Controlling Oil Aeration and Foam
Controlling Oil Aeration and Foam * Print
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
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