Protecting Compressor Oils & Components Against Water
As mentioned earlier in this series, there is an endless supply of water available in the air to contaminate and attack compressor oils and internals and drive-up maintenance costs. But with proper oil selection, monitoring, and maintenance, water can be controlled, and serious problems associated with water contamination avoided.
During normal operation, with the compressor operating at design pressures and temperatures and all compressor components functioning properly, water should remain dissolved in the oil and cause few problems. However, there are many abnormal issues that often arise to affect the oil-water equilibrium, forcing the water out of solution and resulting in undesirable emulsions and free water. Common examples include the compressor running unloaded for extended periods of time, running too cool, intermittent operation (with some oils), malfunctioning thermal valves, oil cooler leaks (water-cooled compressors), leaking discharge check valve, and more. So, here’s some tips on what to look for and how to prevent, protect and avoid water-related problems.
Condition Monitoring
As with most rotating equipment problems early detection is vital. Visual inspections and regular oil analysis are essential tools that every compressor user should utilize to monitor water in their compressor oils. By simply taking an oil sample and allowing it to sit and cool for a while, or looking in the compressor’s oil fill sight glass, users can easily see whether the oil looks translucent or cloudy or a layer of emulsion or free water begins to form. This simple visual inspection normally will provide a pretty good idea if water contamination may be a problem.
Regular oil analysis is also a good way to determine water contamination in the oil. The most common tests oil analysis labs use are the “Crackle” and “Karl Fisher” (KF) tests. The easiest and cheapest test to perform is the Crackle test, which as the name implies is a simple go/no-go test that detects emulsified and free water by putting drops of oil on a hot plate and listening for a “crackle” sound. This test can be performed in the field, and with a little experience users can determine if water levels have become excessive, and whether more sophisticated and accurate water testing is warranted.
The test most often used to accurately quantify water levels in compressor oils is the Karl Fisher (ASTM D1744) test. Usually performed in the lab, this test is more expensive employing wet chemistry titration that detects all forms of water – dissolved, emulsified and free. This method is extremely accurate above minimum water levels (> 200 ppm) which is when water contamination is of greatest concern.
For best oil analysis results and data analytics, oil samples should be taken from the same location every time, and from a flowing source while the compressor’s oil is at normal operating temperatures. Samples should never be taken from stagnant or cold oil sources located in the bottom of the compressor’s sump or its oil filter. Sampling from these locations can load the sample with excessive water, rust and other sediment, and would not be a good representation of the oil flowing through the compressor.
Prevention
As with all oil contaminants, steps should be taken to prevent them from entering the compressor. Unfortunately, preventing all water from entering the compressor is impossible, rotary screw air compressors should still be located in as cool, dry, and clean an area as possible. Special care should be taken not to locate compressors in the vicinity of cooling towers, boilers, steam traps, or other “wet” equipment or areas where excessive moisture or heat are present. The moisture holding capacity of the ambient air ingested by a compressor roughly doubles for every 20°F increase in temperature. So, it’s vitally important to locate air compressors where their inlet air temperature is minimized. Compressors located in hot equipment rooms are known to have more problems, higher maintenance costs, and live shorter lives than compressors ingesting cool fresh air.
In addition to ambient air temperature, the compressor’s discharge oil temperature also has a critical impact on the oil’s water content and the state in which the water exists in the oil. Rotary screw air compressors are intentionally designed to run hotter than most other rotating equipment in an effort to keep water in vapor form and prevent it from condensing inside the compressor. A good rule of thumb for rotary screw air compressors operating at 150 psig or less, is that they should have their discharge temperature around 100°F above the ambient air temperature to keep the water in vapor form and moving through the compressor. Unfortunately, there can be a number of reasons a compressor’s oil runs “too cool” and causes excessive water condensation and oil contamination. When a compressor runs too cool, some of the possible causes worth investigating include determining if the compressor is running unloaded for extended periods of time, if its thermal valve is malfunctioning, if improper cooling water flow rates or temperatures are being used (water-cooled compressors).
More often than not, excessive water problems in compressor oils are temperature related. So, when faced with excessive moisture in the oil, among the first things to look at are the inlet air temperature and discharge oil temperature.
Oil Selection
As mentioned previously, water concentrations in in-service compressor oils vary greatly depending on the oil type, compressor location and ambient conditions (temperature and humidity). For example, the water-tolerant PAG/POE compressor fluids often recommended for use in the hot and humid areas will normally have water concentrations (by KF) range from 2,000 – 5,000 ppm in the milder winter months, and up to 8,000 ppm or higher in the hot summer months. Compare this to the much less water-tolerant PAO and SHC compressor fluids, that when used in the same environment would have water contents of only 200 – 800 ppm throughout the year. This illustrates the importance of selecting a compressor oil that is best suited for the application and environment.
It’s also worth noting that due to these vast differences in the oil’s water holding capacity, it’s imperative that the oil analysis lab knows the type of oil they’re analyzing. Without knowing the oil type, a lab could mistakenly condemn and recommend replacement of a perfectly good PAG/POE fluid thinking it has an abnormal water content.
Removal
Once excessive water or any of its symptoms are detected, compressor users should immediately investigate to identify the source of the water and correct the problem before major damage can occur. Unfortunately, draining and replacing the oil alone will do nothing to solve the problem. Without the source of the water corrected, the new oil will quickly return to the old oil’s waterlogged state.
Similarly, any efforts to try to remove water from the oil using a water-removal filter without correcting the problem will prove ineffective in the near term. The water holding capacity of water absorbing filters is extremely limited compared to the endless supply of water in the air. These filters simply can’t keep up and water levels will soon return to their pre-filtration levels.
Additive Replenishment
One of the least detected and yet most insidious effects of water contamination on compressor oils is additive depletion. Even in its dissolved and relatively benign state, water will chemically react and strip the oil of key protective additives, degrading its performance and dramatically reducing its useful life. Fortunately, these additives are easily replenished to restore the oil’s protection against oxidation and corrosion to its new oil levels. Additive replenishment is especially well suited for the PAG/POE synthetic blends with their high solvency and varnish-free properties. For users of these fluids, additive replenishment is also recommended for normal operation, even with no known water-related issues.
Conclusion
Water is one of the most harmful contaminants found in rotary screw air compressor oils. Often overlooked, water is a leading cause of oil degradation and compressor reliability problems resulting from increased corrosion, wear and fouling – the root causes of most compressor maintenance and repairs. However, armed with information on how to detect and correct water related issues, compressor users can significantly improve their compressors’ reliability, availability, and maintenance costs.