"We employ oil analysis for many of our screw-type, oil-flooded compressors. The original equipment manufacturer (OEM) recommends an oil change after 4,000 hours if a mineral oil is used and 8,000 hours if a synthetic oil is used. It is also recommended to change the oil filter and separator at 8,000 hours. The problem is that when we analyze the used oil, the data indicates that an oil change should take place in much shorter intervals, even half of what the manufacturer recommends. Typically, the acid number and viscosity have increased, there is some moderate water contamination, and the additive concentration has been greatly reduced. All these indicate that an oil change must take place. If this is the case, why do OEMs continue to give the same recommendations regarding oil change intervals?"
OEMs recommend specific oil change intervals based on tests performed during the equipment development or lubricant approval process. The results of these tests on several machines lead to conclusions for the recommended intervals.
If your oil analysis program indicates that the oil should be changed in intervals that are not consistent with the OEM recommendation, there are a number of questions you should ask to identify the reasons.
First, is your oil analysis program using the same tests, baselines and limits to determine the lubricant condition? This is important in order to compare "apples" to "apples."
Are the compressors working in the expected operating conditions (discharge pressure, flow rate, etc.) and environment (ambient temperature, ambient moisture, etc.) according to the OEM's guidelines?
Is the in-service lubricant the same as what the OEM recommends for the specific drain interval? If not, does the lubricant have the same specifications and claims as the recommended one? Lubricant specifications may change over time, even if it is the same name or brand. If there is a significant change in the specifications, there may also be a change in the performance.
Is any cross-contamination occurring as a result of top-ups with a different lubricant, the presence of residue from a previously used oil or a varnish problem?
Also, be sure to consider the maintenance program. Are high-quality oil filters being used? Has there been a major repair or intervention that may be affecting the operation of the compressors?
The answers to these questions should provide guidance for determining the proper oil change intervals. If you still have concerns, you may need to perform a root cause analysis.