Filter selection for hydraulic system


  • Filter element ratio selection
  • Recommendations for locations of filters at your system
  • Beta Ratio understanding

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The next step after the oil filtration rate selection is filter and filter element selection.

1) Filter element ratio selection

Using filter manufacturers tables we can select filter element ratio, for example:

HYDAC  recommendations:


or Hy-Pro Filtration recommendations:


2) Determine filter element efficiency

So, now we know the filter element ratio, but we need one more parameter to select filter element – its efficiency. The main parameter of the efficiency of a filter element is the Beta Ratio – this is a ratio between the quantity of particles upstream (before filter) and the quantity of particles downstream (after filter). For different sizes of particles there are different beta ratio for one filter element.

For example, assume that 50,000 particles, 10 micrometers and larger, were counted upstream (before) of the test filter and 10,000 particles at that same size range were counted downstream (after) of the test filter. The corresponding Beta Ratio would equal 5, as seen in the following example:


Now you can calculate an efficiency, expressed as a percent:


So, bigger value of beta ratio is a bigger efficiency of filter element:


3) Select filter location.

The last step is filter location selection. There are 4 locations in the system where we can install filters:

Suction filters serve to protect the pump from fluid contamination. They are located before inlet port of the pump. But, to protect the pump from vacuum at the pump inlet, suction filters usually take the form of a 150-micron (100-mesh) strainer. This is why they are not used as primary protection, and moreover, some pump manufacturer don’t recommend to use suction filters.

Pressure filters are located downstream from the system pump. They are designed to handle the systempressure and sized for the specific flow rate in the pressure line where they are located. Pressure filters are especially suited for protecting sensitive components directly downstream from the filter, such as servo valves. Located just downstream from the system pump, they also help protect the entire system from pump generated contamination.

Return filters using when the pump is a sensitive component in the system. Ususally, return filter is the last component trough which hydraulic fluid passes before entering the reservoir. In some cases, cylinders with large diameter rods may result in “flow multiplication”. The increased return line flow rate may cause the filter bypass valve to open, allowing unfiltered flow to pass downstream.

Off-Line filtration system is  totally independent of a machine’s main hydraulic system.  Fluid is pumped out of the reservoir, through the filter, and back to the reservoir in a continuous fashion.  An off-line filtration
loop has the added advantage that it is relatively easy to retrofit on an existing system that has inadequate filtration. Also, the filter can be serviced without shutting down the main system.

There is a good table with recommendations what type of filter should we use in different systems (from Parker catalog):


Here: P – pressure filter, R – return filter, O – off-line filtering system.


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