Maintaining the purity of lubricating oil is essential for optimal system performance and extended component life. It is also relatively inexpensive and easy to do if you utilize the correct oil purification equipment.
Water, particulate and gaseous contamination of lube oil is an unfortunate certainty and must be addressed. Failure to do so, will have a material impact on system operation, uptime and reliability. Failure to continuously remove these contaminants is the number one reason for bearing failures that lead to machinery downtime and related costs. “In a study by the Massachusetts Institute of Technology (MIT), it was estimated approximately $240 billion is lost annually (across US industries) due to downtime and repairs to manufacturing equipment damaged by poor lubrication” (Mowry 2).
Particulate contamination will result in equipment failures due to abrasive wear, erosive wear, adhesive wear and fatigue wear, varnishing, valve stiction, worn shaft seals, scoring on shafts, and premature loading of filter elements. Therefore, it is paramount that it is monitored and controlled. The gold standard for benchmarking particulate contamination is ISO 4406. This standard establishes and defines a method of coding the concentration and sizes of particulates in a fluid. This is the ISO Cleanliness Level and can be correlated to machinery service life. Recommended oil cleanliness levels for some common machinery components are shown in Table 2. Although these are good guidelines, actual application conditions and equipment characteristics should be considered before settling on an ISO Cleanliness Level.
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Machine/Element Target ISO Code
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Roller Bearing
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16/14/12
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Journal Bearing
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17/15/12
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Industrial Gearbox
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17/15/12
|
|
Steam Turbine
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18/15/12
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Table 2. Typical Base Target Cleanliness Levels (Particulate Contamination 5)
Whether dissolved, free, or emulsified, water contamination can be as much or more damaging than high particulate levels. Excessive water in lube oil leads to the following major issues:
- Premature component wear
- Depletion or alteration of the fluid’s additive package
- Formation of rust, emulsions, sludge, etc.
- Reduced hydrodynamic film thickness
- Formation of bacterial growth
- Accelerated metal surface fatigue
- Increased dew point in generator, resulting in increased windage losses/generator inefficiency
- Sticking/jamming valves
- Bearing wipes
- Potential icing at low temperatures
- Formation of acids
To get an idea of the magnitude of the damaging effects, consider that “as little as one percent water in oil can reduce the life expectancy of a journal bearing by as much as 90%” (Water).
According to Ametek Brookfield “acceptable levels of dissolved moisture typically range from 50-300 ppm (or 0.0050% – 0.0300%)” (Moisture in Oils). There are three forms of water in oil, dissolved, emulsified, and free. All forms should be controlled. Free and emulsified water are directly related to equipment damage. Dissolved water can react with oil additives causing degradation of its intended properties varnish precursors. If water concentration is not kept well below the saturation point, there is a risk of formation of free and emulsified water during normal fluctuations in oil temperature. Therefore, water concentration should be kept below the saturation point corresponding to the lowest expected oil temperature.
Gases exist in oil in four forms, free, entrained, dissolved and foam. According to Noria, “Entrained air has the potential to impact the oil’s compressibility, heat transfer, film strength, oxidation, cavitation and varnishing (microdieseling)” (Fitch). These issues result in shorter oil life, machinery damage and safety concerns. Therefore, monitoring and control of gases in lube oils should be a standard part of lubricated equipment reliability programs.
Effectively managing contamination and the subsequent issues caused can be far less troublesome than you may imagine. The answer is usually as simple as investing in the right
oil purification system. Oil purifiers are compact cart or skid mounted systems that filter, dehydrate and degas oil. Oil purifiers like PowerFlow’s XLP Series will effectively and efficiently clean (ISO 14/12 or better), dry and degas mineral-based and synthetic oils. They will remove 100% of free water and gases and 90% (or more) of the dissolved water and gases from oils.
To make the most of your investment there are some very important characteristics to consider when evaluating oil purifiers:
Filtration Performance
An
oil purifier should utilize highly efficient particulate filtration meeting a minimum particulate removal rating of Bx(c) = 1,000 per ISO 16889. Many equipment manufacturers provide ISO Cleanliness Code targets for lubricated machine components. The particulate filter should ultimately be able to achieve and maintain this level of cleanliness, while maintaining acceptable service life. Incoming The air used in the dehydration/degasification process should also be filtered as it enters. Here we will focus on the oil filter.
There are a few characteristics about the filter element and housing that should not be overlooked. Foremost, the filter housing and element should be specifically designed for lube and hydraulic applications. This is very important because there are distinct differences from those that are designed for other applications, such as municipal water.
The mechanical design of the housing should meet or exceed the maximum and minimum oil temperatures and pressures that could occur during any mode of operation or standby. The housing, element or both should have an appropriately set bypass valve that will let oil bypass the filter during cold start-ups or other times when the pressure differential across the housing or element exceeds the limits of the system. The filter housing should have a service indicator or pressure gauges that can be used to determine when the filter element needs to be changed. The housing should be positioned with easy access and plenty of overhead room for changing the filter element(s). The access port to the filter element should have a cover that is easy to operate and properly seal.
The filter elements should be absolute rated. This means that the filter efficiency is tested and rated in accordance with a respected standard such as ISO 16889. This also ensures the test results are reliable and repeatable. Common absolute filter ratings will be stated as a beta ratio; for example, β(x)=1000, where ‘x’ is the particle size in microns at which the beta ratio is measured. A nominally rated filter may not have a published filter efficiency and will not have a standard associated with it. Many filter elements are also rated under cyclic stress conditions to provide a better estimate of performance in systems with pressure or flow cycles. To get reliable, repeatable results always invest in absolute rated filter elements and consider cyclic ratings if available.
Other important features of
lube and hydraulic filter elements are related to the design and construction of the cartridge and media pack. When properly designed the collapse rating will be at least 150 psid. They will employ a multi-layer media pack with inner and outer support layers and a filtration layer. Often there will be other proprietary layers that enhance performance under stress or increase service life. The most reliable filter elements will have an O-ring seal, positive pressure seal or a proprietary seal that is very precisely designed for ease of installation and zero bypass. Flat gaskets are not recommended due to the propensity for sealing surface misalignments, hold down mechanisms that are prone to operator installation errors and adhesives that often fail.
More needs to be said about the most important component of the filter element, the media pack. There are many designs and claims. However, there are a few basic principles to keep in mind. The media must be designed to maximize filtration capacity without unloading filtered particles, shedding fibers, compressing or bunching. The most common absolute rated lube and hydraulic filter media material is microglass. There is often an option for anti-static microglass media that helps prevent static discharge within the filter media. This is a great choice if there is evidence of static discharge in your lube system. Cellulose media is very common because it is inexpensive. However, it is rarely absolute rated and has other drawbacks that can result in a higher cost of filtration. For example, it absorbs water causing premature element changes and potential media failure. The most reliable filter elements feature robust design and construction that will hold up to the application demands. When selecting an element, consider the value of protecting your equipment investment with a filter element that has documented performance characteristics, compatibility with contaminants like water and lasts several times longer than cheaper paper/cellulose alternatives.
Oil purification starts with particulate control. Consider the technology and the design and construction quality of the particulate filter element and housing when evaluating oil purification systems.
The Dehydration and Degasification Process
Since much of the water and gas is dissolved in the oil the separation is usually accomplished with some variation of mass transfer optimized by temperature and pressure manipulation. One of two common processes are used by most oil purifiers, flash distillation or vacuum mass transfer. Flash distillation uses relatively high heat (150-160 deg F) and deep vacuum (25-29 inHg (v)) to “flash” or boil off water and gases from the oil, but there are known issues associated with that. “The high temperature and vacuum employed in flash distillation devices can lead to loss of lower boiling base stock fractions and volatile additives and can result in thermo-oxidative fluid degradation” (Day). Vacuum mass transfer is a more efficient process in which the oil is processed through an engineered packed column. The packing creates an ultra-thin film and maximizes the oil surface area, releasing water and gas molecules through diffusion and convection. The vacuum mass transfer process is highly efficient at lower temperatures and vacuum levels.
In both processes, ambient air is drawn into the vacuum chamber where it expands, reducing the relative humidity. The dry air carries the water and gas vapors out through a vacuum pump and exhausts it to the atmosphere.
The bottom line is that the oil purifier should be able to remove 100% of the free water and gases and 90% (or more) of the dissolved water and gases from oils at no more than 120 deg F and 22 inHg vacuum. This is accomplished easily and reliably with an expertly designed vacuum mass transfer process utilizing an engineered packed column.
The Oil Flow Control System
Oil purifiers are typically offline systems. They are installed temporarily or permanently in a kidney loop recirculating on an oil reservoir. Therefore, a means of pumping the oil through the purifier and back to the reservoir is required. Some systems use pumps and others rely on the process vacuum to pull the oil into the vacuum chamber. A pump draws the oil out of the chamber and returns it to the oil reservoir.
There are pros and cons to the various oil feed systems outlined below.
Vacuum Assisted Oil Feed:
Pros
- There may be reduced CAPEX, operating and maintenance cost because there is no oil feed pump; larger systems may require a booster pump.
Cons
- This is a batch process requiring a more complicated control system that is sensitive to changes in the oil temperature, pressure and viscosity. Due to this the system may require more operator attendance as oil conditions change that require machine adjustments.
- These systems tend to require deeper vacuum, creating more potential for foaming. The foaming can result is process destabilization. In many cases this requires more operator attendance, or the system is adjusted to process a smaller volume of oil that will result in less process upsets. In effect, a larger system may be tuned down to the point of processing less oil than a smaller more efficient system
- Some system components such as control valves may be subject to high rates of repetition due to the nature batch processing. These components may require frequent service or replacement.
Constant-Flow Oil Feed Pump:
Pros
- Allows for a continuous process at constant flow reducing processing time. This can result in quicker resolution of an oil contamination event.
- Less complex and more reliable process control.
Cons
- Process control with a constant flow oil feed pump can require more operator attendance as changes in the oil temperature, pressure and viscosity will need to be addressed by manual tuning. Another option would be to use a more complex control system. In either case, the result may be less maximum oil processing capability under some conditions.
- Additional CAPEX, operating and maintenance required for the oil feed pump.
Oil Feed Pump with Variable Frequency Drive:
Pros
- Allows for a continuous process at constant flow, reducing processing time. This can result in quicker resolution of an oil contamination event.
- Improved process control. The VFD can be programed to change the pump speed automatically to maintain a continuous flow rate as oil conditions fluctuate.
- Minimal operator attendance requirements.
Cons
- Additional CAPEX, operating and maintenance required for the oil feed pump and VFD.
The bottom line is that a purifier with a variable speed oil feed pump will provide the most reliable processing with the least amount operator attendance.
The Oil Dispersion Process
Within an oil purifier’s vacuum chamber, the oil is exposed to dry air under vacuum. The dissolved water and gasses transfer from the oil into the air as the vacuum source pulls it though the chamber and exhausts it to the surrounding atmosphere. The rate of transfer of the water and gas molecules from the oil to the air is proportional to the surface area of the oil. Therefore, the dispersion process and its effectiveness of maximizing the oil surface area is the most critical factor in oil purifier design. It is often the most coveted also. There are many designs. Most use some form of dispersal media.
There are two common types, dispersal elements and packing rings. Dispersal elements are cylindrical cartridge elements with a fiber matrix designed to create a thin film of oil within the media pack. Packing rings are small metallic structures with geometries engineered for specific applications. For oil purification the rings are designed to randomly stack in the vacuum chamber/tower with optimum fluid dynamics and mass transfer properties.
Dispersal elements are not the most efficient nor effective means of promoting mass transfer. The fibers that make up the media are not specifically designed for maximizing mass transfer. For a fiber matrix to be most effective the flow and distribution of oil through the element must be very precisely controlled to avoid flooding and optimize fiber utilization. Considering the normal fluctuation in oil temperature, pressure and viscosity, cylindrical geometry of the elements, and the potential for media fouling, expect performance to be inconsistent at best. Due to fouling and fiber damage the elements are considered consumables. This is an undesirable maintenance burden and additional cost of ownership when compared with systems that use packing rings.
Packing rings used by some manufacturers are specifically engineered for their oil purifier vacuum chamber design. For example, some manufacturers test various ring designs until they discover the size and geometry that provides the best performance in a specific purifier model. This results in better mass transfer efficiency, lower energy demand, the elimination of the damaging effects of high heat and vacuum on the oil, and a lower cost of ownership because a properly design packing never needs to be replaced.
The bottom line is that a purifier that utilizes a column of packing rings for dispersal media will process oil more efficiently, will not harm the oil and will have fewer consumables to replace.
The Vacuum Pump
Both vacuum mass transfer and flash distillation units rely on vacuum pumps. The pumps generate the vacuum needed for the dehydration process and pull a small amount of air through the process to sweep out the water and gas vapors. There are three types of vacuum pumps commonly used. These are liquid ring, oil sealed rotary vane and dry claw pumps. Liquid ring pumps have a high utility cost primarily due to the need for a constant supply of sealing water. The sealing water supply may also limit the installation and mobility of the purifier. Oil sealed rotary vane pumps require a commitment to regular monitoring and service work. They are also sensitive to water droplets that can be entrained in the exhaust gases, if not adequately removed upstream of the pump. The dry claw style pump is the most suitable for the application. “A claw pump is approximately 30% more efficient than a comparable rotary vane pump and uses less energy” (Dry). They provide sufficient vacuum with very little utility cost, service or monitoring and less sensitive to liquid contamination.
The bottom line is that a purifier that uses a dry claw style vacuum pump is going to be more reliable, durable and have lower operating and maintenance costs.
We have covered some of the most important features to consider when evaluating oil purifiers. Following these guidelines will help you make a good investment that will greatly extend service life, reduce downtime and lower the total cost of ownership of rotary equipment. Keep our guidelines in mind will lead you to a purifier that will do all this while being very easy to operate and maintain.
Works Cited:
Fitch, Bennett, “Why You Should Be Measuring Air Contamination in Oil”, machinerylubrication.com, Noria Corporation, Dec 2014, Web, 19 April 2022
“Moisture in Oils: The Three-Headed Beast”, brookfieldengineering.com, Ametek Inc., 14 April 2014, Web, 19 April 2022
Mowry, Matt, “The True Cost of Bearing Lubrication”, toolbox.igus.com, IGUS Inc., Spring 2011, Web, 19 April 2022
“Particulate Contamination – Identifying, Eliminating and Removing it”, media.noria.com, Noria Corporation, n.d., Web, 19 April 2022
Day, Mike and Bauer, Christian, “Water Contamination in Hydraulic and Lube Systems”, machinerylubrication.com, Noria Corporation, September 2007, Web, 19 April 2022
“Dry Pumps: Claw Pumps”, vacaero.com, Vac Aero International, 23 March 2016, Web, 25 April 2022
“Water in Oil Contamination”, machinerylubrication.com, Noria Corporation, July 2001, Web, 19 April 2022
