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Vibration, Fluid Analysis, Or Both?

EP Editorial Staff | September 1, 2021

While each has its strengths, combining vibration and fluid analyses will result in optimum condition-based maintenance.

By Mark Barnes, PhD, CMRP, Des-Case Corp.

For critical rotating equipment, the value of transitioning from run-to-failure or time-based maintenance to a condition-based strategy cannot be overstated. Reduced downtime and maintenance costs, increased safety, and improved environmental compliance are the primary benefits that result when companies implement proactive, condition-based maintenance practices.

Condition-based practices require data in the form of one or more data points that make possible informed, educated maintenance. While several data inputs can be used to make decisions—from simple temperature or visual readings to a more complex sensor and systems-based approach—two of the more tried and trusted methods for rotating equipment are vibration and oil analysis. In the hands of a trained, skilled practitioner, these two methods serve as vital predictive (after a failure has started) and proactive (prior to failure initiation) data sources. Which one is better? Should you invest time, effort, and money in one versus the other? The answer is both.

Failure Mode Vibration analysis Fluid analysis Comments
Looseness Strength Weakness Structural looseness, incorrect tightening, and poor component fit can all be detected through vibration analysis, coupled with motion-amplification techniques. 
Machine resonance Strength Weakness By looking at the amplitude and phase of vibration during start-up and slow down, resonance can be detected as a large increase in amplitude as the machine transitions through the speed that corresponds to a natural resonance frequency of the machine.
Alignment Strength Weakness While the effects of long-term misalignment will be seen as an increase in internal wear due to the impact on internal clearances, vibration analysis provides a very early indication of parallel and angular misalignment.
Unbalance Strength Weakness Like misalignment, vibration analysis is an early indicator of unbalance conditions, compared to oil analysis which will only show the long-term effects of unbalanced rotors.
Wrong lubricant  Weakness Strength Depending on the time the wrong lubricant is in use, vibration may show evidence of loss of film strength in the high-frequency (>20 KHz) range. By comparing in-service oil samples with a new-oil reference sample, incorrect lubricant usage can be detected through routine oil analysis as a change in viscosity and/or additive composition.
Degraded lubricant Weakness Strength By comparing the physical and chemical properties of in-service oil samples with a new-oil reference, oil analysis can provide a very early warning of lubricant degradation.
Under/over lubrication Strength Strength Incorrect oil level—particularly too little oil—can be detected as a loss of film thickness through high-frequency vibration analysis.  While active machine wear due to incorrect oil level can be detected using oil analysis, under or over lubrication is best detected through visual oil analysis (checking oil level in the sight glass), in conjunction with thermographic analysis.
Water contamination Weakness Strength While loss of oil film and subsequent bearing/gear wear due to water ingress can be observed using vibration analysis, oil analysis can provide an early, proactive warning of moisture ingression, provided the correct test for water is used.
Particle contamination Weakness Strength Particle contamination can lead to a multitude of wear modes including three-body abrasion, erosion, and particle-induced contact fatigue.  Establishing proper target cleanliness levels and using oil analysis to monitor and control particle contamination is a proactive strategy that mitigates the impact of particle contamination.
Active wear—rolling element bearings Strength Strength Vibration and oil analyses will pick up active rolling-element bearing wear. Provided samples are taken from the right location, oil analysis will typically show a problem earlier while vibration analysis does a much better job of identifying the source of the wear (inner race, outer race). The efficacy of oil analysis is strongly influenced by sump capacity since a large oil volume can dilute wear debris below detectable limits. 
Active wear—journal bearings Strength Variable While vibration analysis can detect certain problem conditions, such as oil whirl, leading to possible oil whip failure, oil analysis is far better at detecting plain bearing wear from small amounts of wear debris formed due to two-body abrasion (rubbing) from loss of hydrodynamic oil film. Oil analysis is best suited to small- and medium-sized assets versus large turbomachinery where proximity probes provide an earlier warning of oil instability.
Active wear—gears Strength Strength While oil analysis and vibration analysis will both detect gear wear, some failure modes, such as broken or cracked gear teeth, are best detected with vibration analysis. Other failure modes usually show up first in oil analysis.  Typically, oil analysis is the earliest indicator while vibration analysis provides far greater localization/specificity of the nature of the failure.
Greased lubricated bearings Strength Strength Issues such as under and over lubrication can be detected through high-frequency vibration analysis, as well as ultrasound. Periodic analysis of in-service grease can be used to trend grease degradation rates and contamination issues.  Grease and vibration analyses will show signs of active wear, though vibration analysis will provide better localization of the problem.

The table provides a critical look at the benefits and efficacy of vibration analysis and fluid analysis when addressing common rotating failure modes. In the past 20 years, several tools, technologies, and strategies have merged, all focused on increasing asset reliability. Two of the oldest—oil analysis and vibration analysis—are still as relevant today as they were when they were first invented. When combined, they offer a comprehensive and unparalleled source of data to drive condition-based decisions. EP

The research shows . . .

Numerous studies have empirically evaluated vibration and fluid-analysis condition-monitoring techniques. In general, they’ve reached the same conclusion: For an effective condition-monitoring program you need both vibration analysis and fluid analysis. Perhaps the most comprehensive study was conducted at the Palo Verde Nuclear Power Station in Arizona (Practicing Oil Analysis magazine, July 1998, Noria Corp.) 

In the study, bearing faults from 750 machines were analyzed and categorized as showing up first in oil analysis, vibration analysis, or both. Based on the data, it may be tempting to suggest that, at 40%, oil analysis had a slight edge. In reality, this study shows that omission of one or the other technology would result in roughly a third of all faults going undetected. Even the 27% of faults that showed up in both techniques is instructive, since corroborative evidence from two difference sources provides a greater degree of comfort that the right maintenance decision can be made.

Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control.

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