While many people may be familiar with particle counters and the techniques for measuring moisture in an oil sample, few are aware of the methods for assessing the amount of air in oil or even that air is a serious form of contamination. In certain situations, air contamination has the potential to be very destructive, and its effects on oil and machinery deserve more attention.
Air can be challenging to quantify. It may exist as a contaminant in the following states: dissolved, entrained, free and foam.
The normal level of dissolved air for mineral oils is 10 percent by volume. High levels of dissolved air from pressurized oil accelerates additive depletion and oxidation.
Entrained air can be characterized as unstable, suspended microscopic air bubbles in oil, which results in clouding of the oil. Entrained air has the potential to impact the oil’s compressibility, heat transfer, film strength, oxidation, cavitation and varnishing (microdieseling). As an oil sample sits on a counter, the entrained air may rise to the surface. This is true for any oil in a machine that allows time for detrainment.
Free air may be found wherever there are trapped pockets of air in dead zones, high regions and stand pipes. It can affect hydraulic compressibility, corrosion, vapor lock (retarded oil supply) and loss of system controls.
Foam occurs when the oil is more than 30 percent air. It may be seen on the fluid surfaces of highly aerated tanks and sumps. Excessive foam can ooze out of a machine and/or cause hydraulic compressibility issues, corrosion, vapor lock and loss of system controls.
Although all states of air in oil can be harmful, entrained air has arguably the greatest potential to cause damage, as it can increase foam potential, oxidation, pump cavitation, varnishing, erratic hydraulic response and fluid flow, and even overheating.
A number of tools are available for measuring air quantities in oil. One type of device is designed to measure the air content within hydraulic lines. It works by creating a vacuum in a collected volume of oil. This vacuum separates the entrained and dissolved air from the oil. Once the air is expelled from the oil, the resultant oil volume is compared to the original volume to calculate the amount of entrained or dissolved air.
Another type of instrument can be used to measure aeration for online monitoring. The principle behind this technology is based on X-ray transmission. Oil circulates through a chamber, which allows the instrument to perform online measurements. Data is reported in standard conditions such as 20 degrees C and 1 bar.
Other devices offer quick testing for entrained air by measuring the pressure changes in a compression piston chamber. These instruments are effective for industrial use to determine where a trouble spot exists on a line.
In addition to measuring the concentration of air in an oil sample, a variety of tests can be performed to assess other air contamination factors. For instance, the air-release test can be used to determine the tendency of an oil to retain entrained air. It is based on ASTM D3427.
Compressed air is blown into an oil sample using a defined method. The time required for the air to be reduced to 0.2 percent by volume is then measured. While the oil’s viscosity will be one of the main factors in the air-release time, other variables related to the base oil formulation can influence these results as well.
Another method measures an oil’s ability to separate from water. In the ASTM D1401 standardized procedure, the test oil is mixed with equal parts water and left in a graduated cylinder to separate. The faster the separation time the better. Although this is the most common test for demulsibility, the ASTM D2711 method is typically more effective for lubricants with viscosities above ISO 220.
Oils with air-handling concerns usually have demulsibility problems. In other words, the causes of impaired demulsibility are often common to air-handling issues as well.
A third alternate test is based on ASTM D892. For this foam tendency/stability test, air is blown into an oil sample to produce foam. The foaming tendency and stability are then measured at 24 degrees C, 93 degrees C and then again at 24 degrees C. The initial foam volume is measured after each blowing (foam tendency) and again five minutes later (foam stability). See the table above for more details on this procedure.
Although testing an oil’s air content may never be part of a standard test slate for routine sampling, this does not mean that it is unnecessary. Because of the nature of aerated oil, the air concentration that can be tested depends on the time in which it is allowed to sit undisturbed. This makes the typical method of testing oil, like collecting oil in a sample bottle, nearly irrelevant for air concentration testing. Testing for air is all about timing, so if any of these methods are to be applied, be sure to know the unique design requirements.
Regardless, a particular machine condition or recognized operating state can be more than enough cause to investigate and quantify air contamination. If foam is a persistent issue or an excess amount of aerated oil is seen through an oil sample inspection or sight glass, a careful investigation should be conducted to determine the source of the issue.
There may be an easy solution, such as using a more appropriate lubricant formulation, or it may be more challenging, such as a machine design error. In any case, it’s important not to overlook this valuable information, because air contamination can have very destructive effects.