The Problems Created by Water – Let’s Count the Ways!
Water is an extremely challenging and complex contaminant. It negatively impacts compressor oils and internals in different ways which require different strategies to prevent or mitigate the damage that water can do. A partial list of the ways that water works to constantly attack air compressor oils and vital internals includes:
- Oxidation & Hydrolysis. Oxidation is a destructive chemical reaction that naturally occurs between oxygen from the air and the oil’s base stocks and additives. It is the leading causeof compressor oil replacement and failure. Oxidation is a compressor killer by producing acids, which along with water, elevated temperatures, and metal particles are catalysts that accelerate a chain reaction of oxidation, additive depletion, acid production, and oil degradation. This auto-catalytic chain reaction progresses at an ever-increasing rate while steadily increasing the oil’s viscosity and TAN (Total Acid Number), lowering its pH and increasing corrosion potential, producing varnish (in mineral oils, PAOs and SHCs), degrading the oil’s ability to lubricate, cool, seal and protect, and dramatically reducing its useful life.
Water will also chemically react with diester base stocks which are commonly blended with PAO and SHC compressor oils to increase their solvency, keep varnish in solution, improve elastomer compatibility, and incorporate critical additives that won’t naturally dissolve into the other base stocks. This reaction between water and diesters, known as hydrolysis, produces sludge which will foul compressor oil filters, coolers and valves to restrict oil flow, inhibit lubrication, reduce heat transfer, and shorten fluid life.
- Additive Depletion. As mentioned, water acts as both a catalyst and a reactant in a series of destructive chemical reactions that deplete the oil’s additives and reduces its protection and service life. Whether through chemical reactions like oxidation and hydrolysis, or other mechanisms like agglomeration or simple water-washing, many of the oil’s vital additives are depleted by water. Some of the additives negatively impacted by water include its antioxidants, corrosion inhibitors, dispersants, detergents, anti-wear, and extreme pressure additives. As these additives steadily deplete, the oil’s ability to perform and protect suffers while water continues to pursue a whole host of other destructive mechanisms.
- Corrosion. Water, acids, corrosion, and rust go hand-in-hand in air compressor oils. We know that water accelerates oxidation and acid production, which increases the oil’s corrosion potential and causes rust of compressor internals. Corrosion of bearing raceways and rolling elements results in pitted and etched surfaces that disrupt elasto-hydrodynamic oil films and promote contact fatigue and corrosive wear. Water can also mechanically wash away critical corrosion inhibitors that bind to metal surfaces to provide a protective coating against water and corrosion. When this occurs, ultra-fine rust particles, many too small to be removed by standard oil filters, accumulate in the oil and circulate throughout the compressor to foul and plug oil filters, separators, scavenge lines, injection orifices, and coolers. This results in elevated operating temperatures, separator pressure drops (energy consumption), and oil carryover.
- Loss of Film Strength. Water can also damage compressor bearings by reducing the viscosity of the oil. Rolling element bearings rely on the oil’s viscosity to provide the film strength necessary to maintain critical bearing clearances. When microscopic water droplets with poor film strength and a viscosity of only 1 cSt enter the load zone of the bearing, these clearances can be lost resulting in metal-to-metal contact between bearing components. As a result, spalling, pitting, and fatigue cracks will occur leading to increased bearing temperatures, vibration and wear, and shortened bearing/air-end life.
- Aeration and Foam. Water also reduces the oil’s surface tension which disrupts its air-handling ability and can lead to excessive aeration and foam. Like water, entrained air can be catastrophic to bearings by weakening the oil’s film strength, promoting bearing wear and reducing oil flow, lubrication and cooling. Excessive aeration and foam will not only destroy compressor oils and bearings but will also result in excessive oil carryover and downstream contamination
- Hydrogen Embrittlement. When water is subjected to the extreme pressures generated in the load zone of rolling element bearings (often > 500,000 psi), the water molecules can be torn apart into their hydrogen and oxygen constituents. Due to their extremely small size, the hydrogen atoms can penetrate the microscopic fatigue cracks in bearing balls, rollers and races, and be absorbed onto bearing surfaces. This process, known as hydrogen embrittlement, results in subsurface changes to the bearing’s metallurgy which causes it to become brittle and prone to more cracking. Once this process begins, the cracks will spread and lead to pitting, spalling, fractures, and ultimately failure.
- Restricted Oil Flow. Water is a highly polar molecule and as such is strongly attracted to other polar materials. These include many of the oil’s protective additives, oxidation byproducts, rust, wear metals and many other contaminants routinely found in compressor oils. This attraction causes these polar materials to agglomerate or cling together to form sludges and emulsions that can foul and plug injection orifices and tight clearances to restrict oil flow and its ability to lubricate, cool, seal and protect vital compressor components.
Don’t miss Part 3, where we will discuss strategies to protect compressor oils and components against water!