IT’S BEEN ESTIMATED that 70% to 90% of hydraulic-system problems can be tied to hydraulic-fluid cleanliness, and with good reason. The combination of high pressures, high flow rates, tight clearances, and compressive heating combines to make hydraulic-system components highly sensitive to the presence of particles and moisture.
For this reason, hydraulic-system designers and fluid-power specialists pay particular attention to controlling the ingress of contaminants, while ensuring that any contamination is immediately filtered out.
Monitoring particle and moisture content in hydraulic systems should be part of routine, monthly oil-analysis testing. Particle contamination is usually measured and reported using the ISO 4406:2017 standard.
ISO 4406 provides a range of particle concentrations in the > 4-micron, > 6-micron, and >14-micron ranges. Other standards, such as the NAS 1638 or SAE AS4059, are used in industries such as marine, military, and aerospace. The degree to which particle contamination should be controlled depends on the type of system, including criticality, operating pressure, pump type, and valve type. Typical limits for ISO 4406:2017 particle counts are provided in the table (right).
Water in a hydraulic system can cause rust and corrosion, promote the formation of sludge and varnish, reduce overall lubricant film strength, and cause hydraulic pumps to cavitate. Ideally water should be kept below its saturation point at all in-service temperatures. In practice, this means below 100 ppm (0.01% v/v) for most hydrocarbon-based fluids.
Controlling contamination means looking at all possible sources of ingression. This includes ingested contaminants (particles and moisture that enter from the outside) and internally generated contaminants from active wear or fluid degradation. Key strategies for reducing ingestion include pre-filtering new oils, using proper vents and breathers, and ensuring that seals and gaskets are well maintained.
To minimize ingested contamination, all critical hydraulic systems need to have permanent filtration that achieves the optimum levels of cleanliness outlined in the table below. However, this is where it can become complex. Adding filtration to any system restricts fluid flow, while creating a differential pressure gradient across the filter element. For hydraulic systems, this can be especially egregious since hydraulic-system performance relies upon stable flows and pressures.
The trick with a hydraulic system is to maintain overall balanced filtration. Balanced filtration simply means that the rate at which contaminants are removed or excluded matches or exceeds the rate at which they are generated or ingested at the specific particle size and cleanliness rating indicated in the table. In doing so, we need to understand how and where particles enter a system so that we can select an appropriate filtration strategy. That strategy should include not just the micron rating of the element, but the rate at which the filter can remove particles, the overall dirt-holding capacity of the element and, most important, where the filter should be located to optimize contamination removal.
Figure 1 below illustrates the available options for filter location. By balancing filter efficiency, dirt-holding capacity, and filter location, optimum filtration performance can be obtained for even the harshest applications.
The most basic of hydraulic filters is the suction strainer. As the name implies, this filter is installed on the suction line inside the reservoir. The intended purpose of a suction strainer is to protect the pump from particle-induced failure. However, most suction strainers offer little more than a 140-micron mesh screen, which provides little-to-no protection from silt-sized (1 to 10 micron) particles that cause most problems. Suction strainers are little more than “rock stoppers” and can actually do more harm than good.
Often ignored and unmaintained, a damaged, degraded, or plugged strainer can introduce particles into the fluid flow, or create a restriction that can lead to pump cavitation. Provided the rest of the system is properly protected and properly sealed, there is little to no value in using a suction strainer.
Nearly all hydraulic systems use a high-pressure inline filter in the supply line. Sometimes supplemented with so-called “last chance” filters, the purpose of a filter on the supply side is to protect contamination-sensitive hydraulic valves and actuators. When a hydraulic valve actuates, fluid flow rates are typically at their highest, meaning that any in-line filter needs to cause as little restriction as possible while still offering fine-particulate filtration. To survive at high operating pressures, in-line pressure filters tend to be large and expensive.
While there’s little doubt that supply-line filters are necessary, they may not always be the most effective way to achieve the desired level of cleanliness, particularly in systems that demand very high levels. Upgrading an under-performing in-line filter to provide a greater level of protection often requires not just a more-efficient filter but a new filter housing, since a large element may be needed to maintain adequate flow rate at a smaller-micron-capture efficiency. This may not be the best choice from a cost perspective.
For open-loop hydraulic systems, a return-line filter is often a good choice. Located on the main return line, these filters can remove a large number of contaminants that may enter the system from the actuators, which is often the greatest source of contamination ingress. Return-line filters need to be selected correctly to withstand the maximum surge pressures and flow rates often seen on return lines, without causing fluid backup. They also must have excellent dirt-holding capacity to reduce frequent changes or prevent the filter from going into bypass. A common method of installing return-line filters is the so-called “in-tank” design. While in-tank designs provide a neat, compact installation, they can cause servicing problems, making an in-line, return-line filter outside the tank a better option.
From a “cost-per-unit-gram of dirt removal” perspective, perhaps the most effective filtration for hydraulic systems is off-line or bypass filtration. Off-line filtration, sometimes called kidney-loop filtration because of its similarity to kidney dialysis, takes a small amount of oil at a low flow rate (typically no more than 10% of the total tank volume) and passes the fluid through a fine filter, returning it back to the sump. Because flow through an offline system doesn’t have an impact on the main fluid flow, offline units can be sized smaller, “valved-off ” for on-the-fly filter changes, and use slow flow rates.
Often used in mobile hydraulic applications, bypass filtration takes a small slipstream of fluid (usually less than 10% by volume) from the main pressurized line and passes the oil through a flow-control valve before returning the fluid to the reservoir. Bypass filtration is a low-cost, effective way of achieving very low levels of particle contamination without compromising the design and operability of the hydraulic system.
A well-balanced contamination-control strategy can result in years of trouble-free operation and is a critical component of any reliability-driven maintenance strategy.
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management and oil analysis.