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The correct way to choose hydraulic filter

October 19, 2021

A proper filter system is an indispensable condition for long-life operation of a hydraulic system, but choosing the right filter is often not an easy task.
Different industries, different components of the hydraulic system have their own requirements, and influence each other, and finally involves how to choose the simple filter element. In order to avoid excessively high costs in system operation, when selecting filter elements, you not only need to consider the pressure, flow and cleanliness requirements, but also need to have an overview of the application conditions. This article lists five knowledge points that must be mastered when calculating and selecting filters.
1. Familiar with application conditions
The first and most important thing is to understand the performance requirements of the filter. When calculating the filter specifications accurately, the performance requirements are usually expressed by the filtration ratio β value. The so-called β value refers to the number of particles larger than a certain size in the fluid at the inlet of the filter and the number of particles larger than a certain size in the fluid at the outlet of the filter. The ratio of the number of particles. Therefore, the larger the β value, the higher the filtration efficiency of the filter. β value can also be expressed as a percentage to express its efficiency, η=1-1/β.

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Today's filter designs generally have higher dirt holding capacity, longer life, and lower pressure drop.
For example, in the application conditions that require high-performance filter elements, usually the β value exceeds 1000, which means that only one particle will flow out of 1000 particles entering the filter. In this case, the filtration efficiency is 99.9%. Generally, regular filter manufacturers will have special documents, which define the filter performance according to the size of the filter element pore size. Therefore, a correct understanding of the application performance requirements can enable you to correctly select the filter specifications and ensure the good operation of the equipment.
2. Know the hydraulic system
Once you have established the specific performance requirements for the application conditions, you can properly evaluate the hydraulic system. According to industry standards, the hydraulic components installed in the hydraulic circuit directly affect the type of hydraulic filter. Such industrial standards are mainly ISO cleanliness grade codes, which help us choose the appropriate filter type. For example, the proportional valve circuit requires at least an ISO cleanliness level of 20/18/15. To meet this requirement, the absolute filtration accuracy of the filter element is at least 3 or 6 μm.

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The selection of the filter requires comprehensive consideration of the dirt holding capacity, pressure drop and economy of the filter element.
If the circuit only contains gear pumps and directional valves, they are relatively insensitive to pollution. Therefore, filters with low performance characteristics can meet their requirements-such as paper or metal mesh filter elements may be economical choose. Knowing what kind of hydraulic components fit what kind of filter, and understanding how they affect the performance of the filter can save some unnecessary costs for long-term operation.
3. Know the viscosity of oil
When selecting filter specifications, a fact that is often overlooked is the oil viscosity. In fact, viscosity has a great influence on the selection of filter specifications. In the initial selection of filter specifications, the oil viscosity is often ignored. As a result, the size of the filter element is selected too large, resulting in unnecessary high operating costs. Failure to understand the importance of oil viscosity may result in the selection of filter element specifications that are too small, the pressure drop is too high, and the pollution indicator will alarm early. On the contrary, it is also possible that the filter element specifications are selected too large, resulting in excessive investment costs and operating replacement costs. The user needs to understand the hydraulic system clearly enough to evaluate its rated operating temperature range so that the most accurate minimum temperature can be used as the basis for selecting filter specifications. The correct selection of the operating temperature range can ensure that the filter element will not enter the bypass mode in advance, and avoid excessive selection of the filter element.
4. Regarding pollution-transmitting devices
It is assumed that the filter is equipped with a pollution signal device (analog or digital). When selecting a filter, the user should have a suitable target requirement for its pressure drop. Under normal circumstances, the pressure difference range shown by the signalling device is between 1 and 8 bar. As a general principle, the ratio of the set pressure of the signaling device to the pressure drop of the clean filter is at least 3:1.
For example, if the filter visual signal device sets an alarm pressure of 5 bar, based on the above ratio, the highest acceptable pressure difference of the selected filter is about 1.7 bar. If you don’t understand this principle, users will be at risk when choosing a filter, either too large or too small. In order to ensure that the hydraulic system is adequately protected, the selection of larger filters is very common in our work, and the result is an increase in costs.
5. Calculate the flow through the filter
When choosing a filter, you must carefully analyze the flow rate, which is very important. It should be noted that the instantaneous flow rate may exceed the maximum flow rate of the pump. The typical working condition is that in an accumulator circuit, the accumulator stores a certain amount of pressure oil. When the accumulator is released, its discharge flow will be superimposed on the pump flow, which greatly exceeds the pump itself. The discharge flow rate. The high-pressure filter after the accumulator circuit must fully consider this. Underestimating the flow through the filter will increase the filter element pressure difference, which will eventually lead to a decrease in loop efficiency and increase operating and maintenance costs.

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This icon illustrates the characteristics of the filter, which is designed to have a low pressure difference even at very high flow rates-the total pressure difference is the sum of the filter element pressure difference and the filter housing pressure difference, and is mainly determined by the filter element pressure difference
The correct selection of filters not only saves costs, but also prolongs the service life of the hydraulic system.