Hydraulic Return Line Filters Explained

Hydraulic Return Line Filter

The return line filter can be mounted to the system in two ways: on the return line before it enters the tank, or somewhere on top of the tank (or inside it).

All of the hydraulic fluid in the return line must flow into the tank through the filter. If the fluid returning to the tank isn’t combined into a single collection line, filters can also be placed on the branch lines.

Filtering on the return line is more economical than filtering on the pressure line. When selecting filter size, the possible maximum flow rate should be taken into account.

To prevent the fluid from foaming in the tank, the system fluid must return below the fluid level in the tank under all operating conditions.

It may be necessary to mount a pipe or flow diffuser at the filter outlet. Care must be taken to ensure that the distance between the bottom of the tank and the end of the pipe isn’t less than 2 or 3 times the pipe diameter.

Properties of Return Line Filters

Operating pressure: depends on the design, up to a maximum of 30 bar.

Flow rate: up to 1300 l/min (for tank-mounted filters), up to 3900 l/min (for large filters mounted vertically on the pipeline)

Filter fineness: 10-25 µm

Allowable pressure differential ∆p: up to approximately 70 bar, depending on the filter’s construction.

A dual-filter system is used to avoid stopping the system for filter maintenance.

In this system, two filters are arranged in parallel, and the contaminated filter is replaced by switching in the second filter without stopping the system.

illustration of a dual-filter system — Illustration of a dual-filter system

Choosing a Hydraulic Return Line Filter

Some points to consider when choosing a hydraulic return line filter can include:

The filter being the right size for the system’s needs.
The filter’s efficiency and operating range (e.g., cleanliness level in microns, flow rate, pressure)
The filter being made of a suitable material (e.g., resistance to conditions like temperature, chemicals, high pressure)
The filter being easy to clean and maintain.
The filter’s price being reasonable.
The manufacturer’s technical support and service.
The filter meeting relevant standards and compliance requirements.

Maintaining the Hydraulic Return Line Filter

If the hydraulic return line filter fails, or is no longer able to do its job, contaminants inside the system can damage the system and reduce its performance. Additionally, a failure of the filter material can allow the system to continue coming into contact with harmful substances. This can shorten the hydraulic system’s service life and lead to larger failures. For this reason, it’s important to service the hydraulic return line filter at the intervals recommended by the manufacturer.

The following steps are generally followed to maintain hydraulic return line filters:

Before running the system, use a pressure gauge or manometer to check the filter’s operating condition.
Periodically inspect the material inside the filter and clean or replace it as needed.
Clean the outside and surroundings of the filter housing.
When replacing the filter material, depressurize the system to reduce the operating pressure of the filter housing.
Correctly install the new filter material, then refill and reconnect the filter housing.
Check the filter’s operating pressure and flow rate, and restart the system.
Maintenance and cleaning should be performed at the intervals specified by the manufacturer.
It’s important to replace the filter material in time once its service life is up.

Low-Pressure Filtration

Return line filters filter the oil returning to the system at low pressure, protecting the pump from high-pressure loads.

Fine Filtration Capacity

Return filters generally provide precise contamination control with fine mesh sizes, such as 10-25 microns.

Preventing Tank Contamination

These filters reduce system contamination by ensuring used oil is cleaned before it returns to the tank.