Effects of Aeration on Industrial Lubrication Equipment
By Dan Freeland, from Machinery Lubrication magazine, July/August 2007. www.machinerylubrication.com
Posted (10-27-08)
Almost all lubricating oil systems contain some air. Air is not typically considered a contaminant, but the presence of air in its various forms may have an impact on the ability of the lubricant to perform. Air is found in four phases: free air, dissolved air, entrained air and foam.
Figure 1. Vented
Figure 2. Closed System
Free Air
Free air is normally found trapped in a system, such as an air pocket in a dead leg of a piping system, 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 startup and free air is drawn into circulating oils. Free air can become entrained or dissolved through direct contact with oil that is being agitated. Dissolved Air
Dissolved air is not easily drawn out of solution. It can be a problem when temperatures increase quickly or pressures drop. Petroleum oils can 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 one millimeter 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
Air can also be temporarily entrained into oil through agitation. This can occur in healthy oil, but is more common in oil containing high amounts of moisture, as well as oil having other forms of contaminants. Air entrained in oil sumps of rotating equipment tends to rise out and re-enter the headspace over time. This can occur quickly in healthy and low-viscosity oil, or take much longer in contaminated and high-viscosity 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 the following:
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pump cavitation
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spongy, erratic operation of hydraulics
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loss of precision control, vibrations
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equipment shutdown when low oil pressure switches trip
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microdieseling due to the ignition of the bubble sheath at the high temperatures generated by compressed air bubbles
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safety problems in turbines if overspeed devices do not react quickly enough
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loss of head in centrifugal pumps
Figure 3. Nonvented
Figure 4. Baffle
Foam
Oil sumps or tanks that have a foaming condition usually experience the sump fluid surface to contain more than 30 percent air. This condition can be caused by the following:
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high oil level where air is whipped into the oil by gears or pumps
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cascading of oil into reservoir
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insufficient resident time to allow foam to break
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air leaking into pump suction though pipe fittings
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contamination of oil by grease from seals
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dirt entering the lube system
Foam and air entrainment can displace the oil and result in oil starvation to machine components which will lead to metal-to-metal contact and high operating temperatures. Foaming can also make it difficult to determine the oil level in a sump.
As we have seen, air in oil can cause many problems for oil's ability to properly lubricate machine components. It can also create issues for auxiliary equipment, such as constant level oilers. The remainder of this article focuses on the occurrence of the misfeeding of constant level oilers due to air entrainment in oil.
Constant Level Oilers
Constant level oilers have been used on industrial rotating equipment for more than 50 years. Trico Corp. introduced the Opto-matic oiler in the 1930s. As the understanding of rotating equipment has grown, the need for different types of constant level oilers has grown as well. There are essentially three different styles of constant level oilers used today: the traditional vented oiler, the closed system oiler, and the nonvented (Watchdog) oiler. Vented (to atmosphere)
How does entrained air affect oilers? In most cases, aeration occurs in the area nearest the bearings, and rises to the top of the oil sump where it escapes back into the headspace. However, some sump designs can push the entrained air toward the constant level oiler. Once entrained air migrates into a constant level oiler, the effect depends on the design of the constant level oiler. In the case of the traditional vented oiler, the aeration will rise to the oil surface within the oiler and escape into the oiler headspace, then vent back out into the atmosphere (Figure 1).
Note: The rate at which entrained air can escape from oil depends on the oil's ability to release entrained air, and the health or condition of the oil. If you know you have a problem with entrained air, your oil supplier may be able to recommend additives which can deter entrainment.
Closed System (to sump headspace)
A closed-system oiler will have a similar effect as the vented oiler. The aeration, upon reaching the oil surface within the constant level oiler, will return to the air volume shared by the constant level oiler and oil sump's headspace by way of a pressure balancing line (Figure 2). Nonvented
A nonvented, Watchdog constant level oiler can be affected more by entrained air. As the entrained air enters the constant level oiler, it rises to the surface of the oil within the oiler, similar to the previous two styles, but has no place to go. Instead of escaping or returning to the sump headspace, the air could travel to the top of the oiler reservoir (Figure 3). This will eventually displace the oil, causing overfeeding of the oil, and ultimately increasing the oil level in the oil sump. This is a slow process. Typically, this would occur on the order of one ounce or 28cc per 100 hours of operation. This rate is only an approximation, because each circumstance will be different.
This condition is rare, (reported on less than 0.1 percent of installations) and can generally be prevented by good oil management practices. To further prevent this occurrence, use a nonvented oiler that has a baffle incorporated into the oil level of the oilers (Figure 4). This baffle will block the migration of entrained air from entering the body of the constant level oiler, and will do this without interfering with normal operation of the oiler. This baffle exists at the top of the oil level where most aeration is found.
The use of constant level oilers is an ideal way to ensure the proper level of oil is being maintained. Oil conditions, such as air entrainment, can affect the oil's ability to lubricate properly, reduce machine component life, and can case auxiliary equipment to be compromised. Identifying and understanding air in oil conditions and working with oil suppliers to recommend proper oil/additive packages is a great start in reducing air entrainment and the problems it can cause. |
