Purpose of Lubricants
Lubricate - The bearings, gears and rotor surfaces need lubrication for the air compressor to run efficiently and to limit maintenance downtime.
Cool - As well as lubricate, the lubricant removes heat from the system.
Seal Rotors - Lubricant also works as a sealant for the rotors to prevent loss of air through the clearance.
Why test compressor Lubricant?
Even though typical fluid life is 8000 hours, there are a number of events that can happen and affect that. Contamination, excessive temperature, and mechanical failure may occur between normally scheduled fluid changes.
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Important Test for Oil Analysis
Relative motion between lubricated parts is always accompanied by friction between the contacting surfaces. This friction causes a gradual wearing away of these surfaces, despite the fact that the parts themselves are usually coated with a thin film of oil. Metal particles rubbed off in this manner are small enough to remain suspended in a circulating lubrication system, and since these wear products are always composed of the same materials from which they originated, the relative level of each metal present in the used oil relates directly to the wear condition of the lubricated assemblies.
Technique for detecting and quantifying metallic elements in a used oil resulting from wear, contamination or additives. The oil sample is energized to make each element emit or absorb a quantifiable amount of energy, which indicates the element's concentration in the oil. These values are classified as "fine" readings on your report, and reflect the concentration of all dissolved metals (from additive packages) and particulates up to 2 microns in size.
The resistance of a fluid to flow. Viscosity is the most important lubricant physical property. Lubricants must have suitable flow characteristics to insure that an adequate supply reaches lubricated parts at different operating temperatures. The viscosities of lubricants vary depending on their classification or grade, as well as the degree of oxidation and contamination in service. If viscosity of the lubricant differs by more than 10% from nominal grade, a change of oil is recommended.
Total Acid Number (TAN)
A titration method designed to indicate the relative acidity in a lubricant. The acid number is used as a guide to follow the oxidative degeneration of an oil in-service. Oil changes are often indicated when the TAN value reaches a predetermined level for a given lubricant and application. An abrupt rise in TAN would be indicative of abnormal operating conditions (e.g. overheating) that require investigation.
Spectrometric technique for detecting organic contaminants, water and oil degradation products in a used oil sample. During a lubricant's service life, oxidation products accumulate, causing the oil to become degraded, and in most instances, slightly acidic. If oxidation becomes severe, the lubricant will corrode the equipment's critical surfaces. The greater the "oxidation number", the more oxidation is present. Similarly, the "nitration number" reflects the level of nitrogen compounds in the oil resulting from nitrogen fixation (common in natural gas fueled engines). Conditions such as varnishing, sludge deposits, sticky rings, lacquering and filter plugging occur in systems with oxidation and/or nitration problems. Infrared spectroscopy also indicates contamination due to free water, glycol antifreeze, soot deposits and fuel dilution.
Usually not desirable in oil, water can be detected visually if gross contamination is present (cloudy appearance). Water contamination should not exceed 0.25% for most equipment, and not more than 100 ppm for turbine lube and control systems. The Karl Fischer method is used for moisture contamination down to levels of 10 ppm (.001). Excessive water in a system destroys a lubricant's ability to separate opposing moving parts, allowing severe wear to occur with resulting high frictional heat.
A method used to count and classify particulate in a fluid
according to accepted size ranges, usually to ISO 4406.
High counts lead to filter plugging, servo valve jamming, and hydraulic piston failure. The test is a good reflector of filter efficiency. Turbine systems typically should not exceed ISO 18/16/12, although some applications require more stringent limits.
A technique which separates magnetic wear particles from the oil and deposits them on a glass slide known as a ferrogram. Microscopic examination permits characterization of the wear mode and probable sources of wear in the machine.