Filtration Principles in Hydraulic Systems

This article is the second part of the following three-part series of articles:

Filtration and Drying Principles in Pneumatic Systems

Filtration Principles in Hydraulic Systems

Filtration Principles of Pneumatic and Hydraulic Systems: A Comparison Study

A hydraulic power system transmits the power in a controlled manner using an incompressible oil medium. Remember, the fluid medium links all the components in the system and is regarded as a critical element in the system. A fluid is prepared from a base stock and additives. Some examples of base stocks are petroleum oils, high-water-based fluids (HWBF), synthetic fluids, and vegetable oils. And some examples of additives are viscosity index (VI) improver, anti-wear additive, oxidation inhibitor, and corrosion inhibitor. Many types of fluids can be formulated by adding a base stock with varieties of additives to meet the exacting requirements of complex hydraulic systems.


Fluids are susceptible to various types of contamination. They are exposed to the following types of contamination: (1) Particulates (dust, dirt, sand, rust, fibres, elastomers, paint chips), (2) Wear metals, silicon, and excessive additives (aluminium, chromium, copper, iron, lead, tin, silicon, sodium, zinc, barium), (3) Water, (4) Sludge, oxidation, and corrosion products, (5) Acids and other chemicals, (6) Sealants (Teflon tape, pastes), and (7) Biological, microbes (in high water-based fluids).

Silt particles (<5 μm) of size corresponding to the typical tolerance in hydraulic components are most dangerous than larger chip particles (>5 μm). Chemical contaminants are formed by the breakdown of additives, due to chemical reactions. The reaction products generate acids and oxidants in the presence of water and heat. They can cause physical and chemical changes in the additive elements. These changes can lead to the deterioration of additives and subsequent fluid breakdown.

Effects of Contamination

Contaminants are the natural enemy of hydraulic components and systems. 70 to 80% of the hydraulic system failures are due to the adverse effects of contaminants, like surface degradation. Even minute particles can damage today’s hydraulic system components due to the existence of minuscule clearances in them. Excessive water contamination is liable to accelerate the ageing process of fluids.

Removal of Contaminants

Particles can be removed by installing correctly sized filters at appropriate locations. The removal of acids, sludge, gums, varnishes, and other oxidation products requires the use of adsorbent filters with active type clay, charcoal, or activated alumina. Magnets can be installed to remove ferrous particles and rust matters. A water-removal filter or a vacuum dehydrator can remove water.

Hydraulic Fluid Cleanliness Standards

Many national and international organizations such as ISO, SAE, National Aerospace Standards (NAS), etc., have developed standards for specifying the particle size classification and contamination concentration levels in hydraulic fluids. The important standards are ISO 4406, NAS 1638, and SAE AS 4059.  The cleanliness classes are based on particle sizes, number, and distribution. All standards specify the contamination level in counts per volume and provide easy methods for converting the particle counts into limits that are simple to interpret.

Applicability of the Standards

The ISO 4406: 1999 standard is widely used throughout the world for determining hydraulic fluid cleanliness. The NAS 1638 cleanliness standard was developed for aerospace components in the US in 1964 and is still widely used for industrial and aerospace fluid power applications. It may be noted that NAS 1638 has now been made inactive for new designs. AS4059 class using differential particle count method applies to those currently using NAS 1638 classes and desiring to maintain the methods/format and results equivalent to those specified in the NAS standard. AS4059 class using cumulative particle counts applies to those using the methods of previous revisions of AS4059 and/or cumulative particle counts.

Methods of Particle Counting

ISO 4406 uses the electron microscope counting method. NAS 1638 uses the optical counting method. SAE AS 4059 uses the optical counting method or electron microscope counting method.  

Particle Size Classifications

ISO 11171 specifies the following cumulative sizes of particles: >4, >6, and >14 µm.

The NAS 1683 system divides particles into five particle size ranges: 5–15, 15–25, 25–50, 50–100, >100 µm.

SAE AS 4059 specifies the following size ranges of particles for the optical counting method: 6-14, 14-21, 21-38, 38-70, and >70 μm. SAE AS 4059 specifies the following cumulative sizes of particles for the automatic particle counting method: >4 (Code A), >6 (Code B), >14 (Code C), >21 (Code D), >38 (Code E), >70 μm (Code F).

Cleanliness levels

ISO 4406 specifies the cleanliness level of a given sample of fluid by a three-number range code representation, based on the cumulative numbers of particles of sizes greater than 4, 6, and 14 microns respectively, present in one millilitre of the fluid.

NAS 1638 specifies the cleanness level of a given sample of fluid by a single figure (from 0 to 12) representing the maximum allowed differential particle counts (i.e. worst case), present in 100 ml of the fluid, for the designated particle size ranges.

SAE AS 4059 specifies the cleanness level of a given sample of fluid by a single figure representing the maximum allowed cumulative particle counts (i.e. worst case), present in 100 ml of the fluid, for the designated particle sizes according to the particle counting method.

Cleanliness Level Targets

Equipment manufacturers, fluid suppliers, and fluid power associations have established target fluid cleanliness levels applicable for the general types of hydraulic components.

Filters, Hydraulic System

An efficient filtration system should be an integral part of every hydraulic system. Filters remove particulate contamination. When fluid flows through the media, it traps contaminants and at the same time allows the fluid to flow through it easily. A filter mainly consists of the following: (1) Filter Element, (2) Filter bowl, (3) Filter head, (4) Clogging Indicator, and (5) Bypass Valve.

Filter Head

A filter head consists of ports for the inlet and outlet, and visual or electrical indicators. It is made of cast Aluminium as a standard material or ductile iron for high-pressure applications.

Filter Housing

A filter housing encloses the filter element. Housing styles are categorised as: removable housing/cartridge unit, spin-on, in-tank, and in-line. It is usually made of ductile iron or stainless steel.

Filter Element

A filter element is usually made of steel wire screen, cellulose media, or synthetic glass fibre media. It consists of millions of tiny pores of micron sizes. A piece of filter media is pleated and assembled in a canister as disposable elements.

Steel Wire Media

Wire-mesh media are made of epoxy-coated stainless steel. The filter captures contaminants in a fluid stream on one side of the wire screen, which faces the fluid flow (surface filtration). This type of filter element is used to make coarse filters, usually known as strainers. Typically wire-mesh filters are used to catch very large, harsh particulate matter that could rip up a normal filter. Wire-mesh media are available in 3 mesh sizes: (1) 100 mesh yields 150 µm filtration, (2) 200 mesh yields 74 µm filtration, and (3) 325 mesh yields 44 µm filtration.

Cellulose Media

Cellulose Media are made from plant fibres and are held together by resins. The pores are microscopic. The thick-walled media absorbs contaminants throughout the depth of the material as the fluid flows through the media (depth media).

Glass Fibre Synthetic Media

Synthetic Media are man-made, consistent, and rounded off to provide the least flow resistance. They are made of inorganic micro-fine glass fibres. They are randomly laid into a multi-layered web with tapered pore geometry (larger pores on the upstream surface and finer and finer pores towards the downstream side). The thick-walled glass fibre media captures contaminants throughout the depth of the media.

By-pass Valve Setting

Bypass valves have cracking pressures typically in the range between 0.1 bar (2 psid) and 7 bar (100 psid). Suction and return-line filters have a lower setting [max. 1 bar (14.5 psid)] than that of the pressure-line filters [max. 7 bar (100 psid)].

Service Indicators

The filter can be provided with a pressure gauge, visual indicator, and/or electrical indicator to point out the need for the replacement of its filter element.


A magnet can be incorporated to attract and hold ferromagnetic particles down to even smaller than 1 micron.

Installation Locations, Filters

Based on the installation locations, hydraulic filters can be classified as: (1) Strainers, (2) Suction filters, (3) Pressure filters, (4) Return-line filters, and (5) Offline filters.


It is installed at the pump suction side. It is a coarse filter, made of a piece of wire mesh, typically having a mesh width of ≥149 μ.

Suction filter

The suction filter is connected to the pump suction side. It is a coarse filter typically having a mesh width in the range from 5 to 149 μ. It is usually mounted outside of the reservoir in a service-friendly manner. It protects the pump from coarse particles, economically.

Pressure filter

It is installed downstream of the pump. It can also be smaller and finer (10 – 20 μ). The main function is to keep the fluid that comes directly from the pump clean. It serves to protect expensive and dirt-sensitive downstream components.

Return-line filter

It is installed in the return-line. The purpose is to trap dirt from the system working components, as well as particles entering the system through the worn piston-rod seals in the system.

Off-line filtration

It consists of a separate pump, filter unit, hoses, and quick-disconnect couplers. The components can be arranged on a mobile cart and retrofitted to an existing system temporarily or integrated into the hydraulic system permanently. In this system, fluid is pumped out of the reservoir, passed through the filter, and allowed to return to the reservoir continuously.

Air Breather

Air breathers provide fast-acting protection against airborne moisture and particulate contamination. They stop solid particulate down to 3 µm at 97% efficiency and prevent moisture from entering the reservoir.

Joji Parambath

Author, Fluid Power Educational Series Books

Hydraulic Books authored by Joji Parambath

37 books in Paperback and Kindle eBook versions on the subjects of Pneumatics and Hydraulics, authored by Joji Parambath, have been published under Fluid Power Educational Series. Joji Parambath is a trainer in the field of Pneumatics, Hydraulics, and PLC, for over 25 years. All the books are available at Amazon marketplaces.

These textbooks deal with the components and circuits of hydraulic systems. The fundamentals required to understand the core topics are given initially. The book describes the topics on hydraulic fluids, filters, power packs (reservoirs, pumps, pressure relief valves) hydraulic actuators, directional control valves, flow control valves, pressure control valves, fluid conductors, and accumulators, in detail. Further, the book presents the maintenance, troubleshooting, and safety aspects of hydraulic systems.

These books separately describe the design aspects of hydraulic systems in the SI system units and the English system units for educational purpose. These books highlight the essential parameters, mathematical relations, and specifications of many hydraulic components such as hydraulic pumps, reservoirs, pressure relief valves, filters, fluids, hydraulic cylinders, hydraulic motors, control valves, accumulators, and fluid conductors. Examples of designing typical industrial hydraulic systems are also given in this book. Patient learners can extract many design concepts from any of these invaluable books.

The book on hydraulic fluids explains, in detail, the functions, types, characteristics, and selection of hydraulic fluids. The subsequent sections present topics on fluid contamination, the effect of contamination on fluids, fluid analysis, fluid quality standards, and the maintenance aspects of fluids.

This book on filters presents the principles of filtration in hydraulic systems. These principles include the materials of filter media, various designs of filters, and the typical locations of filters in hydraulic systems. Further, this book describes the filter element performance ratings, such as the micron ratings, beta ratio, and filter efficiency, and the multi-pass test to determine such ratings.

These books take up a detailed discussion of hydraulic power packs and their constituent parts including reservoirs, pumps and pressure relief valves. These books also give a brief note on the topic of heat dissipation and sound reduction techniques in hydraulic systems.

These books bring out the essential technical information related to hydraulic cylinders, in a simple and easy to understand manner. The topics include the principal parts and body styles, position transducers, piston-rod buckling, classification and types, side loads, installation and mounting, advantages, applications, standards, maintenance and safety, and design of hydraulic cylinders.

These books bring out the fundamentals and other most essential technical information related to hydraulic motors. The topics include basic hydraulic motor working, terms and definitions, constructional features, side loads, classification, comparison, performance characteristics, applications, and maintenance of hydraulic motors.

These books bring out the essential technical information related to hydraulic accumulators extracted, especially from the material available from the manufacturer’s domain. The topics include functions, classification, constructional features, comparison, pre-charging, safety requirements, applications, maintenance, and accumulator sizing. Many hydraulic circuits with accumulators are also presented.

These books present information about the constructional features, performance specifications of pipes, tubes, and hoses and their fittings. The topics include the terms & definitions and design of hydraulic conductor systems, and installation, routing, and maintenance of fluid conductors.

And many more ….

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