Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/325287
28 May - June 2014 | www.machinerylubrication.com vapor in the bubble to combust (during implosion), resulting in thermal/oxidative damage of the oil to form varnish. Electrostatic energy is generated by turbulent oil movement, especially by pushing oil through the pore of a mechanical filter. When too much oil is forced through a small pore space, too much static charge is produced to be dissipated by normal means. This static charge will find a point of least resistance for discharge. This point is usually inside a mechanical filter housing or at the tip of the lubricant return pipe to the reservoir. The sparking creates an oxidation byproduct known as varnish. Additive depletion can be a source of varnish with certain types of antioxidant additives such as phenyl-alpha-naphthyl- amine (PANA). This synergistic additive is good at rejuvenating itself when it depletes, but it does so by creating a soft and polar byproduct. This byproduct, which has also been lumped into the varnish category, is easy to identify since its solubility is very sensitive to temperature. It will cause oil to appear hazy at ambient temperatures and clear at higher operating tempera- tures. Additive incompatibilities can be major or minor and will almost always be present, even when mixing brands or types of oils and within the same base-stock category. Standardized Testing Varnish detection has been difficult in the past not only due to a lack of standardized testing but also because of a lack of understanding of the best type of testing. It was common to see "free oil analysis reports" from oil companies showing normal ranges for the rotating pressure vessel oxidation test (RPVOT), acid number and viscosity, and thus no reason could be given for varnish-related problems. Once the existence of varnish was proven, the search began for an acceptable test method. ASTM D7843 was developed to create a standardized test, titled the Membrane Patch Colorimetric Method. This has been dubbed the MPC test. MPC patch testing involves diluting the oil sample with a strong solvent and vacuum-filtering the mixture through a fine-pore, ultra-white-colored membrane. The color of the residue that is left on the white membrane is measured by the international CIELAB color scale. The CIELAB value between pure white and the residue color is given as the test result. The basic problems with the MPC test method include controlling the temperature of the oil/solvent during preparation and filtration, resulting in a loss of data during filtration and difficulty in differentiating between soluble and insoluble. By measuring the patch residue's weight in some repeatability testing, you can get an idea of just how challenging the MPC method's repeatability can be. Enhanced MPC Method (iMPC) An improved version of the MPC test method has been developed based on many of the same principles, including the same color scale and membrane. The iMPC method represents a measurement of the total varnish present in the oil — both the soluble and insoluble varnish. It uses the capillary pressure of the membrane and the straight oil, i.e., no dilution with a solvent. The pure oil sample is adsorbed by the membrane and allowed to dry into a stain, not a residue. The color of the stain is measured and calculated in the same manner as the MPC patch residue. The CIELAB value between pure white and the residue color is then given. As a result, no color data is lost during the vacuum filtration where soluble color bodies are carried away by the solvent. The simplest way to establish if an oil is functioning normally or is depositing insoluble varnish onto a system's metallic surfaces is to compare the ratio of the iMPC to the MPC. Since the iMPC value represents the total varnish (soluble and insoluble), it should always be higher than the MPC value. The best situation is a 3-to-1 or 2-to-1 ratio (iMPC VArNIsh Correlation of the iMPC/MPC average to the water-separation characteristics of turbine oils 29 8:24 7:12 6:00 4:48 3:36 2:24 1:12 0:00 0 9 35 Days of Filtration Air-Release Time (minutes) Varnish Removal vs. Air Release (ASTM D-3427) 22 Correlation of air-release time with days of varnish-removal filtration