Advanced Monitoring to Improve Combustion Turbine/Combined ...

Vibration Fault Diagnostic System Module
7-44
Figure 7-32
Main Viewing Area for Orbits Analysis Tool
Frequency Domain
Frequency domain features are useful in not only detecting faults but also diagnosing the type of
fault. The magnitude of the frequency content of the raw time domain signal at specific
frequencies can be used to determine resonances of the structure, unbalance, misalignment and
many other mechanical faults.
An FFT is applied to one second’s, windowed data to attain the frequency content of the raw
signals. Applying the window insures periodicity of the signal, a requirement of the FFT
calculation. Magnitudes corresponding to specific frequencies of interest can be extracted from
the FFT by utilizing a “peak-picking” algorithm. These peaks represent the energy content of the
signal at the frequencies of interest. These frequencies (and signals) include shaft speed and its
harmonics, i.e. full and fractional engine orders.
In addition to the peaks at frequencies of interest, the sidebands of the peak are used in
diagnostics. Sidebands are peaks in the FFT magnitude that are near a larger peak. The
existence and magnitude of sidebands is used to diagnose and characterize faults.
The one-second sampling period provides a one hertz resolution on the FFT based diagnostics.
The graphical interface provided for the user is shown in Figure 7-33. The interface consists of
three plotting regions and user controls for selecting the signal to highlight and capture a static
representation of the waterfall plot.