Oil purification can have many interpretations. In lube and hydraulic fluid applications eliminating water, gases and solids from oil is critical for maintaining optimum system performance and oil service life.
Oil gets contaminated during operation, maintenance and storage. Contaminants come from various sources like:
- Leaking coolers
- Rust and corrosion
- Thermal oxidation and other fluid degradation mechanisms
- Environmental sources like air, rain, and dust
- Wear, break-in and damage of parts within the oil circuit
- Contaminated make-up oil
Contaminants from these sources are major catalysts in the cycle of wear; Contamination results in wear that results in contamination that results in more wear and so on.
Types of Contaminants Commonly Found in Oil
There are different types of contaminants found oil. Some of these include:
- Solid and gelatinous particles
- Water, coolants and other liquids
- Air and other gases
- Paraffinic deposits and varnishes
- Acids
- Metals, salts and soaps
Oil purification equipment is used to control and/or eliminate these contaminants, improving the oil’s performance and service life.
Oil Purification Methods
There are different methods used for purifying oil. The type of oil and contaminants dictate the method used. Following is a discussion of common methods.
Gravity Separation:
This form of oil purification involves an engineered system that exploits the differences in densities of solids, water and oil as a means of separation. The equipment is simple, such as a settling tank, a weir-style oil/water separator or a drop leg. Gravity separation can be very slow and is affected by many variables, making it an unreliable method in many applications. It is most useful for bulk separation where time is not a major factor, and it may be followed by other purification steps.
Filtration:
This method is used when the primary contaminant is particulate matter. Filters designed and constructed specifically for
oil and hydraulic applications are used to remove trap contaminant particles. These filters are typically cylindrical cartridges that fit in a sealed housing. The cartridges contain a fibrous filter medium that the oil flows through trapping particles of a specified size. As filters trap particles the pressure drop across the filter media will increase. Once it reached a specified level that filter cartridge must be replaced or cleaned.
If the contaminant load is heavy, there may be a series of filtration steps beginning with course filters followed by finer filters. There are specialized filter media for applications such as highly viscous fluids and fluids that tend to generate and discharge electrostatic energy. Other specialty filters are designed to remove varnish precursors, soot, and gelatinous particles. Filtration is a fundamental component of oil purification and may also serve as a stand-alone process or as a final guard from contamination that breaks through upstream processes.
ISO Cleanliness Levels are used to quantify particulate contamination. Various organizations have evaluated tolerances and recommended ISO Cleanliness Levels for most lubricated machinery and hydraulic systems. Many lube and hydraulic filters have rated removal efficiencies correlated to these for easy selection of the appropriate filter.
Filtration efficiency claims are often misunderstood. Basically, filtration efficiency is a ratio of the number of particles of a specified size (commonly measured in microns) exiting a filter to the number of these particles entering the filter. For filters in lube and hydraulic applications Beta Ratio is a common formula to express this. The formula is:
ß(x)=[(Number of particles of size (x) upstream)/(Number of particles of size (x) downstream)]*100
This can be converted to percentage efficiency as follows:
Efficiency %=[(ß(x)-1)/(ß(x))] * 100
This is a scientific, quantifiable industry standard for measuring filtration efficiency. Any absolute rated filter that is manufactured in a quality-controlled environment will have documentation of the filtration efficiency and the procedure used to measure it.
Hence, it is important to understand the difference between “absolute” and “nominal” ratings. Absolute rated filters have particle removal efficiencies that are measured, repeatable and monitored by quality control. Industry standards will be used and documented for measurements. These filters tend to be manufactured to higher quality control standards and have efficiency ratings of at least 99% or ß(x)=100, although it could be any efficiency. The key is that “absolute” means the efficiency is strictly defined and verifiable.
There is no industry standard for nominally rated filters. At most there may be an understanding based on the manufacturer, media composition, experience and maybe some established empirical data that a particular filter has a specified nominal filtration efficiency within an acceptable level of variability. However, according to the Cambridge Dictionary nominal means “in name or thought but not in fact or not as things really are.” So bare that in mind and expect your filtration specialist to know how each of your filters is rated.
Finally, it is important to understand that every conventional filter media has a filtration efficiency for every particle size. Particle removal efficiency for any filter media will follow a curve on an x-y chart. The filtration rating is arbitrary. Filtration manufacturers consider many factors, including industry, application and standards in deciding where on the efficiency curve to rate a particular series of filters. Consider a filter marketed for compressor
lube oil applications, and the industry filtration standard is 99.5% at 5 micron. It would be reasonable for a filter manufacturer to design this filter to remove 99.5% of 5 micron particles. This does not mean that it will not remove larger and smaller particles. In fact, the manufacturer should have an x-y curve that specifies the filtration efficiency for a range of particles sizes. The filter may remove 80% of 3 micron particles and 99.98% of 10 micron particles. While this may be considered a 5-micron filter by definition, it could be used in an application that requires filtration of 99.98% at 10 micron or any other application fits its efficiency curve. The question to ask is not “what is the filter rating?” The correct question to ask is “what is the filtration efficiency” at a specified particle size. Subtle but important.
Centrifugal Force:
This method could be considered enhanced gravity separation. By imparting centrifugal force on the oil within a centrifuge or cyclonic separator. The inertia of the heavier contaminants forces them out of the clean oil stream into a lower energy section of the device. In this lower energy environment, gravity takes over and the contaminants fall into a collection area. Finally, the contaminant can be manually or automatically discharged.
A centrifuge is the most common example in oil applications. They work as follows:
- A drum filled with contaminated oil rotates
- The rotation imparts centrifugal force on the oil, sending contaminants heavier than oil to the edge of the drum
- The contaminants are removed by various manual and automatic mechanisms and are discarded as oily waste
- Clean oil is moved in the opposite direction toward the discharge
Centrifuges are effective for rapid removal of bulk, free water and particles down to 2.5-20 micron depending on many variables. They are unable to remove dissolved water, emulsions and finer particles that form soot, sludges and varnishes. They also require a significant investment in maintenance and operational resources.
Liquid/Liquid Coalescing:
Liquid/Liquid coalescing is the process of separating two immiscible liquids. The bulk liquid is referred to as the continuous phase and the other liquid is the dispersed phase. These devices work by promoting the coalescence and settling of droplets of the dispersed phase.
Coalescing occurs naturally but can be enhanced with the use of a fiber matrix, similar to a particle filter. However, instead of permanently retaining the contaminant, the filter is designed to provide a quiescent zone where the following mechanism occurs:
- Droplets impact and cling to the media fibers
- As droplets accumulate, they contact and coalesce with other droplets
- The smaller droplets grow until they are larger enough for gravity to cause them to drain out of the coalescing media into a sump area
- The sump accumulates bulk liquid and is drained as needed
Coalescing filters are commonly used as a stand-alone system to periodically remove bulk free water from oil. They are also commonly integrated into multi-step oil purification systems.
Rapid removal of large amounts of free water contamination is often needed in lube and hydraulic system applications. Coalescing filters work well for this. However, their removal capabilities diminish sharply when the free water content drops to about 100-250 ppm(w), depending on many variables. Simple physics suggests fewer droplets offer less probability of droplet/droplet interaction and coalescing. Therefore, the law diminishing returns becomes evident. Removal of free water below this threshold is not in the domain of coalescing filters. Moreover, they are unable to remove dissolved water and most water-in-oil emulsions. These forms of water contamination are handled best by flash distillation or vacuum mass transfer, also known as vacuum dehydration.
The potential for contamination of coalescing filters should be considered carefully. By nature of their fiber matrix, they will filter particulate from oil but should not be used for this purpose. A separate prefilter should be used. Particulate contamination will degrade coalescing filter performance and service life. Also, surfactants coat the fibers of common coalescing media, disarming it and severely degrading performance. If surfactants are present in the oil a filtration specialist should be consulted. Media is available that is not disarmed by surfactants and other chemicals.
Flash Distillation:
Flash distillation is one of two methods used to remove free, emulsified and dissolved water from lubricating and hydraulic fluids. In this context flash distillation refers to the process of heating an oil/water mixture to the point that it will boil after being pumped into a vacuum chamber. This process will yield a vapor and liquid phase in equilibrium. The vapor will consist almost entirely of water since it has a higher vapor pressure than the base oil. The water vapor is removed through the vacuum pump and condensed. The dehydrated oil is pumped out of the bottom of the chamber.
This method also removes any gases and components that are volatile under the chamber pressure and temperature. Removal of dissolved air and gases is beneficial. However, this can also exacerbate foaming. This is addition to temperature and vacuum level must be carefully controlled to minimize operational issues and limit the thermal degradation of the processed oil.
Vacuum Mass Transfer:
The term “vacuum mass transfer” is used in this context to refer to a process that essentially maximizes the transfer of water molecules from the oil to a counter current dry air stream without boiling. This process was developed to address many concerns about flash distillation of oil. These concerns include:
- Acceleration of oil degradation by deep vacuum and high heat
- Severe oil foaming that adds significant operational complexity
- High energy consumption
- Dialing back processing capacity to reduce operation complexities precented at the rated capacity
Vacuum mass transfer optimizes the vacuum dehydration process around three key variables, oil surface area, oil temperature and chamber vacuum level. Using these variables, it is possible to promote a high rate of water mass transfer by dispersing the oil through an engineered packed column/chamber. The very thin oil film and extensive surface area created by the special packing significantly improves the mass transfer efficiency under mild heat and vacuum level.
As water transfers from the oil to air stream it is carried out of the chamber through the vacuum pump, condensed and collected in a tank.
This process can remove 90% of free, dissolved and emulsified water and 80% of dissolved gases without imparting thermal or barometric stress to the oil. The milder vacuum also reduces the foaming, energy consumption and operational complexity. Vacuum mass transfer addresses all concerns raised by flash distillation.
Adsorption:
Adsorption is the physicochemical attachment of contaminants to the surface of a solid material by means intermolecular forces. Adsorbents are typically solid materials with extremely high surface areas. Generally, they are highly porous, providing a labyrinth of passages where the contaminant molecules come into close contact with the adsorbent surface and adhere. These materials are generally used to remove or control trace contaminants at a molecular level. This is often referred to as polishing or finishing.
There are several adsorbents commonly used in the lube and hydraulic oil applications. These include:
- Lenticular filters made with cellulose media impregnated with activated carbon, diatomaceous earth and other activated materials that target specific contaminants. These combine the advantages of depth filtration and selective adsorption in a high flow cartridge configuration. While very convenient and effective for targeted contaminants, they are also the most versatile of the adsorbent category. Still sources, sizes and sealing configurations vary by manufacturer with no industry standard. This means you could start with one product and if your contamination issue changes you may have to purchase another complete lenticular filter assembly or other purification technology to address it. One manufacturer does not necessarily have a complete range of lenticular solutions.
- Clay (Fuller’s Earth) is a fine mineral clay that has a generous adsorptive capacity for organic acidic contaminants that are common degradation biproducts in phosphate ester-based oils. Clay is typically packaged in a disposable canister-style cartridge that is installed in a cylindrical housing. While effective for acid control, Fuller’s Earth must be carefully monitored for performance and can require frequent replacement. It also contains extractable metals that can contribute to oil degradation and can contaminate the oil with clay fines if a post-filter is not installed and properly operated and maintained.
- Activated alumina is aluminium oxide that is formulated (for our purposes) to adsorb organic acidic species. Similar to Fuller’s Earth, this adsorbent can be an effective solution for acid control if properly monitored and maintained. It has similar issues to Fullers Earth with extractable metals and fines.
- Ion exchange media is considered the state-of-the-art for lube and hydraulic oil polishing. This media consists of highly porous resin or silica gel beads that can be functionalized for specific purposes. The beads are packed in disposable cylindrical canisters forming a bed or column. These are installed in a housing like a filter and oil percolates through the resin bed. As oil fills the pores within the beads, contaminants adsorb and/or ionically bond to the surfaces. The resins used are functionalized to control acid and conductivity of lube and hydraulic oil. However, they also absorb a significant amount of water, posing some consideration in system design and operation. On the other hand, some resins contain no extractable metals, and their hard plastic construction minimizes fines.
Electrostatic Varnish Precipitators:
Electrostatic precipitators are designed specifically for varnish mitigation. They charge contaminant particles causing them to bond to rods, plates or other oppositely charged surfaces. When properly operated and maintained, these devices can remove varnish precursors down to a submicronic level. However, they rely on the conductivity of the processed fluid. Since most lubrication and hydraulic oil systems experience periodic water ingression, fluid conductivity must be carefully monitored.
Many precipitators must be manually cleaned. This can be a messy and incessant task, possibly resulting in equipment neglect and performance issues. Obtain references and do some research before investing in any equipment. Electrostatic precipitators are no exception.
A note about varnish removal devices:
There is much said about varnish removal devices removing varnish from a fluid as well as machine components. As with any substance, varnish particles have a solubility limit in fluids. Consequently, removing these particles restores the fluid’s varnish solubility. This promotes dissolution of varnish from machine components that contact with the fluid. By this mechanism, any system that reduces a fluid’s concentration of varnish particles and precursors will also remove varnish from surfaces submerged in the processed fluid.
Fluid Purification/Conditioning Systems:
Throughout this article we have discussed the most common fluid purification methods. The application of any single method is contingent upon understanding the fluid and contaminant characteristics. In most lube and hydraulic oil system applications there will be multiple sources of contamination that may require different methods of purification. For this purpose, multicomponent fluid purification/conditioning systems are common.
For example, steam turbine lube oil is often contaminated with ash and dust, degradation biproducts, water, acids, varnish and more. Therefore, a turbine lube oil conditioner might contain a coalescing filter for rapid removal of bulk water, a vacuum mass transfer oil dehydration system for control of dissolved and emulsified water and dissolved gases and a particulate filter to maintain the recommended ISO Cleanliness Level. Other components could be included as needed, such as a varnish removal pod, particle counters, water sensors, acid control modules and more. Many OEM’s design and build custom systems that meet specific needs.
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