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What is a liquid filter?

November 29,2021

Liquid filter is an indispensable device on the transmission medium pipeline. It is usually installed at the inlet of pressure reducing valve, pressure relief valve, constant water level valve or other equipment to eliminate impurities in the medium and protect the normal use of valves and equipment. When the fluid enters the filter cartridge with a certain specification of filter screen, its impurities are blocked, and the clean filtrate is discharged from the filter outlet. When cleaning is needed, just take out the removable filter cartridge and reload it after treatment. Therefore, it is very convenient for use and maintenance.

Classification of liquid filters

According to filter media:

  1. Air filter

Make the polluted air clean to the state required by production and life, that is, make the air reach certain cleanliness.

  1. Liquid filter

Make the contaminated liquid clean to the state required by production and life, that is, make the liquid reach certain cleanliness.

  1. Network filter

Block junk information through settings to make the information on the computer screen meet the requirements as much as possible. The same absorption principle separates light of different colors

  1. Light filter

Absorb some unwanted light.

According to the installation method:

Oil suction filter

Oil return filter

Pipeline filter

According to performance, it is divided into:

Pipeline filter

Double cartridge filter

High pressure filter

The above should be a mesh filter. In fact, there are many kinds of filters, such as laminated filter, sand filter, carbon filter, etc. the main principle is to use the pore diameter of the filter medium to intercept substances smaller than the pore diameter of the medium. Of course, some filter media also have special effects such as adsorption. Most filter backwashing is not so troublesome. As long as clean water is introduced into the medium in the backwashing filter from one end of the filtered water, a good backwashing effect can be achieved.

General principles of filter selection

  1. Inlet and outlet diameter

In principle, the inlet and outlet diameter of the filter shall not be less than the inlet diameter of the matched pump, which is generally consistent with the diameter of the inlet pipeline.

  1. Nominal pressure

Determine the pressure level of the filter according to the maximum pressure that may appear in the filter pipeline.

  1. Selection of hole number

It mainly considers the particle size of impurities to be intercepted, which is determined according to the process requirements of medium process. The particle size that can be intercepted by various specifications of screen can be found in the following table “screen specifications”.

  1. Filter material

The material of the filter is generally the same as that of the connected process pipeline. For different service conditions, cast iron, carbon steel, low alloy steel or stainless steel filters can be considered.

  1. Filter resistance loss calculation

The pressure loss of water filter is 0.52 ~ 1.2kpa under the general calculated rated flow rate

Filter screen accuracy conversion

Micron 10             25           30           40           50           80           100         120         150         200         400         800         1500 3000

Mesh number 1500           650  550               400         300         200         150         120         100         80           40           20           10           5

Millimeter 0.01   0.025  0.03           0.04        0.05        0.08        0.1          0.12        0.15        0.2          0.4          0.8          1.5

3.0 test bench filter

It is used on the filter performance test-bed. The test-bed must strictly purify the system before the test. Therefore, the test system and the pollution injection system serving the test system need to be equipped with filters. The filter and the tested filter are two different things. Its accuracy requires that the dirt holding capacity is higher than that of the tested filter, In order to distinguish two types of filters in the same test system, this kind of filter is called test bench filter, and the tested filter is called tested filter.

Hydraulic system filter

There are four types of oil filters: coarse filter, ordinary filter, precision filter and ultra-fine filter, which can filter more than 100 according to their filtering accuracy (particle size of impurities) μ m、10~100 μ m、5~10 μ M and 1 ~ 5 μ M impurities

When selecting oil filter, the following points shall be considered:

(1) The filtering accuracy shall meet the predetermined requirements.

(2) It can maintain sufficient flow capacity for a long time.

(3) The filter element has sufficient strength and will not be damaged due to the action of hydraulic pressure.

(4) The filter core has good corrosion resistance and can work permanently at the specified temperature.

(5) The filter element is easy to clean or replace.

Therefore, the model of the oil filter shall be selected according to the technical requirements of the hydraulic system and the conditions such as filtration accuracy, flow capacity, working pressure, oil viscosity and working temperature.

  1. The installation positions of oil filter in hydraulic system are generally as follows:

(1) To install at the suction port of the pump:

Surface type oil filter is generally installed on the oil suction road of the pump to filter out large impurities and particles to protect the hydraulic pump. In addition, the filtering capacity of the oil filter shall be more than twice the pump flow, and the pressure loss shall be less than 0.02MPa.

(2) Installed on the outlet oil circuit of the pump:

The purpose of installing oil filter here is to filter out pollutants that may invade valves and other components. The filtering accuracy shall be 10 ~ 15 μ m. It can withstand the working pressure and impact pressure on the oil circuit, and the pressure drop shall be less than 0.35Mpa. It shall also be installed

Safety valve to prevent clogging of oil filter.

(3) Installed on the oil return circuit of the system: this installation plays an indirect filtering role. Generally, a back pressure valve is installed in parallel with the filter. When the filter is blocked and reaches a certain pressure value, the back pressure valve opens.

(4) It is installed on the branch oil circuit of the system.

(5) Separate filtering system: a large hydraulic system can be specially equipped with a hydraulic pump and an oil filter to form an independent filtering circuit.

In addition to the oil filter required by the whole system, a special fine oil filter is often installed in front of some important components (such as servo valve, precision throttle valve, etc.) to ensure their normal operation

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