Gas Turbine Handbook : Principles and Practices

192 Gas Turbine Handbook: Principles and Practices
hour cost and overhaul cost. Many oxidation, corrosion or erosion
problems initially discovered by gas path analysis can be verified by
a boroscope inspection. Then an evaluation can be made as to how
detrimental that fault is to continued operation of the unit. Once a
fault has been isolated it can be tracked or trended using both gas
path analysis and visual (boroscope) inspection. If the oxidation/corrosion/erosion
is affecting the nozzle airfoils uniformly and the resulting
inefficiency and power loss is tolerable, then the decision to
repair the damage can be delayed while the progress of the damage
is monitored. Figure 12-4 depicts varying degrees of metal loss on a
turbine nozzle with estimated “urgency to repair.”
As with nozzles, corrosion/erosion affects blades initially in the
airfoil mid-span. Because blade loading is more severe than nozzle
loading, less blade material loss can be tolerated. Furthermore, trailing
edge losses are more tolerable than leading edge losses. Figure
12-5 depicts acceptable and unacceptable material loss.
Impact damage is usually first noticed as an increase in vibration.
If the vibration is not severe and stabilizes after the initial
Figure 12-4. Turbine nozzle material loss and urgency of repair.