Water, water, everywhere . . . Water is ever present in the environment. Unless you live in an arid region, it is a fundamental fact of life. Water co-exists in oil in essentially the same way it co-exists in the atmosphere. It starts off in the dissolved phase - dispersed molecule-by-molecule throughout the oil. Just like water present in the air, it cannot be seen in oil, which may appear clear and bright. However, once the saturation point is exceeded, water is typically present in the emulsified phase creating a milkiness or fog in the oil, just like moist air on a cool day. When sufficient water exists, or when the oil has adequate demulsibility, free water will collect. Because water is typically heavier than oil, it settles below the oil, at the bottom of sumps and reservoirs.
The point at which an oil contains the maximum amount of dissolved water is termed the saturation point. The saturation point is dependent on the oil’s temperature, age and additive composition. The higher the temperature, the higher the saturation point and hence more water held in solution, in the dissolved phase. This is the same as being able to dissolve more sugar in hot water, than in cold water. Similarly, the older the oil, the higher the level of water that can be dissolved. This is due to polar by-products of oxidation in the oil, which act as “hooks” holding on to the water molecules and keeping them in solution. Likewise, highly additized oils, like crankcase oils, have a higher saturation point than lightly additized oils like turbine oils, because the additives - many of which are polar - also hold the water in solution.
Why is water considered an evil? Water will affect the oil’s base stock, encouraging oxidation, viscosity increase and foaming.
Water can also affect the additive package through water washing and hydrolysis, leading to acids and additive depletion. Water encourages rust and corrosion and will cause increased wear as a result of aeration, changes in viscosity resulting in film strength failure, hydrogen blistering and embrittlement, and vaporous cavitation. Finally, water is a generator of other contaminants in the oil such as waxes, suspensions, carbon and oxide insolubles and even micro-organisms.
Water ingression is either insidious as a result of atmospheric humidity levels or immediate as in water jet washing or sudden seal failure. Whatever the source, immediate attention is required to remove it. If significant water ingress has occurred over a prolonged period, detailed oil analysis, such as rust and corrosion inhibition characteristics, remaining useful life measurements, demulsibility and foam suppression and tendency may also be necessary to determine the oil’s suitability for further use. Merely replacing the oil will not cure the ingress source. Root cause corrective measures are necessary to resolve or limit water ingression.
Basic measures to address water ingression include the use of desiccating breathers, improved seal technology and training maintenance and operations personnel to avoid direct contact with wash down water on shaft seals and breathers.
Measures to minimize water ingress should start in the oil store. Drums and tanks should be sheltered from the environment. Even indoors, this means they should be sheltered against process water sprays, fire sprinkler tests and general cleaning sprays. Open barrels should also be protected with desiccant-style filters, particularly in humid storage areas, to prevent water build up and oil degradation.
A number of methods or technologies, from inexpensive gravity separation to complex vacuum dehydration, exist to remove water. Which technology is most effective will depend on the target dryness level required, the volume of water that must be removed, the base oil (mineral, synthetic, etc.) and the required flow and processing rate. The following is an outline of technologies that can remove water from oil, together with their relative advantages and disadvantages.
As already mentioned, free water in the system will settle to the bottom of the tank (assuming the specific gravity of water is greater than the lubricant). The time it takes the water to separate will depend on the system’s temperature, as well as the additive formulation, age of the oil and the base oil type. Some oils are designed to hold water in suspension rather than to allow it to separate out, making gravity separation a less-than-effective strategy.
In basic systems, opening the drain valve and allowing the water to drain off may be sufficient. The effectiveness of this action, however, will depend upon how long the system was allowed to stand prior to draining the water, whether the temperature was low enough to lower the saturation point dramatically and the oil’s demulsibility characteristics. Lowering the saturation point helps ensure that as much of the water as possible will exist in the free state. In larger volume systems, a separate settling tank may be employed to allow the oil to cool and demulsify prior to water removal (Figure 1).
Figure 1. Settling Tanks for Moisture and Solids Separation
The major downside to this method is that it removes only free water, so elements of emulsified and dissolved water will remain. The upside is the low cost of water removal.
The principle of the centrifuge (Figure 2) is to separate the oil’s heavier elements by spinning the oil to create high G-forces - often in the tens of thousands of Gs.
Figure 2. Centrifugal Separation
The greater the difference in specific gravity between the contaminant and the oil, the more effective the process. For this reason, centrifuges often work better on low specific gravity and low viscosity oils, like turbine oils, rather than heavier gear type oils. In a centrifuge, both free and emulsified water will be removed; this will depend to some extent on the type of additive package, as some water will be held in suspension in the oil. Just like gravity separation, the lower the oil’s temperature, the more effective the removal process will be, because much of the water will exist in the emulsified and free states.
As a tool, a centrifuge is relatively expensive. However, given that it is also a means of removing other heavier contaminants and has a comparatively high throughput compared to other technologies, centrifugal separators are relatively cost effective.
The downside of centrifuges is that only emulsified and free water will be removed - although this can be partially overcome by keeping temperatures low.
Typically, most filter media will absorb a small amount of moisture from the oil, resulting in swelling of the media. This is particularly true for cellulose-based media. In fact, examination of used filters will often indicate if the presence of water is a concern. Some filter cartridges with an additional wrap consisting of polymer and desiccants are available. These filters are specifically designed to remove water by absorption and remove both emulsified and free water, as well as solids. However, the elements typically have a limited volume capacity and are best fitted to a portable filter cart for minor water ingression problems. In fact, when a small gearbox is being fitted with an expansion chamber type breather, it is worthwhile to filter the gearbox with a water-removing element to remove any trace elements of moisture that may condense out on surfaces within the unit when it cools.
The main disadvantage of absorption removal is that it has a limited capability for water removal per element. The positive aspect is not just its ability to trap solids, but also that it is a relatively cost-effective means of dealing with small systems that require polishing to remove moisture.
The vacuum dehydration process (Figure 3) lowers the partial pressure, which assists in removing the water from the oil. Just like boiling water on top of Mount Everest, lowering the pressure allows water (and other volatile materials) to boil at a much lower pressure.
Figure 3. Vacuum Dehydrator
At the typical pressures used by most vacuum dehydrators (25" to 28" of mercury), water boils at 120°F to 130°F. By heating the oil, typically to 150°F to 160°F, water is vaporized inside the dehydrator, without causing excessive oil degradation due to thermal and oxidative stress. In most dehydrators, the air is warmed and dried prior to being passed over the oil, encouraging the water to transfer from the oil into the air. To maximize the process, the oil is thinned to obtain the greatest amount of surface area exposure possible. This is achieved by allowing the oil to pass over a number of surfaces internally in the vacuum chamber, or by creating an umbrella spray within the chamber through which the dry air passes.
The real benefit of this process is its ability to remove dissolved water and other low-boiling liquid impurities such as fuels and solvents. The removal of dissolved water makes it ideal for systems requiring low target levels of moisture. It is particularly useful in environments where large volumes of oil are at risk from the process or system, such as in steam turbines or paper mills. In fact, for lightly additized oils such as turbine oils and transformer oils, a vacuum dehydrator can remove as much as 80 percent to 90 percent of dissolved water, achieving water levels as low as a few ppm.
The main disadvantage of vacuum dehydrators is their cost and comparatively low flow rate. Because of the cost, many companies chose to rent dehydrators on an “as-needed” basis rather than purchase them.
The main advantage of vacuum dehydrators is that they offer the ability to remove moisture to very low levels. The greater the volume of oil and water, and the lower the target moisture level, the more cost-effective vacuum separation becomes.
An alternative technology to vacuum dehydration is dehydration by air stripping (Figure 4), a process that removes water as well as gaseous contaminants in the oil. Not only does it remove free and emulsified water, but also dissolved water down to less than 100 ppm.
Figure 4. Air Stripping Dehydrator
Because of its ability to degas, it is also suitable for removing hydrocarbons in seal oil systems. Air stripping works by drawing air or nitrogen gas into a stream of heated oil, which mixes in and absorbs the water and gasses within the oil. The oil/air is then expanded to release the air or nitrogen, which takes the impurities with it. Generally, the water removed will be of a reasonable quality, sufficient to allow it to be drained off in the normal network without special disposal requirements. The exhaust air and gasses are also controlled to minimize the oil vapor released.
Just like vacuum dehydrators, cost is an issue with air stripping. However, its advantage is that it costs less to maintain than a typical vacuum dehydrator because it has fewer moving parts. The fact that it can also remove other gaseous impurities, as well as dissolved water, makes air stripping technology an effective alternative to vacuum dehydration.
Some applications are self-cleansing because they run at elevated temperatures and consequently, water is evaporated. The combustion engine is a perfect example of a self-cleaning application. However, some settling tanks (see gravity separation) may also include heating elements to assist with water removal below the saturation point. Whether it is best practice to deliberately heat the oil briefly to drive off moisture to maintain oil health is open to debate. Allowing the water to remain in the oil is usually far more damaging than briefly heating the oil. Therefore, heater units are available as portable water removal systems. In static systems, like reservoirs, it is important to ensure that the power density of such elements remains below 5W/in2 to minimize thermal stress to the oil.
The downside to heating oil is that it must be controlled, particularly with mineral oils, to avoid harm. However, the relative cost is less than the centrifugal or vacuum separation technologies, making this an effective water removing tool in certain circumstances. The decision about which main water removal technology is best will predominantly be based on the volume of oil and the water to be treated. The decision will be further impacted by the need to reach a target moisture level. If target moisture levels are well below the saturation limit, then more complex and expensive methods will be required as necessary if large quantities of free and emulsified water are to be removed.