Find Answers In Oil Filters

Used oil filters can play a major role in your condition-based maintenance strategy.

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

Tools such as vibration analysis, ultrasound, oil analysis, and thermography are mainstays of a condition-based maintenance strategy. Each provides a unique perspective on the health of critical rotating and non-rotating assets. Vibration and oil analysis are particularly useful when assessing rotating equipment such as pumps, gearboxes, and circulating bearing-lubrication systems, while oil analysis is critical when monitoring hydraulic fluids. In the world of condition monitoring, the complementary data provided by these two techniques is a case of one and one equals three. Filter analysis (monitoring circulating lube oils and hydraulic fluids) is the third leg to the stool.

Because of the impact that contamination has on equipment life, any circulating system needs an oil filter. The filter traps any particle in the system larger than the filter’s micron rating. It’s that basic function that makes filter analysis so valuable as a condition-monitoring tool.  

Unlike oil or vibration analysis, which are snapshots of what’s happening within the system at the instant a sample is extracted or the vibration signature recorded, filter analysis is an historical record of everything that has happened in the system since the last time the filter was changed. Since some filters don’t need to be changed more than once a year, that’s a lot of historical evidence.

In general, filter analysis can identify three types of problems:

• large wear debris caused by active machine wear

• ingressed particles from poor sealing, intrusive maintenance, or ineffective breathers

• solid and semi-solid particles created by lubricant upset such as excessive oil degradation, accidental cross contamination with an incompatible fluid, or a reaction between the oil and an ingressed process fluid or cleaning agent.

Effective filter analysis uses many of the same tests involved in oil analysis. These include elemental analysis, which looks for the presence (or absence) of specific elements such as iron, copper, or lead as wear elements or calcium, zinc, or phosphorus, which are common additive elements. Unlike conventional oil analysis, specialized elemental tests are used, such as x-ray fluorescence (XRF) or energy dispersive x-ray (EDX) analysis. These tests are often performed in conjunction with scanning electron microscopy to image specific particles trapped within the filter media.

Fourier Transform Infrared Spectroscopy (FTIR) is often used to look for molecular “fingerprints” of organic materials such as degraded base oils, stripped additives, or chemical contaminants. Since FTIR instruments are often equipped with a “library” of known organic compounds, unexplained inorganic residue within a filter can often be identified.  

Unlike conventional wear-debris analysis, analytical ferrography, which uses wear-particle morphology (size, shape, color texture), can be deployed to look at large wear-metal particles trapped within the filter. This can often be helpful when trying to determine the root cause of active machine wear as part of a root-cause failure analysis (RCFA).

Despite adding this offline filtration system to improve cleanliness and reliability, a food manufacturer continued to experience issues. Deposits in the oil filter provided the answer.

To understand the value of filter analysis, consider the following examples:

Paper machine bearing failure: A paper mill experienced a catastrophic failure of a suction roll bearing. After investigation, the oil supply line to the bearing was found to be plugged with a grease-like material, resulting in lubricant starvation. A similar deposit was found in the main supply-line filter, which had plugged and gone into bypass. The plugged filter was removed and sent to a lab for analysis alongside in-service oil samples.

The oil sample showed no indication of the root cause of the problem. The semi-solid residue from the filter was analyzed using FTIR and it showed a match for a stearate soap material and traces of sulfur, zinc, and phosphorus from the oil’s additive package. Upon detailed chemical analysis, the problem was diagnosed as a chemical reaction between the oil’s antiwear additives and detergent used to clean the paper machine. The detergent entered the lube oil system during routine cleaning.

Lube-oil system filter plugging: A chemical plant was experiencing foaming issues in a circulating lube-oil system. An accelerated number of filter change outs suggested that contamination was causing filter plugging and foaming. To control foaming, an aftermarket foam inhibitor was used with little effect.  Samples of the in-service oil before and after adding the defoamer, along with a portion of the filter and a sample of the antifoam additive were submitted to a lab.

Elemental analysis using rotating disk electrode (RDE) atomic emission spectroscopy and a microscopic examination of the filter sample revealed high levels of silicon in the filter, as well as a visible amorphous white residue within the filter media indicating that filter plugging was due to stripping of the antifoam additive.

Hydraulic filter plugging: A food manufacturer decided to switch from a conventional mineral-based AW46 hydraulic fluid to a synthetic food-grade lubricant. At the same time, an offline filtration system was added to try to improve overall system cleanliness and pump and valve reliability. Despite these changes, cleanliness of the oil, as measured by ISO particle counting, became worse and valve failure started to occur. No evidence of any issue was seen in routine oil analysis, except for elevated particle counts.

After removing and examining the offline filter it was found that the filter media was coated in an inorganic material. X-ray fluorescence showed high levels of zinc and phosphorus and what appeared to be oil-degradation byproducts (chart, p. 20). After comparing the results with a test for oil-varnish potential, it was concluded that the new oil increased solvency levels to the point where old deposits caused by degradation of the old AW46 fluid were entering the oil as “soft” particles and plugging the filter. After a thorough flush/clean, the unit was put back into service with much-improved results.

Filters are often considered consumable items for use and disposal. However, next time you think about throwing out an oil filter, consider the trove of useful data that it may contain and how analyzing it may serve as an invaluable tool, alongside more conventional condition-monitoring practices.

Thanks to Rich Wurzbach and Dylan Kletzing of MRG Laboratories, York, PA (mrgcorp.com), for providing some of the data and images included. EP

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.