Air should always be considered an unwanted contaminant in oil as it can result in several negative effects. For instance, the oil in use can become compressible, an undesirable condition for multiple application like circulation and hydraulic systems. An influx of air also speeds up the oxidation process, which reduces the active service life of the oil.
However, air contamination can have other unwanted impacts. It can produce cavitation and micro-dieseling process, which generate soot within the oil and increase the level of the oil. When air levels rise significantly, it can even cause oil leaks, creating risks to health and safety.
As a rule, air contamination is not usually measured. This is because when contamination is identified, the appropriate action is to eliminate its root cause, whether it is related to the condition of the oil or caused by poor equipment design or excessive turbulence.
However, the standard method used in laboratories to measure the tendency of an oil to foam is ASTM D892. This built-for-purpose test can determine the volume of foam produced after air is blown into an oil sample under controlled conditions. The test also reports how much air volume remains in the sample after a 10-minute-long settling period has passed.
Although many operators may understand the nature of particle counters and techniques designed to measure moisture in a sample of oil, very few know the specialist techniques devised to assess the amount of air present in oil, or are aware that air is an exceptionally severe type of contamination. Under certain circumstances, air contamination possesses the potential to be incredibly destructive, and its impact on oil and the machinery it serves requires critical attention.
In this blog, we’ll put measuring air contamination under the microscope. We’ll examine the different states of air contamination before taking an in-depth look at the tools and techniques employed in measuring.
What are the different states of air contamination in oil?
Air is often a challenging element to quantify. Air contamination can exist in four separate states – foam, dissolved air, air pockets and entrained air.
Foaming will typically occur when the composition of oil is over 30% air. It can be observed on the surfaces of fluids in highly aerated sumps and tanks. Excessive foam can also ooze out of a mechanical systems and cause issues concerning hydraulic compressibility, but also corrosion, vapour lock and even system control loss.
At normal levels there should be no more than 10 per cent dissolved air in oil volume. Higher levels of dissolved air caused by pressurised oil can accelerate the oxidation process, lessening oil lifespan but also increasing additive depletion reducing performance of the oil’s properties.
Free air can be found present where trapped pockets of air exist in dead zones, standpipes, and high regions. It can negatively impact hydraulic compressibility, cause loss of control systems, hinder oil supply and lead to loss of system controls.
Entrained air is characterised as unstable microscopic air bubbles suspended in the oil. Visually, the oil appears cloudy. Entrained air can impact the compressibility of the oil but also reduce its effectiveness as a heat transfer fluid and the film strength it supplies. It also increases the chance of oxidation, varnishing and cavitation.
While all these states of air contamination in oil are potentially harmful, experts consider entrained air to be the most likely to result in damage. This is because it can increase the potential of oxidation, foam potential, pump cavitation, micro dieseling, erratic fluid flow, hydraulic response and system overheating.
What tools are used to measure air contamination?
Several tools measure air quantity in oil volume. One device type is designed to measure air content in hydraulic lines. It creates a vacuum in a volume of oil. The vacuum separates both dissolved and entrained air from oil and, once expelled, the oil volume remaining is compared to the previous volume to calculate the quantity of air contamination.
Another instrument employed to measure air contamination for online monitoring is X-ray transmission technology. Oil passes through a chamber allowing the instrument to take measurements online.
However, many other devices provide quick tests for entrained air, measuring pressure changes inside compression piston chambers. Such instruments are useful in industrial applications to find problem areas on a line.
What are the different methods used to test air contamination?
Along with measuring air concentration per oil sample, several tests exist to assess other factors involved in air contamination.
ASTM D3427
Also known as the air-release test, ASTM D3427 can determine an oil’s tendency to retain entrained air. Testers blow compressed air into an oil sample via a pre-defined method. Then the time needed for air to reduce to 0.2% by volume is measured. Although the Viscosity Index (VI) of the oil is among the key factors affecting air-release time, there are other variables connected to the formula of the base oil that can influence results.
ASTM D1401 and ASTM D2711
Methods also exist to measure how well an oil separates from water. ASTM D1401 is the standard procedure that involves the test oil being mixed with water in a 1:1 ratio and left inside a graduated cylinder where it can separate. The quicker the separation time is, the better. While this is by far the most common method for testing for demulsibility, another method, ASTM D2711, is considered more effective for oils that have viscosities over ISO 220.
ASTM D892
Finally, the ASTM D892 method is a test measure of an oil’s foam tendency and stability. Air is blown into the oil sample to generate foam. Both the oil’s foam stability and foaming tendency are measured at intervals – first at 24 C°, then at 93 C°, and then back at 24 C°. The oil’s initial foam volume must be measured after every time air is blown to test foam tendency and once again five minutes after to test foam.
While testing the air content of oil may not always be included in standard routine sampling, this doesn’t mean it is unnecessary. Due to the nature of oil when it has become aerated, the air concentration available for testing depends on how long the sample is allowed to remain undisturbed. This can make the typical methods of testing oil, such as collecting it in a sample container, almost irrelevant for calculating air concentration. Timing is crucial for testing for air, so operators using these methods must understand the design requirements involved.
However, a specific machine condition or identified operating state often provides more than sufficient cause to both investigate and measure air contamination levels. If foam becomes a persistent issue or operators identify an excess amount of air contamination through either an oil sample examination or sight glass, a meticulous investigation should be immediately performed to determine the origin of the problem.
Fortunately, in many cases, an easy solution may be possible like deploying a more appropriate lubricant, although it can sometimes be a more challenging issue like an error in machine design. Regardless of the root cause and remedy, when signs of air contamination arise, taking measurements is often the first step to eliminating the problem before greater harm occurs.
