ArticlePump Vibrations. Part 2: Analysis
by Kevin R. Guy
This second in a two part series on pump vibration deals with the analysis of common pump problems. A suggested list of common pump fault frequencies is given. As in the first paper on this topic, an emphasis is again made on the need to monitor not only pump vibration, but also its pressure & flow. For pumps operating on fluid film bearings like boiler feed pumps, the use of thrust probes to monitor the position of the rotor is emphasized.
“When a program is used for analysis, the number of data points increases because both trend data and data for analysis – spectrum plots and time plots – are needed. These data provide insight into potential problems and minimize the possibility of having to acquire additional data. Spectrum and time plots are used to analyze 80% of equipment problems if the data base is adequate. Initial fault frequencies should contain the operating speed, two-times operating speed, operating speed harmonics (multiples), and vane pass.
Fault frequencies should be listed for the belt-passing frequency. The calculations are as follows.
belt length ≅ 2L+[(d1*π)/2]+[d2*π)/2]
belt-pass frequency =[d1*π)*N]/belt length
BPL = belt-pass frequency, Hz
L=centerline distance between sheaves, in.
d1 = drive sheave diameter, in.
d2 = driven sheave diameter, in.
N =rotating speed of driveshaft, RPS
Vane-pass problems can occur if the impeller is not centered in the discharge volute (Figure 10). Inadequate clearances between the outside diameter of the impeller and the inside diameter of the pump housing can also cause problems. The vane-pass frequency is the number of blades in the impeller multiplied by the RPM of the shaft. Figure 11 contains plots of spectrum (top) and time (bottom) of a vane-pass frequency from a four-blade impeller operating at 29.75 Hz.
Fault frequencies developed for the analysis of the pump are given in the table. Such information is important in identifying frequencies in a spectrum necessary to solve the problem. The time required to generate a data base helps the analyst become familiar with the machine. It is imperative that the analyst be aware of the components inside the pump; e.g., bearing type, impeller vanes, turning vanes, coupling type. The same spectral data are used in Figures 12 through 16. The figures illustrate the ease of frequency identification when fault frequencies are available.
The analyst must check fault frequencies with time plots to confirm that both are displaying the same data. Peak amplitude from time plots should be compared with amplitudes in the spectrum plots. Data from spectrum plots should never be the sole source of information for an analysis.
Boiler Feed Pumps
Boiler feed pumps are complex. Data should include all operating variables; i.e., flows, pressures, temperatures. Data from turbine-driven boiler feed pumps should be collected in the same manner as for a turbine generator. Phase and amplitude data should be included with normal vibration data. If the pump drive or feed pump is instrumented with proximity probes, periodic data should be collected directly from the probes. Voltage gaps should be part of the data.”
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