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An in-depth overview about filters

• Hydraulic Filters are a very useful and vital device that are used in hydraulic systems.

• The main purpose is to keep contaminants at bay.

• The main issue being that contamination is a major cause of failure in hydraulic systems.

• Such failures cause major downtime which in turn require fluid or component replacement; at time it may even mean high maintenance charges.

• Thus installing hydraulic filters will result in good protections against contamination.

• This will help make sure the hydraulic system as well as the components are safe and work in an reliable manner.

Introduction:

• A hydraulic systems filter consists of two basic units; the housing and the elements which contain the filtration medium.

• Each of these have special features to suit the particular applications in terms of pressure rating, duty, fluid filtered and location in the circuit.

• The user has a choice of which filter assembly to use since this is governed by the requirements of the system in which the filter is to be fitted as well as the level of reliability required.

• The design of the filtration media is of great importance because since the efficiency of the filter removing solid contamination from the fluid counts to a great extent.

Filter Location:

• Where the filter is placed in the hydraulic circuit depends largely on why you are including the filter.

• The fact of the matter is that there are many different sources of contamination. To achieve maximum protection it is necessary to place a filter directly in the upstream of the component.

• This filter should have the correct size and correct rating for it to be very efficient though.

• That said if the sole function of your filter is to control the level of contamination in the circuit it can be placed in any of the major flow lines.

Filter Types:

The filter design is dictated by its location in the circuit; thus there are many different types of filters that can fulfil a wide range of requirements as mentioned here:

Pump Inlet Filter:

• A filter can be placed near the inlet port near a pump. This serves the purpose of protecting the pump in the cases where if may be deemed necessary.

• There filters are unique, in the sense that pump inlet conditions need to be maintained over a wide operating range to avoid capitations damage to the pump as well as loss in output flow.

Pressure line filters:

• The pressure line filters function as wither protection or control filters.

• They need to be designed such that they are capable of withstanding the full system pressure and endure the fatigue producing conditions and high transient pressures inherent during system operation.

• The pressure drop across the assembly is generally not as critical as in other locations, although a high pressure drop will mean increased energy loss.

Return Line Filters:

• The return line filers are usually lower pressure units and can be line or reservoir mounted.

• The lower pressure requirement results in reduced housing costs and provides a cost effective solution where general control over contaminants is required.

• Such filters will successfully control the flow of contaminants generate in the components upstream and not contaminants into the reservoir.

Off-line Filters:

• With off-line filters the circuits is slightly different.

• The fluid is pumped through a filter before being returned directly before being returned directly to the reservoir.

• This type of filtration does not provide direct protection to the components in the system hence should only be used to supplement an existing one.

There a several advantages of using these filters

• They can be left running even when the system is shut down.

• They will continue to filter the system even when variable volume pumps are off load.

• It is possible to change the elements without disturbing the main system.

• They can also be used for changing fluid into the system.

Reservoir Breather Filters:

• A breather filter is essential to prevent the contaminants from being ingested into the system during the operation as a result of the environment.

• Thus small paper filters are commonly seen fitted into integral filter (breather units). However these filters may not be adequate at all times.

• The reason being that they are too course to match the system filtration requirements and their size is such that they block up very easily and restrict the passage of air thus damaging the system.

Filter Assembly:

• The filter housing in general comprises of a head to connect the filter to the system pipe-work and a bowl for containing the filter element.

• The bowl can be made integral with the element and is screwed directly to the head on replacement. Furthermore the filter body needs to be designed such that it limits the flow restrictions.

• It should positively retain the filter element to avoid the leakage of unfiltered fluid. They should be designed to facilitate easy removal of the element.

• The deal with high pressure filters is different because the housing needs to be designed such that it can withstand frequent changes in pressure and high transient pressure.

• Features like flow value options and differential pressure indicators are usually included in with the filter head.

• Bypass valve may be provided to limit the differential pressure across the element to avoid the possibility of damage to the filter medium.

• The blocking of the element with retained contaminants leads to the breakdown of the media thus causing the differential pressure to increase.

• Thus the design of the valve is quite critical as well to the elements as well as the system concerned.

• One of the more important features of these valves is the necessity to direct the flow away from the element to avoid disturbing and en training contaminants embedded on the element and transferring it into the system.

Element Design:

• The elements usually used in fluid power are usually of the cartridge type where the flow is essentially from outside to in.

• The flow direction enables the use of a small though strong support core.

• These elements are manufactured for in-to-out flow directions though the filter medium still requires supporting and to achieve the same burst rating.

• The element consist of a filtration medium. The basic types of medium are fibrous, woven, sintered and membrane.

• Hydraulic systems often make use of membrane filters. They are of the surface acting type with a very narrow pore size distribution, down to sub-micronic.

• The material is usually PVC, PTFE and nylon.

Additionally the filtration medium needs to be adequately supported on the upstream and downstream surfaces so that if can endure the fatigue stresses caused by cyclic flow conditions in the circuit. Unsupported filtration medium is usually subjected to compression under high differential pressure conditions.

Filtration medium Design:

• The filtration medium design is very important and cannot be stressed enough.

• The ability of the medium to consistently perform depends on whether the design criteria are met or not.

• The filtration medium design can be either be of low permeability, fixed pore size, adequate bonding of fibres or medium thickness.

Other Filter Types:

• The fibrous fixed pore element is solely used when the protection of components and control of contamination is required.

• The other forms of filtration devices available are magnetic filters, centrifuges, electrostatic filters.

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