Introduction To Common Oil-Analysis Tests (And How To Take A Successful Sample)


Oil analysis is one of the most valuable condition-monitoring tools available to reliability-focused operations. Why? It can save countless dollars by preventing equipment failures and helping maximize the life of lubricants.

Oil analysis tells us the condition of a lubricant, which, in turn, lets us be proactive about replacing it before equipment damage occurs. Wear debris analysis—a specific type of oil analysis—also can tell the condition of equipment, which allows us to respond to potential failures at an early stage.

In short, the proper use of oil analysis can help:

  • Improve asset reliability
  • Identify and eliminate repetitive equipment problems
  • Reduce unscheduled maintenance
  • Maximize use of lubricants in service
  • Reduce maintenance and lubrication costs
  • Extend equipment component life

Tests for oil condition include:

  • Viscosity measures the resistance of a fluid to flow, is the most important property of a lubricant.
  • Karl Fischer measures all forms of water at low levels and is recommended for industrial equipment. (The Qualitative Crackle test is used for engine oils.)
  • Acid Number measures acid buildup, which denotes oxidation.
  • FTIR measures chemistry changes in a lubricant, which are good indicators of oxidation and nitration.
  • Base Number is for engine oils. It measures the depletion of the detergent, which neutralizes acids.
  • Flash Point is a measure of light components, which lower the viscosity of lubricants.
  • Particle Counts, by size and amount, are determined with the use of a laser counter.
  • Voltammetry measures depletion of antioxidants in lubricants.

Tests for equipment condition through the measurement of wear debris include:

  • Atomic Emission Spectroscopy measures metals in parts per million (limited to particles under 10 microns in size).
  • Ferrous Density, both direct-read ferrogram and particle quantifier, measures ferrous particles without the size limitation of emission spectroscopy.
  • Analytical Ferrography looks at size, shape and color (the three most important physical characteristics of a particle)to determine the wear mechanism and severity in machinery. This is the only common oil-analysis test that can justify equipment shutdown.

Sampling Guidelines
Any successful oil-analysis program begins with sampling. Remember, though, that bad data is worse than no data. Thus, if you're collecting samples incorrectly, an oil-analysis program is a waste of time and money. Key guidelines include:

  • Take warmed-up machine samples while equipment is running. If that's not possible, sample no later than 30 minutes after shutdown
  • Use clean, sealed bottles and flush the system properly (at least 5-10 times the sample-line volume). This is critical when running particle counts.
  • Collect samples from the right location:
    • Sample from live fluid zones. Fluid is moving through the system. The best spot is the return line back to the reservoir.
    • Sample, if possible, from a turbulent region (such as an elbow) to get better particle distribution.
    • Sample downstream from components such as bearings, gears, etc. Never sample after a filter, unless you're trying to determine filter efficiency.
    • Sample from the same location each time. This is difficult with static sampling, especially with the use of a plastic tube and vacuum gun. Use permanent pitot tubes, where possible, when conducting static sampling. Never do drain samples from the bottom of a reservoir.
  • Properly document samples on the sample bottle. It's not uncommon for incorrect information to end up on a sample bottle, which leads to erroneous results.
  • Send samples immediately to the laboratory. An unsent sample may contain information on potential problems that can't be identified until the sample is analyzed.
  • Properly establish sample intervals based on the following criteria:
    • Criticality is the most important factor. The typical interval for critical equipment is monthly.
    • OEM recommendations are important. Some OEMs provide sampling-interval guidelines.
    • Environmental conditions can dictate frequency. Severe environments necessitate more frequent sampling.
    • Current PMs and lube- and filter-changing schedules can also dictate sampling frequency.
    • Historical problems with the equipment (or similar equipment) may call for more frequent sampling.

What's Next?
During 2011, this column will explore some of the most important oil-analysis tests. In the next issue, the focus will be on Particle Count Testing. LMT

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