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Filters, Hydraulic - Description

Filters are necessary in hydraulic systems for filtering out contamination and debris. Contamination and debris in hydraulic systems come from many sources. Contamination includes metal flakes, glass, ash, lint, various fibers, rubber, sand, etc. Contamination is very small – generally in the 1 – 100 micron range (1 micron = 1.0E-06 meter). Over time contamination can impact component reliability and performance of components. The purpose of hydraulic filters is to remove contamination from a system and keep the fluid cleanliness within design tolerances.

During the formation of the hydraulic oil, the fluid will not be pure and contamination will be present. Therefore, when put into the system there will be some amount of contamination in the fluid. This type of contamination includes dust and ash plus small particulates/residues from processing equipment. Small amounts of water may also be present in the fluid. During storage and shipping, dust, paint, chips of various materials and possibly paint can be introduced into the fluid.

Once installed in a vehicle, the main sources of contamination is contamination that exists in components and tubes (from manufacture or introduced during installation) and wear that occurs during normal operation. During manufacture and assembly a common practice is to flush components to remove any debris arising during the manufacturing process. On airplanes, fluid lines can be flushed prior to connection of the hydraulic components. This is also a common practice. The greatest source of in-service contamination is the hydraulic pump. Pumps operate at high speeds in a severe environment and contamination (metal filings and seal pieces) from the rotating components occurs in every pump. This contamination makes its way into the system through the case drain line, which is why case drain fluid must go through it’s own filter or through the return filter. Should a pump lose inlet fluid the pump runs dry and contamination from the pump increases dramatically. Normally after a failure of this nature, all filters are replaced in a system. Other components with moving parts will also create contamination, but usually at a rate less than a pump. Another source of contamination is hydraulic ground power carts. Sometimes carts are not well maintained or the filter on the cart is not to the level required by the airplane. This is why ground service connections always flow directly to the filters before entering the system.

The overall fluid cleanliness for a hydraulic system is defined by a Class number. Class numbers run from 1 to 12. The Class number is determined by the number and size of particulates (contamination) in the fluid. Class number is determined through a laboratory analysis of a representative fluid sample from the system. The lower the Class number the cleaner the fluid. Aerospace vehicles are usually in the 7 to 9 class range. Table 1 shows the maximum amount of particles for a given particle diameter for Class 6, 7, & 8 systems. As shown in the table, Class 7 system will have ≤ 32,000 particles/100 mL in the 5 to 15 μm range, ≤ 5,700 particles/100 mL in the 15 to 25 μm range, ≤ 1012 particles/100 mL in the 25 to 50 μm range, ≤ 180 particles/100 mL in the 50 to 100 μm range, and ≤ 32 particles/100 mL in > 100 μm range. Going to Class 6 divides the number of particulates by a factor of 2. Going to Class 5 halves the allowable particulate count from the Class 6 level (or divides by 4 from the Class 7 levels). Similarly a Class 8 system is allowed twice as many particulates in each size range as a Class 7.


Particle Size

Class 6

Class 7

Class 8

5 to 15 μm

16,000 particles/100 mL

32,000 particles/100 mL

64,000 particles/100 mL

15 to 25 μm

2,850 particles/100 mL

5,700 particles/100 mL

11,400 particles/100 mL

25 to 50 μm

506 particles/100 mL

1012 particles/100 mL

2024 particles/100 mL

50 to 100 μm

90 particles/100 mL

180 particles/100 mL

360 particles/100 mL

> 100 μm

16 particles/100 mL

32 particles/100 mL

64 particles/100 mL


Table 1 Particle Size for Class Rating


A simple filter would consist of fine mesh screen or more appropriately, a number of fine mesh screens put in series so that the fluid has to flow though many meshes. Other methods to filter contaminates would be a membrane, woven wire cloth element, synthetic fiber, cellulose, micro-glass and metal edge element. A mesh filter is shown in Figure 1. In this example, the filter assembles into a container which threads into a filter manifold (not shown). A filter manifold is a machined housing containing pressure, return and case drain (if used) filters. Inlet and outlet ports are also included in the housing for connecting the appropriate pressure and return tubes. The advantage of the filter manifold is all filters are contained in one location for ease of inspection and maintenance.



Figure 1 Mesh Filter


A filter schematic is shown in Figure 2. This schematic shows a common filter arrangement and the flow paths through the filter. An additional feature is shown in Figure 2. This feature is a high Δp “pop up” switch or button. During normal operation – low Δp across the filter – the delta pressure across the switch is not sufficient to overcome the spring force and the button remains recessed. If the pressure drop across the filter increases to a predetermined level – high Δp across the filter – the delta pressure across the switch piston will overcome the spring and push the button up. This is illustrated in the enlarged portion in Figure 2. In lieu of a pop button, an electrical switch can be used. With an electrical switch the high Δp piston will actuate a switch which will provide indication to a ground service panel and/or flight compartment.



Figure 2 Hydraulic Filter Schematic with Δp “Pop Up” Indicator


The main considerations of a filter are filtration level, efficiency rating and flow versus pressure drop characteristic. Filtration capability is listed as a micron level. So a 5 micron filter will filter out particles with a diameter or width 5 microns or greater. Particles smaller than 5 microns will be able to flow through the filter. Filters can be rated as 100% efficient which means that all particles equal to or greater than the filter rating will be caught in the filter. If the rating is less than 100% than some particles in the filters range will make it through the filter. Efficiency ratings are sometimes given in terms of a beta rating. Beta ratings are defined in the following way: For a 5 micron filter, a beta rating of 200 means that for every 200 particles > 5 microns only 1 particle greater than 5 microns will make it through the filter (β = 200/1 = 200) and a beta rating of 2 means that that for every 200 particles > 5 microns 100 particles greater than 5 microns will make it through the filter (β=200/100=2). Thus, higher beta ratings imply a more efficient filter. Filters that are 100% efficient (i.e. β = ∞) are referred to as absolute filters.

Since filters must inherently restrict flow there will be a pressure drop (flow resistance) though the filter. As a general rule, as filter efficiency goes up (i.e., more and/or finer meshes) pressure drop will increase. Also, as filters get dirty flow resistance goes up. When evaluating a filter in the system the pressure drop versus flow rate characteristic should be evaluated. The characteristics should be evaluated at the most dirty condition (i.e., just prior to filter indication). Hydraulic filters are often manufactured as a filter manifold assembly. This means that more than one filter may be part of the filter manifold. Also, the manifold allows easy access to the filter to support periodic filter replacements. Old filters may be discarded. In some filters, it may be possible to send filters back to the manufacture for cleaning and recycling.

Filters normally have a indicator as part of the assembly (see Figure 2). The indicator will use a delta pressure device that senses Δp across the filter. When the Δp is sufficiently high the indicator piston will move to a position where a switch is closed or visual indication shows. The indicator alerts maintenance personnel that the filter needs replacement. Filter indications are often checked on a pre-flight walk around by flight crew and maintenance personnel. If a switch is used, the switch can be connected to crew compartment indication. In some bypass filter designs, an indication is provided when the bypass valve goes into bypass mode.

A filter may include a filter bypass provision, as shown in Figure 3. A filter bypass allows fluid to flow around (bypass) the filter when the Δp across the filter is high (i.e., filter is clogged). A bypass functions similar to a priority valve where the Δp overcomes a spring when the Δp is sufficiently high. A good practice is put bypass provisions on filters in the return line filter. Generally it is preferable to have dirty fluid flow to the pump rather than no flow. No flow to the pump will require the pump to be replaced and will put much more contamination into the system as the pump starts to run dry. Bypass provisions on pressure lines is generally not required nor preferred. If a bypass provision is provided in the pressure line filter then when the filter is clogged and goes into bypass contamination will make its way to all downstream components. This can affect performance of many components in a system and lead to (expensive) replacement of numerous components in a system. If a filter is used on a case drain line, a good practice would be to install a filter with a bypass provision. This will prevent excessive case drain back pressure and premature pump failure.



Figure 3 Hydraulic Filter with Bypass Valve


Another type of filter is shown in Figure 4. This filter is an inline filter and would be representative of a filter used to filter flow just upstream of a hydraulic component, such as a PCU.



Figure 4 Inline Filter


Filter requirements come from component cleanliness requirements. – can be put on main system filter or a separate filter can be put in the lines flowing to the component or internal to the component (as is often the case with PCUs). As discussed in System Design, Hydraulic Power Generation, a table should be built that lists the cleanliness requirements for each component. The worst case requirement should be used to select an appropriate filter.

Tighter filter requirements will lead to a cleaner system and a lower Class rating. However, there is no standard relationship between filter capability and Class rating. To determine Class rating for a given hydraulic system, a fluid sample will need to be taken from the system and sent to a certified lab for evaluation. Care should be taken to not introduce additional contamination into the fluid when gathering the sample. Statistical sampling methods are recommended due to variances in vehicle operation and lab methods. There will likely be a difference in Class rating based on the length of time the vehicle has been in service.


When selecting a filter, the following factors should be evaluated:

Maximum Capacity Rating – capacity rating is the maximum flow that can be accommodated by the filter at the rated pressure drop

Maximum Pressure Rating – pressure rating is the maximum pressure that will be seen by the filter in service. Return pressure spikes should be evaluated when determining the maximum pressure requirement. Proof and burst pressures will be based on the maximum pressure requirement.

Flow vs. Δp Characteristics – filter manufacturers can provide a flow vs. Δp curve for the filter. This curve should be evaluated to ensure that the filter will not restrict flow up to the maximum Δp (clogged filter) indication. If a simulation model for the return system has been constructed this curve should be used for the filter model (see also Filter, Hydraulic – Sizing).

Maximum Temperature – filter should be rated for the maximum combined fluid and environmental temperatures expected in service. This will normally be determined by the seal material used in the filter.

Minimum Particle Size Rating – what is the filter cleanliness capability in microns (e.g., 5 micron filter)?

Efficiency – what is the efficiency rating of the filter, in terms of a β rating or whether is absolute (100% efficient)?

Filter Element (Cartridge) Type – will filter be wire mesh, membrane, woven wire cloth element, synthetic fiber, cellulose, micro-glass or metal edge element? What are specific maintainability issues associated with available filter types? Is the filter compatible with your fluid? What is the expected life of the filter? The pros and cons of available choices in your application should be reviewed with the filter manufacturer.

Reusable Element – is the selected filter element reusable after cleaning or is the filter disposable? If reusable, what is the method for cleaning?

Container (Housing) – what is the material used for the housing? Is the material corrosion resistance?

Clogged Filter Indication – will a high Δp (clogged filter) indicator be used? If so, what are the design characteristics? What type of indication is provided – manual pop up button or electrical switch?

Bypass Provision – will the filter contain a bypass provision? If so, what are the design features of the bypass valve? Does the bypass valve provide indication independent of a clogged filter indicator?

Seal Type & Material – what type of seal is used in the filter? Is the seal material compatible with the fluid and operating temperatures of the fluid?

Fittings – the inlet and outlet tubing fittings need to be compatible with the fittings on the filter. Normally, these are threaded fittings.

Mounting – the method of mounting or supporting the filter within the system should be robust and not allow any loads to be imparted to the connecting tubes.


Filter Qualification

See Qualification - Hydraulic Components for discussion on hydraulic filter qualification and required certification testing. Temperature, proof and burst pressure, vibration, impulse pressure and endurance testing would be important.