Oil degradation is a general term used to describe the destructive mechanisms that cause physical and chemical changes to compressor fluids while in service. Oil degradation results in a deterioration in fluid performance and a dramatic reduction in fluid service life. If ignored, oil degradation will progress and can lead to failures of critical compressor components. Understanding the mechanisms of oil degradation and their impact on fluid properties and performance, enables compressor users to proactively maintain fluid health, maximize fluid life, and avoid costly oil changes and major repairs.
OIL DEGRADATION MECHANISMS
Oxidation is primary oil degradation mechanism and is the chemical reaction that naturally occurs between the oil and oxygen from the air. Rotary screw compressor fluid is extremely vulnerable to oxidation since the compressor’s design provides an extreme, oxygen-rich environment. Oxidation is an auto-catalytic series of chemical reactions that continuously produces acids, depletes additives, and ultimately will form harmful varnish and sludge in many compressor fluids. Oxidation rates and acid production continuously accelerates as operating temperatures increase (doubles for every 180F increase), as acid levels (TAN) increase, as additive levels (antioxidants) decrease, and as fine metal catalysts accumulate in the fluid. Oxidation also impacts compressor internals by promoting corrosive wear, pitting and rust to unprotected metal surfaces.
Additive Depletion is another harmful process that reduces the performance, protection, and service life of compressor fluids. Lubricant formulators rely heavily on performance enhancing additives to control fluid service life, inhibit oxidation, and protect compressor internals from corrosion. While in service, additive levels continuously deplete which reduces the oil’s ability to resist oxidation and limits oil service life. Like oxidation, additive depletion is accelerated by catalysts like acids, water and fine metals. Monitoring fluid additive levels and replenishing the fluid’s additive package when necessary, helps maintain fluid chemistry and performance, and can dramatically extend fluid service life.
Contamination of air compressor fluids by foreign substances (solids, liquids or gases) can significantly accelerate fluid degradation and create a host of additional problems to air compressors. Rotary screw compressors have a “forced-contamination” design that continuously ingests atmospheric contaminants and forces them into the oil. Many of the airborne contaminants found in industrial environments (combustion exhausts, cooling tower chemicals, welding fumes, process vapors) are strong oxidizers that will accelerate fluid oxidation and acid production. Internally generated contaminants, like the acidic byproducts of oxidation, and fine metals (rust and wear particles), also add fuel to the auto-catalytic degradation cycle.
The Oil Degradation Cycle is a self-propagating, auto-catalytic cycle that continuously accelerates at an ever increasing rate. Acids (produced from oxidation), and other contaminants (fine metal particles and water) are catalysts that accelerate fluid degradation, acid production and additive depletion. If not addressed, oil degradation will proceed unchecked until the fluid is no longer fit for continued service.
Proactive maintenance of compressor fluid additive levels, coupled with acid-control oil purification, dramatically impacts compressor fluid performance and service life.