# What You Should Know About Design Ratings

ISO benchmark General indu s tr y pra ctice is to quot e d es ign ratings at ISO condition s on n atur a l g a s fu e l for indu s trial unit s a nd di s tillate I for marine unit s. Power i s ge ne ra ll y qu oted as o utput across th e ge ne r ator terminal s for e lec tri c p owe r p l a nt s a nd as s ha ft co uplin g o utput for m ec hani ca l and m ari n e dri ves. In a ll cases they cons titute g r oss plant o utput rating s without a llowan ce for para s iti c au x il iar y plant loads, gearbox effic ie ncy, inl e t a nd out le t pre ss ur e drop losses. 

### Design ratings The performance s p e cifi c ation s ha ve been format

ted to pro v id e comparable inf or m at ion for a ll production mod e l s:

• Year i s the fir st yea r production de s ign was avai labl e for s hipm e nt. A code numb er or le tt e r at th e end of the model de s i g n a ti o n m ay r efe r to a s i ze c la ss or de s i gn up grade.

• ISO ba se i s th e maximum co nti nu ous power you ca n expect fo r 6,000 hour s o r more of o pe ration per yea r at 59°F se a l e vel 60% re l ative humidit y, wit ho ut losses, a nd ex p e ri e nce n or mal gas turbin e wear and maintenance

.

• Heat rate i s a meas ur e of fu e l e n e rgy input per power output , ba se d o n the low e r heatin g value C LHV) of th e fu e l con s um e d Cas opposed to hi g he r heating value that de te rmin es fuel s uppl y a nd contra ct price).

• Efficiency i s a m eas ur e of p o wer output divide d by th e rate of fuel energy input , ba se d o n g ro ss pow e r output for s imple cyc le plant s a nd ne t po wer for comb i ned cycles.

• Press ure ratio i s a m eas ure of co mpr esso r di sc harge pre ss ure di v id e d by atmosp he r ic pr ess ure. (A ll e l se equal, as d es ign pre ss ur e ra tio in creases so do es e ffi c i e ncy and pow e r ou tput. )

• Flow relate s to a ir ma ss flow throu g h th e g a s turbine s s p ecifie d in pound s per seco nd

. (G e n era ll y as air flo w in c reases so doe s gas turbin e power output.)

64 2009 GTW Handb oo k • Tllrbine speed refer s to power output s haft des ign s peed. Dire c t drive s for pow er ge n e r at ion run at 3000 rpm for 50

- Hz, 3 600 rpm f or 60-Hz , and 3300 rpm a s an op timi ze d compro mi se for 50- or 60-H z fr eq ue ncy.

• Exhaust temp e rature r efe r s to th e g a s turbine ex h au s t flow. (Low exhaust temperatu r es are indicati v

e of hi g h efficie ncy gas turb ine o p e ration. ) 

### Correction factors In s t a ll e d gas turbi n e p e rforman ce i s a ffected b y a mbi en t temperatu r es, si te altitude, a nd fla nge-to

flange pre ss ure drop l oss. Th e fo ll owi ng rule s o f thumb are u se ful for estimating the influ e nce of s ite s p ec ifi cs on ISO d es ign rating s which a r e s pecified at 59 ° F se a leve l :

• Temperatttre. Ga s turbin e o utput will drop 0.3 to 0.5 % fo r eve ry 

### I O F ri se in ambie nt a ir t e mperature, with a proportionate in crease in h ea t rate
.

• Elevatioll. Every 1 ,00 0 ft i n c r

ease i n s ite elevation abov e sea leve l w ill see a 3.3% reduc t io n in gas turbin e p owe r o utput.

-[lIlet loss. For each in c h water ga u ge Cwg) inlet pre ss ure drop, you ca n est im ate a 0.5% reduction in power output a nd 0. 1 % in c r ease in hea t rate.

• Outlet loss. For each inch of o utl e t pre ss ur e drop , yo u ca n e s timat e a 0.15 % re du c tion in p ower and 0.1 % in c r ease in heat rat e.

• Liqllid/Ile[

. Ga s turbin es w ill ha ve a 2 -3% l ower output and 1 -2% hi g he r heat ra te o n di s tillate fuel compa red to n atural gas d es i g n ra tin gs. 

### Performance verification D es i g n rating s publi s hed in th e GTW H a nd book for " n ew a nd cl ea n " performan ce are u s uall y ve ri

fi e d by f ull-l oad facto ry tes tin g prior to s hipm e n t o r comm iss ionin g tests in th e field for large unit s. OEM s pref e r fa c tory tes t ing to ons it e performance ve rifi cat ion be ca u se it i s mor e co nve ni e nt , eas il y d one a nd l ess expen s ive

. It is al so s impler to trouble s hoo t eq uipm e nt problem s that ma y ari se a nd correct them pri or to s hi p m e nt.L arge gas turbin es, howeve r , ca nn ot b e full

- l oa d tes tc d in the fac tor y. Th e y re quire fi e ld ve rifi cati o n tes ts of th e co mp le te d in sta ll a ti o n s as p art of o r imme di at e ly f oll o win g the c o mmi ss ionin g pro cess . If th e pl a nt e nt e r s co mm e rcial se rv i ce b e for e p e r fo rm a n ce ve rifi cat i o n , O E M s u s u a ll y s p eci f y a narr o w wind ow for va lidati o n tes tin g to s tart aft e r a limit e d numb e r o f opc rat i ng ho ur s . 

### In-service degradation Se r v i ce- re l at e d d eg rad at i o n in p owe r o utp u t a nd hea t rate i s u s uall y cla ss ifi e d a s recove rable o r non recove rabl e losses in pe r fo rm a nce. Rec o ve rabl e losses are norm a ll y assoc i a te d with co mpr esso r bl a de a nd va ne f o uling whi c h can b e trea ted b y ro utin e o n-lin e co mpr esso r was hin g (dail y in s om e cases) and m o r e th o rou g h o f f -lin e was hin g, as re quire d

. No n-r ecove rabl e o p e rat ing losses du e to normal e quipm e nt w ea r c an b e t a k e n ca re of durin g sc hed ul e d m ai nt e n a nce fo r i nte rim an d m a j o r ove rhaul a nd re build

. O EM s u s uall y p r esc rib e m a int e n a nce int e rval s b ase d o n num ber o f s tart s a nd s t o p s, o p e ra tin g hour s, duty cy cle and typ e fu e l. Ty pi ca ll y, p owe r a nd hea t ra te m ay d eg ra d e b y 2-6 p e rc enta ge point s durin g th e fir s t 24 , 000 hour s o f o p e ra ti o n ( ro ut inc int e rva l f o r a ho t gas p ath ins p ec ti o n ) ass umin g p art s a re no t r e pl ace d. Re pl ac in g w o rn p a rt s no rmall y w ill res tor e g as turbin es to w ithin I to 1. 5 p e r ce nt age p o int s of o ri ginal f ac tor y- new p e rforman ce leve ls. In so me in s tan ces re pl ac in g o ld p a rt s w ith up g rad e d c omp o ne nt s o r ret ro fittin g new tec hn o logy de s i g n impr o ve m e nt s durin g a maj o r ov e rhaul can s urp ass o ri g in a l perf o rman ce a nd dur ability. 

### Project evaluation Wh e n eva lua t ing co mpe titi ve mod e l s fo r p r o j ec t d evelopm e nt , it i s ad v i s abl e to co nta c t the ga s turbin e O E M s t o con firm th at th e ra tin gs yo u h ave a r e ac c urat e and th e mo s t c urre n t ava il abl e. Y

o u ca n a l s o fi nd o ut i f a d es i g n up g rad e o r n

ew mo d e l i s in th e wo rk s th a t m ay b e tt e r fi t yo ur tim e t abl e a nd perf o rm a nc e r e quire me nt s - es p e cial

l y f o r l o ng -te rm p rojec t s. .

Dire c t co nt ac t w ith O EM s i s th e onl y w a y t o obt a in reli a bl e s it e s p e cifi c data a nd for id eas o n how you mi g ht o ptimi ze gas turbin e p e rfo rm a nce t o mee t s p e cial s it e and proj ec t r e quire ment s .



### Efficiency
Efficien cy %
Efficienc y %



### 1 

### kWh 1 

### hp-hr

kW to hp: hp to kW: Btu to kJ: kJ to Btu :

66 2009 GTW H andbook = 

### 3413 Btu / kWh

860 kcal/kW = 

### 2545 Btu / hp-hr 641 kca l/ hp-hr Equivalent Energy 859.9 kcal 3413 Btu 0

. 746 kWh 2545 Btu
Conversion Factors 1.341 

### x kW 0

. 746 

### x hp 1.055 

### x Btu 0.948 

### x kJ =

=

= 

### horsepower kilo w atts kilojoules thermal units = 

### 0

. 138 lb/ hp-hr

0

. 063 kg / hp-hr

# Combined Cycle Reference Plant Design Ratings

Stand a rd reference plan t r ati n gs Combined cyc le reference plant ratings in the GTW Handbook are s pecified for net p l ant performance at ba se load output on natur al gas fuel at 59 ' F sea 

### level s it e conditions. Rating s include a ll owances for gas turbine losses, para s itic auxiliary power consumption and 10

- inch (wg) HRSG pressure dr op (without ca t alys t). They also assume a set of steam bottoming cycle design co nditions, including s t eam in let tem

### perature , pressure and condenser vacuum pressure, whicht are generally unspecified. 

### Emphasis on maximum efficiency OEM reference plant gas turbine and s team turbine design s are typic ally optimized for maximum combined cycle efficiency and packaged as si ngle- or mu lti-shaft modu l es. The upside to design optimization i s the potentia l to gain up to 1.5% incr ease in efficiency; the 

### downside i s that equipment and in s tallation are cons iderab ly more expensive. Sing le-shaft lxl p l ant configurations are des igned around a si ngle gas turbine, unfired HRSG 

### and s team turbine -- with the gas turbine and steam turbine coupled to opposite ends of a commonly 

### driven electric generator. Multi-shaft plants are configured around one or more gas turbine and HRSG units that supply s team through a common header to a se parate s team tur

### bine generator. Design factors Mo st OEMs adopt a s tate-of-art approach that i s not 

### overly conservative or aggressive and that is co ns iste nt with company's engineeri n g and marketing c ul tur e . The influence of key engi neering f acto r s on performance can b e illu s trated by analyzing the des ign of a typical 2x I F-class combined cycle rated at 530 MW net p l ant output and 60 1 5 Btu/kWh heat rate (56.7% efficiency):



### • HRSG. Increa s ing the pressure drop across an HRSG works to further reduce net plant power output and heat rate. Typically, OEM referen ce plant ratin gs are based on a 10-inch pressure drop for an unfired

84 2009 GTW Handbook HRSG design without catalytic reduction for emis

### s ions control. For the 530-MW p l ant example, the net rating refl ec t s a 6-MW lo ss in gross gas turbine power output (due to the IO-in c h HRSG pr ess ure drop) and a 100 Btu/kWh increa se in heat rate. Pre ss ure drop of that same HRSG equipped with SCR and CO catalyst sect i o n would increase to around 14 inches, for a net lo ss of 8 MW in g ro ss gas turbi ne power output a nd 125 Btu/kWh increa se 

### in heat rate .



### • Gas turbine. Systems de s ign changes in m ass flow and tempera tu re of gas turbine exhaust gases entering th e HRSG ha ve a dir ec t effect on s te a m flow and temperature. 

### For example, a I% 

### decrease in gas turbine exhau st flow is equivale n t to a 1 % 

### red u ction in stea m flow. Conversely, a I % increase in exhaust fl ow will see a corresponding 1 % increase in s team flo w. Rule of thumb on temperature changes i s that a 5 ' F drop in gas turbine ex h aust te mperature will be accompanied by an approximate I % reducti o n in s team flow or 2 ° F drop in s team t empera ture.



### • COl/de l/ ser coo ler . Few OEM s spe cify condenser design parameters or approach temperature in their published plant ratings. And rare ly make refe rence to coo ling water conditions, type cool i ng syste m or condenser de sign for the s tea m turbine s id e of the combined cy cl e plant. In the case of stea m co nden se r design, the "app r

oach" temperature i s the d ifference betwe e n the incoming cooling water an d the outgoing cooled condensa te temperatures
. Ideally , yo u want as sma ll 

### a d e lta as practical within econo mi c co n s traint s. The eq u ipment cost of a 5 ' F approach temp e rature design, for examp l e, i s mu c h hi g her than that of a more economical but le ss effective 10 ' F condenser syste m. I n theory, once-through water flow can b e u se d 

### for direct condenser cooling. But it 

### require s access to ri ver or sea water under st rict re str iction s on allowab l e temperature rise of the returned coo l ant fl ow. l! a l so calls for high vo l ume flow ra tes and lot s of pumping power . Altogether an impra c tical choice, unl ess sited next to a n arctic fjord supply of 

### ice water for cooling purpo ses.• Exhau s t pr ess ur e . U ltim at e l y t he condense r des

i g n goal i s a reasonab l y l ow approac h tempe ratu re fo r goo d p e r fo rm a nce, co nsi s tent w ith reasona bl e 

### cos t. Si nce th e a p p roac h t e mp e ra ture co n t rol s co nde nse r t e m pera ture, and th ereby th e bac k p ressure on th e s t ea m tur bi ne, i t has a d ir ect b ea rin g o n l owpr ess ur e s tea m turb i ne out p ut. 

### Th e l owe r th e ex h au st p ressure, th e bette r th e pe rf orman ce - - w ith th e l owes t pra cti ca l s t ea m tur 

### b i n e ex h a u st pressu r es down at around 1.5 inch Hg ( no t co n se r va t ive but s till rea li s ti c, d e pe ndin g o n 

### co ndit io n s). Ge tti ng b ack to th e hy p o th e ti ca l 53 0

- MW co mbin e d cyc l e pl a nt as a n e x a mpl e : re du c in g t he 

### des i gn ex h a u st pressu r e to 1 .5 Hg f ro m 2.3 inc hes wo uld add abo ut 7 .4 MW to pl ant o utput by (1 .4% in crease) and red uce heat ra t e by 85 B tu (1.4% i m prove m e n t).

• NOll-reh eat vs . reh eat. No n -re heat s tea m cyc l es are t y pi ca ll y u se d for gas turbin e ex hau s t t

e mp era

### tures ( in to th e H RSG) of approx im ate ly 

### I OOO ° F o r less. M ore co mpl ex re h ea t s t ea m cycles, on th e o th e r h and, a r e m ore appr op r ia te for gas turb i ne ex h au s t 

### te mp e ratures o f I

### O O

O

° F a nd hi g h e r. Two and t hree-p r essure re heat s t ea m cycles are m ore effic i e nt th a n non - reh ea t sin g l e- pr ess ure sys

### te m s , bu t th eir in sta ll ed cos t i s h ig her . D esp ite a dd e d ca p ita l cos t , the h ig he r effic i e n ci es cl ea rl y make muhi - pr ess ur e des i g ns th e eco

### no mi c c h o i ce fo r hi g h fue l p ri ces and hi g h loa d fac

t or op e ra t. i o n .

• Auxiliary losses
. 

### Ty pi ca ll y, a u x ili ary l oa d s o n th e o rd e r o f 2.5% of pl a nt out put (o n th e co n serva tive s i de) are inc lud e d in O E M qu o te d co mbin e d cy cl e ra tin gs . Th e 530- MW n e t o utpu t r a tin g o f th e co mb ine d cyc l e r e f

e re nce pl a nt , f o r exa mpl e, incl udes a dedu c ti on o f 132 5 kW fo r au xi li ary losses. H owever, n e

t pl ant ra ti ngs do not norm a ll y i ncl ude t ransfo rm 86 2009 GTW Handbook e r l osses w hi ch can co ntri b ute an o th e r 0.5 % l oss i n powe r . T hi s m ea n s owner op era tors shou ld re du ce 

### O EM co mb in ed cycle ne t p ower rat i ngs by th at 0,5 % to ca lcul at e th e v alu e of o utp ut th ey ca n se ll at 

### th e p la nt bo und ary. S ub se qu e nt l osses in th e sw it ch ya rd or s ub s tat ion are u sua ll y t o th e u t il ity's acco unt. 

### Onsite verification U pon co m p l et io n of i ns ta ll at i o n , l arge co m b in e d 

### cycle pl a nt s r e quire fi e ld test in g to co nfirm O E M s

p

ecified o r g uara n tee d pe r fo rm ance pa ra m e te rs. Co nt rac ts norm a ll y s p

ec ify a ce rt a in t o l e ra nce fo r va ri a t io n s in fi e l d ve rs u s fac tor y ve ri ficat i on resu lt s to a ll ow f or c hanges i n ope ra tin g, l oad a nd tes t co ndition s , Us ua ll y, once a p l an t b eg in s co mm e r c i a l o pe rati on , th e re i s al so a tim e limit on w he n tes tin g s

hould s ta rt So me O E M s in c lud e s t a rt-s t ops a nd opei'a ti on durin g pl a nt co mm iss i o nin g p rior t o th e wa rr an ty tes t i ng as p art o f thi s tim e limi t. 

### Prov id e d eve ry thin g g o es s moo thl y acco rdin g to pl an, w ith o ut a ny hit c hes to pro l o ng th e verificati on t es t pe ri od , th e r e i s no p rob l e m , H oweve r , as a h ed ge, so me ow ner- opera t ors 

### con tr act w ith th e ir OE M o r E P e prov id er to t a k e o n so m e ri sk in th e eve nt of a n exte nd e d co mmi ss i on 

### i n

g period. G enera ll y m ost OEM s will att ac h co nditi o n s in th e wa r ra nt y t est p roced ur es w ith r es p ec t to pe r form an ce meas ure m e nt s, in c ludi ng ge n erous tes t to l e ran ces w he re b y th e d a ta ca n b e a d jus te d s tat i s t ica ll y to t ake in to acco un t t est unce rt a inti es. 

### Ty pi ca ll y, acce pt a bl e co ntr ac t te rm s s ho uld s

p

eci fy a t es t t olerance equa l t o test u ncerta in ty i n 

### acco rd a nce w ith AS M E p owe r tes t co d es a nd p roce dur es, In a n y eve nt , adju stme nt s to wa r rantee p e r forman ce ( de p e nd in g o n co nt rac t ag ree me nt s) ca n a l low fo r a I t o 2 % to l e ra n ce o n sa ti s f y in g qu oted powe r an d hea t ra t e,