Simple Cycle Performance Specs

Standard design performance ratings and adjustments for actual site and operating conditions


GTW simple cycle performance specifications are based on OEM rated gas turbine performance at ISO standard conditions: 59°F (15°C) ambient air temperature, 14.7 psia (1.015 bar) sea level site elevation, 60% relative humidity.


Usually the ratings are specified for operation on natural gas fuel and gross base load output without deductions for inlet and exhaust losses, and power consumed for shaft-driven auxiliaries -- unless otherwise noted.


Some OEMs prefer quoting net output, which considers impacts of installed inlet loss (typically 3.5 inch H2O), exhaust loss (2.5 inch H2O) and auxiliary system losses -- with footnotes to that effect.


Specified ratings are always for a “factory new and clean” gas turbine having less than 200 hours of operation.


Rated performance gradually deteriorates with time depending on severity of operating mode and environment, maintenance practices and type fuel(s) burned. Most performance degradation is recoverable with periodic maintenance as prescribed by the OEM.


For consistency, the tabulation of OEM ratings in the Performance Specs has been standardized to report these engineering design and performance data.

  • Model. OEM gas turbine designation or name for units in production or scheduled for near-term introduction.
  • Year. First year the original gas turbine series was (or will be) introduced into the marketplace. A number or letter suffix at the end of the designation indicates an evolutionary upgrade of the original platform design series.
  • ISO base. Gross power output without losses, at 59°F sea level site conditions Rated for base load operation, i.e. over 6,000 hours per year with normal gas turbine wear and maintenance intervals.
  • Heat rate. Based on the lower heating value (LHV) of natural gas fuel unless otherwise stated.
  • Efficiency. Measure of ‘energy-out” over “energy-in” (LHV) when operating at base load output (equals 3412 But/kwh divided by heat rate, expressed as percent).
  • Pressure ratio. Ratio of compressor discharge pressure to inlet pressure. High pressure ratios (typical of jet derivative units) indicate high efficiency gas turbine design.
  • Flow. Gas turbine mass flow (air plus  fuel)  at  ISO  conditions, base load output, normally without water  or steam injection unless otherwise noted.
  • Shaft speed. Power output shaft coupling rotating speed, for direct drive at 3000/3600 rpm for 50/60 Hz power generation or geared to match load.
  • Exhaust temperature. Average temperature of the gas turbine exhaust flow at ISO conditions, base load output.


Correction factors

As highlighted in the Editorial Box there are handy rule-of-thumb factors that may adjust ISO ratings for estimating installed performance at actual site conditions:

  • Site temperature. For each 1°F (0.85°C) increase in ambient temperature above 59°F (15°C), there is about a 0.4% reduction in power output and a roughly 0.1% increase in heat rate, (The opposite holds true for decreasing temperatures below 59°F)
  • Site elevation. Every 1,000-ft increase in site elevation above sea level will see a 3.5% reduction in rated gas turbine power output. Heat rate is minimally affected by site elevation.
  • Inlet losses. For each inch of  inlet pressure drop (water gauge), you can expect around 0.4% reduction in power and 0.1% increase in heat rate.
  • Outlet losses. For each inch of outlet or exhaust pressure drop, you can expect a 0.1% reduction in power and 0.1% increase in heat rate.
  • Type fuel. Figure on about 2-3% less power and 1-2% higher heat rate when operating on distillate fuel as compared to natural gas.


Factory verification

Design ratings are based on ‘new and clean’ performance, typically confirmed by full-load factory testing prior to shipment or field tests shortly after installation.


OEMs prefer factory testing because it is faster, more easily done and less expensive than field test verification. It is also much simpler to troubleshoot problems in the factory for correction prior to shipment.


Large gas turbines often cannot be full-load tested in the factory, in which case their performance is verified in the field as part of (or immediately following) commissioning.


Usually, OEMs place a limit on how many hours the gas turbine can run once in service. If not completed within a specified time window, performance testing will be deemed  to have been done and requirements met.


Recoverable degradation

All gas turbines experience a loss in power output and efficiency during their operating lifetimes. To a limited extent those losses are recoverable.


Performance degrades gradually as parts wear, critical surface finishes are lost, and clearances open up. While this happens, power and efficiency typically will decline by several percentage points (compared to factory-new ratings) and then stabilize.


Lost performance due to compressor fouling can partially be restored by on-line water washing at regularly scheduled intervals. During shutdown periods when more extensive maintenance is possible, some operators supplement water washing with mechanical methods for cleaning fouled blades and vanes.


Non-recoverable losses

Non-recoverable losses due to worn parts and widened clearances are usu- ally taken care of during scheduled maintenance and overhauls. Normally, OEMs prescribe such maintenance intervals, based on fleet history, as a function of the number of start-stop cycles, fired hours, and fuel type.


Typically, power and heat rate will degrade by 2-6% points during the first 24,000 hours of operation (routine interval for a hot gas path inspection) assuming parts are not replaced. Replacing worn parts normally will restore gas turbines to within 1 to 1.5% points of original factory-new performance levels.


Retrofitting gas turbines with improved technology components during a major overhaul is often done to increase power, efficiency and maintenance intervals, “beating the degradation curve”.


OEM confirmation

GTW’s Performance Specs are based on latest OEM input and are well suited for evaluating and comparing gas turbine design ratings and performance.


Note, however, that listed ratings are not always 100% accurate or up-to-date. Ratings can (and do) change unpredictably without OEM announcement, as the result of a recent field test or design upgrade or modification.


For anything more than routine performance calculations or preliminary evaluation, it pays to contact OEMs for confirmation of the listed ratings of any models being considered for one’s project.


OEM Feedback

Under such circumstances, direct contact is a “must” for checking out specific model ratings and specific site performance.


It is also a good opportunity to find out about any near-term upgrade or a new gas turbine design in the works that may better suit project requirements and still fit your timetable.


OEM project application engineers are also good at optimizing off-design performance of their machines to satisfy special environment and/or operating requirements.



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