Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/120548
COVER STORY Varnish Basics The typical varnish progression begins with a reaction at the molecular level. This generally includes an oxygen molecule. In oil, the oxidized molecule is controlled through additives, which inhibit it from accelerating the degradation of the remaining oil. As more as a varnish cloud of nanometer-sized material. When the cloud density oversaturates the oil, some of the varnish material will settle out within the lubrication system (like rain falling from the sky) in the form of a deposit. In time, the deposit can harden into a solid material, which is commonly known as varnish. Oil Saturation Figure 1. These heavy varnish deposits were found on bearing surfaces. (Courtesy Fluitec International) varnish forms, it becomes distinct particles that can be measured in nanometer-sized particles. As the numbers of these particles increase, this degradation material within the oil can be described Turbine oil is designed to hold and manage a finite volume of varnish material. When this capacity is exceeded, the oil is considered saturated. Deposits can then form and accumulate in the system. Saturation has a relationship with temperature in that oil at a higher temperature is able to retain a greater volume of the nanometer-sized varnish material than oil at a lower temperature. The desired system condition for new oil would be to install the oil into a clean system so the progression of oil degradation and subsequent varnish accumulation would be limited to natural degradation of the oil. This degradation progression should be limited to the influences of new varnish created within the system as opposed to existing system varnish, which is known to accelerate oil oxidation. It is important to install oil into a clean system so the maintenance sensitivity will more appropriately respond to the oil's initial varnish saturation level as an oil failure criterion. This sensitivity should be maintained at the lowest expected system oil temperature rather than at a higher temperature, since deposits will form and collect in a system at this lower temperature, and deposit formation should be considered a lubricant failure mode. Base Oil Influence Varnish accumulation is also influenced by the base oil category, as designated by the American Petroleum Institute (API). Group II base oils have a superior design and can be expected to provide improved performance over Group I base oils, assuming that the system in which the oil is installed is clean. Both Group I and II base oils have inherent solvency, which means that they have a finite capacity to accumulate and hold varnish products. However, there is an important difference in how each does this. Due to the manufacturing process resulting in a more highly saturated molecule, Group II oils have less varnish-retention capacity than Group I oils. As such, Group II oils allow varnish deposits within a system to occur with a lower overall volume of the material present. System Cleanliness When a new charge of turbine oil is installed, it is vital that the system be clean and free of varnish. A common problem is that many existing systems are not cleaned prior to the installation of new/replacement oil. As mentioned previously, turbine oil has a natural 14 | March - April 2013 | www.machinerylubrication.com