Certification Matters, Part V: Fluid Conditioning

EP Editorial Staff | April 11, 2012


When it comes to the cleanliness of the lubricants in your rotating equipment, you can’t be too vigilant.

This article is the last in our year-long series on the most important components of lubrication certification exams administered by the Society of Tribologists and Lubrication Engineers (STLE) and the International Council for Machinery Lubrication (ICM). However, even if you’re not pursuing certification, having a thorough understanding of fluid-conditioning principles is crucial in dealing with rotating equipment. Here, we examine the two most destructive contaminants: particulates and water.

Particulate contamination
It’s been said that 75% of equipment failure in circulated systems is caused by contamination. The major cause of equipment wear is three-body abrasive wear caused by particles—according to some estimates, up to two-thirds of the wear in equipment. And, it’s what you don’t see that causes the major problems. The smallest particle size that can be seen with the naked eye is 40 microns. Unfortunately, the most destructive particles are much smaller in size: you just can’t see them.

In dealing with equipment cleanliness, one must be as proactive as possible. It costs at least 10 times more to remove particles than to keep them from entering a system. The following are some of the ways to be proactive:

  • Properly flush new equipment to remove any built-in particles that may be present.
  • Filter new oil going into hydraulic and turbine systems to a minimum cleanliness standard.
  • Minimize outside ingression.
    • Maintain good practices in dealing with reservoir openings.
    • Utilize good breathers to minimize solid ingression.
    • Maintain rod seals in linear actuators in hydraulic systems.
  • Practice good storage and handling procedures.
    • Properly store and maintain drums.
    • Utilize proper dispensing containers such as sealed plastic ones.
    • Utilize off-line filtration in large storage tanks to maintain proper cleanliness.
  • Conduct training to develop and reinforce the attitude in lubricators that oil cleanliness is vital to equipment life.

General filtration principles
Even if your operations are being proactive in fluid cleanliness, it’s still necessary to have proper filtration to achieve targeted cleanliness levels. Outside particulate ingression is usually not totally controlled, and wear metals from operation need to be removed. The most effective filters have the following characteristics:

  • Synthetic glass or metal with thin fibers
  • Graded density with tapered pore construction
  • Supported elements

The following are key factors in filter selection:

  • Cleanliness required
  • System pressure drop
  • Selection of fiber type to give desired performance
  • Bypass or non-bypass filter
  • Simplex or duplex
  • Location
    • Pressure line
    • Return line
    • Off-line

Filter sizing is extremely important. Pressure drop determines the filter’s size. The following variables determine pressure drop:

  • Filter housing must be sized so flow won’t blast through the element and also to ensure that the filter functions properly during cold startups, where viscosity can be 3-30 times the operation viscosity.
  • Type of media grade
  • Fluid viscosity
    • Oil type
    • Temperature
  • Flow rate

Measuring filter performance…
Filter performance should be based on the absolute filter rating which is determined in a laboratory with the Multi-Pass Filter Performance Test. Dirt in milligrams per liter is introduced in a test fluid at a constant rate. The fluid is circulated at a constant or variable rate through a test filter. The test concludes when the terminal pressure drop of the filter is reached. This is the point when the filter manufacturer designates the filter as no longer operable and needing to be changed. The filtration efficiency of the test filter is expressed as the filtration ratio.


For example, a filter with a beta ratio of ß6=200 indicates that for every 200 particles greater than six microns in size entering the test filter one will pass though. Therefore, 199 will be captured. The efficiency of the filter is calculated as ß-1/ß x 100. The efficiency of a filter with a beta ratio of 200 is 99.5%.

ISO Cleanliness Code…
Fluid cleanliness is expressed as a three-number code according to ISO 4406. The code is expressed as all particles > 4µ[c], > 6µ[c] and > 14µ[c]. The numbers are obtained from the ISO 4406 Chart in Table I.


As an example, consider a fluid where the following particles per milliliter of fluid were measured:



Find the range number that expresses the number of particles per milliliter. For instance, 7500 particles are found at the range number of 20 where the range is 5000 to 10,000 particles. Notice that for every increase in range number, the number of particles can double. Thus, even a moderate increase in the range number can result in a large introduction of particles.

Target equipment cleanliness code…
To extend equipment life, the correct cleanliness must be maintained. Different equipment components require different cleanliness levels. The clearances of the equipment determine the level of cleanliness. For example, a servo valve in a hydraulic system requires cleaner oil than a gear pump. Many tables are available as a guide to equipment cleanliness requirements. For purposes of this article, we use one from from Eaton (Table II).


Utilizing Table II, let’s determine the cleanliness level required for a variable inline axial piston pump operating at a pressure of 3500 psig with a servo valve. The most sensitive component in the system determines the cleanliness code selected. The piston pump requires a fluid cleanliness of 16/14/12 while the servo valve requires a cleanliness of 15/13/10. Thus, the cleanliness required for this hydraulic system is 15/13/10.

Filter location…
Filter location placement is a key variable in achieving target cleanliness. Usually a combination of locations is employed. Work with your filter manufacturer to determine the optimum combination of filter types and location to achieve your desired cleanliness level. 

Figure 1 (from Pall Filters) illustrates the most common filter-placement locations.


Pressure-line filter…

  • Installed downstream of pump before the valves
  • Provides protection to valves which are usually most sensitive components in the system
  • Subjected to variable flows and pressure cycles
  • Operating pressures that can reach 450 bar and higher
  • Bypass or non-bypass dependent on critical nature of components
  • Most expensive filter in the system
  • Additional pressure-line filters (pilot filters) possibly required with sensitive components where finer filtration is required

Return-line filter…

  • Installed between working components and reservoir
  • Usually located inside tank or outside spin-mounted on reservoir
  • All flow from system collected and directed through the return filter
  • Largest and least expensive filter on system
  • Must handle maximum system flow
  • Protects system from wear particles and particles ingressed through retracting piston in cylinders
  • Ensures clean fluid in reservoir
  • Usually the only filter for mobile equipment

Off-line filter…

  • Self-contained system including pump, motor and filter
  • Main purpose is to keep reservoir fluid clean
  • Can run 24/7
  • Easily serviced without disrupting operation
  • Very fine filter elements can be used
  • Help extend useful life of pressure- and return-line filters
  • Target cleanliness codes most easily achieved with these types of filters
  • Water-removal elements can be used
  • Can be permanent-mount or filter carts

Water contamination
The second most destructive contaminant—water—is introduced in the system the following ways:

  • Humid air
  • Reservoir and tank condensation
  • Bearing-housing breathing
  • Steam leaks on dryers
  • Heat-exchanger leaks
  • Leaking pump packing and turbine steam glands
  • Rain
  • Washdowns

Water contamination manifests itself as follows:

  • Reduced lubricant effectiveness resulting in increased wear
  • Corrosion of machine components
  • Premature filter plugging
  • Increased oxidation of lubricant
  • Depletion of oil additives
  • Viscosity increase

Small amounts of water can reduce rolling-element bearing life via hydrogen blistering and embrittlement. Aim for 200 ppm or less; close to 100 ppm is better. Journal bearings in turbines should be maintained not to exceed 200 ppm.

The three water forms are dissolved, emulsified and free. Dissolved water usually isn’t a problem, unless the saturation point is reached at a lower temperature, resulting in free water which can become emulsified and can only be removed though vacuum dehydration or passing hot air through a special type of filter or system. The most destructive form is emulsified water, which can be removed through centrifuging, coalescing filters and vacuum dehydration. Free water is the easiest to remove: Reservoirs and sumps should be routinely drained of free water, which, if circulated, can become emulsified.

The following proactive practices can help prevent water from entering a system:

  • Hermetically seal bearing areas, if possible.
  • Repair rotary steam joints on dryers.
  • Routinely inspect heat exchangers, steam coils and packing.
  • Build shelters to prevent water from falling on equipment.
  • Use desiccant breathers on vents in tanks and reservoirs to reduce condensation.
  • Train operators on proper use of cleanup hoses.
  • Keep hatches and covers closed on reservoirs.

The following are ways to remove and minimize effects of water ingression in a system:

  • Monitor and drain reservoirs frequently to remove  any free water.
  • Install vapor extractors on wet reservoirs.
  • Install centrifuges or filter-coalescers on wet systems to remove gross water.

This entire five-installment series of articles was designed to help prepare you for the various certification exams through the STLE and ICML. Be aware, though, that “help prepare” is the operative term. Reading this series of articles will be a step in the right direction, but that alone won’t be enough. Much greater preparation—through significant self-study and formal training—will be required to seriously pursue certification. For complete details on STLE and ICML certification, go to: and LMT




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