Gas Turbine Handbook : Principles and Practices

Parameter Characteristics 109
Note that one mil amplitude is considered smooth at 10 cycles per second
and rough at 100 cycles per second. The most uniform measurement
is velocity, which is always smooth below .02 inches per second and is
always rough above 0.11 inches per second. Velocity can be obtained
from amplitude and running speed by multiplying amplitude in mils,
times π, times rpm, divided by 60,000.
V = MILS • π • RPM/60,000
EXHAUST GAS TEMPERATURE (EGT)
Exhaust gas temperature is one of the most critical parameters in
a gas turbine, in view of the fact that excessive turbine temperatures
result in decreased life or catastrophic failure. In older machines, when
temperatures were not as high, turbine inlet temperature was measured
directly. In the current generation of machines, temperatures at the combustor
discharge are too high for the type of instrumentation available
and, so, intermediate stage or exhaust gas temperature is used as an
indication of turbine inlet temperature.
The quality of the temperature measured is a function of the number
of probes, their positions, and their sensitivity. Since the temperature
profile out of any gas turbine is not uniform, the greater the number of
probes, the better the representation of the exhaust gas average temperature
and profile. A greater number of probes will help pinpoint disturbances
or malfunctions in the gas turbine by highlighting shifts in the
temperature profile.
As metal temperature increases, creep also increases. At any given
power, exhaust gas temperature increases with the increase in ambient
temperature. Therefore, to remain within the safe temperature envelope
as outside temperature increases, output power must decrease (Figure
7-3).
ROTOR SPEED
Rotor speeds are commonly used as control functions on almost
all gas turbines. At any constant power, rotor speeds will increase with
an increase in outside air temperature. Therefore, most control functions