In a perfect world, every stage of a hydraulic system would have its own filter, both before and after each component. In reality, budgets, space constraints, and pressure loss through the filters mean hydraulic systems must balance protection with practicality.

That’s why understanding the different types of hydraulic filters—and what they protect—is key to maximizing system life and minimizing downtime.

What Affects Filter Placement?

Filtration design varies depending on factors such as:

• Component sensitivity and cleanliness requirements

• Ambient contamination levels

• Duty cycle of the equipment

• Ease of servicing and part replacement

• System pressure and tolerance levels

• Critical value of the machine

Now, let’s look at the main types of filters you might find in a hydraulic system.

1. Suction Strainers

Located inside the reservoir, suction strainers block large contaminants like cable ties, nuts, bolts, or debris (150+ microns) from reaching the pump.

Why they matter:

• Inexpensive protection against catastrophic pump damage

• May minimise air bubbles from entering the circuit

• Acts as a final safeguard from unexpected large debris

2. Suction or Low-Pressure Filters

These are usually spin-on filters installed between the tank and the pump. Designed to protect the pump from harmful contaminants without causing cavitation, they are often steel mesh or open medias. Low-pressure spin-on filters are easy to service and less expensive than many other types of filters.

Key points:

• Easy to service and affordable

• Feature a broad range of micron ratings

• Minimise the risk of pump cavitation

3. High-Pressure Filters

Installed after the pump, these filters clean fluid before it reaches critical components like actuators or servo valves.

Why they’re essential:

• Protect high-value parts

• The most important filter in an advanced high-pressure system

• Critical applications may not include bypass valves

Downsides:

• Withstanding pressures up to 450 bar (6,500 psi) , housings are heavy and costly

• Non-duplex designs require a full system shutdown for servicing

4. Spin-On vs. Cartridge Filters

Spin-On Filters:

• Quick and easy to remove and replace

• The media and filter can be discarded during service

• May be a higher element cost and more waste

Cartridge Filters:

• Only the internal element is replaced

• Lower element cost and reduced waste

• Multiple complex servicing steps may result in greater fluid loss or disposal

Choosing between them often comes down to balancing service ease, environmental considerations, and total cost of ownership.

5. Reservoir Breathers

Breathers filter the air entering a reservoir as fluid levels rise and fall during machine operation.

What they protect against:

• Airborne particles

• Moisture ingress (in the case of Donaldson T.R.A.P. breathers)

Tip: Breathers are often overlooked but offer exceptional value for their cost. Regular inspection and replacement can significantly reduce system wear and contamination.

6. Off-Line Filtration (Kidney Loop Systems)

These standalone systems can be used to continuously filter fluid, whether the main system is running or not.

Advantages:

• Provide high-efficiency filtration

• Do not interfere with the existing filtration or equipment performance

• Extend hydraulic fluid life and service intervals

• Ideal for critical systems where uptime is crucial

Limitations:

• Do not directly protect in-line components

• Require an external power source

Bottom Line: Know Your Filters

Whether you realize it or not, if you own or operate hydraulic equipment, filtration is a critical part of your system. Filters work behind the scenes, protecting your biggest investments.

Best practices:

• Follow your manufacturer’s filter specifications

• Maintain a regular service schedule

• Choose high-quality filters and breathers to avoid costly repairs and downtime

If you have questions or need help selecting the right filter for your machine, contact a Donaldson expert—we’re here to help.