Hydrolysis can occur at temperatures as low as 90☌, forming low molecular weight by-products, mostly acids and alcohols. High-risk lubricants include phosphate esters, diesters and polyol esters. Risk is limited to nuclear power plants and is generally rare.Ĭertain ester-type synthetic lubricants, when contaminated with heat and water, can hydrolyze. Prolonged exposure to high doses of gamma radiation can cause molecular cleavage and loss of viscosity. Common examples are microdieseling, hot spots (for instance, leaky steam flowing across on oil line), high Watt-density heater elements, and close-proximity furnaces. Oils exposed to high localized temperatures can crack oil molecules into progressively smaller fragments which thins viscosity considerably. These high-temperature events can fracture molecules resulting in gas evolution (release into the oil), causing a loss of viscosity. High temperature will swell the VII molecule, making it more vulnerable to shear.Īrcing electricity can occur for several reasons including improperly grounded electric motors/generators, welding activities and electrostatic discharge. This is influenced by the quality of VII, its concentration in the oil, the oil's operating temperature and the shear rate. After exposure to mechanical shear within a machine, the average MW of these VIIs can fall to 50,000 or less. Some VIIs have molecular weights ranging more than one million. Because the viscosity of an oil can be referred to as the average molecular weight of an oil's blended population of molecules, viscosity will decrease when large molecules break into numerous smaller molecules (like crushing a rock into gravel). Mass change is probably the most common explanation for a low viscosity reading and can occur due to numerous mechanical, electrical and chemical reasons. The separation can occur during storage, centrifugation or filtration, but in other cases may simply form sludge zones in tanks or become released to form deposits, varnish or reservoir bathtub rings. Such impurities can separate or stratify from the oil due to cold temperature insolvency, loss of dispersancy, chemical coagulation/agglomeration or water washing. These include waxy suspensions, sludge, oxide insolubles, decomposed additives, soot and gels. Many contaminants and soft impurities that might have originally elevated viscosity can later be removed, causing a noticeable drop in viscosity. These high molecular-weight polymer additives used in many lubricant formulations may become separated from the base oil due to (1) insolubility by sustained exposure to very cold temperature, (2) insolubility when mixed with an incompatible base oil (such as olefin copolymers in Group II base oils for instance), or by (3) mechanical filtration (at higher temperatures, some VII are said to be able to plug extremely fine filters). The following are a few examples of how this could occur: This would cause a loss of viscosity because it would destabilize the blend. The selective removal of high molecular-weight suspensions in a lubricant is less common, but still plausible. Examples of such contaminants include natural gas, solvents, diesel fuel, degreasers, process chemicals and refrigerants. Also, when certain nonlubricants contaminate a fluid, a similar end result occurs. In plant or field equipment, this could happen when a low viscosity lubricant is accidentally used as makeup fluid which drops viscosity through dilution. This type of blending is often performed by formulators and blend plants to bring a high-viscosity oil into a target viscosity grade (ISO VG 46, for instance) of a branded product. Lubricant viscosity can be thinned or cut back by adding a low viscosity fluid or dissolved gas to the mix. When it comes to an abrupt loss of viscosity or a downward viscosity movement, the following are common contributing factors that we now know: The oil analysis community is aware of the usual suspect conditions or events, but some remain undiscovered or at least are not fully understood. It's safe to say that viscosity will not change without a forcing event or condition that incites the change. What does it mean and why did it occur? Let's explore the many possible causes of low viscosity. What's not so good is when viscosity moves suddenly with no obvious explanation or warning. After all, when viscosity has not changed, you can rightly conclude that the many known viscosity-altering factors are probably not happening - a good thing for sure. The list of root causes that can alter a viscosity reading is quite extensive hence the reason why viscosity has become such an information-rich measure of used oil condition. Viscosity can go up, down or remain unchanged.
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