Offline Filtration: Key To Establishing And Maintaining Oil Cleanliness

EP Editorial Staff | April 2, 2010


While it’s an especially cost-effective approach to one of your biggest lube-handling challenges, it must be done right.

An entire five-part series of articles dealing with the importance of oil cleanliness in machinery reliability and Best Practices in achieving these goals has been published in LMT (starting in 2008 and concluding in Jan./Feb. 2009). In this article, we revisit the topic by focusing on offline filtration as it applies to hydraulic systems and gearboxes.

Offline filtration of hydraulic systems
A typical hydraulic circulation system normally has three filter placement options: pressure line, return line and offline (Fig. 1). In some cases, a control-circuit filter is installed before the servo valves (Fig. 2).


Fig. 1. A typical hydraulic circulation system normally has
three filter placement options: pressure line, return line and offline.
(Source: Hy-Pro Filtration)


Fig. 2. In some hydraulic systems, a control-circuit filter is
installed before the servo valves. (Source: Hy-Pro Filtration)

Many people once considered the pressure-line filter to be the most important in the circuit—because it protects the valves, the most sensitive components in a hydraulic system. These days, it is widely accepted that the proper use of offline filters, in conjunction with pressure-line and return-line filters, can achieve high levels of cleanliness at a reasonable cost. By maintaining a constant level of cleanliness in the reservoir, the pressure-line filter’s main function becomes protection against pump failure. Therefore, a coarser, less-expensive filter can be used. Offline/kidney-loop filters offer the following advantages:

  • Constant flow through the filter (which is not the case in some hydraulic applications involving variable displacement pumps)
  • Low pressure resulting in the selection of finer media at a reasonable cost (which allows for target cleanliness levels to be achieved)
  • Serviceability where the filter can be changed without interrupting system flow
  • Reliability (because the system will see constant flow at low pressure)
  • Application flexibility
    • Self-contained systems that are capable of being permanently mounted or used on a filter cart
    • Minimal floor space required
    • Adaptable to filtration of oil entering or leaving reservoir
    • Adaptable to filtration of new oil in tote or drum that is being added to a reservoir
    • Ideal places to add temperature-control devices
    • Use of water-removal elements as part of the system
  • Positive impacts on overall system
    • Allows meeting of cleanliness targets
    • Extends life of pressure- and return-line filters

Positive-impact examples…
A recent example of the effectiveness of using an offline filter in combination with other filters was in a hydraulic system using a 3 micron control-circuit filter and a 10µ return-line filter. By changing to a 3 micron return-line filter, the particles ≥4 micron were reduced tenfold and the control-circuit filter could have been changed to a coarser grade to achieve the previous cleanliness target.

An injection-molding plant had 10 machines that were down an average of 10% of the time because of servo-valve problems. The pressure-line filter sent 21+/18 oil to the servos. Since installing a 30-gal. recirculating loop supplying 13/10 oil to the servo, and installing an offline system in the 550-gal. working reservoir supplying 14/11 oil to the system, the machines have seen less than 1% unscheduled down time. This has resulted in over $1,000,000 in yearly savings, not including reduced waste from unexpected shutdowns.


Fig. 3 Example of permanently mounted offline filtration systems

Types of offline systems…
There are three types of offline systems to clean fluids. The most common is the permanently mounted system that can operate 24/7. Once installed, it is the easiest to work with. There is, however, some equipment where installation may be difficult because of available space. Some suppliers offer skid- or panel-mounted systems that can be attached to the equipment as illustrated in Fig. 3 and Fig. 4.


Fig. 4. Example of permanently mounted offline filtration systems

Two types of portable systems are utilized. The most common is the filter cart (available from different suppliers in a range of sizes). Another portable system utilizes large totes with a mounted offline filter system. These totes are transported to the equipment where they can supply clean oil from the tote—or have oil from the reservoir pumped into the tote where it is cleaned during recirculation through an offline filter until the cleanliness target is met. As shown in Fig. 5, there also is an offline system that can be installed on the tops of drums or small totes to filter new oil through dual filtration.


Fig. 5. Example of an offline system that can be installed
on the tops of drums or small totes to filter new oil through dual filtration
(Source: Des-Case)

A filter cart represents the most common type of portable offline filtration system. Manufactured by a number of suppliers, these carts also come in a range of sizes (Fig. 6 and Fig. 7).

Filter carts normally have two filters—A and B. The A filter is coarser than the B filter. As an example of filter selection, one manufacturer of servo valves requiring ISO cleanliness of 15/13/11 recommends the use of ß12­=200 and ß3=200 filters. The A filter is coarser at 12 micron while the B filter is finer at 3 micron. In some cases, a water filter is used as the A filter to remove free and emulsified water. Reaching the target cleanliness code calls for the fluid to be turned over seven times in the reservoir. (The amount of turnover is related to the initial cleanliness of the oil.) Filter carts for hydraulic fluids with ISO VG ≤ 68 commonly have flow rates of 2, 5, 10 or 20 gpm.


Fig. 6 and Fig. 7. Examples of filter carts that are available
in range of sizes from various manufacturers

Although filter carts appear easy to use, they have the following disadvantages:

  • They’re often purchased, stored and never used.
  • Cross-contamination can result from using the same cart for different fluids without proper flushing.
  • Carts designed for low-viscosity hydraulic fluids are sometimes used inappropriately on high-viscosity gear oils.
  • Problems can arise from not having the proper connections or hooking up a cart improperly.
  • Poor contamination-control practices, such as storing carts in dirty environments and not properly preventing contamination of the hoses, can cause problems.

Offline filtration of gearboxes
In the past, clean oil was not considered to be an important issue with gearboxes. Because so many of them run in dirty environments with high-viscosity oils, users have assumed that effective filtration is impossible. Times have changed, however, as evidenced by the importance gearbox OEMs have placed on clean oil for longer life. Wind-turbine gearbox manufacturers want to see new oil that is 16/14/11 or cleaner—and want oil maintained in the system at no more than 18/16/13.

Offline filtration systems (both permanent and portable) have been able to filter high-viscosity oil down to very good cleanliness levels. Still, keeping gear oils clean is a challenge because of the following factors:

  • Oils are typically high-viscosity (ISO VG 220-460).
  • Most don’t have filtration (or only have strainers).
  • Many are used in dirty environments (in coal-mill pulverizers, for example).
  • Many are not fitted with breathers.
  • Dirty oil and gearbox failures are accepted as a common practice.
  • There can be problems in draining and refilling gearboxes with new oil, including:
    • Gearbox drain is 1~2″ above gearbox sump low point.
    • Before a drain machine is turned off, suspended contaminant falls out of the oil and settles in sump.
    • Oil is drained without many of the suspended solids.
    • New oil (dirty?) is added and the vicious cycle continues.

Selecting the right type of filter and housing is critical for effective gearbox filtration. The key factor in the selection process is viscosity. Viscosity is expressed either as centistokes (cSt) or with the old system, which is Saybolt Universal Seconds (SUS). The conversion factor from one system to the other is SUS/5~cSt. For example, a 1500 SUS fluid has a kinematic viscosity approximately 300 cSt and has an ISO VG of 320. You need to consider the temperature at which the fluid will be filtered. Even small temperature differences can have a major impact on viscosity. For example, if a mineral gear oil with a viscosity of 460 cSt at 40 C (104 F) is filtered at 30 C (68 F), the viscosity doubles to 927 cSt. Viscosity affects port sizes, but affects the element size and selected media even more. If your housing selection is on the borderline of being too small—as determined by the clean element pressure drop—go up a size. This will allow for a finer filtration if needed, or handle a higher-viscosity fluid. Consult closely with your filter manufacturer during this selection process.

Successful examples…
When it comes to gearbox filtration, how quickly you reach your targeted goal will depend on the flow rate, the initial fluid cleanliness and the viscosity. The following examples are based on real-world situations.

An ISO 460 VG gear oil in a 275-gal. sump in a coal-pulverizer gearbox filtered with a 6 micron absolute filter at a flow rate of 6.5 gpm produced the results in Table I.


In many cases, if the correct filter is selected, the target cleanliness can be reached quickly depending on the size of the reservoir and the flow rate though the system. Typically, turning the sump over at least seven times will reach the cleanliness goal.

One of the disadvantages of a portable system—such as a filter cart—is that once it is removed from the system in a dirty environment, the oil will quickly become contaminated. Therefore, proactive measures, such as installing desiccant breathers to minimize moisture and particles, should be taken. Despite such actions and removing the filter cart, in the case of the coal-pulverizer gearbox, oil quickly jumped two ISO codes in the > 6 micron particles (see Table II ).


The economics…
What does clean oil do to the economics of an operation? Consider the example of a coal mill where a skid-mounted offline filtration unit was installed on a pulverizer gearbox that had been utilizing only a strainer for filtration.

A ß5=1000 filter was installed for filtration of an ISO 460 gear oil. As a result, the initial oil cleanliness level of 21/15 was brought down to 13/11—a 99.42% decrease. The box below shows yearly savings following installation of the filtration system and desiccant breathers. (By the way, each of this mill’s five coal-pulverizer gearboxes had previously required an annual rebuild.)


Clearly, there are major cost savings from keeping gear oil clean through proper proactive measures and the right filtration system—savings that can go straight to your bottom line.

Clean oil makes a big difference in equipment reliability, resulting in major bottom-line savings. This was illustrated with a few examples in this article. There are many more.

Utilize offline filtration to help meet your cleanliness goals economically. Work closely with your filter manufacturer to establish a cost-effective program. Be sure to set meaningful goals before embarking on your program, and measure progress by monitoring your oil cleanliness through particle counts. Ideally, have your own particle counter. If this is not possible for your operations, work with your oil-analysis laboratory to obtain timely reports. Closely monitoring and improving the program is essential for success.

No longer accept the notion that gear oils are necessarily dirty. The technology through offline filtration is now available to give you very clean oil. Work with your filter manufacturer to develop the right system for your operations.

Finally, while filter carts can be important equipment in achieving your cleanliness goals, they MUST be used properly (something that comes through proper training). There are many suppliers of filter carts, so choose wisely—especially when it comes to filtering gear oils. LMT

The author wishes to thank Mike Boyd, of Fluid Solutions; Mike Skuratovich, of Eastern Oil; and Brian Gleason, of Des-Case, for their assistance with this article.

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. Telephone: (281) 257-1526; e-mail:

Meet Critical Lube-Related Training Requirements At MARTS 2010

Contributing Editors Ken Bannister and Ray Thibault are two of the Applied Technology All-Stars who will be up at bat at MARTS this April. For more information on or to register for their special lube workshops (including one that helps participants prepare for the CLS Certification Exam), visit:


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