36
Measurement and Testing
WHY THE AIR RELEASE PROPERTY OF AN OIL IS AN IMPORTANT CHARACTERISTIC TO MEASURE AND UNDERSTAND FOR LUBRICANTS
Introduction The ability of lubricating oils to manage entrained air is a critical performance characteristic, as excessive air entrainment can lead to various issues, such as reduced lubrication eff ectiveness, increased wear, accelerated oxidation, and cavitation damage. Koehler’s Automated Air Release Value Analyzer provides a standardized and controlled testing environment conforming to methods like ASTM D3427 to quantify this crucial property. Air can coexist in oil in three states: dissolved, entrained, and foam. Dissolved air refers to the microscopic air bubbles dispersed throughout the oil at the molecular level, accounting for up to 10% of the oil’s total volume [9]. This contamination is common in new and in-service lubricants, becoming problematic as high levels of dissolved air from pressurized oil accelerate additive depletion and oxidation [9]. Entrained air consists of bubbles smaller than 1mm dispersed in the oil, causing a cloudy appearance. Entrained air bubbles act as nucleation sites for dissolved air to form larger bubbles, which rapidly collapse, generating shock waves that can severely damage metal surfaces. This type of contamination is potentially the most damaging, negatively aff ecting the oil’s compressibility, heat transfer, fi lm strength, oxidation, cavitation, and varnish formation [9]. The third state, foam, refers to 1mm bubbles accumulating as a stable layer on the oil’s surface. While surface foam may not cause signifi cant damage in some systems, it can lead to hydraulic compressibility issues, corrosion, vapor lock, and loss of system control when it overfl ows the reservoir[9]. Air can signifi cantly interact with lubricants, presenting itself in all three coexistence states and negatively aff ecting the lubricant’s physical and chemical properties and the system’s performance.
Koehler’s Automated Air Release Value Analyzer
The Automated Air Release Value Analyzer, which can be seen in Figure 1, automates determining air release properties [1]. The instrument consists of a test vessel with airfl ow control equipment that delivers heated air at a specifi ed rate to an oil sample maintained at a constant temperature. An integrated touchscreen guides the user, provides density calculations, and measures timing as depicted in Figure 2. The oil is heated (commonly to 50°C) and compressed air is blown through it [6]. After stopping airfl ow, the apparatus measures the time for the entrained air content to reduce to 0.2% volume. This separation time is the air release value [1]. Accessories which include drying ovens, circulating baths, compressed air heaters, and over-temperature/pressure protection circuitry, support the testing process. The drying oven allows pre-warming test oils up to 100°C. A circulating bath and air bath ensure temperature control for the sinker component used to detect the 0.2% air level. The results will depict a graph that can give information on the target density time and value as seen in Figure 3.
The Air Release Value Apparatus uses a borosilicate glass test vessel consisting of a jacketed sample tube fi tted with an air inlet capillary, baffl e plate, and air outlet tube This sample tube and an accompanying set of components (inlet capillary, baffl e plate, outlet tube) are marked and intended to be used as a pair, though interchanged parts can be used if the resulting vessel conforms to the stated dimensions. A pressure gauge spanning 0 to 35 kilopascals with 2 kilopascal divisions and 1.5 kilopascal accuracy measures compressed air pressure as seen in Figure 4. Two thermometers are employed: an air thermometer ranging from -20°C to 102°C for monitoring compressed air temperature, and a sample thermometer for tracking the oil sample’s temperature during preparation and trials[6]. A heater brings the compressed air up to the desired measurement temperature before introduction into the sample. This carefully designed apparatus, with its instrumented test vessel, pressure and temperature monitoring, and controlled heating, enables precise, standardized testing of lubricating oils’ air release properties.
Importance of Air Release Testing
Air contamination in lubricating oils and hydraulic fl uids can signifi cantly impact machinery performance and longevity. Excessive air in these fl uids can cause sponginess and lack of sensitivity in the control of turbine and hydraulic systems [1]. Air bubbles dispersed in the oil affect its physical properties, reducing its bulk modulus and ability to transmit pressure effectively. This spongy behavior results in poor component response [4]. Entrained air bubbles also act as nuclei for cavitation, where rapid expansion and collapse of the bubbles from pressure changes create shock waves that can severely damage metal parts like engines, plain bearings, pump inlets, and other sliding surfaces [4,5]. Checking the air-release properties of lubricants is crucial because it expresses their ability to separate
Figure 1: Above is an image of the Air Release Value Apparatus [10]
Figure 2: This displays the apparatus’ integrated touchscreen [12]
Figure 3: This image represents the graph that is generated while running the apparatus[12]
PIN AUGUST / SEPTEMBER 2024
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
