How to Choose the Correct Hydraulic Filter?

The right filtration system is essential for the long life operation of a hydraulic system, but choosing the right filter is often not a breeze.

Different industries, different components of a hydraulic system have their own requirements and influence each other, and in the end it comes down to how to choose that seemingly simple filter element. To avoid excessive costs in system operation, when selecting a filter element you need to consider not only the pressure, flow and cleanliness requirements, but also an overview of the working conditions applied. This article lists five points of knowledge that must be mastered when calculating and selecting filters.

Familiarity with Application Conditions

First and foremost, it is important to understand the performance requirements of the filter. When calculating filter specifications precisely, the performance requirements are usually expressed in terms of the filtration ratio β. The so-called β value is the ratio of the number of particles larger than a certain size in the fluid at the filter inlet to the number of particles larger than a certain size in the fluid at the filter outlet. Therefore, the larger the β value, the higher the filtration efficiency of the filter. β value can also be expressed as a percentage to indicate its efficiency, η=1-1/β.

Today's filter designs typically have higher dirt holding capacity, longer life and lower pressure drop.

For example, applications requiring high performance filter elements usually have a β value of over 1000, meaning that of 1000 particles entering the filter, only 1 particle will exit. In this case, the expression in terms of filtration efficiency is 99.9%. Generally, regular filter manufacturers have special documentation, which defines filter performance according to the size of the filter element pore size. Therefore, a proper understanding of the application performance requirements allows you to be able to select the correct cartridge size and ensure good operation of the equipment.

Knowing the Hydraulic System

Once you have established the specific performance requirements for the application conditions, you are ready to properly evaluate the hydraulic system. According to industry standards, the hydraulic components installed in the hydraulic circuit directly influence the type of hydraulic filter. Such industry standards are mainly the ISO cleanliness class codes, which help us to select the right type of filter. As an example, a proportional valve circuit requires a minimum ISO cleanliness rating of 20/18/15. To meet this, the absolute filtration accuracy of the filter element is at least 3 or 6 μm.

Filter selection requires a combination of cartridge holding capacity, pressure drop and economy.

If the circuit contains only gear pumps and directional valves, they are relatively insensitive to contamination, so a filter with low performance characteristics can meet its requirements - a filter element such as a paper or metal mesh filter may be an economical choice. Knowing what hydraulic components are suitable for what filters and understanding how they affect filter performance can save some unnecessary costs for long-term operation.

Understanding Oil Viscosity

A fact that is often overlooked when choosing cartridge specifications is the viscosity of the fluid. In fact, viscosity has a great influence on the selection of cartridge specifications. In the initial selection of cartridge specifications, oil viscosity is often overlooked. The result is that the cartridge specification is chosen too large, resulting in unnecessarily high operating costs. Not understanding the importance of oil viscosity may lead to the selection of too small a cartridge specification, too high a pressure drop, premature alarm of the contamination sender, etc. On the contrary, there is a possibility that the specification of the filter element is too large, resulting in excessive investment costs and operating replacement costs. The user needs to understand the hydraulic system well enough to evaluate its rated operating temperature range so that the most accurate minimum temperature can be used as the basis for selecting the cartridge specification. Correctly selecting the operating temperature range ensures that the cartridge does not go into bypass mode prematurely and avoids over-selection of the cartridge.

About the Pollution Transmitter

Assuming that the filter is equipped with a contamination sender (analog or digital), the user should have a suitable target requirement for the pressure drop when selecting the filter. Typically, the differential pressure indicated by the sender is in the range of 1 to 8 bar. As a general principle, the ratio of the set pressure of the transmitter to the pressure drop of the clean filter is at least 3:1.

For example, if the filter visual sender sets an alarm pressure of 5 bar, the maximum acceptable differential pressure for the selected filter is approximately 1.7 bar based on the above ratio. if this principle is not understood, the user runs the risk of selecting a filter that is either too large or too small. To ensure that the hydraulic system is sufficiently protected, it seems very common in our work to select filters towards the larger ones, with the result that the cost is increased.

Accounting for the Flow Through the Filter

When choosing a filter, the flow rate must be carefully analyzed, its quite important. It is important to note that the instantaneous flow rate may exceed the maximum flow rate of the pump. A typical operating condition is that in an accumulator circuit, the accumulator stores a certain amount of pressure oil, and when the accumulator is released, the flow rate when it is discharged will be superimposed on the flow rate of the pump, thus greatly exceeding the flow rate of the pump itself. The high pressure filter after the accumulator circuit must be fully considered. Underestimating the flow rate through the filter will increase the differential pressure across the cartridge, ultimately leading to a reduction in circuit efficiency and increased operating and maintenance costs.