Part II: Oil Cleanliness: The Key To Equipment Reliability

Cleanliness can impact equipment life in a big way. For example, in hydraulic systems, with servo valves, a typical new unfiltered hydraulic fluid usually has an ISO cleanliness code of 25/22/19. The system requires a 16/14/11 based on pressure. By filtering the oil to the proper cleanliness level of 16/14/11, the life of the valve can be increased by four times.

As noted in Part I of this series, cleanliness is crucial to all equipment components—not just hydraulic equipment. In general, the tighter the clearances, the cleaner the fluid must be. Rolling element bearings have tight clearances with thin lubricant films. Table I illustrates rolling element life extension based on fluid cleanliness. Knowing that fluid cleanliness is important in equipment longevity, the next question is how clean does my fluid need to be and what kind of filtration is needed to achieve these levels.

Cleanliness targets The most sensitive components in a hydraulic are the directional control valves that usually dictate the cleanliness standards for the whole system. If only valve type and pressure conditions are known, a generalized ISO cleanliness code, as illustrated in Table II, can be used as a reference. Bearings also require clean fluids. ISO cleanliness requirements for bearings are shown in Table III.

Mike Boyd of Fluid Solutions has developed a simplified way of being more specific, based on equipment conditions, in determining system cleanliness requirements for both hydraulic systems and gearboxes. This simplified method can be seen in Fig. 1 and Fig. 2 (see page 10).


Both Pall and HY-PRO filter companies have a method for calculating hydraulic system ISO cleanliness codes based on a large number of variables. Adapted from the British Fluid Power Association, a summary of this method is illustrated in Table IV.

Filtration requirements Selecting the most optimal cleanliness program requires the following:

  • Select the ISO cleanliness code to be achieved (previously discussed)
  • Select the correct placement for the filters
  • Select the correct filter sizing to achieve the desired cleanliness code

Hydraulic systems have a number of options on filter placement, including the three illustrated in Fig. 3. A fourth filter placement option with servo valves involves a control circuit filter placed just before the valves.

Cleanliness targets can be achieved with a pressure line filter alone or with a combination of various filter locations. As provided by HY-PRO, Table V notes the filter sizing to achieve various cleanliness codes with one filter containing a Beta ratio of 1000.

It must be must be emphasized that Table V is solely for illustrative purposes—and ONLY for HY-PRO filters. Each filter manufacturer has tables for its own cleanliness guidelines. Filter ratings based on the new test dust (>4μ, 6μ, 14μ) will be coarser for the same cleanliness code than the old test dust (>2μ, 5μ, 15μ).

Filter manufacturers use the Multi-Pass Filter Test to establish filter ratings in achieving cleanliness codes. (This test was discussed in Part I of this series.)

The Multi-Pass Filter Test is run at both constant and varying flow to simulate a hydraulic system as closely as possible. It can be run under many different conditions, including viscosity of fluid, amount of test dust added, flow rate, terminal pressure drop, etc. This testing is used to develop filter media to achieve different cleanliness targets. Because high levels of test dust are constantly added in the Multi-Pass Filter Test, high beta ratios compared to what is actually measured in the system are usually obtained. The same filter rated on an actual system may show a much



lower efficiency. As an example, a filter on the Multi-Pass test rated at 6μ showed a beta ratio of 3000. When analyzed in the system, the actual beta ratio was only 3, but the cleanliness targets were met with the filter because a steady state condition had been reached in particle removal. Not many new particles were entering. The point here is that the way to evaluate filters is not strictly on beta ratios, but rather on how they perform in the system.

Achieving your requirements The most effective way to achieve your cleanliness code requirements is to optimize your filter placements and their size. The Parker Hannifin handbook is a particularly helpful reference in that it lists cleanliness requirements for various components and the filter requirements. As an example, for servo valves the handbook lists three different filter placement combinations: pressure, pressure & return line and pressure & offline. Many times, using a lower priced return line and/or offline filter will be more economical with the same results because a less expensive, coarser pressure line filter may be used. The four different filter options are as follows:

Pressure line filter…

  • High cost
  • Protects sensitive components downstream of pump
  • Sees total system flow
  • Bypass options


Control circuit filter…

  • Directly before sensitive valve and
    will only filter fluid to that component
  • Protects sensitive valves
  • Cost effective when used in combination
    with return and/or offline filters
  • High collapse option

1107_contaminant_img5Return line filter…

  • Typically sees total system flow at low pressure
  • Cleans ingressed and generated contaminants
  • Low cost to weight of dirt removed

Offline filter…

  • Low pressure
  • Constant flow
  • Capable of optimizing system at low cost
  • Filter changed without system interuption

The impact that various filter options can have on a system can be seen in the following example. Here, a hydraulic system was using a 3μ control circuit filter and a 10μ return line filter. By changing to a 3μ return line filter, the particles >4μ were reduced tenfold and the control circuit filter could have been changed to a coarser grade to achieve the same previous cleanliness target.

The key to any reliability-based program is to develop a successful cleanliness control program. This is done by minimizing contaminant ingression through a cost-effective filtration program based on the following criteria:

  • Setting of cleanliness requirements based on objectives and equipment type
  • Selection of optimum filter placements
  • Selection of filter sizing

Utilize your filter manufacturers in evaluating your systems and building the cleanliness control program(s) to meet your requirements.

Coming up next time 
The final article in the series will present case histories on the economics of effective filtration programs.

The author thanks HY-PRO, Pall and Parker Hannifin for providing useful information for this article. Particular thanks go to Mike Boyd of Fluid Solutions for his mentoring on filtration principles and providing valuable information.

(Editor's Note: In Part I of this series— Sept./Oct. 2007, pgs. 34-38—Fig. I and Fig. II were provided by Parker Hannifin. They were referenced incorrectly, and we regret the oversight.)

Contributing editor Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training in a number of industries. E-mail:  This e-mail address is being protected from spambots. You need JavaScript enabled to view it ; or telephone: (281) 257-1526.