Oil Analysis: An essential part of engine monitoring programs

An essential part of engine monitoring programs By Joe Escobar Routine oil analysis has become quite the standard in our industry. Many of the engine manufacturers recommend regular sampling as part of a preventive maintenance program. The...

Filter analysis
Rory Hammond, owner of Aviation Laboratories in Kenner, LA, stresses the importance of oil filter analysis. "The oil filter analysis is able to detect and identify the larger contaminants that are present in the oil," says Hammond. "If an engine is experiencing wear that produces particles larger than 10 to 15 microns, a spectrometer analysis is not going to detect all of it, since most turbine oil filters remove debris down to 10 microns in size. Unless the customer includes an oil filter analysis, some types of problems may go undetected."

Performing an oil filter examination is a structured inspection. The lab technician removes the oil filter from the shipping container and plugs it to prevent any contamination from entering the interior of the filter. He then places it in a clean container, adds a solvent to it, then shakes it to dislodge any debris in the filter. The particle-laden solvent is then passed through a patch filter and the step is repeated. After these initial solvent washes, the filter and container are placed in an ultrasonic cleaner to remove any remaining contaminants. This solution is also passed through the patch filter. The oil filter is then carefully inspected for any debris that may still be present and it is removed as necessary. Now the particle analysis begins.

Analyzing the particles
Hammond states that performing an analysis on a patch filter entails a triage of steps. First, the filter pad is visually inspected to get a brief overview of what is present. Any large particles are separated when possible for later analysis. Chemical spot tests can then be done to aid in material identification. Once the larger materials have been separated and materials have been identified using chemical spot tests when possible, the electron microscope analysis begins.

The electron microscope is a high magnification microscope able to magnify up to 20,000 times - far more magnification than is possible with an ordinary optical microscope. An X-ray spectrometer in the microscope aids in the identification of the debris. So the technician is able to determine from the spectra present the exact Aerospace Material Specification (AMS) number of a particle - thereby determining exactly what alloy it is. The technician is not just able to determine that there is aluminum or stainless steel present, but exactly what alloys or grades they are. This specific identification comes in handy in determining origin of the particles.

There are four criteria that are evaluated when analyzing the particles present in a sample. These are type, form, amount, and condition. By using all of the information from these four evaluations, the inspector is able to determine debris origin.

  • Type - This is the specific identity of the material. Determining the identity is the first step in evaluating particulates.
  • Form - This is the specific form of the particles. The forms can vary from fine wear, machining chips, and slivers to chunks and platelets. The form of the material is crucial in determining its origin. For example, carbon steel platelets can indicate bearing wear, whereas carbon steel fine wear can be indicative of shaft wear.
  • Amount - This is the amount of the material present in the test sample. It can be expressed by weight of the material or by particle count.
  • Condition - The most important factor on particle condition is oxidation. If the particles exhibit signs of oxidation, they probably are not recently generated particles. Particles that were generated in the past but are no longer being produced can lead to different disposition when considering corrective action.

Check that chip detector
The chip detector can be a valuable tool in monitoring engine health. Just because there aren't any large chunks present doesn't mean that the fuzz present can't be analyzed. The process of removing the debris is fairly simple. It involves using a strip of tape provided by the laboratory to pull the debris off the chip detector. Although this tape looks similar to transparent tape, it is different in its composition with lower adhesive properties. This allows the technicians at the laboratory to easily remove the particles from the strip and analyze them using an electron microscope as discussed earlier.

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