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.
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:
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.
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 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”.
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.
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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