Gas Turbine Handbook : Principles and Practices

Detectable Problems 183
Chapter 12
Detectable Problems
As with any piece of mechanical equipment, the gas turbine
is susceptible to a wide variety of physical problems. These
problems include such things as dirt build-up, fouling,
erosion, oxidation, corrosion, foreign object damage, worn bearings,
worn seals, excessive blade tip clearances, burned or warped turbine
vanes or blades, partially or wholly missing blades or vanes, plugged
fuel nozzles, cracked and warped combustors, or a cracked rotor disc
or blade. Gas turbine problems may be detected by concentrating on
four general areas: 1) the thermodynamic gas path, 2) vibration of
bearings, rotors, etc., 3) lubrication, and 4) controls. The information
gathered will often be the result of an interdependent parameter relationship
and will assist in verifying a diagnosis and more precisely
isolating a fault.
Some of these problems will become evident as vibration increases,
others will be detected by a change (increase or decrease) in
lubrication oil temperature. However, some problems (often the most
serious problems) can be detected only through gas path analysis.
The gas path, in its simplest form, consists of the compressor(s),
combustor, and turbine(s). The operation of each component follows
predictable thermodynamic laws. Therefore, each component will
behave in a predictable manner when operating under a given set of
conditions.
Detectable faults make themselves evident through changes in
observed engine parameters. Actually these faults are the result of
changes in component geometry. Therefore, it is necessary to identify
the geometry changes in order to evaluate the severity of the problem.
Lou Urban, in his book “Gas Turbine Parameter Interrelationships” 1 ,
defined the measurable engine parameters (rotor speed, temperature,
pressure, fuel flow, and power output) as dependent variables and
pumping capacity, efficiency, and effective nozzle areas as independent
variables. To further simplify matters it is usually not necessary
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