The selection error of filter in hydraulic system

Hydraulic Filter as the main component of hydraulic system pollution control, its design and selection is reasonable, daily use (maintenance) is correct is directly related to the safety and reliability of the system. In practical applications, many users still have many misunderstandings about the selection and use of filters, which will affect the normal and reliable work of the hydraulic system if not corrected.

1 Filter selection errors in hydraulic system

1.1 Misunderstanding 1: Choosing a high-precision Oil Suction Filter can both effectively protect the pump and ensure the cleanliness of the system

Because the particle pollutants in the oil will aggravate the wear of the pump and thus affect the performance and life of the pump, large particle pollutants may also jam the pump, seriously affecting the safety and reliability of the system. Therefore, some users choose high-precision oil suction filters, thinking that it can protect the pump and ensure the cleanliness of the system. However, the high-precision oil suction filter is easy to clog due to excessive pollutants, resulting in poor pump oil absorption, resulting in suction, accelerating pump wear, and seriously affecting system safety. Therefore, the pressure drop of the oil suction filter should be strictly controlled. General hydraulic systems can consider installing low-precision oil suction filters to protect the pump, and install filters in front of contaminant-sensitive components to protect them to control the impact of particle pollution. In order to most effectively intercept contamination in the loop due to component wear or external intrusion, it is recommended to install a Return Oil Filter to control it to improve the cleanliness of the entire system. At the same time, pipelines and fuel tanks should be thoroughly cleaned before the operation of the system to ensure the degree of oil pollution. In this way, the oil pollution of the whole system is basically controlled, which protects the pump and the whole system.

1.2 Misunderstanding two: The rated (nominal) flow of the filter is the actual flow of the system

The rated flow rate of the filter is the flow rate through the clean filter element under the specified original resistance when the oil viscosity is 32cst. However, in practical applications, due to the different media used and the different temperature of the system, the oil viscosity will also change at any time. If the filter is selected according to the rated flow rate and the actual flow rate 1:1, when the viscosity of the system oil is slightly larger, the resistance of the oil through the filter will increase (such as the viscosity of No. 32 hydraulic oil at 0 ° C is about 420cst), and even reach the pollution blocking signal value of the filter, and the filter element is considered blocked. Secondly, the filter element of the filter is a wearing part, the work is gradually polluted, the actual effective filtration area of the filter material is constantly reduced, and the resistance of the oil through the filter soon reaches the pollution blocking signal value. In this way, the filter needs to be cleaned or replaced frequently, increasing the user's cost.

At present, the domestic filter manufacturers have stipulated the rated flow of their filter production, according to the author's previous experience and many customers use, the system uses oil for general hydraulic oil, it is recommended that the filter in the selection of the following multiple of the flow :① the rated flow of the oil suction, return Oil Filter is more than 3 times the actual flow of the system; The rated flow rate of the pipeline filter is more than 2.5 times the actual flow rate of the system. If the oil is not general hydraulic oil or high viscosity hydraulic oil, please consult the manufacturer for selection.

1.3 Misunderstanding 3: The higher the accuracy of the filter selection, the better

The solid pollution in the hydraulic system is the main reason for the failure of the hydraulic system, so the high-precision filter is selected to control the pollution. In fact, this not only increases the manufacturing cost of the system, but also shortens the service life of the filter element. Then how to choose the accuracy of the filter reasonably? It is mainly determined by the pollution requirements of the components of the hydraulic system on the oil, and the higher the cleanliness of the components, the higher the precision of the filter selection.

When the wear particles enter the gap between the moving pairs of the components, a chain reaction of wear will occur. So wear it down
To minimize and maximize the life of the component, it is necessary to filter out particles that are close to the gap size. The level of oil contamination required for typical hydraulic components and the recommended value of filter selection accuracy are shown in Table 1.

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1.4 Misconception four :(Xμm) filter element with filtration accuracy can filter out all particles greater than its accuracy

Because the hydraulic pollution control technology is still in the development stage in our country, many users are not very understanding of the definition of filter precision, that the system as long as the installation of (Xμm) precision filter can ensure that there are no pollutants greater than (Xμm) particles in the system oil, in fact, this is wrong. The national standard GB/T20079-2006 stipulates that the filtration capacity of the filter is expressed by the filtration ratio βx(c), which is defined as the ratio of the number of pollutant particles in the unit volume of the oil above and below the filter that is greater than a given size x(c).

That is, the filtration accuracy is defined as the minimum particle size x(c) that can be effectively captured by the filter (βx(c)≥100), in microns as a unit of measurement, expressed in μm. At present, the size of the filter ratio βx(c) value is not uniform among filter manufacturers. Since the filtration accuracy is determined according to the filtration ratio, the same filtration accuracy is completely different because of its actual βx value. Therefore, the filter element with (Xμm) filtration accuracy can not completely filter out particles greater than its accuracy. If the system selects a filter with a smaller filtration value than βx, the oil pollution degree is difficult to control.

2 Daily use (maintenance) filter misunderstandings

2.1 Misunderstanding 1: The filter with a bypass valve can not be cleaned or replaced for a long time after the filter element is blocked

Many users will think that the bypass valve of the filter and the safety valve of the system have the same function: after the filter element is blocked, the bypass valve is opened, and the full flow of the system oil passes through, which has no effect on the system. When the bypass valve of the filter is opened, the pollutants blocked by the filter element (the filtered pollution particles) will re-enter the system through the bypass valve, and the pollution concentration of the local oil is the highest at this time, which has great damage to the hydraulic components, and the previous pollution control will lose meaning. Unless the system requires very high continuity of work, it is best to choose a filter without a bypass valve. Even if a filter with a bypass valve is selected, when the pollution of the filter blocks the transmitter, it is necessary to clean or replace the filter element in time, which is the way to ensure the safe and reliable operation of the system.

2.2 Misunderstanding 2: To judge the filter performance by the service life of the filter Many users because there is no oil pollution detection equipment, to judge the filter performance by the service life of the filter. The filter plugging speed shows the good and bad performance, and these two ideas are one-sided. Because the filter performance of the filter is mainly reflected by the filter ratio, pollution capacity, original pressure loss and other performance indicators, only in the same working conditions and to ensure the cleanliness of the hydraulic system requirements, the longer the service life, the better.
3 Closing remarks

Whether the filter can be correctly selected and used in the hydraulic system is the key to the pollution control of the hydraulic system, and also the reliable guarantee of the safe operation of the system. In order to make the system and components have an ideal working life, it is necessary to carry out pollution control on the oil, configure reasonable different types of filters to achieve the most economical and reliable effect, and perform good daily maintenance on the filter to ensure the safe and reliable operation of the system