Oil analysis is an essential tool used in monitoring and maintaining the health and reliability of rotary screw air compressors. By regularly analyzing your compressor’s oil, you can save money by optimizing your oil change intervals, identifying potential problems, and taking corrective actions before they lead to costly maintenance, repairs, and downtime.
But compressor users should know that not all oil analysis test packages are the same. The standard industrial oil analysis packages often provided for free by oil suppliers, are inadequate and not specifically tailored for the unique design and challenges of rotary screw air compressors. To get the most out of your compressors’ oil analysis, we recommend the following oil analysis tests and methods be employed.
Viscosity: Viscosity is a measure of the oil’s resistance to flow and is one of the most important parameters in compressor oil analysis. If the oil becomes too viscous, it can lead to reduced flow, increased friction, and higher operating temperatures. If the oil‘s viscosity is too low, it may not provide adequate film strength, lubrication, and protection against wear.
While in service, oil viscosity will normally increase 10% – 20% from its new oil value as its more volatile components evaporate and ultra-fine solid contaminants accumulate. An increase greater than 20% or a decrease in viscosity is considered abnormal and needs to be investigated.
Certain compressor oils, like those formulated with mineral oils, synthetic hydrocarbons (SHCs), or PAO base stocks are susceptible to forming varnish, which is normally preceded by an increase in viscosity. So, by monitoring the oil’s viscosity you can detect varnish early and avoid more costly problems with the oil and compressor.
Acid Number: The oil’s acid number (AN) or TAN is a measure of the concentration of acids present in the oil. It is a leading indicator of the oil’s oxidative state and a reliable predictor of its remaining useful life. For maximum accuracy and repeatability, the preferred AN test method is the ASTM D664 “potentiometric” test. Many oil analysis labs offer the faster, cheaper, less accurate ASTM D974 “colorimetric” test, which is more subjective, less repeatable, and generally not recommended for air compressor oils.
As compressor oils oxidize from exposure to air and their protective oxidation inhibitor additives steadily deplete, they produce weak acids that accumulate in the oil to increase its AN. In “normal” environments, absent any acid-gas contaminants, the oil’s AN will steadily increase until it reaches its condemning limit, which is typically 1.0 above its new oil value.
The primary benefit of the AN test is that it provides early indication of how far and how fast the oil has oxidized to allow users to plan their next oil change. However, AN only tells part of the “acid story” since it does not differentiate between weak and strong acids or provide any information on the oil’s corrosion potential. This limitation becomes evident when air compressors ingest “acid-gas” contaminants that are often found in industrial environments and are typically stronger and more corrosive. Even the slightest concentration of strong acids ingested from the air can make the oil highly corrosive to compressor internals. But because their concentration is normally low, the oil’s AN may not increase to alert the user of pending corrosion. Relying on the AN test alone can give users a false sense of security, that their compressor’s oil is still “normal” and safe to operate. A good oil analysis test package for air compressor oils should always include additional acid tests, like pH or SAN (Strong Acid Number) to be viewed in tandem with the standard AN.
pH and SAN: Unlike the AN test, the pH (“apparent” pH) and SAN tests are specifically designed to detect strong acids in compressor oils. Strong acids are not formed in the compressor but are ingested as trace acid-gas contaminants in the air. Examples of acid-gases that can destroy air the oil and compressor internals include diesel engine exhausts, cooling tower exhausts (water treatment chemicals), process vapors, welding fumes, herbicides, and insecticides.
New compressor oils contain no strong acids and will normally have a pH between 7.0 – 8.0 (slightly basic) and a SAN value of zero. Then once the oil is put into service, mixes with the air, and acids begin to accumulate, the oil’s pH will gradually decrease (becoming more acidic), and its SAN will gradually increase. In “normal” environments absent acid-gas contaminants, the oil’s pH and SAN values will gradually move in their respective directions but will never reach their condemning limits. Meanwhile in these environments the AN will reach its condemning limit first and be the normal determinant of when the oil needs to be replaced.
However, in “abnormal” environments where strong acid-gases are present and ingested by the compressor, the oil’s strong acid indicators of pH and/or SAN, will normally reach their respective changepoints long before the AN reaches its. Depending on the oil’s formulation, the typical changepoint before the oil becomes corrosive is a pH of 4.5 and a SAN of 1.0.
Water Content: Water is a harmful contaminant that is present in all air compressor oils. It exists in different states (dissolved, emulsified, and free) and in amounts that vary greatly from 100 ppm to 8,000 ppm or higher. Consult your oil supplier to learn the normal range and maximum water level (saturation point) of your oil.
As noted, water levels will vary depending on many factors including the oil’s base stocks, the time of year, the compressor’s location, operating pressure, and temperature. Excessive water levels in compressor oils can lead to serious problems resulting from reduced lubrication, increased wear, increased corrosion, accelerated additive depletion, reduced oil life, and the formation of harmful sludges. Monitoring the oil for water enables users to correct and avoid these problems before they occur.
The two most common tests used for detecting water in oils are the “Crackle” and “Karl Fisher” (KF) tests. While the KF test is much more precise, the Crackle test is more than adequate for routine testing of air compressor oils where all you need to know is whether the water level is approaching its saturation point or condemning limit. Even visual inspections of an oil sample will allow users to determine if the oil has reached this limit. Unsaturated oil samples will have no visible water layer on the bottom and will be translucent, like coffee without cream. On the other hand, oils that are saturated with water will look cloudy or milky in appearance and will have a water layer settle to the bottom given time to separate.
When excessive water is detected, the first step should not be to change the oil, but rather investigate to determine why the water levels are too high. More often than not, the answer to high water levels is related to the compressor running too cool, or unloaded for extended periods of time, or problems with the compressor’s condensate drains. Simply changing the oil without first identifying the source and correcting the problem only ensures that the new oil will quickly return to its saturated state and money will have been wasted.
Spectrochemical or Elemental Analysis: Spectrochemical or Elemental analysis measures the concentration of 20 or more metallic elements that are dissolved or suspended in the oil. It can detect elements up to about 8 microns in size and reports them in ppm. Spectrochemical analysis is a fast and inexpensive test, and while it produces a lot of data and takes up a lot of space on the report, there are only a few elements worth noting.
The elements reported in Spectrochemical analysis are typically grouped into one of three categories – wear metals, contaminant metals, and additive metals. Within these categories, normally only the following elements deserve your attention:
- Wear Metals
- Iron, Chromium, Molybdenum
- Contaminant Metals
- Aluminum, Boron, Calcium, Copper, Magnesium, Potassium, Silicon, Sodium, Zinc
- Additive Metals
- Barium, Calcium, Phosphorus, Silicon
Metallic elements should be viewed individually, compared to their new oil values, and monitored for any abnormal changes. These elements should also be viewed alongside the oil’s other properties to see if there are any correlations. Many of the oil properties shown on an oil analysis report are interrelated, with a cause-and-effect relationship where the movement of one parameter can be explained by the movement of another. For example, if an oil suddenly turns acidic as indicated by high AN or low pH, and at the same time you see a large jump in the contaminant metal boron, don’t assume the two are isolated events and not related. Boron is used to make boric acid, which is commonly used in weedkillers and if ingested by the compressor will introduce acids into the oil. There are many other interrelated oil properties that when viewed collectively can provide even greater insight to your oil and compressor’s health.
ISO Particle Count: The ISO particle count test is a measure of the oil’s “cleanliness” and is determined by counting the number of ferrous and non-ferrous solid particles, across various micron size ranges. The two most common test methods used for ISO particle counting are the “Laser” Light Scattering method, and the “Pore Blockage” method. While the Laser-light method is preferred by many labs, dark oils and/or oils with entrained air or water (most air compressor oils) can interfere with the optical counters used and produce erroneous results. For this reason, the Pore Blockage method which is unaffected by air, water or color is recommended for air compressor oils.
The relationship between oil cleanliness and the life of the oil, bearings, gears, and shaft seals, is well documented – they all last longer with cleaner oils. Similarly, an air compressor’s air-oil separators, oil filters, and oil coolers also last longer when its oil is cleaner.
In air compressor oils solid contaminants can originate externally from the air ingested by the compressor, and internally generated as a result of corrosion and wear. If the compressor is located in a dirty-dusty environment as many of them are, or if it has a history of shortened air-end or separator life, you will want to consider adding the ISO Particle Count test (Pore Blockage method) to your oil analysis test slate.
Direct Reading (DR) Ferrography: Alternatively, compressor users who suspect excessive bearing wear or corrosion may already be occurring should consider the Direct Reading (DR) Ferrography test. This test, which is a little more costly than the ISO Particle Count but arguably more useful, measures the amount of ferromagnetic wear and corrosion debris in an oil sample. It reports these particles as dimensionless numbers “DL” for larger particles greater than 5 microns (normally associated with wear) and “DS” for smaller particles less than 5 microns (normally associated with corrosion).
These values are viewed individually to indicate the relative amount and size (large or small) of ferrous particles present. They are also viewed as a ratio of DL/DS to indicate the severity of any active wear. For example, an oil with DL and DS values of 5 and 5 respectively would suggest low levels of ferrous contamination from either wear or corrosion. Compare this to a sample taken months later that shows an increase in the DL and DS values to 50 and 15, which would suggest accelerated active wear and increased corrosion.
Summary
Oil analysis is a cost-cutting tool that every compressor user should utilize to monitor and maintain the health and reliability of their rotary screw air compressors. A well-designed test package specifically tailored for these compressors should include Viscosity, Acid Number (D664), pH or SAN, Water Content (Crackle), Spectrochemical analysis, and when warranted by the compressor’s environment or concern over wear – ISO Particle Counts (Pore Blockage)or DR Ferrography.
For maximum benefit, oil samples should be taken from the same “flowing” location each time, at regular intervals at least every 2,000 hours (in normal environments), or more frequently in acid-gas environments or where typical oil life is less than the oil’s rated life – typically 8,000 hours.
Oil analysis parameters are best viewed individually, as a snapshot of the oil’s actual condition, and over time to look for any alarming trends. Many parameters, like viscosity, AN, pH, and element metals, should be viewed collectively when any one of them is flagged as “abnormal.” With a little training and practice, compressor users can become experts at interpreting their oil analysis results and maximizing the reliability-enhancing and cost-saving benefits of compressor oil analysis.