Gas Turbine Handbook : Principles and Practices

210 Gas Turbine Handbook: Principles and Practices
blades. Mid-turbine boroscope ports can be used to view the turbine
blades both upstream and downstream of the boroscope position.
Compressor Blades And Stators
Except for some aero-derivative gas turbines, boroscope ports
are rarely provided within the compressor section of the gas turbine.
However, the compressor inlet guide vanes and first stage compressor
blades are accessible via the clean air compartment immediately
upstream of the gas turbine. For a thorough examination, a boroscope
can be inserted through the inlet guide vanes and all of the first stage
blades inspected as the compressor rotor is turned. A boroscope view
of the first stage compressor blades is shown in Figure 13-4. Note:
the tar-like substance on the leading edge of the airfoil demonstrates
the detail that can be achieved. This amount of contamination does
affect compressor performance. Contamination such as this can be
removed with a wash solution as recommended by the gas turbine
manufacturer. However, if the wash/rinse length is inadequate, the
residue will be distributed over a larger portion of the airfoil surface.
This is shown in Figure 13-5. As the contamination accumulates the
compressor efficiency would continue to deteriorate. The first stage
compressor stators can also be partially viewed through the inlet
guide vanes and the first stage blades. It is advisable to use extreme
caution when inserting the boroscope probe into the blade path (be
sure the rotor is locked so that it cannot be turned).
Figure 13-6 is a boroscope view of the last (eleventh) stage stators
and blades of a 5,000 brake horsepower gas turbine. All of the
boroscope photographs were taken using a 35mm single lens reflex
camera attached to a flexible fiber optic boroscope. As there was
not a boroscope port provided on this particular unit, a fuel nozzle
was removed to inspect this area. After removing a fuel nozzle, the
boroscope was snaked into the diffuser case toward the stators and
blades. Note that with proper lighting the view is clear and sharp.
While only a few stator vanes can be viewed from this position, by
turning the rotor all the compressor blades can be viewed. To view
all stator vanes, continue removing fuel nozzles around the circumference
of the engine. Figure 13-7 is a photograph of the last (eleventh)
stage compressor blades similar to that of Figure 13-6. The contamination
visible in the compressor, specifically between the blades in
Figure 13-7, would have been obvious when viewed through a boro-