Lubrication: Oil Contamination and Wear Debris Analyses

Quick and frequent lubrication oil tests, particularly those related to contamination, are vital in maintaining the health of turbomachinery. In many cases, contamination can occur suddenly as the result of a malfunction or a developing problem. And oil contamination can quickly damage sensitive parts of these machines. Therefore, a fast response is essential.

Monitoring techniques for the detection of contaminants have advanced rapidly in past decades. Many methods can provide data on lube oil cleanliness and contamination level.

Online sensors are permanently installed in large-scale turbomachinery. Medium and small machines often rely on portable sensors and instrumentation. Two sets of sampling ports are typically identified as primary and secondary, used respectively for routine analysis and troubleshooting.

Ease of operation and application are important factors in the selection of monitoring instruments and contamination-detection methods. These are as vital as technical requirements. A moderate level of precision is often enough when coupled with the ability to rapidly and easily conduct regular analyses.

Wear debris analysis

Wear of sliding lubricated components results in debris discharged into the oil. Wear debris analysis, therefore, has proven useful in the monitoring of bearings, gear units and other turbomachinery systems. It can reveal true tribological conditions.

Monitoring techniques and other analysis methods should be used in combination. One method, used in isolation, can sometimes be misleading. False detection reports are common when people attempt to draw a premature conclusion from incomplete information.

Common technologies used for screening purposes include ferrous density analysis, elemental spectroscopy, particle counting and patch testing. Complementary monitoring and fault-detection methods include:

• Filter debris inspection

• Magnetic plug analysis

• Sump sediment analysis

• Ferrography

• Acid-dissolution spectroscopy

• Particle heat treatment

• Particle impaction testing

• Chemical microscopy

• Digital shape profiling

• Rotrode filter spectroscopy

• Gravimetric analysis

• Ultracentrifuge (separation of soluble metal fraction)

• Pore blockage particle counting

Reliance on only one of these methods may lead to damage going unnoticed or to misleading conclusions. However, when two, three or more trend lines show the same result, it is usually a reliable sign that could be used for further monitoring, investigation or even corrective action.

The cleanness of lubrication oil can strengthen the signal-to-noise ratio in a wear debris analysis. Without the background noise of dirty oil, weak signals can be detected.

Thus, correct positions should be selected for sampling points to identify particle or debris generating sources and potential locations for incipient problems. By sampling immediately downstream of a wear-generating source and upstream of filters or reservoirs, data is not stripped by filtration or muted by dilution.

The size and shape of particles is another important element in wear debris analysis. It is best to detect particles in their original size and shape close to their generating sources, such as bearings, gears and other particle generation points. Circulated particles produced long ago are not useful.

Wear mode

The identification of the wear mode and the location of a particle generation source is difficult for reworked and recirculated particles. Good places to find particles in their original form are in the sump sediment, magnetic plugs, chip collectors and filters.

Proper condition monitoring of lubrication oil in rotating machinery is rapidly becoming an accepted part of world-class operation, reliability and maintenance programs. It requires the examination of many parameters and factors, such as oil properties, contaminants, wear debris and more.

It is necessary to look carefully at many aspects to determine the condition and health of turbomachinery. As a rough indication, more than 50% of all failures and operational issues can be traced back to lubrication oil problems. ■