Fluid Handling

Clean up hydraulic circuits

Selecting the proper filter leads directly to greater uptime, lower maintenance expense and longer machine life.

By Phillip Johnson

If you think that replacing hydraulic fluid filters is merely necessary maintenance or that filters are nice-to-have accessories, you’re probably not giving enough technical consideration to the matter. Consider that the proper filter leads directly to greater uptime, lower maintenance expense and longer machine life.

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Clean oil needed
Fluid power circuits come in all shapes and sizes, both simple and complex in design, and each is subject to contamination damage. Abrasive particles enter the system and, if left to circulate freely, damage sensitive components like pumps, valves and motors (Figure 1). The hydraulic filter captures these particles to help minimize component wear and system failure (Figure 2). As hydraulic systems become increasingly sophisticated, the need for reliable filtration protection becomes ever more critical. The filter, after all, costs only a fraction of the value of the machine it protects.

Achieving proper hydraulic system cleanliness requires the successful capture and removal of contaminants without disrupting the oil flow or unduly increasing the system’s pressure drop. Convenience of servicing is another big design factor. It’s all a delicate balance between system design and efficiency.

Contaminant types
Contamination has many sources, but the main culprits are new, unfiltered oil and environmental contamination in the plant (Figure 3). For most hydraulic systems, contamination prevention is more cost effective than the cure. Typical contaminants include:

• Particulates (dirt, sand, rust, fibers, elastomers, paint chips)
• Wear metals (chromium, iron)
• Water
• Sealant (Teflon tape, pastes)
• Sludge, oxidation and other corrosion products
• Acids and other chemicals
• Biological or microbial

Studies show that surface degradation is responsible for about 70% of hydraulic component replacements and mechanical wear causes most of that degradation. Filtering hydraulic fluids properly can lengthen component life.

Selection criteria
When purchasing a new filter or selecting a replacement element, begin by answering basic questions about your application. For example, what type of oil are you filtering? Where will the filter be used? What is the required cleanliness level (ISO code) of your system? Is there sufficient service access?
Consider the viscosity of the fluid. Operational temperatures can affect this. Cold oil is thicker and therefore flow is restricted through a filter. Although heating and polymer adjuncts can reduce viscosity to make the fluid flow easier, a better option is to install larger filtration units, perhaps mounting several filters in parallel.

Next, think about duty cycle and flow issues. Flow consistency is important. Filters that handle steady, continuous flow will function better than filters that must endure cycles of pulsating flow. Components such as cylinders often produce wide flow variations that degrade filter performance. On the other hand, dedicated off-line filtration—also called kidney loop—operates at a consistent flow that maximizes filter life and performance.

An important consideration is filter servicing. For instance, removing a yard-long filter cartridge from a yard-long housing will require almost 7 ft. of space—and probably a step ladder—to complete the job. A large cartridge loaded with sludge can weigh 70 to 80 lbs. Try this when it's oily and slippery. It might be wiser to use banks of smaller filters mounted in easy-to-reach locations.

Finally, do the math. Calculate how much it costs to replace failed components and include downtime in your calculations. Large numbers suggest a need for protecting the components with proper filtration. For example, high-performance servo valves are extremely sensitive and costly components that need to be protected with finer filtration (media with lower micron size ratings). Remember, the lower the filter’s micron rating, the more often it is likely to need changing because it’s trapping more particles faster.

Engineering for success
Selecting the right filter might involve unique factors. See the Media Selection Guide to walk through the considerations. Of course, to get the best filter to suit your requirements, consult a fluid power specialist or filter manufacturer.

Determine location and type
Every industrial hydraulic circuit offers many possible places to install filters. Much depends on component cleanliness requirements, environment, equipment duty cycle and other variables that vary by application. It’s best to position filters where the oil flow is uniform and where filters can be serviced easily. Typically, this means in the reservoir, before or after the pump, in the return line, or off-line. You have a choice of a variety of filter types to consider.

Pressure filter: Also known as “last-chance” filtration, pressure filters offer large benefits. High-pressure filters clean the oil as it exits the pump, which better protects more expensive downstream components such as valves and actuators. Pressure line filters also offer protection from catastrophic pump failure. Pressure filters are a worthwhile investment for high-value systems typically used in the aircraft industry, paper and steel mills, plastic injection molding and die-casting machines.

Service also can be more difficult because of the heavy-duty construction—as anyone who’s ever tried to change a slippery, 200-lb. cast-iron filter can attest.

Kidney loop filters: This high-benefit concept is sometimes referred to as off-line filtration. They achieve fine filtration by maintaining steady-state flow that is independent of hydraulic circuit action. With kidney loop filtration, the entire hydraulic system can operate while the kidney loop filter is being serviced.
A kidney loop filter can use low-pressure housings that are easily accessible and serviceable. These filters can be integrated into the main hydraulic reservoir mounted on mobile filter carts. Kidney loop filters don’t protect components directly. Rather, their main function is to keep the oil polished to a very clean condition.

Return-line filter: The advantages and benefits of return-line filters are many. They can generally use low-pressure housings, which are less expensive. Their purpose is to collect contamination as the oil returns to the reservoir. Much like the kidney loop, the return-line filter provides high flexibility in positioning—it can perform almost anywhere in the return line circuit, either mounted inline or built into the reservoir.
Downsides are few, but worth noting. Return-line filters often are subject to flow surges that contribute to poor filter performance.

Filler/breather: Another high-benefit approach is the tank breather. Installed on hydraulic reservoirs, these units prevent atmospheric contamination from entering while keeping the inside of the reservoir at atmospheric pressure. Breathers should prevent moisture, as well as particles larger than 3 µm, from entering the reservoir. It’s a sensible, affordable addition to any hydraulic system, but don’t rely on it to serve as the only filter on the system. Use a breather filter that also captures moisture. Given that most contamination enters through the reservoir, filter-breathers are a good value for the money.

Suction strainer: Many hydraulic fluid reservoirs have suction strainers, or sump-type filters. Considered to be a low-benefit accessory, their only real value is keeping cigarette butts, moths, fasteners and the like out of the pump mechanism. A better way to eliminate such contaminants is to keep the reservoir sealed with a filler/breather and install a return-line filter.

Suction filter: These medium-benefit accessories are normally placed between the reservoir and the pump. They’re easier to service and less expensive than many other filters—but because restriction in the suction line must be minimized, filter housing size tends to be larger than similar flow return or pressure filter housings.

The most popular application for suction filters is in variable-speed hydrostatic pumps commonly found in off-road mobile applications and industrial variable-speed drives. A serious negative is that unless these filter are correctly serviced, they can cause pump cavitation and catastrophic pump failure.
The bottom-line best practice is to prevent dirt ingress and to remove contamination via steady-state flow with filters that can be easily serviced.

Philip Johnson is global liquid product director at Donaldson Company, Inc., Minneapolis, Minn. Contact him at PHJohnso@Mail.Donaldson.com and 952-887-3131.

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