Whether in automotive or industrial applications, filters find widespread applications and high efficiency filters are able to removal of particles down to 1 micron possible and at time less than this.
While this has had tremendous benefits to machine reliability, there are some hidden downsides. Much of the debris that would have been detected in oil analysis is now retained by the filtration system, potentially reducing the efficacy of an oil analysis program. If this were a crime scene, the clean up crew has processed the site before the investigation team could collect evidence. If the oil analysis laboratory knows the type of filtration in use, limits and expected rates of change can be adjusted to help compensate for the impact on results. There is however another option that is often overlooked, filter analysis.
Filter analysis is particularly helpful if there is a known or suspected issue on a piece of equipment or if it is on critical equipment. An example of the latter is filter analysis is used extensively in aircraft engine condition monitoring programs, however any end user using filtration can benefit.
In its most basic form, a filter can be cut open using fit for purpose filter cutters onsite and inspected. A significant amount of wear metal from a component not expected to wear significantly would be cause for concern. Depending on the level of expertise available, the extent of the problem as well as wear modes may be determined. The latter typically requires a microscope and increased skill levels.
In many cases, it would be a better option to speak to your oil analysis laboratory for testing options. ALS has several laboratories well equipped for such analysis and either a piece of the filter element or the whole filter may be submitted. The latter option allows the filter to be opened using the correct tools in a clean environment so is often preferred.
The following testing options should be considered:
A very good starting point would be a Microscopic Particle Evaluation (MPE), where particles are washed from the filter element and either deposited on a glass slide using magnetic fields to assist in concentrating ferrous wear in different size classes or on a membrane filter. An expert then uses a high powered optical microscope to carefully examine and report on the material observed with are taken to differentiate between different wear modes for wear metals present. This is known as Analytical Ferrography. Magnified photographs of the debris are included on the report.
Elemental Analysis also provides a wealth of information. Debris collected form the filter is digested in powerful acids and after dilution, the solution is run through a ICP spectrometer to determine the elemental composition. This provides insights into the types of metal trapped by the filter and can assist in determination of the specific part that is wearing in the system.
The concentration of debris from the filter can be measured using the Retained Solids test. The filter material is washed with solvent and the debris is collected. This is then refiltered on a laboratory grade filter membrane and the mass of debris precisely determined. This would assist in determining the extent to which a filter is loaded.
The PQ Index can also be determined on an area of filter material. This gives an index of the amount of ferrous wear present and should filter analysis be routinely conduced on a compartment, trend analysis would be available.
Additional tests including SEM EDS may be available in your region. SEM EDS is an electron microscope coupled with an x-ray analyzer that allows for the elemental composition on a particle by particle basis.
For further information or to establish a proactive testing program for your oil, coolant, or fuel in critical operating equipment feel free to contact ALS Tribology laboratory or any of our regional testing labs.
Paul Swan, MLA I, MLA II, BSc (Hons) MSc (Chemistry)
Regional Manager, Tribology