Filtration Solutions

Hydraulic Components Need Protection
Fluid power circuits are designed in all shapes

How Contamination Damages Critical Parts
In normal operation, the spool slides back and forth in the valve body,
diverting oil to one side of the valve or the other. If a particle lodges
between the spool and valve body, it will erode small flakes from the
metal surfaces. As these flakes are moved back and forth by the
action of the spool, they can roll into a burr that jams the spool
and disables the valve.

Types of Contaminant

• Many different types of contamination may be present in hydraulic fluid, causing various problems. Some are:
• Particulate (dust, dirt, sand, rust, fibers, elastomers, paint chips)
• Wear metals, silicon, and excessive additives (aluminum, chromium copper, iron, lead, tin, silicon, sodium, zinc, barium, phosphorous)
• Water
• Sealant (Teflon®* tape, pastes)
• Sludge, oxidation, and other corrosion products
• Acids and other chemicals
• Biological, microbes (in high water based fluids)

Typical Factors in Component Life

Studies show that most (typically 70%) of hydraulic component replacement is necessary because of surface degradation, and most of that is due to mechanical wear. Proper filtration of hydraulic fluids can lengthen component life. There are a surprising number of different sources of system contamination in hydraulic filtration.

 
New Hydraulic Fluid
Adding new fluid can be a  source; even though it’s fresh from the drum, new hydraulic fluid isn’t clean. (It may look clean, but, remember, the human eye can only see a particle the size of about 40 μm.) Oil out of shipping containers is usually contaminated to a level above what is acceptable for most hydraulic systems: typically, new fluid has a cleanliness level about the same as ISO Code 23/21/19, and water content is typically 200 to 300 ppm. Never assume your oil is clean until it has been filtered.

Built-In
Built-in contamination, also called primary contamination, is caused during the manufacture, assembly and testing of hydraulic components. Metal filings, small burrs, pieces of Teflon tape, sand and other contaminants are routinely found in initial clean up filtration of newly manufactured systems.

The major source of contamination are the pump and actuators, the hydraulic cylinder, or the hydraulic motor. Wear-generated contaminants are a hazard during normal hydraulic system operation. The circuit actually generates additional particles as the fluid comes into contact with the precision machined surfaces of valves, motors and pumps. Contaminant levels can keep doubling with every new particle generated. The result can be catastrophic if these contaminants are not properly filtered out of the system.

Fluid Conditioning
Fluid Conditioning is the term for the overall conditioning of the fluid in the hydraulic system, and encompasses particulate removal via filters along with other various methods for removing silt, air, water, heat, acid, sludge or chemicals.

Particulate Removal
Particulate removal is usually done with mechanical filters. A well designed reservoir that allows settling will also help in keeping particulates out of the mainstream fluid. For ferrous particulates and rust, reservoir magnets or strainer band magnets can also be used. Other methods such as centrifuging or electrostatic filtration units can also be used, particularly in continuous batch processing and fluid reclamation.

Removal of Silt
Silt, defined as very fine particulate under 5 μm in size, requires very fine filtration or “oil polishing.”

Air Removal
Getting air out of the system is best done by adding 100 mesh screen in the reservoir, approximately 30° from horizontal to coalesce entrained air and allow larger bubbles to rise to the surface when reservoir velocities are low.

Water Removal
A number of techniques exist to prevent water or moisture ingression or to remove water once it is present in a hydraulic or lube oil system. The best choice of technique for removal is dependent on the whether or not the water exists as a separate phase (dissolved or free), and also on the quantity of water present. For example, the presence of water or moisture can be reduced or prevented from entering a fluid reservoir through the use of absorptive breathers or active venting systems. However once free water is present in small quantities, water absorbing filters or active venting  systems usually provide adequate removal means. For large quantities of water, vacuum dehydration, coalescence, and centrifuges are appropriate techniques for its removal. However, as each of these techniques operates on different principles, they have various levels of water removal effectiveness. The chart below provides comparative information on these techniques and their relative effectiveness. Care should be taken to apply the best technique to a given situation and its demands for water removal.

 
Chemical Removal
Removal of acids, sludge, gums, varnishes, soaps,  oxidation products and other chemicals generally requires an adsorbent (active) filter with Fuller
Earth, active type clays, charcoal, or activated alumina.

Heat Removal
Removing heat is important to maintain viscosity and prevent fluid breakdown. Usually performed with heat exchangers, including air-to-oil and water to-oil types, finned coolers, or refrigerated units.

Heat Addition
Added heat is used for cold temp start-up to get fluid viscosities within operational limits. Use heaters, immersion or in-line.

Kidney Loop Filtration
One very effective way of ensuring thorough fluid conditioning is with a dedicated

Proper Filter Application
When selecting a filter or replacement element, it’s important to first answer some basic questions about your application. Where will the filter be used? What is the required cleanliness level (ISO code) of your system? What type of oil are you filtering? Are there specific problems that needed to be addressed?

It’s also important to think about the viscosity of the fluid in your system. In some machinery lubrication applications, for example, the oil is very thick and has a tougher time passing through the layer of media fibers. Heating techniques and the addition of polymers can make the liquid less viscous and therefore easier to filter. Another option is to install a filter with larger media surface area, such as the Donaldson W041 or HRK10 low pressure filters, that can accommodate more viscous fluids.

Next, think about duty cycle and flow issues. Working components such as cylinders often create wide variations in flow—also called pulsating flow —that can be problematic for filters with higher efficiency ratings. On the other hand, dedicated off-line filtration (also called “kidney loop”) produces a very consistent flow, so it makes sense to use a more efficient filter. Filters used in applications with steady, continuous operation at lower pressures will last longer than filters that must endure cycles of high pressure pulsating flow. Generally, the lower the micron rating of a filter, the more often it needs to be changed since it is trapping more particles. Finally, it’s wise to ask yourself, “How much is my equipment worth?” Calculate how much it would cost to replace the equipment in your system, in case of component failure, and make sure those areas are well protected with proper filtration. (For example, high performance servo valves are very sensitive, costly components that need to be protected with finer filtration media.) Minimizing maintenance costs through good contamination control practices requires proper filter application based on the specific contamination problems. Good contamination control means cost-effective filtration. When looking for a filter, first assess the needs of your system and any problem areas.

Combining the ISO Rating and Filter Performance Ratings
While filter manufacturers publish beta ratings

Micron Sizes of Familiar Particles Grain of table salt          100 μm Human hair                    80 μm Lower limit of visibility   40 μm White blood cell             25 μm Talcum powder              10 μm Red blood cell                8 μm Bacteria                          2 μm Silt                                 <5 μm

For more information please refer to Donaldson "Technical Reference Guide" Blue Pages

http://www.donaldson.com/en/ih/support/000721.pdf
Information provided courtesy of Donaldson