The wide variety of hydraulic grades, viscosities, and operating conditions makes selection of the right pump an important decision. If the heart of an off-line filtration system is the pump, the heart of a pump is its rotating group. The pump’s rotating group is turned by a motor to create a partial vacuum on the suction side of the pump. Vacuum draws the oil out of the reservoir through the suction hose to the pump inlet and fills the cavities of the rotating group. The rotating group carries the oil over to the pressure side of the pump and forces it through the filters, through a return hose, and back into the reservoir. The performance and efficiency of the pump is determined by the amount of oil that completely fills each cavity in the rotating group as it turns.

This establishes four external factors that will influence the performance and efficiency of the off-line filtration unit:

• inner diameter of the suction hose,
• length of suction hose and number of connections in it,
• type and configuration of the pump’s rotating group, and
• the speed of the pump’s rotating group.

Figure 3

Gear pumps, Figure 2, are usually specified for off-line filtration loops. They work well with most hydraulic fluids, but their design limits their use with higher viscosity oils, especially in cold weather conditions. The small cavities created by the their spur gears, the knife edge gear tooth form, the close tolerances required for higher pressure, and the fixed outer gear housing all combine to severely limit its efficiency in an off-line application.

The rotating group of a gearotor pump, sometimes called a crescent pump, Figure 3, is better suited for higher viscosity fluids, especially when used under cold operating conditions. In these pumps, an internal gearotor drives an external gearotor. Inherent to the design are larger and fewer cavities with rounded corners that open and close more smoothly. This rolling action of the two gearotors delivers ripple-free flow and reduces shearing of the oil in the pump, which means less noise, less aeration, and less cavitation.

The ripple-free flow also puts less stress on the oil filter and increases dirt holding capacity of the filter media. Turning the rotating group at a lower speed improves the pump’s performance even further, particularly in cold conditions with higher viscosity fluids.

The most important feature of a filter cart is to have an adjustable relief valve built into the pump housing. This eliminates the need for a bypass valve in the filter head and ensures that only polished oil can pass through to the reservoir. When the relief valve is set at 60 to 80 psi, it significantly increases the dirt holding capacity for an oil filter that would otherwise bypass at 25 psi.

Filtration increases productivity

One feature on an off-line filtration system that pays huge dividends is a selector valve and a secondary output flow path. When a reservoir must drained, its fluid must be transferred to an oil drum for removal and subsequent recycling or other processing. A selector valve that offers a secondary, unfiltered flow path can be used to transfer this dirty oil without fouling the main oil filter. A secondary outlet can also be fitted with a backup or duplex filter so the off-line filtration can continue without having to return to the maintenance shop and service the filter cart.

For more information, contact JLM Systems Ltd. at (604) 521- 3248 or visit

A checklist for off-line filtration

• Make it easy to do a better job. Pre-install hose connection points and oil sampling valves on all hydraulic reservoirs.
• Locate kidney loop connection points where they are safe and convenient for service technician.
• Standardize maintenance procedures throughout a facility to improve productivity.
• Keep suction lines as short and use the fewest number of fittings as possible.
• Position a filter cart as close to the reservoir as possible. Accessibility is the key to maintenance procedures that are performed well and on schedule.
• Color code machinery and clearly identify the type and grade of fluid for each machine.
• Avoid cross-blending of hydraulic fluids. Consider dedicating a filter cart to each type of fluid used.
• Confirm the working condition and accuracy of the filter condition indicator.
• An oil filter is less stressed and more effective when fluid flow low and ripple-free.
• Seven to 10 exchanges of the reservoir volume per kidney loop procedure is recommended.
• True kidney-loop filtration is independent of the normal operation of a hydraulic system. It can be turned on or off to meet a proactive maintenance schedule.
• A filter head bypass valve begins to bypass unfiltered oil at a cracking pressure, well below the rated bypass pressure.
• Circulating unfiltered oil runs contrary to the ultimate purpose of kidney-loop filtration.
• Use the type of pump that is best suited to the viscosity of the oil, the location, and operating conditions, i.e. indoors or outdoors, summer or winter, exposed or sheltered.
• Choose a filter unit with a pump that has a built in adjustable relief valve.
• A single non-bypass filter with a condition indicator takes the uncertainty and the guess work out of a filter changeout.
• A single filter element reduces the amount waste oil for disposal.
• Choose a filter cart with a selector valve that can bypass the main particulate filter.
• A second flow path can accommodate an auxiliary filter significantly increasing productivity.
• For higher viscosity oils in cold conditions, consider an electric motor that operates at a lower speed, such as 1150 rpm instead of 1750 rpm.