Gas Turbine Handbook : Principles and Practices

Parameter Characteristics 107
velocity (inches per second) and acceleration (inches per second per
second or “g”). These measurements are taken with probes, such as
seismic probes that measure amplitude, proximity probes that measure
primarily velocity, and acceleration probes or accelerometers that measure
acceleration. By the use of integrating or differentiating circuits,
amplitude, velocity or acceleration readouts are possible from any of
the three probe types. However, more accurate information can be
obtained from direct measurements using the proximity probe and accelerometer.
Typical sources of vibration that are usually found in a gas turbine
package are depicted on a real time spectrographic plot (Figure 7-1).
Note that rotor imbalance is seen predominantly at the running speed,
whereas misalignment can be seen at two times the running speed and is
seen on readouts from both the horizontal or vertical probe and the axial
probe. Looseness due to excessive clearances in the bearing or bearing
support is also seen at two times speed, except the axial probe does not
show any excursions. Hydrodynamic journal bearing oil whirl is seen at
0.42 running speed and is both violent and sporadic. Very close to that, at
0.5 running speed, is baseplate resonance. Note that baseplate resonance
is picked up predominantly on the vertical probe. Gear noise is seen at
running speed times the number of gear teeth. Amplitude is not the best
criteria to judge excessive or acceptable vibration since a high amplitude
for a low rpm machine might be acceptable whereas that same amplitude
for a high rpm machine would not be acceptable.
These same guidelines apply to the lube oil and fuel supply systems.
Pumps and motors exhibit the same relative indicators of unbalance.
Even the piping of the lube oil and fuel forwarding systems have
their own unique natural frequencies.
VIBRATION MEASUREMENT
A seismic probe, which consists of a spring mass system, is primarily
used where the frequency measured is well below the machine’s
natural frequency and that of the spring-mass system.
A proximity probe, which is mounted on the equipment case, measures
the relative movement of the shaft to the case. Proximity probes are
used where the frequency measured is well below the machine’s natural
frequency.