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184 Gas Turbine Handbook: Principles and Practices
to actually calculate pumping capacity, efficiency, or effective nozzle
areas. It is sufficient to consider only the changes in the dependent
variable parameters that define the independent variables. It should
also be noted that a change in any one dependent parameter does
not necessarily indicate a particular independent parameter fault.
For example, a change in compressor discharge pressure (CDP) does
not necessarily indicate a dirty compressor. The change could also be
due to a combined compressor and turbine fault, or to a turbine fault
alone.
The objective of the gas path analysis is to detect as many problems
as is sensibly and economically feasible through the observation
of suitably chosen parameters. To be detectable, the problems must be
of a nature and magnitude to produce an observable change. Cracks
in combustors, stators, nozzles, blades, or discs are not detectable
through gas path analysis. They are, however, detectable through boroscope
inspection. Corrosive attacks on airfoils may also be detected
by boroscope inspection. Severe corrosion, as it changes turbine airfoil
geometry, is detectable through gas path analysis. Physical problems
that are the result of changes in component geometry will degrade
component performance, which will produce changes in certain measurable
parameters. The changes in these measurable parameters
will lead to the identification of the degraded component and subsequent
correction of the physical problem.
While input data accuracy is a concern, what is of major importance
is data repeatability. Measurement inaccuracies can be
eliminated (or at least significantly reduced) as an error source by
periodically comparing data to an initial baseline and trending that
data. Care must be taken when replacing sensors to re-establish the
baseline.
GAS TURBINE—GAS PATH ANALYSIS
In determining whether gas turbine engines are operating normally,
data taken from the engine instruments must be corrected to
a standard condition and compared to baseline data or data known
to be correct for that specific engine model. The baseline data usually
consists of a sufficient number of data points to generate a series
of curves representing the engine’s thermodynamic cycle signature.