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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:
spongy, erratic operation of hydraulics,
loss of precision control; vibrations,
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 stability of foam.
Additives Used to Control Foam Tendency
The most common antifoam additive used in detergent oils is based on silicone in the form of polydimethylpolysiloxane (Figure 1).
Figure 1. Basic Chemical Structure of Polydimethylpolysiloxane
Silicones have very low surface tension of about 21 mN m-1 and tend to accumulate at air/oil interfaces. In order to be most effective as an antifoamant, they must be insoluble in oil, while the silicone particles must be less than 5 to 7 microns to ensure long-term foam performance.
The mechanism by which antifoam additives work is simple, as can be seen in Figure 2. Antifoamant contacts a bubble at its film and spreads around the bubble. As it spreads, the shearing force thins the bubble wall until it ruptures.
Because they are surface active, silicone additives attach themselves to bubble walls whether the bubble is on the surface or if it is under the surface. Silicone is much denser than surrounding fluid, and much denser than any bubble, so it slows the bubble’s progress to the surface. This can cause air entrainment in stagnant systems. From the standpoint of air entrainment, as much as half a part per million of silicone can contaminate a turbine oil system, to the extent that air entrainment becomes a problem.
Turbulent systems impart enough energy to allow bubble walls to break when they collide with other bubbles, particularly with antifomants present that increase the surface tension at the air/oil interface. This results in the formation of larger bubbles that can rise to the surface and break. Silicone antifoam additives are effective in turbulent systems for this reason and have been used successfully in concentrations up to several hundred parts per million, although typically most oils contain between 5 to 10 parts per million.
Figure 3 shows the experimental effects of silicone on air release. It demonstrates that silicone limits the amount of bubbles formed, but may retain them longer during a settling phase.
This is consistent with practical experience that shows that silicone additives are effective in turbulent systems but cause air entrainment in stagnant systems.
Casual exposure to silicone can have a significant effect. There are reports of air entrainment resulting from oil passing through hoses that had been formed on a silicone-coated mandrel. In one instance in a turbine application, all sources of air were removed, and the system was carefully evaluated, component by component, to check for sources of contamination. After an exhaustive search, the culprit was a silicone coating on electrical cables that were immersed in oil.
A similar effect may be expected from silicone sealant leaching, commonly used to seal gaskets and other components.
Silicone antifoam additives do not hydrogen bond, but tend to agglomerate at an oil/water interface and may be removed by filtration of water more readily than an ester antifoam additive. They also tend to plate out on fresh metal surfaces, which makes them less available to suppress foam. In rare instances, new equipment depleted of additive can be refreshed by the careful addition of silicone that is well dispersed in a solvent.
Silicone additives have been added successfully to turbulent systems to correct foam. However, they must be added properly and only if the root cause of the problem cannot be corrected. Both the molecular weight and method of mixing silicone additives are critical if they are to be effective. They should be pre-mixed using a high shear mixer into oil or, preferably a solvent carrier such as kerosene or alkyl benzene. They should be misted, or added very slowly to the most turbulent part of the system. It is strongly recommended to abstain from using aftermarket silicone sprays, such as those found at automotive parts stores, not only because they tend to drop out of the oil, but also because they may contribute to air entrainment.
Acrylate copolymers are often found in the range of 0 to 400 ppm in industrial oils that do not contain detergents or dispersants. They are not as likely as silicones to cause air entrainment. Even so, gross overadditizing may lead to air entrainment and may react with other additives. They have been shown to react with poly butenes typically used as viscosity index improvers and for this reason are not commonly used in VI improved oils.
Figure 4. The Structure of Acrylate Copolymers
Most studies show no incompatibility between silicone and acrylates and their effects seem to be additive. Acrylate antifoam additives can be hypersensitive to contamination. They may react with polar molecules, or even with themselves, to form larger molecules called micelles. This renders them less effective, sometimes within six months. The micelles may be broken and the additive restored to its original strength by vigorous agitation. Acrylate copolymers are completely organic in structure and do not contain any other elements, other than carbon, oxygen and hydrogen (Figure 4). As such, there is no easy way to measure their concentration, unlike silicone-based antifoamants, which may be monitored by determining the silicon concentration in new and used oils by atomic emission spectroscopy.
Contaminants and Effects of Foam Tendency
Depending on application, lubricating oils are susceptible to contamination from grease, particulates, sealing materials, other lubricants, process fluids, rust preventives, cleaning compounds and airborne contaminants. Several researchers have tested the effects of contaminants on foam properties of lubricants.
Lithium and calcium grease have a significant effect on foam stability. Both lithium hydroxide and calcium hydroxide thickeners are polar and stabilize foam. They cause characteristically large sticky bubbles of foam that adhere to the walls of a container. Polyurea thickeners are not polar, and therefore do not have such a strong effect
Particulates act as seeds or nucleation points on which bubbles grow. Antifoam additives may also be attracted to their surface, reducing their effectiveness in the bulk oil. In particular, cement dust can cause copious foam. Not only does the dust act as a nucleation site for bubbles, but also calcium seems to bind the antifoam additive, rendering it less effective. In practice, dirty oil can foam more than clean oil, although a review of more than 100 used turbine oil samples showed no correlation between ISO cleanliness and foam tendency, indicating that the effects of particulate contamination are much less pronounced in these systems.
Small amounts of silicone can contaminate a stagnant system. Common sources include gasket sealants, or the seals themselves. Their presence should be monitored and avoided.
Other sources of increased foam tendency are soaps and surfactants, commonly used for machine wash down. Any kind of surface-active contaminant can cause foam and air entrainment and again their presence should be monitored and steps taken to prevent their ingress.
Water has a destabilizing effect on foam. In practical applications, this means that more foam may be generated by the presence of water, but it will dissipate faster. The net effect may be a slight increase in unstable foam. Several investigators have found a direct relationship between foam and water content, although used oil samples showed no correlation. This may be because many water sources contain not only pure water, but also dissolved polar compounds such as process water treatment chemicals.
Oxidation by-products have been shown in the laboratory to produce very stable foam. However, a comparison of the Acid Number of used turbine oil samples failed to show any relationship between increasing Acid Number and foam tendency or stability. In practice, it is likely that other factors would dictate an oil change in this application before oil oxidation results in foaming problems. However, in other oils, particularly highly additized oils, oil and additive breakdown may result in an increased foaming tendency.
It is common practice to coat turbine components with a viscous pre-lube to provide initial lubrication during startup. Pre-lubes, particularly ones that contain polybutenes, can react with acrylate antifoam additives. To prevent this, petrolatum (petroleum jelly) or ISO 460 turbine lubricant should be used as a pre-lube to avoid contamination of the working fluid.
Alkaline detergents can react with acid-based rust inhibitors to form a soap that can wreak havoc with the foaming characteristics of the oil. Automotive engine oil detergents are mentioned in the literature and in field reports as potential pro-foamants, although the addition of dispersants seems to control their effects. Dispersants concentrations 1.2 times and higher, relative to detergent concentrations, provide the best foam control in oils containing these additives.
Resolving Foam Problems
Good problem solving involves asking lots of questions. First, evaluate the overall system. Then take a look at the fluid. Finally, see if bubbles are being introduced through, or generated by, any of the components. Like all lubrication related problems, it is important to treat the root cause, not the symptom.
Evaluate and Treat the System
When looking at system design, ask the following questions:
Is this a new system or an established system?
Did foaming just start to occur or had it been increasing gradually?
Have any changes or modifications been made recently?
If the system is new, make sure that lines are bled properly. If that does not eliminate air pockets, evacuate the system and refill slowly under vacuum. Preheating the oil, or delaying full-speed operation until the oil temperature is high enough to release air that comes out of solution may also help reduce or eliminate foaming problems. It is also important to make sure the lines are flushed free of machining swarf and scale.
If foaming recently started in an established system, ask the question, “What has changed?”. Foaming that initiates just after an oil change, particularly a change in brand or manufacturer, may be a direct result of the new oil. Some oils that contain detergents may dislodge deposits left behind by a nondetergent product. Foam that is produced while the system is being cleaned of old deposits will foam, but the foam goes away with time. Foam caused by an incompatibility between the two oils will be more persistent, however, and is a more serious issue.
In an established system, check the oil level at working elements or in the reservoir when the equipment is at rest. Ensure that make-up oil is added slowly, or place the outlet of the hose under the surface of the fluid to prevent splashing. Check vents to see that they are not plugged, and clean them as required.
Evaluate and Treat the Fluid
To evaluate the fluid as the root cause, take a sample and shake it well, or observe the condition of the lubricant when the machinery shuts down.
Does the foam dissipate rapidly?
Are relatively large bubbles floating on top of the fluid, or is air dispersed throughout the fluid?
A visual examination of this kind is usually sufficient to tell the condition of the oil; and oftentimes it is not necessary to perform the ASTM D892 foam test. If the foam dissipates rapidly, the oil is doing its job and it is likely a mechanical problem that is the cause. If the foam does not dissipate, the oil is probably contaminated.
Evaluate and Treat the Equipment
Figure 5 represents a typical circulating oil system. Start at one end of the circulating system and follow the fluid flow around the circuit, watching for areas where bubbles could be generated. In particular, look for air leaks on the suction side of a high-pressure oil pump or for working elements that may churn the air into tiny bubbles.
Reservoir design plays a significant role in controlling foam and air entrainment. Keeping the reservoir inlet below the surface of the fluid to prevent splashing is an obvious way to prevent foaming. If this is not possible, install an angled plate so that the oil gently slides into the reservoir. Also effective is a wire screen (60 mesh) that acts as a nucleation site for bubbles and prevents foam from entering the outlet (suction line).
Scavenger pumps often mix large amounts of air into the fluid. Instead of keeping the inlet below the level of the fluid in the reservoir, raise the inlet so that air has time to dissipate before it reaches the bulk oil. Use the designs in Figure 6, or add strainers or plates to allow the oil to cascade through the air. It is always important in this type of system to make sure the tank and screen are properly grounded.
If the inlet is too close to the outlet, install baffles or wires to increase residence time.
Maximize surface area and residence time in the reservoir and make sure the oil level is high enough to prevent the exiting oil from creating a vortex and sucking air. Makeup oil should be added to a sump through a hose extended below the surface of the fluid to minimize splashing.
Loose fittings on the suction side of a pump can be a common source of air. It is easy to check for leaks by coating the fittings with foamy shaving cream and watching for dimples. Very often there will be a number of leaks, so it is important to perform this test over all points where air may enter the system.
Filters generally do not contribute to foam, although one study showed that synthetic fiber filters could remove antifoam additives, though this is generally not an issue. Filters that are plugged, in by-pass can allow particulates to pass through the system, which can exacerbate foam formation.
Also watch for and avoid silicone impregnated filter media when foaming is an issue. Entrained air may explain a filter that appears to be plugged, yet there is no sign of physical contamination. Bubbles are surrounded by a sheath that has relatively high surface tension and these bubbles can block a filter, and then disappear when the filter is dismantled for inspection.
Sharp bends in piping can cause a pressure drop that pulls dissolved air out of solution. The same phenomenon can occur if there is a dramatic increase in the diameter of a pipe. Make sure that fluid conductors are properly bled and evacuated prior to start-up.
Also check for leaks around valves and sharp bends or areas of turbulent flow. One field problem with foam was caused by oil flow through a bearing. The oil was forced to make a 90-degree change in direction as it exited the bearing. Chamfering the internal edges solved the problem.
The working elements can beat air into the bulk of the oil if the level is too high, and they can churn foam onto the surface if the level is too low. Check sight gauges regularly, and adjust flow rates to ensure optimum oil levels. Lower the outlet if it is too close to the surface.
Wear in turbine thrust bearings can be caused by dissolved air coming out of solution. One guide recommends using positive pressure of 0.7 bar for turbine thrust bearings to avoid this problem.
Some equipment inherently foams. Electric motors may foam if they are mounted vertically but not if they are mounted horizontally, and vice versa. The equipment manufacturer can usually indicate if their equipment has a tendency to induce foaming more if mounted in one configuration or another.
When diagnosing foam problems, it is important to use a systematic approach to discover the root cause of foam and air entrainment problems. First, determine whether the air-in-oil problem is foam, or if it is actually air entrainment. After that, look at the overall system, the fluid and the components to eliminate the sources of air problems. Avoid treating the symptoms rather than the cause. If it is necessary, however, the use of aftermarket antifoam additives can in some circumstances resolve the problem, provided the guidelines outlined in this article are followed.