How Kidney Loop Filters Bolster Fluid Cleanliness

Feb. 18, 2021

Maintaining clean hydraulic oil is crucial to extending machine component life. Without proper maintenance, the smallest particulate (including dirt, metal, and water) can cause serious damage after entering a machine’s oil reservoir. Valves may become scratched beyond repair, resulting in a costly expense for equipment owners.

As insurance against premature system failure, some fleet managers use supplemental filters designed to trap these particulates before the damage is done.

One common solution is a kidney loop filtration system. Kidney loops filter hydraulic oil through a supply and return line allowing it to draw oil from a machine reservoir or bulk container, run it through a filter, and return it to its source. On-equipment filters typically remove 10 to 30 micron particulates, whereas kidney loop filters can catch around 3 to 7.

This results in a range of benefits including better component protection and meeting or exceeding oil-drain intervals.

“The hydraulic systems in today’s machines typically operate at high pressures and have very tight tolerances,” says Jason Gerig, off highway sector manager at Chevron. “This makes them very sensitive to contamination. Particulates that enter the hydraulic system can make control valves stick, seize up, or scar them to the point of leakage. Kidney loop filtration prevents contamination from escalating into a catastrophic failure.”

Over 70 percent of hydraulic component failures are attributable to mechanical wear and corrosion—a direct result of contaminants in the oil, according to Paul Klick, mobile hydraulics, Donaldson.

Contamination manifests as valve failure and equipment downtime,” says Klick. “These costs could mean spending a few thousand dollars on replacement parts and thousands of dollars per day in productivity loss.”

Klick says a replacement directional control valve—the hydraulic system’s most sensitive component—can cost between $10,000 and $15,000 to replace if damaged by contaminant.

“After factoring in man-hours in the shop, downtime while waiting for the replacement part, installation, and getting the equipment back into owners could lose hundreds of dollars an hour per machine,” Klick says.

One of Donaldson’s customers—an aggregate producer—found their production process was causing significant amounts of dust, exposing its crushers to wear and premature component failure. An analysis revealed that the oil was 125 times dirtier than the industry standard (around ISO 16/14/11). Donaldson recommended installing a 7-micron offline kidney loop filtration system to remove the debris and extend component life.

Offline kidney loops utilize suction and return line hoses to connect to hydraulic reservoirs. They contain their own pump and motor, allowing them to run independently while the machine is idle.

“The 7-micron filters became plugged with contaminants in two weeks—hardly surprising, given the extreme dirtiness of the current working conditions and gearbox oil,” Klick says.

A second analysis showed the oil had now become 125 times cleaner, extending certain component life by 3.5 times, according to Donaldson.

“It is critical that you’re kidney looping hydraulic fluid into the machine that meets or exceeds an OEM’s cleanliness specification,” Klick says. “Although cleanliness specifications vary across the board, the best thing you can do is ensure the filters on equipment last for the extent of that preventative maintenance (PM) schedule and never plug.”

Users can configure how many times to filter the oil after determining how much is in the system.

“One complete cycle would mean all the oil in the machine has gone through the filter,” says Robert Bieterman, sales manager at Eaton.

Clean hydraulic oil also ensures a piece of equipment reaches its recommended oil-drain interval, and protects components in the process. If PM is 1,000 hours, for example, kidney looping hydraulic oil at 500 hours can help equipment owners meet or exceed that interval.

“Proactive maintenance with kidney loop filtration has extended drain intervals from 500 hours to 2,500 hours on many of our customers’ equipment,” Klick says. “With that alone, we’re talking $500,000 a year in savings just on oil.”

Since premium synthetic hydraulic fluids are expensive, kidney looping the existing oil rather than replacing it can also be more cost-effective.

What is a Micron?

Contaminant particles in hydraulic systems are measured in microns. A micron is an extremely small unit of measurement—one-millionth of a meter. A single micron is invisible to the naked eye.

“One grain of table salt is about one hundred microns,” says Paul Klick, mobile hydraulics, Donaldson. “Your eye can see about half of a grain of table salt. The particles that are doing damage to high-pressure hydraulic circuits are between 2 and 10 microns in size.”

“A haul truck might have anywhere between a 100- and 300-gallon reservoir, and the synthetic oil is within the $50 per gallon range,” Klick says. “When you crunch those numbers, the ROI is substantial.”

Gerig has seen similar results working with Chevron’s mining customers.

“One customer extended their hydraulic system rebuild cycles from 20,000 hours to over 40,000 hours after starting with clean hydraulic oil and implementing a regular kidney loop program,” he says. “This provided cost savings of just over $3 million.”

Most OEMs also recommend filtering freshly delivered hydraulic oil before it enters the equipment.

“A lot of people think new oil automatically meets the OEM’s cleanliness specifications, and in reality it doesn’t,” says Gerig. “By filtering oil through a kidney loop before it enters the reservoir, you’re starting out with lubricant that is very clean, and the equipment is not suffering.”

In recent years, clean oil has become such a focus that some suppliers have started offering pre-filtered oil. Chevron now provides certified clean oil directly to

its customers.

“We focus on getting that lubricant clean at the point of delivery to make it easier on the customer,” Gerig says. “We also help them maintain the cleanliness in that piece of equipment.”

Some manufacturers also offer onboard kidney loop filters that are affixed directly to the machine. Rather than filtering oil through an external system, onboard filters are added to the OEM’s filter line. They rely on the machine’s oil flow to capture particulates alongside the original filters.

Though most OEMs recommend filtering oil before it enters the reservoir, Ed Vittoria, CEO at Puradyn Filter Technologies, says its onboard filters are capable of effectively filtering new oil.

“Our point of view is that it’s not necessary to do preliminary cleaning if you’re going to have our system on a machine,” he says. “Our filters have a 97 percent efficiency and constantly clean out contaminants right from where they’re being generated. If you’re filtering oil through the reservoir, it’s already going through the system for several passes.”

Similar to an offline kidney loop, online systems eventually pay for themselves, Vittoria says. In a scenario where an equipment owner has a system with a 1,000-gallon reservoir, for example, he says replacing the oil could cost around $18 a gallon.

“Instead, by paying $2,000 for an onboard kidney loop, your costs are maybe only another $1,000 to pay your people to install it,” he says. “If that’s going to help you skip one change of hydraulic oil—an $18,000 expense—you can see why it pays off.”

Perfect Filtration Systems Inc. also advocates for permanently mounted kidney loop filters.

“The majority of the wear on these high-pressure, tight-tolerance components is solid particles 10 micron and below,” says Keith Nye, Perfect Filtration Systems. “Our systems range from $900 to $1,200 on a piece of heavy equipment. If you could just extend the life of the oil by a couple service intervals, your payback is pretty quick.”

Leigh Dennis, CEM, fleet manager at Carolina Sunrock, installed an onboard kidney loop system on some of the company’s haul trucks. According to an oil analysis, filtration eliminated over 20 times the particulates at 6 micron or smaller.

“Our reduction of particulate contamination should allow us to experience hydraulic system component life of three to four times longer than if we had relied on standard OEM filtration,” he says.

The significant reduction of particulate contamination also resulted in lower ppm (parts per million) of wear metals in the affected systems.

“This is quite significant,” Dennis says. “It ensures sustained performance, greater reliability, longer service intervals, fewer rebuilds, and lower overall operating costs.”

One caveat for those fleet managers looking at onboard kidney looping: most OEM service contracts do not allow the use of third-party onboard filters. Nye says reasons are mixed.

“I have an OEM that buys about 100 pieces from us a year to put on equipment,” he says. “Then, I’ve had other OEMs say, ‘Why should I put that on? We make a heck of a lot of money on the parts.’ The philosophies are vast, but the benefits have been proven.”

Vittoria says overall, users forget how much they can save just by filtering the oil they have. 

“The U.S. is one of the top countries with a concentration in filtration,” he says. “The rest of the world is still behind. Kidney looping has a lot of potential.”