The key to a reliable hydraulic system lies in keeping it cool, clean and dry. The industrial hydraulic environment is a piece of cake compared to what the mobile hydraulic industry is put through. Industrial hydraulics are typically cleaner, more temperate and better outfitted with luxuries, such as coolant systems and advanced contamination removal equipment. Still, if the industrial hydraulic environment is so peachy, why do so many machine failures occur, especially as related to the reliability of their hydraulic systems?
Regardless, industrial hydraulic systems aren’t immune to issues related to reliability, and it’s quite unfortunate because that doesn’t have to be the case. Industrial hydraulics still overheat, still have contamination issues and they still have moisture related failures.
The industrial machine environment provides the environment and resources to ensure hydraulic machines are extremely reliable, but even the most stable machine environment cannot compensate for a poor machine design. Intelligent and well-engineered hydraulic design should be a top consideration for any machine, regardless of environment. The three primary factors to be designed into a hydraulic machine are the control of the three types of contamination: heat contamination, particle contamination and water contamination.
Keep it cool
Controlling the temperature of the industrial hydraulic machine is generally quite easy, and when considered from a design phase, overheated oil should never be a factor reducing reliability. The stable ambient temperature of the plant environment ensures oil is typically within its ideal range of operating viscosity. When an industrial hydraulic machine overheats, the cause is most often generated internally from either poor design or a failed heat-generating component, such as a pump or relief valve.
When hydraulic oil overheats, it loses its lubricity—in addition to the damage it suffers from oxidation. As hydraulic oil rises in temperature, its viscosity reduces inversely proportionate to that increase in temperature. When viscosity is critically low, the oil can no longer maintain a boundary layer of lubrication, and metal-to-metal contact and wear is imminent. Next to particle contamination, heat is the second most common cause of hydraulic component failure.
A hydraulic circuit designed with efficiency in mind is also one designed to run cool. An efficient hydraulic system makes best use of input horsepower by converting more of that energy into useful work, rather than wasting it. Energy wasted in the conversion to hydraulic energy is turned to pure heat. For example, a pump that is 80% efficient, and being run with 10-hp input, will produce 2 hp of pure heat. This 2 hp worth of heat will have to be removed from the hydraulic system. By choosing a pump with 90% efficiency, the heat load generated will be halved. On top of choosing efficient components, the circuit should be designed to reduce heat load, although that is a topic unto itself.
If an industrial hydraulic machine is not designed with optimal efficiency, its environment still allows for a great place to run a reliable machine. Because of access to electrical resources, efficient and powerful forced-air electric coolers can be permanently installed to keep oil within its optimal temperature range. These liquid-to-air coolers can also use thermostatic control of oil to maintain a range of optimal temperature and viscosity.
Should a hydraulic system be particularly inefficient or massive enough to generate high heat waste, running a liquid-to-liquid cooler may be required. Because cooling water can remove heat
from hydraulic oil with much more efficiency, high heat loads can be removed from a relatively small cooler, which is important if plant real estate is at a premium. Shell and tube coolers are common and efficient, but the brazed plate cooler is the king of this realm. The brazed plate cooler design provides a high surface area for heat transfer to take place, allowing a lot of heat to be removed from a small cooler with relatively low coolant flow.
The downside to any liquid-to-liquid cooler is that it requires an expensive cooling infrastructure, with plant-wide plumbing, filtration systems, cooling towers and/or chillers. Most large plants take advantage of this type of process and cooling equipment, so the addition of a liquid-to-liquid cooler will be seamless. If your plant is without the cooling systems in place, you will have no choice but to use a forced air cooler. Regardless of the option you use, what’s important to the reliability of your hydraulic system is to prevent the fluid from overheating.
Keep it clean
Particle contamination is often cited as the cause of most hydraulic equipment failures. The high pressure of hydraulic systems can force particles between and against tiny clearances, which under normal circumstances, would never cause a problem. This is why ultra-high pressure hydraulic systems require ultra-high quality filtration. But every hydraulic system needs filtration on some level, and I can tell you I’ve never seen a machine with too much of it.
The industrial environment has the primary advantage of real estate. In a mobile application, for example, the machine often does not have the space or weight capacity to add multiple stages of high-quality filtration. However, industrial floor space is often available to add large filtration systems, such as basket or bag filters. These filters can hold massive volumes of particulate, and although they’re not often highly fine, they can trap the bulk of the contamination to prevent the finer filters from being clogged quickly. These filters can be used as the first stage in an offline kidney loop, where a second high-efficiency filter is used to achieve target oil cleanliness.
The kidney loop, or offline, filtration system is typically only seen in the industrial domain. It contains a separate pump/motor group whose sole position in life is to circulate oil through a filter (and sometimes a cooler, as well). Hydraulic systems with kidney-loop filtration systems are often the cleanest around, because their filter element quality can be extremely fine with no concern for backpressure related to high flow, as can be seen with return line filters. The extra pump and electric motor require extra space, plumbing and an electrical connection, which is easier to come by in a plant.
Another benefit to the extra space available to industrial machines is the capacity to use pressure filters. A pressure filter is sometimes used in mobile machinery, but the space and cost don’t always allow it. When taken advantage of, the pressure filter prevents downstream contamination-related failures due to either pump disintegration, or during periods of high dirt ingression to the reservoir. Although they cannot prevent a pump failure, such a failure will not damage the downstream components. When particle contamination is protected against, the industrial hydraulic machine really never should see a reliability problem.
Keep it dry
Where industrial hydraulic systems are absolutely superior to their mobile hydraulic cousins is in regard to the prevention and elimination of water contamination. Because of the stable environment and lack of rain, free water intrusion is rare within a manufacturing plant. If it exists, it’s typically in the form of humidity, or during catastrophic failure of a liquid-to-liquid cooler.
I’ve been in enough plants to know they’re hot and humid in the summer, which is sometimes enough to allow humidity to make its way into hydraulic fluid, allowing water to dissolve into the liquid straight from the air. When water saturation within oil becomes high enough, the oil performance can be reduced, which is in addition to the accelerated oxidation of steel hydraulic components, such as pumps and valves.
Humidity can be reduced from hydraulic oil by way of desiccant breathers, which act as a water absorbing barrier between the volume of air in the reservoir and the humidity in the ambient air. As the tank breathes in air, it must pass by the desiccant material, pulling away moisture and leaving the tank air dry. These desiccant breathers are large and relatively expensive, and must be changed much more frequently than traditional filter-breather caps.
Should water contamination be excessive to the point of free water, water absorbing filter elements can be added to the system. These filters are often high-micron elements with lower efficiency in removing particles. They will absorb free water as it passes through, but they’re limited in the volume that can be trapped before they themselves become clogged. Often during times of water contamination removal, multiple filter elements must be used to get the majority of the water out of the oil. It should be noted that water absorbing elements can only remove free water and provide no protection against humidity.
To remove humidity from oil, special machinery is required. The vacuum dehydrator is a machine, often on a large, wheeled cart, which can be trucked around the plant to any system requiring its attention. It will circulate the oil in the reservoir, just as with a kidney-loop system. It heats the oil while at the same time applying a vacuum. The heat and reduced local pressure allow water to boil at a low temperature, and the vacuum pulls the moisture away. This system can remove both dissolved and free water.
Just as you would imagine, the vacuum dehydrator is expensive, bulky and requires an ac power connection, so it generally exists in the industrial realm alone. But the value of maintaining dry oil is often overlooked, so few plants use them. Dry oil has better lubricating properties, oxidizes less and is more stable over an extended period. As you can imagine, dry oil is absolutely required when reliability is a primary concern in the industrial environment.
The reliability of any hydraulic system is improved when particle, heat and water contamination are controlled. Because of the mega-dollar value of some industrial machines, often costing millions of dollars, the extra investment of protection systems should seem like a no-brainer. With the resources available to some industrial plants, reliability should never be a concern.
Filed Under: Components Oil Coolers, Fluid Power Basics