How to Choose a Hydraulic Filter

Filters are crucial to keeping the hydraulic system clean and extending its service life. How do you choose a suitable filter?

Each machine and component in the hydraulic system has its own set of requirements, each of which affects each other and of course affects the choice of filter. To determine the size of the filter and choose the right filter, you must understand the following key information:

1. Filter performance requirements

Filter performance requirements are usually expressed in terms of the filtration ratio β, which is the ratio of particles entering the filter to those exiting the filter.

Today’s hydraulic filters are designed with media that can capture and hold more contaminants than before, thereby reducing pressure drop and extending service life. For example, applications that require high-performance components will typically have a filtration ratio β in excess of 1000 (i.e., 1000 particles enter the filter and only one particle passes through).

2. Hydraulic system

After determining the performance requirements of the filter, we need to understand the hydraulic components installed in the hydraulic system circuit. According to industry standards, these hydraulic components require a corresponding standard filter type. The standard drives the ISO cleanliness code requirements, which helps determine the correct type of filter element.

For example, a proportional valve in a circuit may require a 20/18/15 ISO cleanliness code. In order to meet the standards set by the ISO code, the circuit needs a filter with 3 µm or 6 µm absolute filtration.

When selecting the size of the filter, factors such as dirt holding capacity, pressure drop, and cost must also be considered. Knowing which hydraulic components are attached to the filter and how these hydraulic components affect the performance requirements of the filter can save costs.

3. Oil viscosity

Oil viscosity has a great impact on the size of the filter. When selecting a filter, many people tend to ignore this factor and often choose a larger filter than actually needed, resulting in higher operating costs.

The filter size is too small and the pressure drop is too high, causing the clogging indicator to trip prematurely.

The filter size is too large, which will lead to problems such as high cost, large floor space, and high replacement cost. We should fully understand the normal temperature range of the hydraulic system and select the filter according to the accurate minimum temperature.

4. Clogging Indicator

Assuming the system’s filter has a clogging indicator (analog or digital), the user should have an appropriate target for pressure drop when selecting the filter size.

Typically, the pressure differential for clogging indication ranges from 1 to 8 bar, and the ratio of the clogging indicator setting to the clean pressure drop is at least 3:1. For example, assuming the filter has a 5 bar clogging indicator, the maximum acceptable clean pressure drop target is about 1.7 bar based on the 3:1 ratio.

Without understanding this principle, we may incorrectly select the filter size. Although oversizing the filter can ensure the highest amount of protection, it will increase costs.

5. Filter flow

Flow must be considered when selecting the filter size, and we need to consider the maximum flow.

Note that intermittent flow may exceed the maximum flow of the pump. For example, in an accumulator circuit, where the accumulator stores a certain amount of pressurized fluid and releases it into the system to supplement the pump flow, the flow through the pressure filter connected downstream of the accumulator should take into account this additional flow.

Underestimating the flow through the filter will increase the pressure drop through the filter element, increasing operating and maintenance costs.