Bruce Shane
Technical Services Manager
Parker Hannifin Corp.
Hydraulic Filter
Chances are you wouldn’t drink water out of rusty pipes or straight from a nearby pond or stream. For obvious reasons most of us prefer tap water that has been extensively treated, filtered, and tested. Hydraulic fluids need the same attention.
Oil serves many functions in hydraulic and lubrication systems. It transmits power, acts as a lubricant to reduce friction and wear, and cools bearings and running parts. Oil also prevents rust and corrosion, helps seal out dirt, and carries contaminants to the filter for capture. Water, air, solids, and oxidation products, such as varnish and sludge, hamper these important functions. In short, contaminants are the reason oil and hydraulic components fail. Detecting and eliminating them will maximize oil life and help prevent unexpected shutdowns or system failure.
Weaknesses of oil
There is no way to keep out all contaminants, and the fact that they come in a wide variety makes the job of keeping a system clean even more difficult. For instance, water is a concern with any hydraulic system. It may condense inside oil drums or enter a system through worn seals or reservoir breathers. Water will dissolve until it reaches the oil’s saturation point. Additional water remains in a free or emulsified state, often making oil appear milky. As temperatures rise, however, cloudy fluids may clear up because warmer hydraulic fluids hold more water than colder ones. For this reason visual checks for milky oil are not reliable.
Water changes fluid viscosities and accelerates abrasive wear. Many oil additives break down and form acids that lead to corrosion and pitting. Below freezing, ice crystals form. This produces sluggish, inconsistent performance. Water also weakens a fluid’s insulating properties, increasing electrical conductivity. Several methods remove water from oil. Opening a drain valve is the simplest approach because free water is heavier than oil. More advanced methods include absorption, centrifugation, and vacuum dehydration.
Absorptive materials transform free water into a gel that gets trapped and removed. Absorptive elements fit into standard filter housings, but only remove small quantities of free water. Centrifugation uses a spinning motion to separate water from oil and can remove large volumes of water. Vacuum dehydration is the most complete solution. It removes large volumes of both free and dissolved water.
Air, either dissolved or entrained, is another contaminant. Dissolved air generally does not pose a problem. But entrained air diminishes power transmission, reduces pump output, and raises operating temperature. Because air is up to 20,000 times more compressible than fluid, pumps must work to compress the air rather than contributing to system output. Air is also an oxidation source in liquids, which accelerates corrosion. Air infiltrates oil through system leaks, pump aeration, and reservoir fluid turbulence. Prevent it by bleeding the system, flooding suction pumps, and installing return-line diffusers.
Air and water in oil tend to accelerate corrosion and oxidation, and oil additives may oxidize as well. The oxides that result form particulates or sludge and, if not removed, will ultimately lead to wear, interference, and failure.
Solid particulates are generally classified as silt or chips. Silt particles are smaller than 5 µm and gradually lead to failure. Chips are larger than 5 µm and can interfere with moving parts causing immediate catastrophic failure. Both can be built into the system during manufacturing and assembly processes. Dust, welding slag, rubber particles from hoses and seals, sand from castings, and metal debris from machined components are all examples of built-in external contaminants. Avoid these by thoroughly flushing the system after assembly. During operation external contaminants can enter through openings in breather caps and worn seals. Internal contamination, on the other hand, results as components wear and chemicals react.
Excessive contaminants wreak havoc on hydraulic systems. Warning signs include burned-out solenoids and leaking, off-center, or chattering valve spools. Other indications are frequent pump replacements, scored, leaky cylinders, and servosystem hysteresis.
Experts recommend several precautions to minimize solid-particle ingression:
• Use dessicant or spin-on filters for reservoir air breathers.
• Use rod wipers and replace worn actuator seals.
• Always cap hoses and manifolds during handling and maintenance.
• Filter new fluid before adding it to the system. Even fluid right out of the drum is not necessarily clean enough for many hydraulic or lubrication systems.
Fighting grime
While designers must overcome a number of contaminant-related obstacles, many products are available to maintain optimum oil condition. One such example is high capacity and high-efficiency synthetic or silt-control filters. These provide finer filtration with significantly more dirt-holding capacity than conventional filters. The filters have media with fiber diameters down to 0.1 µm. This fine-fiber construction creates greater void volumes in media matrices, resulting in finer filtration without increasing filter size, flow resistance, or pressure drop.
High-capacity filters typically have two filtration layers. The coarse upstream layer has a graded-pore structure which maximizes filter life and efficiency. The downstream layer has a fine, densely packed fiber structure with binder resin that fixes the pores in place to maintain filter efficiency during high-pressure loading and cyclic-flow conditions. The filters remove particles smaller than 1 µm.
Coreless nonmetallic filter elements use the same high-capacity filter media with reinforced polymer endcaps in place of steel stampings. These elements feature a reusable inner support core, which reduces element weight by 60%. Nonmetallic elements also crush easily to reduce disposal volume and they can be incinerated and used in waste-to-energy systems. Nonmetallic filters should become even more important as industry moves toward water or water-based hydraulics.
Unfortunately, even with the best filters, there may come a time when ingression rates outpace removal rates. When this happens contaminants overload oil and a simple filter change won’t do. Replacing the fluid is an option, but it can often be postponed by thoroughly cleaning existing oil. Leading manufacturers offer a number of options to the industrial hydraulics user. For example, Parker’s PVS purification system uses vacuum dehydration to remove dissolved and free air and water. It also incorporates a polishing filter that removes solid contaminants smaller than 1 µm. The system heats the oil under vacuum to 150°F to convert liquid water to vapor. A vacuum pump draws out the vapor and the water-free gas-free oil passes through a final particulate-removal filter. This process also removes atmospheric oxygen and nitrogen that mix with hydrocarbons to form varnishes, sludge, and carboxylic acids. The fine-particulate filter removes existing varnishes and sludge as well. Vacuum dehydration is essential in industries such as paper and steel mills that require large volumes of water.
Watching for particles
Oil-monitoring devices have also improved significantly over the past few years. Patch tests and laboratory particle counters with white-light sources were once the norm, but these were time consuming and prone to error. Portable particle counting with laser optics solved many of these problems. Portable counters let users see immediate results and download them to a software package for trend analysis in raw particle counts and ISO and NAS cleanliness codes. Users can take bottle samples or connect the unit directly to a system diagnostic port for on-line counting.
Although portable counters give quick results, experts generally recommend an on-board particle counter for continuous monitoring. On-board counters permanently connect to hydraulic and lubricating systems and catch unexpected problems as soon as they happen. Predetermined triggers warn of significant changes in the oil. Counters and flow sensors mount on the machine and can transmit data to a handheld device containing firmware and software. The device can log all oil-condition data or feed information directly into a data-acquisition system.
Components that indicate when a filter element needs to be changed also cost less and are more reliable. Pressure gages, dial indicators, and pop-up style indicators were once the accepted means to determine filter-element condition. But pressure gages raised system costs because measuring pressure differential meant installing two expensive gages, one upstream and one downstream of the filter. Dial indicators linked to internal bypass valve motion, were expensive to manufacture, and often unreliable. And pop-up style indicators told nothing more than if an element was about to clog or had already clogged.
Solid-state element-condition indicators use a double sided or two single-sided silicon strain gages to measure differential pressure. This eliminates moving parts such as pistons, springs, and magnets and provides infinite measurement resolution, excellent accuracy, and repeatability with little or no hysteresis. The indicator can measure the degree of element clogging, oil temperature, and provides LED or light-bar readout and analog output to a remote data-acquisition system.
Although advanced filter elements, oil-monitoring devices, and indicators each improve system performance, industrial-filtration manufacturers often recommend an off-line system with a complete oil conditioning and monitoring package using high-capacity elements, on-board particle counting, and oil monitoring, and filter-element-condition indicators. The unit is typically installed on industrial hydraulic systems as a kidney-loop filtration package, a secondary setup that pulls oil from the reservoir, processes it, and returns it to the system.