Polaris Lubricant Analysis
As a general rule, an effective oil analysis program should focus on leading indicators that anticipate and help prevent compressor problems rather than trailing indicators that only serve to alert the user to an existing problem. These leading indicators can vary depending on lubricant type but several parameters are universally important. They include TAN (Total Acid Number), pH, viscosity, and contamination. Since most of today’s new compressors are filled with Polyglycol based lubricants, the following comments are specific to Polyglycol based synthetic fluids and are intended to be general in nature.
TAN - Total Acid Number is a measure of the oil’s acid level and is a good indicator of the remaining useful life of the
fluid. New fluids will have a TAN value of around 0.10 and this value will increase as the fluid oxidizes and as a result of acid gas ingestion. The recommended oil change value based on TAN varies from 1.0 to 2.0 depending on the supplier. During normal oil oxidation, TAN values will increase gradually at a steady rate to a value of 1.0. Above 1.0, TAN values will begin to increases exponentially and can run-away in a relatively short period of time. It is a safer and more cost-effective practice to change the oil when TAN values are lower (between 1.0 and 1.2) rather than taking the risk that TAN values will run-away. If TAN values reach 2.0, it is generally recommended to drain and flush the compressor with a half charge of new fluid to remove any remaining acidic oil and minimize premature degradation of the replacement fill.
pH – pH is also a measure of the oil’s acid level with new oils having a pH value around 8.0 which gradually decreases towards a value of 5.0 as the oil oxidizes. A rapid or excessive drop in pH, while TAN values are otherwise normal, is an indication that external acidic gases or other oxidizing agents are being ingested from the atmosphere. When the oil’s pH value has dropped below a value of 4.5, the fluid should generally be changed to prevent corrosion.
Viscosity – Viscosity is a measure of the resistance of a fluid to flow. As oil degrades, its molecules will polymerize into larger molecules increasing its viscosity. A viscosity increase within 10% of the new oil viscosity is considered normal. An increase greater than 10% or a decrease in fluid viscosity is usually an indication of contamination or varnish or sludge formation with non-polyglycol based fluids. It is rare to change compressor fluids solely based on viscosity without other abnormal indications.
Contamination – Contamination monitoring is a broad term that from the standpoint of routine oil analysis can include ISO Particle Counts, DR Ferrography, Spectrochemical Analysis, Water Concentration (Karl Fisher or Crackle), Methanol Insolubility and many others. Of these, Spectrochemical and Water Concentration are low cost tests that are most commonly performed by oil analysis labs. In rotary screw compressor applications, testing for contaminants can often cause confusion in the interpretation of the results and should probably be left to very experienced users.