Filtration modes: the problem of cleaning hydraulic oil in a hydraulic system

Hydraulics is a technique for transmitting and regulating mechanical energy that is widely used in industry, the maritime sector and earth-moving machinery. The ability to transfer high power over long distances, as well as to control and regulate it, has led to its widespread adoption across various sectors.
One aspect that must be taken into account is the cleanliness of the hydraulic oil used in the system. As well as transferring power, hydraulic oil is responsible for cooling and lubricating all the components of a hydraulic system. The correct level of cleanliness helps to extend the service life of components and lengthen maintenance intervals, aspects which are of primary importance, as emphasised by the Industry 4.0 initiative.
All hydraulic systems are equipped with one or more filters designed to remove impurities, metal residues and other substances that may be present in the oil.
The most commonly used filtration technique is return-line filtration. A suitably sized filter is installed on the system’s return line and close to the reservoir, with the aim of collecting all impurities which, in the absence of the filter, would settle in the reservoir, with the risk of being sucked in by the pump.
When it is necessary to protect a component on the delivery line (e.g. servo valves) or when it is necessary to intercept the delivery line (in order to protect the actuator), the delivery line filtration technique is used. In this case, the filter will have different mechanical characteristics and filter elements compared to a return line filter. Suffice it to say that, on the return line, the oil pressure can be a maximum of 2–3 bar, whilst on the delivery line the pressure is in the order of hundreds of bar. A delivery line filter has a steel body that must be capable of withstanding the system’s operating pressure. Finally, the filter cartridges have higher filtration grades than those used for return line filtration.
Another widely used technique, which normally complements those just mentioned, is off-line filtration. In this case, a dedicated motor-pump unit is provided which draws oil from the tank and sends it to a filter, which then returns the oil to the tank. It is common to use a heat exchanger in series. This technique improves the oil cleanliness ensured by the return filter and, if present, by the delivery filter.
Finally, there is a combined technique known as return-suction, in which a valve is fitted on the outlet of the return filter to generate back pressure (of approximately 4–5 bar). Upstream of this valve (and therefore downstream of the filter), the pump’s suction line branches off. By doing so, the pump draws in freshly filtered oil at a higher pressure (atmospheric) than that prevailing in the reservoir.
In some cases, a filter is fitted on the pump’s suction line. This filter is often fitted inside the reservoir. This filter, which is designed to protect the pump, can cause excessive suction vacuum in the pump.
Some of the concepts illustrated and other technical descriptions are set out in the technical documentation of one of the leading filter manufacturers, Hydac, from which we present an example of a contamination guide.