Gas Turbine Handbook : Principles and Practices

Detectable Problems 201
Figure 12-11. Plugged fuel nozzle.
overhaul, or break loose after operation is initiated. In a relatively
small number of cases, damage is the result of tools left in the inlet
or the formation of ice in the inlet downstream of the filters. There
have even been several cases where the inlet silencers failed and bits
of silencer debris entered the engine. In any event, FOD is almost
always accompanied by an increase in vibration.
Compressor FOD symptoms are similar to compressor fouling.
Severe Object Damage (OD) will also affect the combustors and turbine.
Combustor damage or blockage is evident as temperature pattern
distortion similar to plugged nozzles. Turbine FOD will cause a
decrease in turbine efficiency similar to blade and vane erosion.
WORN AIR/OIL SEALS
Pressure balanced labyrinth and positive contact carbon-faced
air/oil seals are the most common type used in gas turbines. Worn or
damaged seals may result in oil leaking into the gas path. Sometimes
this leakage will only occur during start-up and shut-down, when the
air pressure is at its lowest and the oil pressure at its normal level.
The first clue to oil leakage, depending on the size of the lubrication
oil system, could be a decrease in lube oil level. For large, combined,
lube oil systems, this may not be easily detected. Internal engine
leaks can be detected by smoke in the exhaust during operation.
Detection is also possible when the engine is shutdown by inspecting
for oil streaks in the exhaust duct and along the compressor ID hubs.
During operation, worn or damaged seals are more likely to leak hot
air or gas into the oil system. This is normally detected by an increase
in oil temperature and possibly oil frothing.