Donaldson FLK90/110/125 Hydraulic Cartridge Filter

A reusable-housing hydraulic cartridge filter in three sizes, to 85 gpm

The Donaldson FLK 90, 110 and 125 series are in-line cartridge filters for medium-pressure hydraulic systems. Each assembly is a robust, reusable housing with a disposable cartridge. Raised grip handles remove the housing from the head without special tools, an oil drain port allows cleaner servicing and an anti-dust seal keeps the threads free from contamination.

RadialSeal sealing prevents cross contamination, and Synteq media gives lower pressure drop, superior cold-start filtration and longer life. Applications named in the sheet are hydrostatic charge pumps, hydrostatic transmissions and pilot control circuits.

This page summarises the manufacturer sheet for quick reference. Compare the figures with your drawing, the existing nameplate and the operating conditions of the installation, and tell us anything that differs so the correct variant can be confirmed in writing before any order is placed.

Technical specifications

Heads and cartridges

• P574994: Size FLK90; Port SAE-12; Bypass 50 psi
• P574995: Size FLK90; Port SAE-12; Bypass none
• P574996: Size FLK90; Port SAE-16; Bypass 50 psi
• P574997: Size FLK90; Port SAE-16; Bypass none
• P766831: Size FLK110; Port SAE-20; Bypass 50 psi
• P767009: Size FLK110; Port SAE-20; Bypass none

FLK90 housings are P766990 (short) and P766961 (long); its cartridges (6, 11 and 15 micron) include P767128 to P767131 and P766959 and P766987.

Service notes

Lubricate the entire thread and O-ring surface with a molybdenum-containing gear oil or anti-seize paste when installing a new filter, as the sheet instructs. Minimum service clearance is 65 mm (2.56 inches). Assembly weights run from 1.01 kg (short FLK110) to 4.76 kg (long FLK125).

The page covers the three-size family because the sheet is one document; give the FLK size, port, bypass and micron rating with your enquiry. Electric and visual service indicators are listed for the 50 psi bypass (for example P165194 and P575335).

Other points from the sheet: the FLK110 takes filter lengths of 7.4 and 10.43 inches, the FLK125 a single 10.85 inch length, and the FLK90 lengths of 4.21 and 8.23 inches. A closed end cap eliminates contamination of the clean side during service, and the filter tabs lock into place, which simplifies positioning during reassembly. Heads have one side machined and plugged for an indicator, and a robust four-point mount is standard with an optional two-point mount.

Selecting the right FLK size and configuration

Choose the filter by the real operating conditions of the circuit, not only by port size. Take the maximum flow through the filter position, including any return surges, and compare it with the maximum flow of each size. Then check the working pressure at that point of the circuit against the working pressure of FLK90, FLK110 and FLK125, because the three sizes differ. Pick the port type that matches the existing pipework, so that adapters are not needed in the line.

Decide whether the circuit needs a 50 psi bypass or no bypass. A bypass protects the cartridge and the system from excessive differential pressure when the element is loaded or the oil is cold, while a non-bypass configuration keeps all flow passing through the media and is used where component protection has priority. Select the micron grade from the cleanliness level required by the most sensitive component downstream, such as a hydrostatic pump or a pilot-operated valve, and consider whether a service indicator should be fitted to the head.

Installation in the hydraulic circuit

Install the head in the flow direction shown in the Donaldson documentation and support the pipework so that the head does not carry piping loads or vibration. Leave at least the minimum service clearance of 65 mm below the housing so the cartridge can be removed without disconnecting lines. Before installing a new filter, follow the sheet instruction to lubricate the thread and O-ring surface, then tighten the housing as described in the Donaldson instructions.

On new or rebuilt systems, flush the circuit before final commissioning and expect the first cartridge to load faster than later ones while built-in contamination is removed.

Service routine

Change the cartridge when the indicator shows that the element is loaded, or at the interval set by the machine maintenance plan if no indicator is fitted. Stop the system and relieve pressure first, use the drain port to empty the housing into a suitable container, and inspect the old cartridge for unusual debris that could point to wear elsewhere in the system. Fit only cartridges with the correct length and micron rating for the housing, and record the change in the machine log.

Hydraulic filtration in Oman’s climate

Mobile equipment, presses and hydraulic power units in Oman often work in high ambient temperatures with fine dust in the air. Hot oil ages faster, and dust drawn in through cylinder rods and reservoir breathers increases the load on the filter. Keep the area around the filter head clean before opening the housing, check oil temperature and cooler condition during summer, and consider oil analysis to confirm that the chosen micron grade is keeping the fluid clean enough for the components it protects.

Keeping spare cartridges on site

Hydraulic downtime often comes from a loaded element with no replacement nearby. List every FLK housing on the plant with its size, cartridge length and micron grade, and agree with the maintenance team how many spare cartridges should be held for planned changes. Store them sealed in their packaging, in a clean and dry place, until they are fitted.

Documents and quotation

FLK90/110/125 is quoted to order. Download Donaldson FLK90, FLK110 and FLK125 in-line cartridge filter data sheet (Hydraulic Filtration pp. 74-83) from this page, check the FLK size, port, bypass setting and micron grade, then send the quantity you need. More products: Filter Housings.