MRO Today - PdM: Analyze this!

Analyze this
Detect problems before they occur by teaming oil analysis, vibration analysis

by Drew Troyer

The power generation and petrochemical industries view vibration analysis as the technique of choice for monitoring the condition of
large, critical pieces of rotating equipment.

Conversely, the fleet industries rely on oil analysis to make effective maintenance decisions.

In industries such as primary metals, and pulp and paper, it is common to use both techniques.

In general, vibration analysis and oil analysis are the most effective techniques for monitoring the health of machinery. The techniques are natural allies due to the complementary nature of their respective strengths. Unfortunately, companies rarely combine the two.

Vibration analysis activities typically reside in the condition monitoring or vibration monitoring group, while oil analysis usually
resides with the lubrication team.

Making matters worse, the oil analysis program usually consists of submitting occasional samples to a laboratory in exchange for results that frequently look more like chemistry than machine condition monitoring. And, too often, oil analysis is used to schedule oil changes while equipment condition assessments are left primarily to vibration analysis.

This is changing in many organizations. For example, the Palo Verde Nuclear Power Generating Station in Arizona made a dramatic change in its approach to condition monitoring. The station combined vibration analysis and oil analysis into a common group, brought its oil analysis on-site and began working as a team. The results have been remarkable.

In an assessment of bearing defects detected by technology, Palo Verde found oil analysis was responsible for 40 percent of the defects found, vibration analysis responsible for 33 percent and both techniques converged on the remaining 27 percent of defects found. Subtracting either technology reduces their detection resolution and ability to control the root causes of machine failure.

Getting all the information
Research conducted at Monash University in Melbourne, Australia, found the correlation between oil analysis and vibration analysis to be generally very good. However, there are instances when one technique indicates a fault while the other shows no change or even a contradictory result.

For example, in applications where sliding wear is prevalent, one might detect increasing rates of wear generation and decreasing rates of vibration. This is caused by what the researchers termed a "lapping" effect. Essentially, the sliding wear slowly hones the surfaces smooth, reducing the overall vibrations until the point at which it induces looseness and mechanical vibration. The presence of abrasive dirt intensifies the effect.

On the other hand, the study found vibration analysis effectively identifies the presence of a fractured gear tooth, but because the size of the debris generated is so large, wear particle analysis is ineffective. The debris falls to the bottom of the sump, never finding its way into a sample bottle until it oxidizes and leeches into the oil, a process that takes months.

The study concluded both techniques are required to effectively monitor and diagnose the condition of plant machinery because each technique evaluates different and complementary symptoms.

An example where both techniques are necessary to effectively solve a problem is the case of a gearbox with increasing vibration at the gear mesh frequency. Inspection of the particle count and ferrous percentage reveal an increase in both categories, improving confidence that a problem exists. The problem's true nature is not revealed, though, until assessment of the oil's viscosity trend. There is a drop in viscosity from 220 centistrokes (cSt) at 40 degrees C to 70 cSt at 40 C.

A review of the work history shows an oil change occurred two weeks earlier. In all likelihood, the worker performing the oil change used the wrong oil, leading to the wear and vibration. Without the combination of condition monitoring technologies, the root of the problem goes undetected.

Conclusions
In general, the following conclusions can be made about combining oil analysis and vibration analysis in detecting and analyzing machine faults:
1) Both techniques are necessary to determine and control the root causes of machine failure.
2) Often, one technique serves as the leading indicator of machine failure while the other serves as the confirming indicator.
3) Oil analysis is generally stronger in detecting failures in gearboxes, hydraulic systems and reciprocating equipment.
4) Vibration analysis is generally stronger in detecting failures in high-speed journal bearing systems.
5) Vibration analysis is often better at localizing the point of failure, depending on the application.
6) Oil analysis is often stronger in determining which wear mechanism is inducing failure.
7) The correlation between oil analysis and vibration analysis is very good, but there are contrary instances.

The tools to succeed
Oil analysis and vibration analysis are natural allies in achieving machine reliability. They offer complementary strengths in controlling the root causes of machine failure and in identifying and understanding the nature of abnormal conditions. Success depends on making changes in the organization to foster the development of condition monitoring and machine diagnostic generalists in lieu of technology specialists.

A carpenter goes to the site with all the tools necessary to complete the job. While it may be possible to cut a board with the claw of a hammer, the carpenter is more likely to draw his saw, a more effective tool for the task.

We in condition monitoring must view technologies as enabling tools. We need the right tools in our bag to complete the job of ensuring machine reliability.

Drew Troyer is the director of technical services for Noria Corp., an Oklahoma-based provider of oil analysis information and educational opportunities.

This article appeared in the December 1999/January 2000 issue of MRO Today magazine. Copyright 2000.