"At our location, we have 18 large pulverizer gearboxes that hold nearly 175 gallons of oil. The oil temperature is approximately 120 to 150 degrees F. Some of these gearboxes have been in operation for almost 30 years before being rebuilt, with oil changes only when required by oil analysis.
"The gearbox manufacturer recommends a 40-micron filter, but we use a 20-micron filter. We previously tried a 10-micron filter but had problems with higher oil differential across the filter and were very close to bypassing.
"We have an external lube system that filters the oil prior to it reaching the bearings and gears. The system is a filter inside a canister, which is 12 inches in length. We are considering a larger filtering system to lower the oil differential because with the 20-micron filters, the differential for a new filter is 14 to 15 and the bypass of these filters opens at 25. What would be your recommendation for an appropriate filter micron size?"
Pulverizer gearboxes are very robust pieces of equipment, but they still benefit from additional filtration and having cleaner oil. From a design standpoint, if you would like to move to finer filtration without increasing the pressure differential across the filter to the point of bypass, a few changes should be made in order to achieve additional surface area and a lower flow rate.
There are multiple ways to get more surface area. The most popular would be to add additional filter housings to the system parallel to the current housing. Another would be to simply procure a larger filter housing (if it fits the application). The last and often forgotten solution is to change the filter media type itself. Materials such as synthetic fiberglass have a much smaller fiber than wood cellulose, and by default, the fiber consumes less space on the media face. This gives the fiberglass media more pores per unit area, inherently increasing the perceived surface area.
In addition, if it is possible to reduce the flow rate of the pump, finer filtration can be used without an increase in differential pressure.
If you want to be considered "world class," the cleanliness level of your pulverizer gearboxes should be ISO 17/15/12. Depending on the flow rate, a B6=100 filter or multiple, parallel filters should enable you to achieve and maintain this level in a 175-gallon sump.