When selecting filtration for high-viscosity gear oils, you should first determine the optimum target cleanliness level for that specific gearbox and ensure adequate breathers are fitted, as any attempts at cleaning the oil will be lost quickly.
Ensure that for each type of lubricant in use, there is a dedicated filter cart to avoid cross contamination of fluids.
Because filter carts are fluid power-generating devices, ensure they comply with all the safety requirements and have pressure venting safety valves in the event of dead-heading the pump.
Ensure the carts include a by-pass loop to the filters, and incorporate a sampling connector for the use of online instruments or bottle sampling.
The design (pump and filter selection) of filter carts is dependent 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 allow 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. So if the gearbox is located outdoors, assume the worst case winter temperature when dealing with the viscosity issue.
Of course, slowing the flow rate to avoid high differential pressures will increase the time to filter the gearbox, and depending on the Beta ratio of the filter, 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 versus the time available for filtering.
In regards to filter rating, experience has shown that a 10 micron filter is capable of achieving better than ISO 17/15/12 oil cleanliness level. 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 the possibility of 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. 3 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, and the availability of power sockets near to the equipment, as well as 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.