Gas Turbine Handbook : Principles and Practices

Parameter Characteristics 105
Chapter 7
Parameter Characteristics
In controlling, monitoring, or analyzing gas turbines the basic
objective is to maximize performance, reduce maintenance, and
reduce unit downtime. To accomplish this, the indicators of gas
turbine health (or condition) must be identified and understood. In general,
a gas turbine may be viewed as consisting of accessory equipment
(controls, lube oil pumps, ignition system, etc.), rotational mechanical
equipment (rotor bearings), and thermodynamic gas path elements (gas
containment path, compressor, combustor, and turbines).
A controlling or monitoring system must pay attention to the three
areas of the gas turbine system:
• The thermodynamic gas path
• Vibrations of bearings, rotors, and gearboxes
• Lubrication, control, and other accessory subsystems
Furthermore, the information gathered from these three areas may
often be used in a cross complementary fashion to verify diagnoses and
more precisely isolate faults.
Table 7-1 shows the key engine parameters and their symbols used
in controlling, monitoring and analyzing gas turbine operation.
VIBRATION
Every piece of equipment, regardless of size or configuration, has
a natural or resonant frequency. Gas turbines are no exception. If the
resonant frequency is below the operating frequency or speed, the unit
is considered to have a flexible shaft. If the resonant frequency is above
the operating speed, the unit is said to have a stiff shaft. Almost all gas
turbines are considered to have a flexible shaft; that is, the normal op-
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