"What methodology should be used when deciding if you can extend preventive maintenance (PM) intervals?"

There are many methods for safely extending intervals. Each depends on the exact PM task you are trying to extend, but at the core of most is oil analysis. Repeatable and representative oil samples with the proper test slates are essential for evaluating the current practices and the possibility of extending those practices.

The first step is to take a deep look into the historical data you have for the component or group of equipment. Groups of equipment should be fundamentally the same and operate within the same environmental conditions. Depending on the type of equipment and PM task you are evaluating, certain trends and data will need to be analyzed. For example, if you want to extend drain intervals for a fleet of diesel engines, look at historical trends of spectrochemical analysis, fuel dilution, soot levels, oxidation, water, viscosity and base number.

Next, you will need a good way to compare/compile the data, such as with a spreadsheet. Once the data is in a spreadsheet, check the number and type of abnormal samples as well as any trends that develop as the oil ages and how soon after a change-out that those trends appear.

If the majority of results are deemed normal when the change-out occurs, the equipment may be a good candidate for extending the service. A good rule of thumb is to use a 25-percent increase in increment while verifying with more oil analysis. You want to ensure the new interval has not overextended the service, so a sample should be taken at the old interval and just before the change-out at the new interval.

The decision to extend intervals typically is driven by cost or savings. However, you will want to justify these changes. To do so, consider the following: oil capacity, oil cost, disposal cost, labor rate, oil analysis cost and how they are affected when the interval is changed.

There is always a risk associated when extending these service intervals. This risk can be mitigated somewhat with careful planning and a solid oil analysis program. Extending intervals can provide great opportunities for maintenance cost savings as long as it is done carefully and based on sound data.

One of the most popular interval extensions is the oil drain. Below are some guidelines for extending an oil drain based on oil analysis.

  • Set water maximum to 0.2 percent by volume.
  • Viscosity is within 10 percent of baseline.
  • Fuel dilution should be a maximum of 2 percent.
  • Soot levels are 25 percent below the manufacturer's maximum recommendation.
  • Base number should not fall below 50 percent of new.
  • Acid number should not increase 1.5 percent above new.