Gas Turbine Handbook : Principles and Practices

Detectable Problems 193
Figure 12-5. Turbine blade material loss.
increase, a gas path analysis may help define the extent of the damage.
This should be followed by a visual or boroscope inspection. If a
boroscope inspection verifies that:
1. the damage is the result of a blade or nozzle being impacted by
an object originating upstream of the turbine section and the
compressor or combustor is not the source of the object damage,
or
2. the impact damage area is at or near the blade tip, and vibration
is stable and not over limits,
then the unit can be run (until a replacement can be obtained or an
overhaul scheduled) while being closely monitored. If blade damage
is between the blade root and mid-span (Figure 12-6), regardless of
vibration level reached, the unit should not be restarted and should
be overhauled. Corrosion over the same parts of the turbine blade as
shown in Figure 12-5 and 12-6 can be tolerated. However, if corrosion
is evident under the tip of a shrouded blade, the unit should be
shutdown and overhauled. If damage was the result of an object from
within the turbine stage (such as a piece of turbine blade or nozzle
airfoil), the unit should not be restarted.
Changes in blade airfoil dimensions will affect the blades natural
frequency. Damage or material loss occurring at the blade tip will