Industrial systems, whether pneumatic or hydraulic, must prevent a variety of contamination sources to run reliably.
By: Josh Cosford, Contributing Editor
Fluid power systems experience many problems. Poor designs, inferior quality components, maintenance neglect, severe climate, and operator error can all inflict trouble on your innocent machine. However, most of these problems are rare cases because poorly designed machines with cheap parts lose their market quickly, and modifications can be made to suit your machine’s environment.
Industrial facilities rely heavily on fluid power systems. Contamination is still the biggest threat to both hydraulic and pneumatic machines, so proper prevention of risks like water, heat and particles is crucial.
So, where does contamination appear in the problem rankings in the fluid power world? Is it still at the top? You bet it is. And let’s be clear that contamination comes in many forms, not just particles. Marriam-Webster (marriam-webster.com) circularly defines contamination as “a process of contaminating.” As far as I’m concerned, dictionaries using the query word in the definition is contamination, but I digress. Digging deeper, they say that to contaminate means “to make unfit for use by the introduction of unwholesome or undesirable elements.”
Using the above definition, how many causes of contamination can you think of in fluid power? There are particles, of course, and they are what comes to mind immediately when most people think of contamination. And because particle contamination is so common, every single compressed air system and hydraulic power unit contains one or more filters.
Wet air breeds contamination
If you thought this article was only about hydraulics, you just found out you’re incorrect. The pneumatic system professionals already know the second source of contamination, and that is water. Water separators are a common in-line accessory in air lines through any facility, as are air dryers for high-demand shops. Pneumatic valves, actuators and tools are often more resilient from water contamination than hydraulics because they’re designed with suitable materials under the assumption of the presence of water.
Air filters are placed throughout industrial automation systems to protect the actuators and valves controlling the processes from contamination.
Water is in the air that both we and air compressors inhale, and when the compressor takes a cubic foot of air and squishes it down to a tenth of its original volume, water “rains” out and settles in the air lines. Water control in pneumatics is a bigger task than the control of particles. You find a large filter directly after the compressor but also nearly every station in the plant requiring connection to the shop air grid, where FRLs (Filter Regulator Lubricator) are installed. However, the very design and layout of pneumatic plumbing revolves around the control of water.
The first defense in the battle on water is the various devices engineered to remove it before it becomes a nuisance to downstream equipment. Refrigerated, desiccant, membrane, deliquescent, and aftercoolers are all types of drying devices with their own particular benefits and flaws used to remove water. Which one you use will depend on the volume of air your facility uses and the dryness of air required. Food & beverage, medical and pharmaceutical manufacturers, for example, will use multiple devices especially desiccant dryers, for ultimate dryness. However, refrigerated dryers are the most common because of their flow capacity and reasonable price.
Coolers, like the Tsunami Aftercooler, pre-cool compressed air before it reaches downstream equipment, maximizing moisture removal
and improving efficiency.
The layout of air system piping is thoughtful of the control and elimination of water. You don’t simply run air pipes or tubes to the stations around your plant requiring it, but instead, install the plumbing up high and install drop lines at machines or work areas requiring compressed air. Distribution lines mounted up high take advantage of gravity so condensed water will settle downward. And rather than simply drain through the valves, actuators and tools, downward extensions are installed where small volumes of water can settle. Installing a ball valve or drain at the bottom of the tube or pipe provides a method to drain the collected water, which should be a task done daily, depending on the efficiency and capacity of your drying equipment.
Wet oil wreaks havoc on hydraulics
If water is bad for pneumatic systems, it’s downright horrific for hydraulics. Water contamination in hydraulic fluid results in corrosion, oxidation, accelerated wear, microbial growth, and in extreme cold, possible freezing damage. Excessive water, especially in its free form, can result in rapid and catastrophic failure as metal-to-metal contact under high pressure will shear, shred and sunder even the most durable pump designs.
Prevention is always the best medicine, so provide water no opportunity to enter your reservoirs, power units, actuators or plumbing. It’s much more difficult to remove water from hydraulic oil than it is from air, so your efforts are better used with intelligent design that inherently prevents water intrusion. A raised filler cap prevents any potential reservoir-top water from leaking into the breather-filler cap, for example. Better yet, installing a desiccant breather cap also ensures that a high-humidity atmosphere isn’t inhaled into the reservoir.
Also, keep in mind that water also exists as humidity within hydraulic oil, just as it does in our atmosphere, and just like our atmosphere, it can “rain” out when the temperature drops. So, in a humid plant or location, your machine may run warm enough to hold a generous amount of saturated water. Be mindful that cooling will induce localized pools of free water. That water will circulate during the subsequent machine startup, offering up the potential for harm.
Once water contamination occurs, you can use water-absorbing filter elements to remove free, circulating water. However, the humid oil requires more effort to dry, and expensive machinery such as vacuum dehydrators might need to make an appearance to avoid replacing all your oil. These machines will heat up the oil under a vacuum, lowering the water’s boiling point and then sucking away the humidity.
The dangers of heat
Although less of a popular opinion in fluid power circles, heat is also a form of contamination that damages both pneumatic and hydraulic systems. Pneumatic systems create heat simply by taking atmospheric air and squishing it down to a fraction of its original volume. And if you squish ten cubic feet down to one cubic foot, the total heat energy in that ten cubic foot space now exists concentrated into the smaller volume. The higher your shop air pressure the more heat you create, and there is no way around it.
Heat in compressed air systems can damage seals and O-rings, oxidize lubricants, and degrade the plastics that constitute plenty of pneumatic valves and components. Aftercoolers placed immediately after the compressor will reduce the heat entering the receiver (the storage tank), and sizing your receiver large enough to create dwell time provides a period where heat can radiate away. Otherwise, the long runs of pneumatic plumbing, often made from metals, provide an excellent heat sink that radiates any remaining heat from the compression stage.
In hydraulic systems, excessive heat has the potential to allow total devastation more quickly than any other form of contamination. Hot oil thins out dangerously where it can allow metal-on-metal contact, where pumps, motors, valves and cylinders can all fail prematurely.
Heat exchangers serve as coolers to remove heat from systems.
A cool hydraulic machine starts with an efficient design, where thoughtful consideration for matching output energy with input energy is made a priority. High-quality pumps such as piston or inside-gear designs are ideal here, and so are versions employing load-sensing or horsepower control technology. Each design wastes little energy while creating closer to the actual pressure and flow demanded by actuators.
Additionally, the creative selection of valves also reduces leakage and, therefore, heat. If you can achieve a function with a poppet valve, do so since spool valves are inherently leaky. Avoid valves with high-pressure drop, such as priority-type flow dividers, where fluid is lost as heat before achieving useful work.
If all else fails, a cooler can be added to any machine. Depending on the heat load, your options range from small heat exchangers that mount near the back of the electric motor to take advantage of the motor’s cooling fan. The most common are simple liquid-to-air coolers that use electric or hydraulic motors to power fans to increase the cooling rate. For ultimate heat removal, liquid-to-liquid coolers can remove hundreds of horsepower of heat, especially if your plant has cooling towers or access to open-loop cooling.
A common problem: particle contamination
This brings us to the form of contamination that gets the least attention in this article because it gets the most attention every other day: particles. The problem with particles is that machines generate them just from daily operations. Although manufactured (built-in) contamination is highest at machine startup, and ingested contamination can also cause dramatic spikes in circulating nastiness, these can either be anticipated or avoided. Meanwhile, internally generated particle contamination will occur, even at low rates, no matter what is done to prevent it.
Coalescing filters remove solid particles in compressed air.
Always remember that particles can act like a lapping compound, essentially sanding away at valves, pumps, motors and cylinders, increasing contamination exponentially if your filtration quality is low. There is no such thing as a hydraulic filter that is too fine, and it’s been proven that expensive and efficient filters will pay for themselves over time.
No matter the form of contamination, being mindful that they can and will conspire to damage your precious and expensive machines allows you to waylay each type before catastrophe strikes. Keep your machines clean, cool, and dry, and you can sidestep tradition by avoiding what is still the biggest problem for fluid power systems — contamination.
Filed Under: Components Oil Coolers, Contamination Control, Engineering Basics, Filtration, Filtration/Contamination Control
