Controlling Oil Aeration and Foam
Marianne Duncanson, ExxonMobil
Tags: contamination control
Foam and air entrainment problems are quite common, but are traditionally hard to treat. Previously, the standard procedure was to run an ASTM D892 foam test on the offending oil, and then indiscriminately add an aftermarket additive, usually silicone-based. Generally foam went away quickly, only to return. More antifoam was added, and the cycle repeated until the system became so overloaded with antifoam additive that the oil has to be dumped. Today, there are more practical methods of searching out and treating the root cause of foam problems so that it is usually unnecessary to use aftermarket antifoam additives. Different Kinds of Bubbles
Almost all lubricating oil systems contain some air. Air is found in four phases: free air, dissolved air, entrained air and foam. Free air is trapped in a system, such as an air pocket in a hydraulic line, and may have minimal contact with the fluid. It can be a contributing factor to other air problems when lines are not bled properly during equipment start-up and free air is drawn into circulating oils. Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12 percent dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are less than 1 mm in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils, causing air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam, and is most often a completely separate problem. Some of the potential effects of air entrainment include: * pump cavitation,
* spongy, erratic operation of hydraulics,
* loss of precision control; vibrations,
* oil oxidation,
* component wear due to reduced lubricant viscosity,
* equipment shut down when low oil pressure switches trip, * micro-dieseling due to the ignition of the bubble sheath at the high temperatures generated by compressed air bubbles, * safety problems in turbines if overspeed devices do not react quickly enough and * loss of head in centrifugal pumps.
Foam on the other hand, is a collection of closely packed bubbles surrounded by thin films of oil that float on the surface of the oil. It is generally cosmetic, but it must be treated if it makes oil level control impossible, if it spills onto the floor to create a safety or housekeeping hazard, causes air locks at high points, or is so extreme that equipment is lubricated with foam. Small amounts of foam do not necessarily need to be treated unless the system suffers from the conditions listed above, although the presence of the foam may be symptomatic of a more serious problem. Base Oils Effects
Base oils inherently have very good foaming tendency and stability, although there is some variation depending upon crude source and processing. Tests have shown a linear relationship between foaming tendency and surface tension. In a system where foam is generated mechanically, switching to synthetic oil may help. * Polyalphaolefin and hydrocracked oils, by virtue of their high surface tension, show relatively low foaming tendency compared to petroleum hydrocarbons. * Unadditized organic esters are essentially nonfoaming, but are highly susceptible to contamination or to effects from additives. * Phosphate esters show foam build-up at low temperatures, but above 122ºF (50ºC) they show very little foam tendency.
* Polyglycols are difficult to categorize because they absorb water, which can influence foaming tendency.
Several studies show that base oils foam the most at 280 cSt. Either lower or higher viscosity can reduce the amount and...
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