This article is the third 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
The filtration principles of pneumatic and hydraulic systems were given in the previous articles. The third part of the three-part series of articles on filtration principles in fluid power systems attempts to make a comparison of the filtration principles, as given in Table 1. A comparative study of the filtration principles would bring out the essential similarities as well as differences in the filtration principles and would provide an overall view of the subject matter.
Table 1 | A comparison of the filtration principles in pneumatic and hydraulic systems
Parameter
Pneumatics
Hydraulics
Medium of energy transfer
Compressed air
Oil
Contaminants
Particles, moisture, oil, +
Particles, chemical contaminants, moisture, air, +
Effect of contaminants
Abrade surfaces Corrode internal parts Reduce functionality and service life of components
Abrade surfaces Corrode internal parts Reduce service life of oil
Cleanliness standards
ISO8573
ISO 11171 & ISO4406 NAS1638 SAE AS4059
Particle removal
Filters
Filters, Magnets
Moisture removal
Dryers, Filters
Vacuum dehydrator
Oil (air) removal
Coalescing filters Adsorption filters
Air bleeds Diffuser (Baffle screen)
Filter types
General purpose filters Coalescing filters Adsorption filters
Pneumatic and 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 books deal with pneumatic system components and circuits. The fundamentals required to understand the core topics are given initially. These books describe the topics on compressed air generation and contamination control, pneumatic actuators, and control valves, in detail. Further, these books present the maintenance, troubleshooting, and safety aspects of pneumatic systems. Many single-actuator pneumatic circuits are given in multiple positions. Many critical positions of pneumatic single-actuator circuits are given to make the reader understand the control circuits easily.
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 explain the basic principles of hydrostatic transmissions. The concepts of open-circuit and closed-circuit HSTs are explained in the book. The topics also include configurations, advantages and disadvantages, and applications of HSTs.
These books enlighten the details of components required for load sensing systems. The operation of load sensing systems in their various operating modes is described in a simplified manner.
The book ‘Electro-hydraulic proportional valves’ explores the technology used in proportional valves. The book also describes the construction of electro-hydraulic proportional valve systems, the details of various types of control elements, characteristics, and applications of proportional valve systems.
The book ‘Electro-hydraulic servo valves’ describes the technology used in state-of-the-art servo valves. The book also describes the construction of electro-hydraulic servo valve systems, the details of various types of control elements, and the applications and characteristics of servo valve systems.
These books explain the functioning of solenoid valves and various electrical control components such as pushbuttons, electro-magnetic relays, limit switches, reed switches, proximity sensors, electronic timers, and counters. The development of many typical single-actuator and multiple-actuator electro-pneumatic and electro-hydraulic circuits are also separately explained in these books. Many circuits are given in multiple positions for a quick understanding of the working of each circuit.
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.
Contamination
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.
Magnet
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.
Strainer
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.
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.
The cleanliness of hydraulic fluids needs to be monitored for maintaining the components of hydraulic systems at a satisfactory level. 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. All standards specify the contamination level in counts per fluid volume and provide easy methods for converting the particle counts into limits that are simple to interpret. Here is an overview of these standards.
The cleanliness classes are based on particle size [differential (e.g. 5 – 15 µm) or cumulative (e.g.>6 µm)], number, and distribution. Before proceeding further, the knowledge of the following terms and definitions would be of great help in understanding various cleanliness standards.
Methods of particle counting
There are two basic methods of counting particles in hydraulic fluids.
By using an optical microscope where the longest dimension sizes the particles. [See Figure 1(a)]
By using automatic particle counters (APCs) calibrated as per ISO 11171, where the particles are sized by the area rather than the length. [See Figure 1(b)]
Figure 1 | Sizing of particles
Particle size analysis
Several methods and instruments, based on different physical principles, are used to determine the size distribution of the particles suspended in a given sample of hydraulic fluid. The numbers of particles found in the different size ranges characterize this distribution. A particle to be analyzed can be sized by: (1) the longest dimension, as sized by a microscope, or (2) the equivalent projected area, as sized by APC calibrated as per ISO 11171.
The ISO System
In ISO 4406: 1999, particle counts are determined cumulatively, for particle sizes > 4 μm(c), > 6 μm(c), and > 14 μm(c) as per the size classification standard ISO 11171, using particle counters and allocated to measurement codes. In 1999, the previous version of the standard, i.e., ISO 4406:1987, was revised and the size ranges of the particle sizes redefined. The contamination code rating system as per ISO 4406: 1999 is given in Table 1.
The NAS System
The NAS 1638 was originally developed in 1964 to define classes for the contamination present within aircraft components. The application of this standard was extended to industrial hydraulic systems as no other standards existed at the time.
In the NAS 1638 classification, the code number refers to a maximum quantity of particles within a specific size class. The NAS system divides particles into five particle size ranges. A series of 14 classes were specified, covering very clean to very dirty levels. The method of counting the particles referenced the optical microscope method. The NAS code specifies a single code number based on the highest particle count in any of the size ranges. As the code number goes high, the degree of contamination increases for any size range. This standard is now considered obsolete. However, it is still widely used in old systems. The cleanliness codes as per NAS 1638 is given in Table 4.
The SAE System
The SAE aerospace standard AS4059 was developed in 1988 as a replacement/equivalent to the obsolete NAS 1638 format. Since then this standard has undergone six revisions and is now at issue ‘F’. This standard specifies contamination classes and levels of particulate contamination in hydraulic fluids. This standard offers two classifications. One classification, based on the microscopic counting, applies to those currently using NAS 1638 classes and desiring to maintain the methods/format. The other alternative, based on the automatic particle counting, applies to those using the methods of previous revisions of AS4059 and/or cumulative particle counts. The cleanliness classes for differential particle counts are given in Table 2, and the cleanliness classes for cumulative particle counts are given in Table 3.
Meaning of Index (c)
Particle size specifications usually contain the index(c), [Example: 4 μm(c)]. This notation is used to indicate that the calibration material used is certified and traceable to a national standard.
A Comparative Study
A comparative study of these standards in terms of the methods of particle counting, particle size classification, contamination concentration levels, and the applicability of these standards is undertaken in the following sections.
Method of Particle Counting
ISO 4406: 1999 uses the electron microscope counting method
SAE AS 4059 Rev F uses the optical counting method or electron microscope counting method
NAS 1638 uses the optical counting method
Particle Size Classification
ISO 11171 specifies the following three-dimensional cumulative sizes of particles:
>4 µm(c)
>6 µm(c)
>14 µm(c)
SAE AS 4059 specifies the following size ranges of particles for the optical counting method:
6 -14 μm(c)
14 -21 μm(c)
21 -38 μm(c)
38 -70 μm(c)
>70 μm(c)
SAE AS 4059 specifies the following cumulative sizes of particles for the automatic particle counting method:
> 4 μm(c) (Code A)
> 6 μm(c) (Code B)
> 14 μm(c) (Code C)
> 21 μm(c) (Code D)
> 38 μm(c) (Code E)
> 70 μm(c) (Code F)
NAS 1638 specifies the following differential sizes of particles:
5 – 15 µm
15 – 25 µm
25 – 50 µm
50 – 100 µm
> 100 µm
Contamination Concentration Levels
In ISO 4406: 1999 specifies the cleanness 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.
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.
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.
Applicability of the Standards
In ISO 4406: 1999 standard is widely used throughout the world for determining the hydraulic fluid cleanliness.
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 NAS 1638.
AS4059 class using cumulative particle counts applies to those using the methods of previous revisions of AS4059 and/or cumulative particle counts
The NAS 1638 cleanliness standard was developed for aerospace components in the US 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.
For information on a complete range of Paperback & Kindle eBooks on Pneumatics and Hydraulics, authored by Joji Parambath, please visit: https://jojibooks.com
Particle Measurement Technology in Practice. From Theory to Application, HYDAC Filtertechnik GmbH, Industriegebiet 66280 Sulzbach / Saar, Germany, www.hydac.com
Swift-JB International, LLC is a division of Swift Filters, Inc
Note: The Tables of cleanliness standards are not included here. The complete article with the Tables can be downloaded by clicking the following link:
