Clean up hydraulic circuits

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

By Phillip Johnson

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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.

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