What You Should Know 

About Design Ratings 

ISO benchmark 

General industry practice is to quote design ratings 

at ISO conditions on natural gas fuel for industrial 

units and distillate I for marine units. 

Power is generally quoted as output across the 

generator terminals for electric power plants and as 

shaft coupling output for mechanical and mari ne 

drives. 

In all cases they constitute gross plant output 

ratings without allowance for parasitic auxil iary 

plant loads, gearbox efficiency, inlet and outlet 

pressure drop losses. 

Design ratings 

The performance specifications have been formatted to provide comparable information for all production models: 

• Year is the first year production design was 

available for shipment. A code number or letter at 

the end of the model designation may refer to a size 

class or design upgrade. 

• ISO base is the maximum continuous power 

you can expect for 6,000 hours or more of operation 

per year at 59°F sea level 60% relative humidity, 

without losses, and experi ence normal gas turbine 

wear and maintenance. 

• Heat rate is a measure of fuel energy input 

per power output, based on the lower heating value 

CLHV) of the fuel consumed Cas opposed to higher 

heating value that determines fuel supply and contract price). 

• Efficiency is a measure of power output divided by the rate of fuel energy input, based on gross 

power output for simple cycle plants and net power 

for combined cycles. 

• Pressure ratio is a measure of compressor discharge pressure divided by atmospheric pressure. 

(All else equal, as design pressure ratio increases so 

does efficiency and power output.) 

• Flow relates to air mass flow through the gas turbines specified in pounds per second. (Generally as 

air flow increases so does gas turbine power output.) 

64 2009 GTW Handbook 

• Tllrbine speed refers to power output shaft design speed. Direct drives for power generation run 

at 3000 rpm for 50-Hz, 3600 rpm for 60-Hz, and 

3300 rpm as an optimized compromise for 50- or 

60-Hz frequency. 

• Exhaust temperature refers to the gas turbine 

exhaust flow. (Low exhaust temperatures are indicative of high efficiency gas turbine operation.) 

Correction factors 

Installed gas turbi ne performance is affected by 

ambient temperatures, site altitude, and flange-toflange pressure drop loss. 

The fo llowi ng rules of thumb are useful for estimating the influence of site specifics on ISO design ratings which are specified at 59°F sea level: 

• Temperatttre. Gas turbine output will drop 0.3 to 

0.5% for every IOF rise in ambient air temperature, 

with a proportionate increase in heat rate. 

• Elevatioll. Every 1,000 ft increase in site elevation above sea level will see a 3.3% reduction in gas 

turbine power output. 

-[lIlet loss. For each inch water gauge Cwg) inlet 

pressure drop, you can estimate a 0.5% reduction in 

power output and 0. 1 % increase in heat rate. 

• Outlet loss. For each inch of outlet pressure drop, 

you can estimate a 0.15% reduction in power and 

0.1 % increase in heat rate. 

• Liqllid/Ile[. Gas turbines will have a 2-3% lower 

output and 1-2% higher heat rate on distillate fuel 

compared to natural gas design ratings. 

Performance verification 

Design ratings published in the GTW Handbook 

for "new and clean" performance are usually verified by full-load factory testing prior to shipment or 

commissioning tests in the field for large units. 

OEMs prefer factory testing to onsite performance verification because it is more convenient, 

easily done and less expensive. It is also simpler 

to troubleshoot equipment problems that may arise 

and correct them prior to shi pment. 
Large gas turbines, however, cannot be full-load 

testcd in the fac tory. They require field verification 

tests of the completed installations as part of or immediately following the commissioning process. 

If the plant enters commercial service before 

performance verification, OEMs usually specify a 

narrow window for validation testing to start after a 

limited number of opcrating hours. 

In-service degradation 

Service-related degradation in power output and 

heat rate is usually classified as recoverable or nonrecoverable losses in performance. 

Recoverable losses are normally associated 

with compressor blade and vane fouling which can 

be treated by routine on-line compressor washing 

(daily in some cases) and more thorough off-line 

washing, as required. 

Non-recoverable operating losses due to normal equipment wear can be taken care of during 

scheduled maintenance for interim and major overhaul and rebuild. 

OEMs usually prescribe maintenance intervals based on number of starts and stops, operating 

hours, duty cycle and type fuel. 

Typically, power and heat rate may degrade by 

2-6 percentage points during the first 24,000 hours 

of operation (rout inc interval for a hot gas path inspection) assuming parts are not replaced. 

Replacing worn parts normally will restore gas 

turbines to within I to 1.5 percentage points of original factory-new performance levels. 

In some instances replacing old parts with upgraded components or retrofitting new technology 

design improvements during a major overhaul can 

surpass original performance and durability. 

Project evaluation 

When evaluating competitive models for project development, it is advisable to contact the gas turbine 

OEMs to confirm that the ratings you have are accurate and the most current available. 

You can also fi nd out if a design upgrade or 

new model is in the works that may better fi t your 

timetable and performance requirements - especially for long-term projects. . 

Direct contact with OEMs is the only way to 

obtain reliable site specific data and for ideas on 

how you might optimize gas turbine performance to 

meet special site and project requirements. 

Efficiency 

Efficiency % 

Efficiency % 

1 kWh 

1 hp-hr 

kW to hp: 

hp to kW: 

Btu to kJ: 

kJ to Btu: 

66 2009 GTW Handbook 

= 

3413 

Btu/kWh 

860 

kcal/kW 

= 2545 

Btu/hp-hr 

641 

kcal/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 

kilowatts 

kilojoules 

thermal units 

= 0.138 

lb/hp-hr 

0.063 

kg/hp-hr 
Combined Cycle Reference 

Plant Design Ratings 

Standard reference plant ratings 

Combined cycle reference plant ratings in the GTW 

Handbook are specified for net plant performance 

at base load output on natural gas fuel at 59' F sea 

level site conditions. 

Ratings include allowances for gas turbine losses, parasitic auxiliary power consumption and 10-

inch (wg) HRSG pressure drop (without catalyst). 

They also assume a set of steam bottoming 

cycle design conditions, including steam inlet temperature, pressure and condenser vacuum pressure, 

whicht are generally unspecified. 

Emphasis on maximum efficiency 

OEM reference plant gas turbine and steam turbine 

designs are typically optimized for maximum combined cycle efficiency and packaged as single- or 

multi-shaft modules. 

The upside to design optimization is the potential to gain up to 1.5% increase in efficiency; the 

downside is that equipment and installation are considerably more expensive. 

Single-shaft lxl plant configurations are designed around a single gas turbine, unfired HRSG 

and steam 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 steam 

through a common header to a separate steam turbine generator. 

Design factors 

Most OEMs adopt a state-of-art approach that is not 

overly conservative or aggressive and that is consistent with company's engineering and marketing 

culture. 

The influence of key engi neering factors on 

performance can be illustrated by analyzing the design of a typical 2x I F-class combined cycle rated 

at 530 MW net plant output and 6015 Btu/kWh heat 

rate (56.7% efficiency): 

• HRSG. Increasing the pressure drop across an 

HRSG works to further reduce net plant power 

output and heat rate. 

Typically, OEM reference plant ratings are 

based on a 10-inch pressure drop for an unfired 

84 2009 GTW Handbook 

HRSG design without catalytic reduction for emissions control. 

For the 530-MW plant example, the net rating 

reflects a 6-MW loss in gross gas turbine power output (due to the IO-inch HRSG pressure drop) and a 

100 Btu/kWh increase in heat rate. 

Pressure drop of that same HRSG equipped 

with SCR and CO catalyst section would increase 

to around 14 inches, for a net loss of 8 MW in gross 

gas turbine power output and 125 Btu/kWh increase 

in heat rate. 

• Gas turbine. Systems design changes in mass 

flow and temperature of gas turbine exhaust gases 

entering the HRSG have a direct effect on steam 

flow and temperature. 

For example, a I % decrease in gas turbine exhaust flow is equivalent to a 1 % reduction in steam 

flow. Conversely, a I % increase in exhaust flow will 

see a corresponding 1 % increase in steam flow. 

Rule of thumb on temperature changes is that 

a 5' F drop in gas turbine exhaust temperature will 

be accompanied by an approximate I % reduction in 

steam flow or 2°F drop in steam temperature. 

• COl/del/ser cooler. Few OEMs specify condenser design parameters or approach temperature 

in their published plant ratings. 

And rarely make reference to cooling water conditions, type cooling system or condenser design for 

the steam turbine side of the combined cycle plant. 

In the case of steam condenser design, the "approach" temperature is the difference between the 

incoming cooling water and the outgoing cooled 

condensate temperatures. Ideally, you want as small 

a delta as practical within economic constraints. 

The equipment cost of a 5' F approach temperature design, for example, is much higher than that 

of a more economical but less effective 10' F condenser system. 

In theory, once-through water flow can be used 

for direct condenser cooling. But it requires access to 

river or sea water under strict restrictions on allowable temperature rise of the returned coolant flow. 

l! also calls for high volume flow rates and 

lots of pumping power. Altogether an impractical 

choice, unless sited next to an arctic fjord supply of 

ice water for cooling purposes. 
• Exhaust pressure. Ultimately the condenser design goal is a reasonably low approach temperature 

for good performance, consistent with reasonable 

cost. 

Since the approach temperature controls condenser temperature, and thereby the back pressure 

on the steam turbine, it has a direct bearing on lowpressure steam turbine output. 

The lower the exhaust pressure, the better the 

performance -- with the lowest practical steam turbine exhaust pressures down at around 1.5 inch Hg 

(not conservative but still realistic, depending on 

conditions). 

Getting back to the hypothetical 530-MW 

combined cycle plant as an example: reducing the 

design exhaust pressure to 1.5 Hg from 2.3 inches 

would add about 7.4 MW to plant output by (1 .4% 

increase) and reduce heat rate by 85 Btu (1.4% improvement). 

• NOll-reheat vs. reheat. Non-reheat steam cycles 

are typically used for gas turbine exhaust temperatures (into the HRSG) of approximately IOOO°F or 

less. 

M ore complex reheat steam cycles, on the other 

hand, are more appropriate for gas turbine exhaust 

temperatures of I OOO°F and higher. 

Two and three-pressure reheat steam cycles are 

more efficient than non-reheat single-pressure systems, but their installed cost is higher. 

Despite added capital cost, the higher efficiencies clearly make muhi-pressure designs the economic choice for high fuel pri ces and high load factor operat.ion. 

• Auxiliary losses. 

Typically, auxiliary loads on the order of 2.5% of 

plant output (on the conservative side) are included 

in OEM quoted combined cycle ratings. 

The 530-MW net output rating of the combined 

cycle reference plant, for example, includes a deduction of 1325 kW for auxiliary losses. However, 

net plant ratings do not normally include transform86 2009 GTW Handbook 

er losses which can contribute another 0.5% loss in 

power. 

This means owner operators should reduce 

OEM combined cycle net power ratings by that 

0,5% to calculate the value of output they can sell at 

the plant boundary. 

Subsequent losses in the switchyard or substation are usually to the utility's account. 

Onsite verification 

Upon completion of installation, large combined 

cycle plants require field testing to confirm OEM 

specified or guaranteed performance parameters. 

Contracts normally specify a certain tolerance 

for variations in field versus factory veri fication results to allow for changes in operating, load and test 

conditions, 

Usually, once a plant begins commercial operation, there is also a time limit on when testing 

should start 

Some OEM s include start-stops and opei'ation 

during plant commissioning prior to the warranty 

testing as part of this time limit. 

Provided everything goes smoothly according 

to plan, without any hitches to prolong the verification test period, there is no problem, 

However, as a hedge, some owner-operators 

contract with their OEM or EPe provider to take on 

some risk in the event of an extended commissioning period. 

Generally most OEMs will attach conditions in 

the warranty test procedures with respect to performance measurements, including generous test tolerances whereby the data can be adjusted statistically 

to take into account test uncertainties. 

Typically, acceptable contract terms should 

specify a test tolerance equal to test uncertainty in 

accordance with ASME power test codes and procedures, 

In any event, adjustments to warrantee performance (depending on contract agreements) can allow for a I to 2% tolerance on satisfying quoted 

power and heat rate,