Comparing the vibration performance of identical or like machines can be used to detect defects and judge their severity.

What is Comparative Analysis?

Comparative analysis is a simple yet powerful technique used to compare the performance of identical or similar machines to detect anomalies and assess the severity of their condition. The crux of the technique is comparing measurements between like components operating in similar conditions to discern whether the components are operating satisfactorily. In layman's terms, we are performing an "Apples to Apples" comparison.

If the operating conditions are not similar, then external influences may cause changes in performance. This article focuses on applying this technique to vibration and other PdM technology measurements, but it equally applies to process parameters.

Why Use Comparative Analysis?

A maintenance engineer at a power plant is often requested to investigate components that exhibit "unusual" noises or vibration. If the engineer is lucky, the component in question is a piece of rotating equipment within the scope of the PdM program and historical data is readily available for trending purposes.

In the other 99 percent of these cases, the first questions are "What is it?" and "Where is it located?" The component may be a static structure like piping, grating or a snubber or a piece of rotating equipment like a motor, pump or fan. This technique provides the most value to components that have never received a PdM inspection. The following list provides some situations where comparative analysis is beneficial:

  • Limited or no historical data available to assess equipment
  • No applicable "standard" alarm limits or lack of confidence in existing alarms
  • An observation of "unusual" noise, vibration, etc.
  • No additional resources required-uses existing data and measurement capabilities

How is Comparative Analysis Used?

Comparative analysis can be applied to vibration data, including overall, spectral, time-waveform (TWF) and enveloping/demodulation measurements. Additionally, it can be applied to other PdM technologies and process parameters. The key point is comparing measurements from identical/similar machines operating at identical/similar conditions. Unfortunately, no standard definition for similarity exists. Common sense and engineering judgment are required to assume that the similarity condition applies.

Parameters to Consider

The following list provides a number of parameters to consider when deciding if components and conditions are similar. Realize that the list is not exhaustive, and all parameters may not be applicable to the components in question.

  • Flow, pressure, temperature, load, speed, force
  • Conditions: ambient temperature, location (indoor/outdoor), nearby machines (in-service/out-of-service), atmospheric pressure
  • When the equipment is operated: time of day, portion of cycle, seasonal (summer/winter), high/low temperature service, tide level (high/low)
  • Layout: suction/discharge piping, structure (baseplate, grouting, pipe-supports), component orientation (vertical/horizontal)

Case History

The following case history provides an example of how comparative analysis was successfully used at San Onofre Nuclear Generating Station.

The component in question is a 40-gpm, centrifugal, overhung, make-up pump, designated at 3P1019. The pump is driven by a two-pole, 7.5-hp, AC induction motor. This pump is operated on a quarterly basis, and is tested within the plant's inservice testing (IST) program for safety-related pumps.

Prior to 2007, the vibration measurements collected from this pump were limited to an overall velocity measurement and a 1-kHz spectrum due to software limitations. In 2006, a plan was developed to add high-resolution spectrums, TWF and spike-energy measurements. The baseline measurements were made in early June 2007 for this pump.

A review of the vibration data showed that the overall amplitudes were below all alert limits and the trend and spectral plots appeared normal. However, the spike-energy (Figure 1) and TWF plots (Figure 2) suggested a pump bearing problem. At this point it was necessary to confirm if the pump bearings were degraded and determine the severity of the degradation.

Figure 1. A waterfall plot comparing the baseline spike-energy spectrums from make-up pump 3P1019. The pump bearing defect frequencies, harmonics and sidebands are indicative of early bearing wear/damage.

Figure 2. Baseline time-waveform plots from 3P1019 on June 4, 2007, indicating impacting at the pump bearings.

This pump is an excellent candidate for comparative analysis due to the following conditions:

  • No historical data for TWF and spike-energy measurements
  • Two pumps per unit (four total) with similar layout/orientation and operating conditions
  • Parameters measured/controlled during IST: suction pressure, flow rate, discharge pressure and differential pressure

Table 1 summarizes the process parameters collected during the baseline testing of each pump. Note that the flow rates are identical and the differential pressures are within a few percent of each other.

Table 1. A comparison of process parameters for each of the four make-up pumps during their baseline testing following the addition of advanced vibration measurements.

The spike-energy spectrum and TWF plots from the PUMP-IB-HORZ position of the four pumps are compared in Figures 3 and 4, respectively.

Figure 3. Spike-energy spectrum plots from the PUMP-IB-HORZ position of the four make-up pumps.

Figure 4. TWF plots from the PUMP-IB-HORZ position of the four make-up pumps.

Although the comparative analysis of the vibration data was conclusive in identifying a problem with the 3P1019 pump bearings, additional PdM data was reviewed for applicability. An oil sample from the pump is collected and analyzed on an annual basis. The results of the inspection did not indicate any alarms or abnormal trends regarding spectrochemical composition, viscosity, water content or total acid number (TAN). Figure 5 compares infrared thermography images from the pump bearing seal location on the make-up pumps. Note that pump 3MP1019 has the hottest seal temperature of approximately 208-deg F.

Based on the results of the various PdM inspections, the condition of the pump bearings were diagnosed as early bearing wear (Stage 2). The agreed course of action was to trend the condition (Figure 6) until the pump could be overhauled during its normally scheduled work window in February 2008.

Following the pump overhaul, the spike-energy measurements showed dramatic improvement as shown in Figure 6. The peak acceleration decreased 85 percent from 14.8 g on December 11, 2007, to 2.2 g on February 11, 2008 (Figure 7).

Figure 5. Thermography images from the pump bearing seal area of the four make-up pumps.

Figure 6. A spike-energy waterfall plot trending the 3P1019 pump condition. Note that the post-maintenance test (PMT) performed on February 11, 2008, showed dramatic improvement.

Figure 7. TWF plots from the PUMP-IB-HORZ position of 3P019 prior to and following the overhaul of the pump.

To finish this investigation, the pump bearings removed during the overhaul were analyzed to determine the cause of their failure. Figure 8 shows a number of needle-shaped dents on the radial bearing (6309) inner race (left) and a large scratch on one of the thrust bearing (5309) balls (right). The suspected failure mode was solid-particles liberated from the pump bearing seal via contact as evidenced by elevated temperatures. The solid-particle was stuck on a ball and imprinted dents on the bearing races.

Figure 8. Pictures from the pump radial bearing (left) and thrust bearing (right) showing evidence of solid-particle contamination.

Conclusion

Comparative analysis is a powerful technique used to assess the condition of equipment when limited historical data is available or standard alarm limits do not apply. This technique can be used whenever a plant or company has multiple assets that are identical or similar in construction and operation. The tool uses existing data and measurement capabilities and does not require any additional monitoring equipment. A case history illustrated the fundamentals of this tool and demonstrated the results from applying it in a practical application.

Pumps and Systems, May 2009

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