ISO/TR 15640:2011

Hydraulic fluid power contamination control - General principles and guidelines for selection and application of hydraulic filters

ISO/TR 15640:2011 is applicable to contamination control principles for hydraulic fluid power systems and includes guidelines for the selection and application of hydraulic filters. Although control of non-particulate contamination, e.g. air, water and chemicals, is important, and is briefly discussed, the primary focus of ISO/TR 15640:2011 is the control of particulate contamination and the selection and application of filters for that function.

Vérification de la contamination des transmissions hydrauliques — Principes généraux et lignes directrices pour l'application et la sélection des filtres hydrauliques

General Information

• Status: Published
• Publication Date: 13-Dec-2011
• ICS: 23.100.60 - Filters, seals and contamination of fluids
• Technical Committee: ISO/TC 131/SC 6 - Contamination control
• Drafting Committee: ISO/TC 131/SC 6 - Contamination control
• Current Stage: 9093 - International Standard confirmed
• Start Date: 15-Nov-2018
• Completion Date: 13-Dec-2025

Overview

ISO/TR 15640:2011 - Hydraulic fluid power contamination control - provides general principles and practical guidelines for controlling contamination in hydraulic fluid power systems, with a primary focus on particulate contamination and the selection and application of hydraulic filters. Published as a Technical Report (informative guidance) in 2011, the document supports cleanliness management across system design, commissioning, operation and maintenance. Non‑particulate issues (air, water, chemicals) are acknowledged but treated briefly.

Key topics

• Contamination types and sources: built‑in, ingressed, generated and maintenance‑induced contaminants; solid, liquid and gaseous contaminants.
• Effects of particulate contamination: component wear, failures and reduced system reliability (note: typical correctly filtered systems produce mostly particles <15 µm).
• Cleanliness evaluation and monitoring: particle size ranges of interest, measurement and monitoring methods for hydraulic fluids.
• Coding systems for cleanliness: introduction to ISO 4406 and other coding schemes (NAS 1638, SAE AS4059, ISO 11218) for expressing particulate levels.
• Setting Required Cleanliness Levels (RCLs): factors and process for defining RCLs for systems and components.
• Cleanliness management: design choices, monitoring strategies and maintenance practices to manage contamination.
• Filters and filtration mechanisms: filter media, types of filters and elements, accessories, filter location, sizing and installation considerations.
• Filter evaluation and selection: laboratory test methods, candidate assessment, verification of correct filter choice.

Practical applications

• Use ISO/TR 15640:2011 to establish or verify filtration strategies, choose appropriate hydraulic filters, and define RCLs for new or existing systems.
• Apply the guidance when specifying filters for mobile equipment, industrial hydraulic systems, off‑highway machinery, and industrial automation to improve reliability, reduce downtime and extend component life.
• Integrate the document into maintenance procedures for clean fluid handling, filter change practices and cleanliness monitoring programs.

Who should use it

• Hydraulic system designers and OEMs
• Maintenance and reliability engineers
• Filtration product engineers and suppliers
• Fluid power consultants and commissioning teams

Related standards

• ISO 5598 (vocabulary) - normative reference within the report
• Cleanliness coding and test methods referenced: ISO 4406, NAS 1638, SAE AS4059, ISO 11218

ISO/TR 15640:2011 is a practical, non‑prescriptive resource for engineers and technicians aiming to implement effective contamination control, select suitable hydraulic filters, and manage fluid cleanliness for improved system performance.

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 15640
First edition
2011-12-15
Hydraulic fluid power contamination
control — General principles and
guidelines for selection and application
of hydraulic filters
Vérification de la contamination des transmissions hydrauliques —
Principes généraux et lignes directrices pour l’application et la sélection
des filtres hydrauliques
Reference number
©
ISO 2011
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2011 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Types and sources of contamination . 1
4.1 General . 1
4.2 Solid contaminants . 2
4.3 Liquid contaminants . 3
4.4 Gaseous contaminants . 3
5 Effects of particulate contamination and the benefits of its removal . 3
5.1 General . 3
5.2 Failures caused by particulate contamination . 4
5.3 Benefits of filtration to reduce solid particulate contamination . 4
6 Evaluation of cleanliness . 4
6.1 General . 4
6.2 Particle size range of interest . 5
6.3 Methods of measuring and monitoring solid particulate contaminants . 5
7 Coding systems for expressing level of solid particulate contamination . 6
7.1 General . 6
7.2 ISO 4406 coding system . 6
7.3 NAS 1638, SAE AS4059 and ISO 11218 coding systems . 7
8 Setting required cleanliness levels (RCLs) for a hydraulic system . 7
9 Cleanliness management concepts . 9
9.1 System design considerations . 9
9.2 Monitoring system cleanliness . 9
9.3 System maintenance for cleanliness management .10
10 Filters . 11
10.1 Mechanisms of filtration . 11
10.2 General filter concepts .12
10.3 Types of filters and filter elements .14
10.4 Filter accessories .15
11 Filter evaluation .16
11.1 General .16
11.2 Laboratory filter test methods .16
12 Filter selection process .18
12.1 General .18
12.2 System definition and setting of the RCL .18
12.3 Selecting the minimum recommended filter rating .19
12.4 Filter location .20
12.5 Filter sizing .23
12.6 Assessment of candidate filters .24
12.7 Verification of correct filter selection .24
13 Summary .25
Annex A (informative) Types of filters and separators .26
Bibliography .28
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no longer
valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 15640 was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 6,
Contamination control.
iv © ISO 2011 – All rights reserved

Introduction
Hydraulic systems transmit power by means of a pressurized liquid in a closed circuit. Foreign materials or
contaminants present in the fluid can circulate around the system, cause damage to the component surfaces,
and reduce the efficiency, reliability and useful life of the system. Hydraulic filters are provided to control the
number of particles circulating within the system to a level that is commensurate with the degree of sensitivity
of the components to the contaminant, and the reliability and durability objectives of the hydraulic system.
The selection and application of filters takes into account the filter design and performance, the system design
and function, the required cleanliness level (RCL), the severity of the system operation and the standard of
maintenance. The only way to confirm whether the correct filter has been selected is to monitor the cleanliness
level in the fluid, and the reliability and durability of the system.
These guidelines are intended to introduce the concepts of cleanliness management and filter selection and
application to both system designers and users. Although this guide cannot make one an expert on filter
selection and use, it does seek to educate and thereby assist the reader in making informed decisions about
filtration, and to improve the communication process.
TECHNICAL REPORT ISO/TR 15640:2011(E)
Hydraulic fluid power contamination control — General
principles and guidelines for selection and application
of hydraulic filters
1 Scope
This Technical Report is applicable to contamination control principles for hydraulic fluid power systems and
includes guidelines for the selection and application of hydraulic filters. Although control of non-particulate
contamination, e.g. air, water and chemicals, is important, and is briefly discussed, the primary focus of this
Technical Report is the control of particulate contamination and the selection and application of filters for that
function.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 5598, Fluid power systems and components — Vocabulary
NOTE The other documents mentioned and referenced in this document in a non-normative way are listed in the
Bibliography.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5598 and the following apply.
3.1
contaminant
any material or combination of materials (solid, liquid or gaseous) that can adversely affect the system
3.2
ingression
introduction of environmental contamination into the system
NOTE Contamination introduced through ingression is referred to as ingressed contamination.
3.3
filter medium
part of the filter structure that removes and retains contaminant
3.4
filter media
collective layers that make up a filter element
4 Types and sources of contamination
4.1 General
Contaminants in a hydraulic fluid are any material or combination of materials (solid, liquid or gaseous) that can
adversely affect the system.
4.2 Solid contaminants
4.2.1 General
Solid contaminant particles come from four main sources as shown in Table 1 and can vary considerably in
material, hardness, shape and size from sub-micrometre to millimetres.
Contaminant shape varies widely and debris can appear as granular (cube-shaped), acicular (rod-shaped),
platelets (very thin, nearly two dimensional), irregular fragments and fibres. Shape affects the way that particles
are aligned in the moving fluid and thus the likelihood of the particles becoming lodged in a small clearance
or trapped within the filter medium. Although quite important, particle shape is rarely reported because of the
difficulties involved in its determination.
Table 1 — Primary sources of particulate contamination
Built-in Ingressed Generated
Maintenance
(manufacturing
(service debris)
Process Atmosphere Surfaces Fluid
debris)
– burrs – initial fluid fill – ingestion – mechanical – re- – repairs
via reservoir wear entrainment
– machining – addition of
– preventive
breather
swarf incorrect fluid – corrosive – filter maintenance
– ingestion via wear desorption
– weld spatter – compressed – new filter
seals
air or gas – cavitation – additive
– abrasives – new fluid
– reservoir precipitation
– pulp – exfoliation
opening
– drill turnings – dirty hose,
– sludge
– pulverized
– hose connector,
– filings – rock dust
coal materials – insoluble components
– mill scale oxides
– dust
– ore dust – filter fibres – top-up
– contaminated – quarry dust – carbonisation containers
– aggregates – break-in
components
– foundry dust debris – coke – incorrect fluid
– cement
– dust from
– slag particles – elastomers – aeration – cleaning rags
grinding – catalysts
– dust from – varnishes – dust from
– incompatible – clays
welding and welding and
fluids
– process grinding grinding
– paint chips chemicals
– dust from
atmosphere
and
workplace
4.2.2 Built-in contaminant
All new systems contain some contaminant left during manufacture and assembly. This can consist of fibres
(from rags, etc.), casting sand, pipe scale, cast iron or other metal particles, jointing material or loose paint.
When a system is operated at an unusual load or if there are high pulsations in the flow, it is likely that built-in
contaminant becomes dislodged.
4.2.3 Ingressed contaminant
Systems can also be contaminated during normal operation, through openings in the reservoir, inadequate air
breather filters, through worn seals in vacuum conditions and by intrusion through the fluid film on piston rods.
Worn seals increase the likelihood of ingression. These ingressed contaminants can be highly abrasive.
4.2.4 Generated contaminant
When a normal system has been run for a reasonable period of time, a quantity of solid contaminant can be
present in the form of small metallic platelets, created by the normal wear process. For correctly designed
2 © ISO 2011 – All rights reserved

systems, which are provided with suitable filtration, the majority of these particles are smaller than 15 µm. If
a filter blockage indicator is ignored, previously retained contaminant can be dislodged from the filter element
(see 10.4.1). However, if abnormal wear occurs, both the size and quantity of particles increase and, if not
detected by monitoring, wear rates can accelerate and the wear mode can change from benign fatigue wear to
abrasive wear. With abrasive wear, substantial amounts of surface material can be removed.
4.2.5 Maintenance-induced contaminant
Contaminants can easily be introduced during routine system maintenance unless the maintenance is performed
in a clean environment, and precautions are taken to prevent contaminant from getting on serviced items. For
example, topping up the system with new fluid can add contaminants unless the fluid is filtered upon addition.
4.3 Liquid contaminants
After damage caused by solid particulate contamination, damage caused by the presence of liquid contamination
is the next highest cause of contamination-related problems. This damage is caused either directly through
corrosion or indirectly through the interaction of the liquid contamination with the hydraulic fluid. This either
reduces the fluid’s effectiveness and thereby increases component wear rates, or reacts with it to produce
insoluble products that can block filters, clearances, etc. Blockage under these circumstances is often rapid
and unless it is detected and rectified, filtration ceases.
Water is the most common liquid contaminant in systems using mineral or synthetic fluids. Water can enter the
system from the atmosphere, leaking coolers and condensation. Although most hydraulic fluids are formulated
to cause water to separate so that it can settle in the reservoir and be drawn off, it is essential that the water
content is maintained at levels well below the solubility or saturation level of the fluid used, at the minimum
operating temperature.
Contamination by even small amounts of water in the fluid significantly lowers the load-sustaining capabilities
of the fluid. This deterioration of lubrication ability is of great importance to many components in hydraulic
systems. One example is that of rolling-element bearings, in which very high pressures are generated. If water
is present in the hydraulic fluid, even in dissolved form, the viscosity increase required for the form of lubrication
required in the bearing might not be achieved, and wear can result.
4.4 Gaseous contaminants
Nearly all fluids contain some dissolved gases. At atmospheric pressure, hydraulic fluids normally contain
about 8 % of their volume as dissolved air, which, at this pressure, causes no problem. Increasing the pressure
in the hydraulic fluid causes an increase in the amount of air that can be dissolved, and in low-pressure parts
of the system, some of this dissolved air can be liberated in the form of bubbles, a situation frequently found
downstream of pressure relief valves.
The presence of air bubbles in a system almost always causes erratic operation of the system, as it affects
the stiffness (bulk modulus) of the fluid and thereby system response. Air bubbles in an inlet (suction) line of a
pump reduce the volumetric efficiency and cause damage to most kinds of pumps through cavitation. Another
effect often seen in high performance systems is the sudden compression of the fluid in the high pressure
section of the pump, which causes the air bubbles to implode, and causing the vapour to ignite momentarily.
The very high temperatures generated cause thermal stress on the fluid, leading to oxidation and nitration. A
similar condition can exist downstream of metering valves; the process is known as “dieseling” and leads to
the formation of gums, varnishes and even microscopic “coke” particles. These in turn can lead to lacquering
of valves and plugging of filters.
5 Effects of particulate contamination and the benefits of its removal
5.1 General
It has been demonstrated that, in the majority of hydraulic systems, the presence of solid contaminant particles
is the main cause of failure and reduced reliability. The sensitivity of components to these particles depends
on the internal working clearances in these components, the system pressure levels and the quantity, size and
hardness of the contaminants.
5.2 Failures caused by particulate contamination
Failures arising from contamination fall into three main categories:
a) sudden or catastrophic failure, which occurs when a few large particles or a very large number of small
particles enter a component and cause seizure of moving parts (e.g. pumping elements or valve spools);
b) intermittent or transient failure, which is caused by contamination momentarily interfering with the function
of a component. The particle(s) can be washed away during the next cycle of operation. For example,
particles can prevent a valve spool from moving in one of its positions but are washed away when the valve
spool is moved to a new position; or a particle can stop a poppet valve from closing properly but is washed
clear during the next operation; and
c) degradation failure, which generally happens over time and shows up as a gradual loss of performance.
The main causes are abrasive wear inside a component and erosion caused either by cavitation or by
impingement of contaminated fluid at high velocity, all of which can cause increased internal leakage. If
degradation failure is allowed to continue, it can eventually lead to catastrophic failure.
5.3 Benefits of filtration to reduce solid particulate contamination
The objective of filtration is to reduce the level of solid particulate contamination present in a system and
maintain an acceptable level of cleanliness, no matter what contamination is being generated and ingressed
into the system. Maintaining an acceptable level of contamination achieves the following benefits:
a) extended component life — the wear in components is reduced thus extending the useful life of the
system;
b) enhanced system reliability (see 8.1) — maintaining fluid cleanliness minimises intermittent failures caused
by particles jamming in critical components;
c) reduced downtime and servicing costs — the cost of replacing components is often far outweighed by lost
production time and servicing costs. By increasing component life and reliability, contamination control
contributes to production efficiency and reduced maintenance costs;
d) safety of operation — safety of operation results from consistent and predictable performance.
Contamination control ensures that the conditions that lead to inconsistent and unpredictable operation
are greatly reduced; and
e) extended fluid life — by minimizing the number of particles in the system, operating with a clean fluid
can extend the life and serviceability of the system fluid by reducing oxidation, which is catalyzed by the
presence of reactive particles. For example, it has been shown that the catalytic effect of a mixture of
copper particles and water results in 47 times more oxidation (ageing) of the oil. This is of considerable
importance when the lifecycle costs of fluid (initial, operational and disposal) are significant.
6 Evaluation of cleanliness
6.1 General
The level of cleanliness in a system varies depending on its design, assembly and operation. Later clauses
describe the control necessary to maintain acceptable cleanliness levels. However, it is important to know what
level of contamination is reasonable for the required reliability and life of the particular system and how these
levels can be categorized.
4 © ISO 2011 – All rights reserved

6.2 Particle size range of interest
A wide range of particle sizes can affect the performance of hydraulic components and systems. The smallest
size of concern can range from 1 µm or smaller, when considering particles that cause wear by penetrating
the clearances of components, to well over 1 000 µm (1 mm) in the case of large particles jamming the moving
parts of components. Table 2, which is adapted from the American Society of Mechanical Engineers (ASME)
Wear Control Handbook (see Bibliography), shows typical dynamic operating clearances for common hydraulic
components.
Table 2 — Typical dynamic operating clearances
Component Clearance Component Clearance
piston pump servo valve
piston to bore: 5-40 µm  spool to sleeve: 1-4 µm
valve plate to cylinder: 0,5-5 µm  orifice: 130-450 µm
gear pump  flapper wall: 18-63 µm
tooth to side plate: 0,5-5 µm roller element bearings: 0,1-1 µm
tooth tip to case: 0,5-5 µm journal bearings: 0,5-25 µm
vane pump hydrostatic bearings: 1-25 µm
vane sides: 5-13 µm gears: 0,1-1 µm
vane tip: 0,5-1 µm dynamic seal: 0,05-0,5 µm
actuators: 5-250 µm
The particle size range of interest presents some difficulties in perceiving and understanding the size of these
particles. For most of the sizes, scientific instruments are needed to both size and count particles, as the
smallest particle that can be seen with the unaided human eye is about 40 µm; see Figure 1.
Grain of salt Diameter of a human hair Limit of human 5 µm 2 µm
≈ 100 µm on each side ≈ 75 µm vision ≈ 40 µm wear wear
particles particles
Figure 1 — Relative sizes (diameter or longest dimension) of common particles and objects
6.3 Methods of measuring and monitoring solid particulate contaminants
Several analytical methods are commonly used to measure and describe solid particulate contaminants.
Each technique produces a different piece of information about the contaminant. None produces a complete
description nor is there a convenient way to translate data of one type into another. These principal methods
are:
a) gravimetric concentration: determines contaminant mass per volume of fluid; ISO 4405 provides a method;
NOTE The inaccuracies inherent in this method make it unsuitable for evaluating the fluid cleanliness of modern
hydraulic systems. It is more suited to analyze samples in which relatively large weights are involved, for example, the
evaluation of component cleanliness.
b) optical particle counting (automatic or manual): determines size and number; ISO 11500 and ISO 4407
provide methods;
c) ferrography: primarily indicates the magnetic metal content of the contaminant;
d) spectroscopy: determines elemental composition of the contaminant;
e) filter blockage: semi-quantitative determination of size and number of particles; ISO 21018-3 provides a
method.
ISO 21018-1 provides a more comprehensive list of contaminant monitoring techniques and the advantages
and limitations of each method.
7 Coding systems for expressing level of solid particulate contamination
7.1 General
The output of most of the particle monitoring instruments is the number of particles at certain sizes. In hydraulic
systems, these can vary considerably from single figure values in the case of larger particle sizes in very
clean systems to many millions in the case of dirty systems. The communication of these varied numbers at
the different sizes is often confusing, and to overcome this, several coding systems have been developed to
simplify the reporting of contamination data. The basis of these codes is the sub-division of the counts into
broad based bands and assigning a code number to each band. The most commonly-used methods currently
in use in industry are described in the following subclauses.
7.2 ISO 4406 coding system
For industrial applications, the ISO 4406 coding system for expressing the level of contamination by solid
particles is the preferred method of quoting the number of solid contaminant particles in a fluid sample. The
code is constructed from the combination of three scale numbers representing the concentration of particles
at three specific particle sizes.
The unit of particle size depends on the sizing parameter used in the analysis, whether it is the longest
dimension (optical microscopic method) or equivalent spherical diameter (automatic particle counter method).
In the ISO 4406 coding system, particle sizes expressed in µm indicates that the particle size distribution
was determined using a microscope, and particle sizes expressed in µm(c) indicates that the particle size
distribution was determined using an automatic particle counter (APC) calibrated in accordance with ISO 11171.
In the ISO 4406 coding system:
a) the first scale number represents the number of particles in a millilitre sample of the fluid that are larger
than 4 µm(c);
b) the second number represents the number of particles larger than 5 µm or 6 µm(c); and
c) the third number represents the number of particles that are larger than 15 µm or 14 µm(c).
Because not every application requires that all three sizes be specified, or in those cases where the
contamination monitor is unable to provide this information, there are three variants on the three-number code,
in accordance with the following examples:
a) 22/19/14, which indicates that all three particle sizes are have been counted;
b) */19/14, which indicates that there are too many particles equal to or larger than 4 µm(c) to count; and
c) -/19/14, which indicates that the application does not require that particles equal to or larger than 4 µm(c)
be counted.
6 © ISO 2011 – All rights reserved

It is recognised that in very clean fluids, the number of particles being counted, even in 100 mL of fluid sample,
can be too low to be statistically reliable. Therefore, if fewer than 20 particles are counted for a particular size,
ISO 4406 requires that the scale number be preceded by ≥ to signify this, e.g. 16/12/≥10.
Note that the ISO 4406 system differs from other cleanliness coding systems in that it does not include codes
for particle sizes larger than 15 µm (see 7.3). This is because most hydraulic systems incorporate filters and,
as a result, there are very few of these larger particles. Because there are so few, it is unlikely that the number
of these larger particles counted will be consistent from sample to sample, so trending or comparing data at
these particle sizes becomes meaningless.
Microscope counting examines the particles by longest dimension (not equivalent area, as an APC does) and
the code is given with a dash and two scale numbers, e.g. -/19/14. The particle sizes reported are at 5 µm and
15 µm, which are equivalent to the 6 µm(c) and 14 µm(c) sizes determined using an APC. See ISO/TR 16386
for a description of differences in particle counting and sizing methods.
7.3 NAS 1638, SAE AS4059 and ISO 11218 coding systems
The NAS 1638 cleanliness coding system was originally developed in 1964 to define contamination classes
for the contamination contained within aircraft components. The application of this standard was extended to
non-aerospace hydraulic systems mainly because nothing else existed at the time, and it is still widely used.
NAS 1638 was made inactive in May 2001 in favour of SAE AS4059, which is SAE’s modernization of the
NAS 1638 coding system and which has been adopted as ISO 11218 for aerospace hydraulic systems.
As with the ISO 4406 code, the SAE AS4059 and ISO 11218 codes are based on the number of particles at
specific sizes. The coding system defines the maximum numbers of particles permitted in each size range, and
the result is usually expressed as a single digit class number based on the highest class level obtained for all
particle sizes measured.
ISO 4406 is recommended for expression of contamination levels for non-aerospace hydraulic systems, so
users of this technical report can consult the respective standards if further details are desired about the other
methods.
8 Setting required cleanliness levels (RCLs) for a hydraulic system
8.1 The amount of contamination that a system can successfully operate with depends upon two factors:
a) the relative sensitivity of the components to contaminant, and
b) the level of reliability and component service life required by the system designer and user.
This was established in a UK Department of Trade and Industry (DTI) survey of hydraulic systems (see
Bibliography), which showed an inverse relationship between the contamination level in the hydraulic fluid and
the system reliability experienced, as shown in Figure 2.
A system designer needs to select the most appropriate fluid cleanliness level for the system, known as the
required cleanliness level (RCL). The importance of the RCL cannot be overemphasised, as it is the basis of
system cleanliness management; it
a) is used to dictate the cleanliness of components at the build stage;
b) defines the level of cleanliness at delivery; and
c) provides the basis for setting maintenance action levels in service.
The subsequent subclauses describe four methods for selecting an RCL.
Key
x reliability (MTBF, hours)
y ISO Code
1 DTI Study, 1984
Figure 2 — Relationship between ISO 4406 code and reliability
8.2 The first and preferred method for selecting the RCL is a comprehensive method based on guidelines
incorporating the specific system operating conditions. The advantage this method has over others is that
it accounts for the life and reliability required by the user for his system, and not the general requirements
for a machinery group. This method is described in British Fluid Power Association Document P5. The RCL
determined is based on the following parameters:
a) operating pressure and duty cycle;
b) component sensitivity to contamination;
c) life expectancy;
d) cost of component replacement;
e) downtime;
f) safety liabilities; and
g) environmental considerations.
The attributes of the system and its operation are reviewed and used to establish a weighting or score. These
are then accumulated to give the RCL. The level selected is considered to be the maximum permissible level
for reliable operation under the circumstances. If it is exceeded, then corrective actions can be implemented,
otherwise the contamination control balance is disturbed, and component wear rates can accelerate.
8.3 The second method is for the system designer to use his own experience or consult with others with
experience and similar requirements. In many cases, this information is supplied by a filter manufacturer that
has access to such information for such benchmarking. The system designer uses caution when applying
others’ experience, as the operating conditions, environment, and maintenance practices might not be the
same.
8 © ISO 2011 – All rights reserved

8.4 The third method is to use studies and resulting data such as Figure 2. Again, the system designer uses
caution, as these recommendations are usually very general in nature and might not address the application
specifically.
8.5 The fourth method is to contact the manufacturer of the most contaminant-sensitive component in the system
for recommendations. Most component manufacturers know the detrimental effect that increased contamination
levels have on the performance of their components and issue maximum permissible contamination levels. They
state that operating components with fluids that are cleaner than those stated can increase life. However, the
diversity of hydraulic systems in terms of pressure, duty cycles, environments, lubrication required, contaminant
types, etc., makes it almost impossible to predict the component’s service life with more precision than what can
be reasonably expected. Furthermore, without the benefits of significant research and the existence of standard
component contaminant sensitivity tests, manufacturers that publish recommended cleanliness levels that are
cleaner than their competitors might be viewed as having a more sensitive product.
9 Cleanliness management concepts
9.1 System design considerations
9.1.1 Although hydraulic filters are critical in maintaining the cleanliness of an operating system, simply
applying a filter and ignoring other recommended practices is inadequate for completely protecting the system
from contamination. Since contamination comes from a wide variety of sources, a comprehensive cleanliness
management program addresses each of these sources and emphasizes methods of minimizing contaminant
entry and damage as well as methods for contaminant removal.
9.1.2 Typical processes used to minimize contaminant entry and damage include:
a) design and manufacture of components and systems that are tolerant of contamination;
b) selection and use of adequate filters; see Clause 12;
c) use of effective reservoir air breather filters, joints, and external seals;
d) repair and maintenance procedures that minimize the entry of contamination; and
e) maintenance of cleanliness throughout manufacture and assembly; ISO/TR 10949, ISO 16431 and
ISO 18413 cover component and system cleanliness measuring and reporting methods; ISO 23309 covers
flushing methods of cleaning lines in hydraulic systems.
9.2 Monitoring system cleanliness
9.2.1 General
It is recommended that the system cleanliness level is monitored after one week’s initial operation and thereafter
in two month’s time. The selected filtration rating is acceptable if the RCL is achieved within this time. If the RCL
is not achieved, then either a filter with a finer filtration rating or additional filters can be required. The frequency
of monitoring is then selected on the basis of the stability of the system cleanliness level.
Regular checks on the fluid cleanliness level ensure that the contamination control system is working
satisfactorily, and that the specified level of cleanliness is being maintained. This can be achieved either by
connecting an on-line cleanliness monitor or by taking a fluid sample from the system and analysing it off-line
in a laboratory.
It is also recommended that the physical and chemical condition of the hydraulic fluid, in addition to its
cleanliness, be monitored. This provides information necessary to ensure that the necessary fluid properties
are maintained.
9.2.2 Fluid sampling
In order to establish the condition of the fluid, it is necessary to extract a representative sample of the system
fluid. ISO 4021 provides recommended sampling procedures. When monitoring is done on a regular basis,
samples are taken from the same sampling point, and after approximately the same operating sequence or
time, beginning typically more than 1 h after start-up.
The importance of correctly taking the sample cannot be overemphasised because modern hydraulic
systems are typically designed to operate with very clean fluids. As a result, there is a high probability that
contamination level could be measured inaccurately if inappropriate techniques are used. This could greatly
increase operational costs if corrective actions result from a non-representative result. Improper sampling
also makes trending data almost impossible. As modern hydraulic systems have very clean fluids, on-line
monitoring is recommended to minimize errors introduced from sample valves and bottles. See ISO 21018-1
for on-line monitoring considerations.
If bottle sampling is used, it is essential to use only sample bottles that have been cleaned and verified in
accordance with ISO 3722. Modern hydraulic systems featuring highly effective filters have fluid cleanliness
levels that approach that of the pre-cleaned sample bottles themselves. The use of inadequately cleaned
bottles can greatly increase the contamination measured.
9.3 System maintenance for cleanliness management
9.3.1 Responsibilities
Regular and effective maintenance is essential if the desired system reliability is to be achieved. It is the
responsibility of the original equipment manufacturer to provide adequate maintenance procedures for the
equipment being supplied. Effective detailed procedures take into account the duty cycle of the system on an
individual basis and the environment in which the system is operating.
The end user is responsible for ensuring that adequate servicing facilities are available and that routine
maintenance is carried out.
9.3.2 Disciplines
The maintenance of a hydraulic system for cleanliness management is usually covered by seven basic
disciplines:
a) inspection of the system, while operating, for leaks and filter blockage;
b) maintaining the level of the fluid in the reservoir within the limits stated by the manufacturer;
c) changing disposable filter elements or cleaning strainers;
d) taking samples of the fluid;
e) monitoring the fluid condition;
f) monitoring the mechanical condition; and
g) identifying corrective action.
9.3.3 Maintenance procedures
Written maintenance procedures typically accompany every new piece of hydraulic equipment and contain at
least the following information related to filtration:
a) types and quantities of filter elements;
b) filter element change procedures with an appropriate recording system for such changes;
c) required system cleanliness level and intermediate action levels;
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d) recommended method of fluid sampling;
e) frequency of fluid condition checks;
f) type of fluid required;
g) instructions about addressing leaks; and
h) corrective actions to be taken in the event that an action level or the RCL is exceeded.
9.3.4 Changing or cleaning filter elements
It is most important that the disposable filter element is changed and or the strainer is cleaned as soon as
its differential pressure device shows that the element is becoming clogged or that a specified differential
pressure has been reached.
It is recommended that filters always be fitted with a differential pressure device or some means of showing
when they are becoming blocked. However, if a filter does not contain such a device, it is vitally important
to change the element at intervals recommended in the maintenance manual, and it is always preferable to
change such an element too frequently rather than to allow it to run in a bypass condition that might remain
unnoticed.
Ensure that replacement elements are of the correct type and rating. Using an element with too coarse a rating
cannot maintain the desired fluid cleanliness level. If an element that is too fine or whose differential pressure
rating in the clean state is too high is installed, it can start bypassing prematurely and become ineffective.
If a fill filter is fitted on the reservoir, it is typically inspected regularly and if significantly contaminated, it is
removed, cleaned, dried and refitted. A damaged fill filter is usually discarded and a new one installed.
Filter elements made of wire mesh and designated as cleanable, including pump inlet strainers, can be cleaned
by reverse flushing with a very clean solvent. However, if the mesh is fine, immersion in an ultrasonic bath can
be required to loosen contaminant trapped in the mesh before final reverse flushing. Ensure that contamination
is not transferred from the dirty to the clean side. The filter manufacturer’s recommended cleaning procedures
can be followed. A cleanable filter element can be cleaned only a finite number of times before replacement
is necessary. C
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