Gas Turbine Handbook : Principles and Practices

Detectable Problems 199
COMBUSTOR DISTRESS AND PLUGGED FUEL NOZZLES
Plugged nozzles and/or combustor and transition piece failures
will always result in distorted exhaust gas temperature patterns.
Sometimes an increase in fuel flow can be seen, depending on the
severity of the damage. Changes in burner efficiency produce no appreciable
change in other parameters (Figure 12-10).
An obvious indicator of plugged nozzles or combustor damage is
a distorted temperature pattern or profile accompanied by a change
in this profile with power changes. This is a result of the swirl effect
through the turbine from the combustor to the exhaust gas temperature-measuring
plane (Figure 12-11).
Distortion in the temperature pattern or temperature profile not
only affects combustor performance but can have a far reaching impact
on the turbine. Turbine blades can be excited both thermally and
structurally by radial variations in temperature or flow. For example,
a dramatic failure of a gas turbine with only 800 operating hours was
due to the failure of a portion of the combustor liner. The failed liner
material lodged against the first stage turbine nozzle and blocked
airflow to that quadrant of the turbine. The subsequent cycling, with
each revolution of the turbine wheel, led to thermal fatigue failure
of forth stage turbine blades at the blade root. The failed blades so
unbalanced the rotor as to dynamically fail the bearings resulting in
further damage and fire. The damage, after the liner had failed, could
have been avoided if operations had been monitoring the exhaust gas
temperature profile. Fortunately disaster was averted on a similar
unit when the distortion in the temperature profile led to the shutdown
and inspection of this unit. The combustion liner had failed in
the same place and lodged against the first stage turbine nozzles.
FOREIGN/DOMESTIC OBJECT DAMAGE
Foreign Object Damage (FOD) is defined as material (nuts,
bolts, ice, etc.) ingested into the engine from outside the engine envelope.
Domestic Object Damage (DOD) is defined as objects from
any other part of the engine itself. Statistically foreign or domestic
object damage accounts for 10.5% of gas turbine failures. 3 Most of
the impact incidents are the result of small loose parts within the
engine. These parts are either left in the engine during the build or