Ensure that for each type of lubricant in use, there is a dedicated filter cart to avoid cross-contamination of fluids.

For fluid power-generating devices, ensure it complies with all the safety requirements and has a pressure-venting safety valve in the event of dead-heading the pump. Lastly, ensure the cart includes a bypass loop to the filters, and incorporates a sampling connector for the use of online instruments or bottle sampling.

The design (pump and filter selection) of filter carts depends on two factors: the lubricant's viscosity grade and the temperature at which the cart will be used. A higher viscosity, such as an ISO VG 220 oil, will require a lower flow rate in the pump to avoid high differential pressures across the filter. But this will be affected by the ambient and operating temperatures. While the use of quick connectors allows the cart to be used while the gearbox is operating (this is the optimum filtering condition), the lubricant's viscosity will also be affected by the ambient temperatures. If the equipment is located outdoors, assume the worst-case winter temperature when looking at the viscosity issue.

Of course, slowing the flow rate to avoid high differential pressures will increase the time to filter the box. Depending on the Beta ratio, the rule of thumb is to allow the volume of the gearbox to circulate seven times through the filter for effective cleanup. For example, a gearbox with 50L sump capacity and a filter cart with a 10L/min flow rate will take five minutes for one pass and approximately 35 minutes to clean up. Keep in mind the flow rate vs. the time available for filtering.

As for the filter rating, experience has shown a 10-micron filter is capable of achieving better than ISO 17/15/12. However, if your optimum target cleanliness level is lower than this, consider a 6-micron filter. There are various ways to strike an optimum balance between flow rate and filter rating, and this includes putting several filters in parallel to increase the flow area. As a simple guide, the differential pressure can be halved by doubling the length of the element or putting two elements in parallel. Three-micron filters will work with ISO VG 220 oils, but check the temperature conditions, and whether your target cleanliness levels requires such fine filters. The cost of these elements should be considered.

Electrical power considerations include the use of single- or three-phase, the availability of power sockets near to the equipment, and ensuring that the unit is intrinsically safe for use in potentially explosive areas. Finally, consider using water-absorbing elements if the gearbox suffers from free and emulsified water, in addition to the use of desiccant breathers.

From the book Lubrication Excellence/Reliability World 2005 Conference Proceedings

A vanishing standard is the filler/breather cap with a mesh-type strainer to capture particles down to 40 microns. Other than capturing moths and large dust particles, this type breather cannot effectively control contamination that causes damage to bearings, pumps or valves. The fact that these type vents were identified as perhaps the single largest culprit for contaminant ingression (both moisture and particulate) will make this option obsolete in the not-so-distant future, especially in forward-thinking maintenance organizations. These companies will request a retrofit to a higher quality breather on new equipment purchases.

A step above the OEM cap is a breather with substantially lower micron ratings (1 to 3 micron) and higher airflow ratings. In the past, retrofitting a standard spin-on filter provided enhanced protection around 10 microns. Today, breathers are commercially available to meet this need.

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