As explained here, several factors, other factors can influence a system’s integrity-proper handling and a solid system design go a long way toward solving dirty oil problems. Each situation is different — with varying climates, environments and cleanliness requirements to take into consideration.

However, a knowledgeable distributor or consultant will be able to accurately diagnose systems and recommend which media choices, filter types, and filter positioning are right for every application.

Without question, the single biggest improvements to hydraulic filtration have come from firstly installing clean fluids, secondly preventing further ingress of contaminants through breathers and seals and thirdly installing filters on equipment in positions where the flow remains as constant as possible during operation of the equipment. Avoiding “stop start” typical return flow conditions can significantly improve system cleanliness levels. If flow is continually starting and stopping through a depth type filter media it will render the filter virtually useless in capturing particulate below ten microns. 

Stopping contamination at its entry point — the reservoir

The amount of generated contamination in a hydraulic system is usually fractional compared to contamination ingression. Much of this ingression occurs at the reservoir, so good reservoir design targets the dirt at the source of handling errors.

The modern trend in reservoir design is to keep the reservoir as small as practicable. This reduces the cost of construction and amount of fluid needed, and enables the oil to be more regularly recycled through the hydraulic system’s filters. The more frequently oil is cycled through a filter, the cleaner it becomes. Therefore, with a smaller reservoir, the oil tends to complete the filter cycle more often. One disadvantage to a small reservoir is that the oil can become too hot unless auxiliary cooling is provided. As the reservoir becomes too hot, oil oxidizes, which can shorten the life of the oil.

One reason for a large reservoir may be to ensure there is sufficient oil to shut down an operation in case of an emergency. This is more typical on heavy rotating machinery where it can take several minutes to stop. Not having enough fluid reserve could cause catastrophic failure. Large reservoirs generally require larger filters and regular recirculation to effectively control contamination.

Breathers — Tank breathers are a critical component for good filtration on most hydraulic systems. At inlet sites, where the system is open to the atmosphere, there is significant potential for contamination. Sometimes, sealed and pressurized reservoirs can negate the need for breathers altogether by preventing dirt from entering the system. However, pressurized tanks are more suitable for smaller reservoirs with stable flow and level conditions. In most cases, breathers will be necessary to maintain the cleanliness of a hydraulic circuit to prevent ambient dirt from entering the system.

Although most systems are designed with the breather inlets on the top of the reservoir cover, it is often more effective to design the tank with inlets on the side (above oil level). With gravity, it is easier for dust, water and other contaminants to fall into top openings — especially during cleaning, filling, etc. This tendency is naturally reduced when the inlet is on the side of the tank. Although side inlets require the additional expense of a filling device, this can be a small investment compared to the added benefits of reducing the contamination.

Ideally, a breather should prevent atmospheric particles larger than 5 μm from entering the system. Typically a good quality liquid filter with a 10 μm absolute rating will perform well at removing 5 μm and smaller dust particles from air.

A properly sized breather should also allow sufficient air movement. An undersized reservoir breather, especially on large tanks, can easily cause a tank to split or collapse by restricting air flow.

In applications where condensation is common — paper mills, bulk storage tanks, food processing plants, and a variety of other environments — many engineers rely on breathers containing a desiccant that absorbs moisture before it can enter the reservoir. However, if the breather’s filter element and desiccant are not kept clean, the desiccant can become the contaminant if the separating filter media is not capable of capturing dirt particles larger than 5 μm.

The next time you conduct a reservoir inspection, see if your breather has a filter inside it. A wire mesh that you can see through is not adequate to control 5 μm dirt particles, Check to see if the seal on the filler cap actually seals air tight, and check to see if there any open pipes, bolt holes, or unsealed covers. Dirty air will take the path of least resistance into the reservoir — eventually causing damage to the hydraulic system.

Suction filters — In the hydraulic circuit, suction filters are normally placed between the reservoir and the pump. Typically, they are designed to remove dirt particles ranging in size from 5 to 10 μm. Suction filters should not be confused with suction strainers, which serve little purpose other than to keep nuts, bolts, weld spatter, cigarette butts, and other large objects out of the pump.

It’s critical to size and maintain suction filters well. If the filter lets contaminants pass through into the pump, the pump will fail. If the filter becomes blocked, it can cause the pump to cavitate. The result will be the same — pump failure and further damage downstream of the pump.

The most popular application for suction filters is with variable-speed hydrostatic pumps, often found in mobile equipment and industrial variable-speed hydraulic drives. Suction filters used in conjunction with this type of pump will normally cause the pump to stall when blocked — before any damage
is caused by the inlet flow restriction.

Each pump manufacturer has strict criteria regarding inlet flow restrictions. These should be followed carefully when applying suction filtration. Generally, try to avoid suction filters with high flow rate pumps, particularly gear and vane pumps. Instead, clean the oil before it gets to the pump.

Offline (kidney loop) filters— Kidney loop filtration can be one of the most effective ways of cleaning a hydraulic system and keeping it clean. One advantage to this type of filtration is that the oil flow is nearly constant, which promotes the most efficient and reliable filter performance. Another is that the filter circuit can be serviced without having to shut down the entire system.

When using a kidney-loop filter, an additional pump is usually required at the inlet site. Considering this additional cost, built-in kidney loop filters are perhaps better suited for applications with large reservoirs of oil, or in hydraulic systems with particularly expensive components, such as servovalves. For small-tank applications, it probably makes more sense to use a mobile filter cart, which can move from one system to the next.

Careful positioning and connection of the kidney loop filter is very important to avoid stirring up dirt that may already be in the system. Ensure that the tank has proper inlet and outlet fittings that connect to the filling hoses.

With careful design, a kidney filtration unit can also be used to filter oil during the filling of the reservoir. Because a kidney filtration system does not take the full flow of the hydraulic system, and simply dilutes the dirt in the oil reservoir, aim to use the finest filtration practicable. Generally, 5 μm absolute filtration is optimal, depending on the application and the oil viscosity.


Philip Johnson is Industrial Hydraulics Director, Donaldson Co. Inc., Minneapolis. For more information, visit