Modern gas turbine instrumentation and control systems provide advanced monitoring and diagnostics designed to prevent damage to the unit and to enable it to operate at its peak performance. The following sections describe the various measurements and instrumentation used in gas turbines.
Machine vibration is monitored using:
Displacement probes
Velocity pickup detectors
Accelerometers (measurement of acceleration)
Displacement probes are used to measure the movement of the shaft in the vicinity of the probe. They cannot measure the bending of the shaft away from the probe. Displacement probes indicate problems such as unbalance, misalignment, and oil whirl.
Velocity pickup detectors have a flat response of amplitude as a function of frequency. This means that their alarm setting remains unchanged regardless of the speed of the unit. Their diagnostic role is somewhat limited. The velocity pickup detectors are very directional. They provide different values for the same force when placed in different directions.
Accelerometers are normally mounted on the casing of the machine. They pick up the spectrum of vibration problems transmitted from the shaft to the casing. Accelerometers are used to identify problems having high frequency response such as blade flutter, dry frictional whirl, surge, and gear teeth wear.
Figure 17.1 illustrates a chart used to convert from one type of measurement to another. The vibration limits are also shown on this chart. It demonstrates that the velocity measurement is independent of the frequency, except at very low frequencies where the displacement amplitude...
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