almost always volumetri ow in afm at te state sution tem-erature, ressure, an moleular weigt. e Yaxis is eiter te isarge res-sure in sia, te ratio of isarge ressure to sution ressure (imensionless),or te ifferential ressure rise from sution to isarge in si. e omressorarateristi urves sulie by te omressor manufaturer over te negativesloe region an en were te sloe aroaes zero. ( negative sloe meanste ressure will ange in te oosite iretion for a ange in ow, an a zerosloe means te ressure will not ange at all for a ange in ow.) f a urve israwn troug te oint of zero sloe for ea arateristi urve, te region tote left of tis line is were te instabilities of surge an stall our. n te litera-ture, tis line is alle eiter te stall line or te surge urve. n tis text it will bealways alle te surge urve. e arateristi urve to te left of tis urve isifult to obtain, but its general sae resembles tat sown in Figure 3-1. enegative ow ortion of te urve an be foun by sulying ressurize gas tote omressor isarge to fore a steay negative ow. e ositive sloe or-tion of te urve is obtaine by smootly onneting a tir-orer olynomialurve between te negative sloe negative ow an te ositive ow ortions ofte urve.

urge an be better unerstoo by visualizing a blok or trottle valve losingownstream of te omressor. s tis valve loses, te sution ow ereases,an te oerating oint moves to te left along a arateristi urve. f te om-ressor see or te guie vane osition oes not ange, te oerating ointeventually asses by te oint of zero sloe on te arateristi urve. Just to


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Description of Surge 23

te left of tis oint, wi is te intersetion of te surge urve wit te ar-ateristi urve, te ressure eveloe by te omressor is less tan te res-sure in te iing between te omressor isarge an te losing valve. eforwar ow troug te omressor stos an reverses iretion. e gas owsfrom te isarge iing bak troug te omressor to te sution iing.e ressure in te isarge iing starts to ro. Wen tis ressure is belowte ressure eveloe by te omressor, forwar ow starts again. f te gasin te isarge iing is still trae by te losing blok or trottle valve, tegas ressure buils u an te surge yle is reeate. Note tat te alternatingressure builu an eay yle oes not require te ow to atually reverseiretion but only to alternately erease below or inrease above te ow ratetroug te blok or trottle valve. nly in severe surges oes te ow trougte omressor atually reverse iretion.

Figure 3-1 comressor Ma


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Weter a omressor system is stable or unstable eens on ow te oer-ating oint reats to a isturbane. f te oerating oint returns to its initial valueafter a isturbane, te system is stable. f te eviation of te oerating ointfrom its initial value grows or ontinually osillates after a isturbane, te systemis unstable. uniform growing eviation is lassie as a stati instability an anosillating eviation of growing or onstant amlitue is lassie as a ynamiinstability. urge is a ynami instability.

Key Concets

urge an stall our in ynami omressors. tall onsists of loalize ow osillations aroun te rotor. urge onsists of total ow osillations aroun te wole rotor. e surge urve is te zero sloe oints of te arateristi urves.

3-2 aC aBlY

tati instability is a uniform (non-osillatory) growing eviation. tati instabil-ity will our wen te sloe of te omressor arateristi urve is greater tante sloe of te eman loa urve.

s reviously mentione, te eman loa urve is a lot of ow te ressurero in te system (ue to iing, equiment, an valve resistane) inreases witow. e general riterion is:For stati instability,

Sc> S

l(3-1)

were:

Sc= sloe of te omressor arateristi urve (si/afm)

Sl= sloe of te eman loa urve (si/afm)

Figure 3-2 illustrates ow te riterion for stati instability an be eke byrawing te eman loa urve on a omressor ma. e omressor ma sowsstability at oerating oint 1 but stati instability at oerating oint 2 were teomressor urve sloe is steeer tan te loa urve sloe. ine te omressorurve an te loa urve are bot steay-state urves, tis instability riterion an


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be eke at one steay state or a suession of steay states witout knowlegeof te system ynamis; ene te name, stati instability. n eletrial iruitanalogy onsists of a voltage or urrent soure an a resistane. n tis analogy,voltage reresents ressure, urrent reresents ow, an eletrial resistane re-resents iing, equiment, an valve resistane. f te omressor arateristiurve is nearly orizontal, wi is tyial for entrifugal omressors, te iruitas a nonieal voltage soure. f te omressor arateristi urve is nearlyvertial, wi is tyial for axial omressors, te iruit as a nonieal urrentsoure. is iruit analogy as no ynami omonents su as inutors oraaitors so te urrent ow oes not vary wit time if te system is stable. eiruit evelos stati instability if te sloe of te soure urve is greater tan tesloe of te resistane urve for voltage versus urrent. f te sloe of te resis-tane is always ositive (voltage ro always inreases as urrent inreases) an

Figure 3-2 tati nstability criterion


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te sloe of te soure urve is always negative (voltage rise always ereases asurrent inreases), te iruit is stable. f te sloe of te soure urve is ositivean greater tan te sloe of te resistane urve, te following sequene of eventsours for any isturbane tat auses an inrease in voltage outut:

(1) nrease in soure voltage(2) nrease in voltage (riving fore) aross te resistane(3) nrease in urrent(4) nrease in soure voltage (sequene reeats)

e orresoning sequene for a omressor system is:

(1) nrease in omressor ressure

(2) nrease in ressure (riving fore) aross te loa(3) nrease in ow(4) nrease in omressor ressure (sequene reeats)

similar sequene an exonential growt of any ositive eviation oursin te oen-loo resonse of temerature for runaway exotermi reators an inte oen-loo resonse of ell onentration for runaway biologial reators (ef.21). n a omressor system, stati instability ours wen te loa urve is nearlyat an intersets te negative sloe ortion of te omressor urve. e loa

urve is nearly at if a iee of equiment wit ressure ontrol su as a reator,onensor, or absorber is onnete to te isarge of a omressor by a sortut or ie witout a trottle valve. Figure 3-2 illustrates tis ase. e ressureat te starting oint of te loa urve at zero ow is te stati ressure or set ointof te ressure ontrol loo for te ownstream equiment. e sloe of te loaurve is roortional to te ow multilie by twie te resistane of te ut orie. ine tis ut or ie is sort, its resistane is low, an te sloe of te loaurve is low at low ow.

Sl = 2 Kl Q (3-2)

were:

Kl= resistane of loa (si/(afm afm))

Q = volumetri ow at sution onitions (afm)

Sl= sloe of te eman loa urve (si/afm)


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f te ut or ie is long, te resistane oefient Klan still be small if te

ie or ut iameter is large wit reset to te ow aaity of an iniviualomressor. is situation ours wen multile arallel omressors are suly-ing a istribution eaer. e eaer iameter is selete base on te total owaaity of all te omressors. e ange in ressure ro in te eaer wit aange in ow from an iniviual omressor is small, an tus te sloe of teloa urve on te omressor ma is small.

tati instability ours only if te omressor system an rea an oerat-ing oint on te ositive sloe ortion of te omressor arateristi urve.Normally te ow ros reiitously wen te oerating oint reaes te zerosloe ortion of te omressor urve an surge osillations evelo, wi is aynami instability.

Key Concets

tati instability is more likely to our for arallel omressors. dynami instability usually ours before stati instability.

3-3 dYamC aBlY

dynami instability is araterize by growing osillations. dynami instabil-ity an our if eiter te isarge lenum volume or te omressor imellersee is large enoug to ause a system resonse arameter to exee a minimumvalue an te sloe of te omressor arateristi urve is ositive. e generalriterion is:

For ynami instability,

B > Bm

(3-3)

Sc > 0 (3-4)B = {N V

p/(A

c L

c)} / (2 a) (3-5)

were:

a = see of soun in te gas (ft/se)

Ac= ross-setional area of ow at in omressor (sq ft)


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B = system ynami resonse arameter (imensionless)

Bm

= minimum B for ynami instability (tyially 0.1 to 1.0)

Lc= lengt of ow at in omressor (ft)

N= omressor imeller see (rev/se)V

p= volume of te lenum (see Figure 3-3) (u ft)

Figure 3-3 sows a simle omressor system wit te omonents ientiefor te ynami instability riterion. e lenum is any enlose volume of gas.n a omressor installation, te lenum may be a vessel or istribution eaerbetween te omressor an te blok or trottle valve. f te valve is installeiretly at te en of a setion of ie or ut onnete to te omressor is-arge, te volume of tis setion of ie or ut an be use as an equivalent

lenum volume witout seriously egraing te auray of quation 3-3. eomressor ow at lengt an area are small enoug an te lenum volumeis large enoug in most inustrial installations to satisfy te riterion for ynamiinstability wen te oerating oint rosses over from te negative to te ositivesloe ortion of te omressor arateristi urve. however, te osillationamlitue stos growing ue to te noniealities an nonlinearities of om-ression in inustrial aliations. e resulting onstant amlitue osillation

Figure 3-3 imle comressor ystem


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is alle a limit yle. urge is a ynami instability tat evelos into a limityle. tall is not a ynami instability beause te amlitue of te osillationsereases (eays) wit time. us te minimum B value (tyially 0.1 to 1.0) iste bounary line between surge an stall. urge ours wen B is above te mini-mum B, an stall ours wen B is below te minimum B. lso, te severity ofsurge is roortional to te magnitue ofB. Note tat tis minimum B arametereens on te omressor see an te system imensions an is ineenentof omressor manufaturer an moel number. us, ifferent omressors anbe omare on a onsistent basis.

Key Concets

urge is a ynami instability (sustaine osillations). e severity of surge inreases wit see for a given installation. e severity of surge inreases wit volume for a given installation.

3-4 CHaaCC f

dee surge starts wit a reiitous ro in ow. e ow will tyially rofrom its set oint to its minimum (ossibly negative) in less tan 0.05 seon. No

oter ysial enomenon an ause su a ro in ow.Figure 3-4 sows te reiitous ro in ow measure by bot a slow neu-

mati transmitter an a fast eletroni / ow transmitter. ine tis reiitousro in ow is unique to surge, it an be use as a trigger to atuate an interloktat will oen te surge valves or start a surge ounter. is ro an be eteteby taking te erivative of te / signal if te ow measurement signal is notnoisy an te transmitter is fast enoug (see etion 6-4 for more etail on surgeetetion metos an etion 7-4 for te effet of transmitter time onstant onsurge etetion).

e erio of te surge osillation (time between suessive eaks in ow)is sorter tan te ontrol loo erio unless fast instruments are use ante ontroller is tune tigt. e surge erio is tyially less tan 2 seons,wile te loo erio is usually greater tan 4 seons. ine te surge erio issorter tan te loo erio, by te time te ontrol loo reats to a artiularortion of te surge yle, tat ortion of te surge yle is long gone. e or-retive ation by te ontrol loo may beome in ase wit subsequent surge


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yles an an aentuate te severity of te surge. f te surge erio is less tan

one-fourt te loo erio, te surge yle an be onsiere as equivalent tounontrollable noise (ef. 21). e minimum loo erio is aroximately equalto 4 times te summation of te instrument time onstants an ea times. (eeetion 12-1 for more etails on ow to estimate te loo erio for tuning tesurge ontroller.) e minimum surge erio tat ours wen B is about equal tote minimum B for ynami stability an be estimate by te following equation:

Ts= {2 (L

c V

p)/A

v}/a (3-6)

were:a = see of soun (ft/se)

Ac= ow at ross-setional area in omressor (sq ft)

Lc= ow at lengt in omressor (ft)

Ts= surge osillation erio (se)

Vp

= volume of te lenum (u ft)

Figure 3-4 preiitous dro in Flow Measure by wo ransmitters(Courtesy Compressor Controls Corporation)


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quation 3-6 sows tat te surge osillation erio inreases as te lenumvolume inreases. Most inustrial omressor installations ave a ek valveinstalle on te omressor isarge to revent bakow uring te oening ofte surge valves or uring te ow reversal ause by surge. is ek valve isartiularly imortant for uiize be reator aliations to revent te bak-ow of atalyst or ammable mixtures. t is also imortant for arallel om-ressor aliations to revent uming gas from one omressor bak trouganoter. e ek valve will slam sut uring te start of surge an will effe-tively reue te lenum volume to tat of te iing between te omressor ante ek valve. us, surge osillation erios are not large even wen tere isa long eaer or vessel between te omressor an te blok or trottle valve.

e lenum ressure oes not reiitously ro as oes sution ow at testart of surge. lso te osillation amlitue in ressure is usually less tan tatin ow uring surge. erefore surge ounters or baku ontrol systems tat tryto etet surge by ressure measurement are liable to exeriene false or missesurge ounts or tris. is is artiularly true if neumati ressure transmittersare use or if te ressure is noisy or swings wit loa.

e ow osillation eaks orreson to te lling of te lenum, an tevalleys orreson to te emtying of te lenum. e time uration of te eaksan te valleys is tyially mu longer tan te time uration of te transitionbetween te eaks an valleys. e time uration of te eaks an valleys eenson te iing system frition (resistane) an volume (aaitane), wile te timeuration of te transition eens on te ui inertia (inutane). e systemynami resonse arameter B is roortional to te ratio of te ressure rivingfore to te inertial imeane. B is reresentative of te aability to aeleratete gas. s arameter B inreases, te amlitue of te osillations beomes largeran te sae beomes more non-sinusoial.

Figure 3-5a sows te at trae out by te oerating oint on a omres-sor ma for severe surge were B is about 6 times te minimum B for ynamistability as a trottle valve is lose ownstream of te omressor. Figure 3-5bsows te osillations in sution ow an isarge ressure tat orreson tote at trae out by te oerating oint in Figure 3-5a. Notie tat te osil-lation amlitue is extremely large an te sae is non-sinusoial. n Figure3-5a, te oerating oint starts at oint an moves to te left along te om-ressor arateristi urve as te trottle valve loses. Wen te oeratingoint reaes oint B, wi is were te omressor arateristi urve sloeis zero, te oerating oint jums to oint c. is jum orresons to te re-iitous ro in ow tat signals te start of te surge yle. e oerating oint


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annot follow te ositive sloe ortion of te omressor urve beause tereue ow out of te lenum, ause by te losing trottle valve, requires tatte lenum ressure inrease instea of erease er te urve. (e lenum res-sure must inrease if te ow into te lenum exees te ow out of te lenum.)e ow attern aroun te imeller breaks u, an te ow reverses iretionto bakwar ow from te isarge volume to te omressor sution. fter tisjum to oint c, te oerating oint follows te omressor urve from oint cto oint d as te lenum volume is emtie ue to reverse ow. Wen te oer-ating oint reaes oint d, wi is were te omressor arateristi sloeis zero again, te oerating oint jums to oint . e oerating oint annotfollow te ositive sloe ortion of te omressor urve beause te reueforwar ow into te lenum volume requires tat te lenum ressure ereaseinstea of inrease er te urve. (e lenum ressure must erease if te owout of te lenum exees te ow into te lenum.) e ow attern aroun teimeller is establise an te ow forwar raily inreases. fter te jum to

Figure 3-5a erating point pat an comressor curve (evere urge)


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oint , te oerating oint follows te omressor urve to oint B as te le-num volume is lle. e surge yle reeats itself unless te trottle valve or asurge ontrol valve is oene. e oerating oint follows te omressor urveonly uring te eaks an valleys of te surge ow osillations. e jums on teomressor ma orreson to te rai transition between te eaks an valleys.e osillation erio is about 1.5 times te erio reite by quation 3-6.

Figure 3-6a sows te at trae out by te oerating oint on a omres-sor ma for te transition between surge an stall. B as been erease until itis about equal to te minimum B for ynami stability by ereasing te seeas a trottle valve is lose ownstream of te omressor. Figure 3-6b sowste osillations in sution ow an isarge ressure tat orreson to teat trae out by te oerating oint in Figure 3-6a. Notie tat te osillationamlitue is an orer of magnitue less tan tat in Figure 3-5b, an te sae isnearly sinusoial. e B value is atually sligtly less tan te minimum B beausete osillation amlitue is graually ereasing, an te at trae out by te

Figure 3-5b ution Flow an disarge pressure sillations (evere urge)


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oerating oint in Figure 3-6a is graually siralling inwar. e surge osillationerio is about equal to tat reite by quation 3-6.

Figure 3-7a sows te at trae out by te oerating oint on a omres-sor ma for stall were B as been erease to about 0.01 times te minimum Bfor ynami stability by ereasing te see as a trottle valve is lose own-stream of te omressor. Figure 3-7b sows te osillations in sution ow anisarge ressure tat orreson to te at trae out by te oerating ointin Figure 3-7a. Notie tat te osillation amlitue is 2 orers of magnitue

less tan in Figure 3-6b an eays to zero. e at trae out by te oeratingoint in Figure 3-7a raily sirals inwar an onverges to te oint of zeroow. e oerating oint aroaes zero ow beause te trottle valve own-stream lose omletely, wi stoe ow out of te system exet for a smallleakage ow. f te trottle valve a remaine sligtly oen, te siral woulave onverge to a oint of ositive ow on te ositive sloe ortion of teomressor urve. e stall osillation erio is about 1/2 te erio reite by

Figure 3-6a erating point pat an comressor curve (urge to tall ransition)


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quation 3-6. is arateristi of omressor resonse, were te osillationerio ereases as te amlitue ereases, is te oosite of te arateristiof general ontrol system resonse. e ultimate erio of a ontrol system oursat te transition from stable to unstable oeration (similar to te transition fromstall to surge). however, for unstable oeration (growing osillations) te ontrolloo erio ereases, an for stable oeration (eaying osillations) te looerio inreases.

e lots in Figures 3-5 an 3-6 were generate by an avane ontinuous

simulation language (cL) rogram oumente in enix B. e rogramis base on te reitzer moel of surge tat was esribe an exerimentallyteste by K.. hansen et al. (ef. 16). e cL rogram integrates te ifferen-tial equations for te momentum an mass balanes an omutes te stabilityarameter B, te surge erio, an te system ows an ressures. e user mustenter ata on omressor urve sae, omressor see, system geometry, surgevalve, an trottle valve.

Figure 3-6b ution Flow an disarge pressure sillations (urge to tall ransition)


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Key Concets

dee surge starts wit a reiitous ro in total ow. e surge erio is mu sorter tan te ontrol loo erio. e surge erio inreases wit volume for a given installation. e surge erio inreases sligtly wit surge severity. e surge sae beomes more non-sinusoial wit surge severity.

3-5 CQC f

e rai ow reversals ause extensive raial vibration an axial trust islae-ment. e reeating of te same mass of gas uring ea surge yle auses a largetemerature inrease. e gas temerature rise is artiularly ramati for axialomressors, wit an observe inrease in temerature of 3000F after 10 surge

Figure 3-7a erating point pat an comressor curve (tall)


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yles for one installation (ef. 26). Vibration an trust monitors ave built-ineletroni elays to revent false alarms or tris ue to noise. emerature sen-sors ave termal lags ue to te resistane to eat transfer of te sensor antermowell onstrution (see etion 7). s a result tese instruments will usu-ally atuate an alarm or sutown after several surge yles ave ourre. nte interim, te inrease in vibration, trust, an temerature an ause extensiveamage to te omressor. e reair osts an range from tousans to millionsof ollars. f te omressor is te sole sulier of gas for a lant unit, te busi-

ness interrution loss an be mu greater tan te reair osts, artiularly if arelaement rotor must be manufature, te lant rout was sol out at tetime of te interrution, or ustomers swit ermanently to a ometitor. ven ifte amage is not notieable, te surge an ange te internal learanes enougto erease te efieny of te omressor. U to a 0.5 erent loss in efienymay result from a few yles of surge (ef. 26). eeate surges will ause a sig-niant aumulate loss in efieny.

Figure 3-7b ution Flow an disarge pressure sillations (tall)


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e rai unloaing of te imeller uring ow breakown an ause over-see of te omressor. Fiel measurements (wit an osillograi reorer) ofte see of one omressor uring te start of surge iniate tat te omres-sor aeleration inrease to 2000 rm er se until ower was remove. isinrease in te erivative of te see (aeleration) is arateristi of a runawayor ositive feebak oen-loo resonse (ef. 21). e runaway resonse was sorai tat te see ontroller for te teste omressor oul not reat an re-vent oversee amage at te start of surge. consequently, te only alternative wasto sut own te omressor wen te outut of a erivative moule iniatete start of a runaway see resonse (700 rm er seon). e same moulestarte te osillograi reorer at 300 rm er seon aeleration to aturea reor of te oerating onitions just rior to te sutown. e ower wasremove from te omressor by losing te steam turbine an exaner suly

valves in less tan one alf seon.e ow reversals uring surge an roue a booming noise lou enoug to

make surge a memorable exeriene for ersonnel in te viinity of te omressor.e booming noise an originate from ollasing gas vois witin te omressor,te exing of te sution lter walls, or te slamming sut of te ek valve.

KY CCp

hig vibration, trust, temerature, an see an our uringsurge.

Many instruments are too slow to etet surge quikly enoug. nternal amage an loss in efieny an result from surge.

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Description of Surge

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