Technical Information

Hydraulic Fluids and Lubricants
Guidelines for Cleanlinesswww.danfoss.com

Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness

Revision history

2 | © Danfoss | Sept 2018

Table of revisions
Date

Changed

Rev

Sept 2018

Rebrand to Engineering Tomorrow

0607

July 2014

Layout, backpage change

GG

Mar 2014

Converted to Danfoss layout – DITA CMS

GFNov 2011

Minor changeGE

Oct 2010

New backpage, change of a layout, no technical change.

GD

Jun 2002

First editionAA

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Contents
Initial questions and answers
Purpose................................................................................................................................................................................................ 4
Why is filtration necessary?...........................................................................................................................................................4
Where does the dirt come from?................................................................................................................................................ 4
How can the required cleanliness level be achieved?.........................................................................................................5Definition of β-ratio, efficiency, filter fineness ...................................................................................................................... 6
β-ratio versus filter efficiency..................................................................................................................................................7
Fluid cleanliness
Definition of cleanliness levels per ISO 4406..........................................................................................................................9
Fluid cleanliness........................................................................................................................................................................ 10
Fluid cleanliness requirements................................................................................................................................................. 10
Selection of an appropriate filter and filtration system
Closed Circuit...................................................................................................................................................................................13
Design of a filter in the CC suction line.............................................................................................................................13
Design of a filter in the CC charge circuit.........................................................................................................................13
Open Circuit .................................................................................................................................................................................... 14
Design of a filter in the OC suction line............................................................................................................................ 14
Design of a filter in the OC return line...............................................................................................................................14
Filtration in OC with multiple pumps................................................................................................................................14
Recommendations for a new filter element.........................................................................................................................14
By-pass filtration.............................................................................................................................................................................14
Dirt absorption capacity, maximum differential pressure.............................................................................................. 14
Why a bypass?................................................................................................................................................................................. 16
Contamination indicator............................................................................................................................................................. 16
Air breather ..................................................................................................................................................................................... 17
What is to be Done if the Required Cleanliness Class is not Achieved? .................................................................... 17
Why Loop Flushing?......................................................................................................................................................................18
Taking of Fluid Samples
Fluid samples...................................................................................................................................................................................19
Sampling according to ISO 4021 from a system in operation....................................................................................... 19
Sampling device........................................................................................................................................................................19
Sampling method.....................................................................................................................................................................20
Sampling from a tank according to CETOP RP 95 H.....................................................................................................20
Working hints
Scavenging and running in........................................................................................................................................................ 21
Monitoring of contamination....................................................................................................................................................21
Permanent...................................................................................................................................................................................21
Cyclical..........................................................................................................................................................................................21
Topping up hydraulic fluid....................................................................................................................................................22
Changing the element............................................................................................................................................................22© Danfoss | Sept 2018

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessInitial questions and answers
PurposeThis leaflet is intended to assist the designer of an installation, a set or a hydrostatic drive to ensure thatthe requirement for a specific minimum cleanliness of the hydraulic fluid is met by means of design
measures such as the selection of an optimal filter, or preferably of an economically efficient filtration
concept. This includes start-up, operation and topping up of hydraulic fluid.
Why is filtration necessary?
In hydrostatic systems a series of sliding surfaces act as hydrostatic-hydrodynamic bearings with gap
heights in the range of 10 µm. That is why dirt is the greatest enemy of hydraulic systems, sincedepending on its nature and composition it generates wear and thus shortens service lives.
This is true for all fields of mechanical engineering and it cannot be repeated often enough. At thepresent time it is not possible to predict the length of the service life of a hydrostatic unit as a function of
the cleanliness of the hydraulic fluid. The fact that these constraints are not known for roller bearings
either, even though very many parameters have been researched for these parts in particular, shows justhow complicated these wear mechanisms are.
Although more effort is currently being concentrated on trying to measure dirt sensitivity of hydrostatic
units in short-term contamination tests, such experiments are unsuccessful because contamination
sensitivity cannot be measured like pressures or speeds. The leading manufacturers of hydrostatic
equipment have therefore decided to give priority to investigating the fundamentals of wear caused bycontamination in hydrostatic units within the framework of a joint research project.
It is uncertain to what extent service life prognoses will be possible at the end of the research project, if at
all. It can, however, be stated that the cleaner a system, the higher its service life expectancy.
A satisfactory service life is achieved if the cleanliness level as required below is maintained.
Where does the dirt come from?
We distinguish between two essential sources of dirt:• contamination occurring during assembly – assembly dirt
• contamination occurring during operation – operating dirt
Assembly dirt
Different kinds of dirt occur during the various production operations: chips, moulding sand, coreresidues, cleaning-rag lint, welding beads, scale etc. Products supplied by Danfoss are therefore cleaned
in modern cleaning installations after completion of machining operations on the individual parts.
Careful attention is also paid to cleanliness when these clean individual parts are assembled to form
highgrade hydrostatic units. However, since dirt occurs during the final assembly in a vehicle, of a set etc.,
especially during the piping work, it is advisable to flush the whole system prior to commissioning.
The basic contamination of the “clean” hydraulic fluid supplied must also be added to the assembly dirt.
As investigations have shown, new hydraulic fluid can contain basic dirt levels in excess of the cleanlinesslevel admissible for optimum operation, see Fluid cleanliness requirements on page 10. That is why asystem should always be filled up via a filter assembly. Particles of the same order of magnitude as thegap widths are to be considered as especially critical.
Operating dirt
Fine dirt from the surrounding environment is drawn into the hydraulic system during operation via
piston rods or other moving seals. Abrasive particles from the components are also pumped through thesystem with the fluid. A frequently underestimated source of contamination is from unsuitable venting
facilities of fluid tanks. Fluctuations in volume cause fine dust to be drawn into the tanks, from where it
causes abrasion of the sliding combinations in the system.

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessInitial questions and answers
How can the required cleanliness level be achieved?
A filtration system must be designed in such a way that it is able to retain the new dirt entering theoverall system in the filter in order to maintain the required cleanliness level throughout the wholeoperating life.
An example description:
If a filter is used in the suction line or the charge circuit, the charge pump size selected - in our example
17 cm3 – determines the volume flow available for filtering as a function of the speed. The following
theoretical calculation serves to illustrate this (see Schematic of Series 90 variable pump with suction filter on
page 6).
Assuming that the pump runs at a nominal speed of 1500 min-1, then at an assumed volumetric efficiencyof 90 %, a charge pump volume flow of approx. 23 l/min results.
A contamination level of 230 particles larger than 10 µm/ml has developed in the oil tank and a filter with
β35 = 75, β10 = 2 (= 50 % filtration efficiency) is fitted in the suction line. The contamination is also
distributed uniformly in the fluid. Approximately 5.3 x 106 particles larger than 10 µm per minute are now
drawn from the tank with the charge pump flow. The filter element holds back 50 % of the particles larger
than 10 µm, so that 2.65 x 106 particles larger than 10 µm reach the charge pump each minute.
If 2.65 x 106 particles larger than 10 µm are also passed to the system per minute (via ventilation filters,
piston rods or abrasion), there is no change in the cleanliness level. If fewer than 2.65 x 106 particles larger
than 10 µm are passed to the system, a lower i. e. a better cleanliness level is achieved. However if morethan 2.65 x 106 particles larger than 10 µm are passed to the system, a higher i. e. a worse cleanliness levelis achieved.
As mentioned at the beginning, this calculation is very close to conditions encountered in practice, butthese will never be so exact because particles will settle at the bottom of the tank in areas with a low flow
velocity. Nor are the particles so uniformly distributed when they pass through the filter. This example ismerely intended to illustrate the principle and it can be established that:
The cleanliness level is improved if:
the filter fineness is improved (higher β-value for a certain particle size, i. e. β10 = 2 becomes β10 = 2.5)••

the volume flow* via the filter is increased

The cleanliness level deteriorates if:• the filter fineness deteriorates (lower β-value for a certain particle size, i. e. β10 = 2 becomes β10 = 1.8)•

the volume flow via the filter is reduced. (Note the change of the β-value shown in the diagram
Retention rate as a function of the differential pressure. The alteration of the volume flow also changes
the differential pressure and hence the β value too. However the volume flow has more influence on
the cleanliness level.)

The flow volume cannot generally be selected freely since it is determined in a closed circuit by the sizeof the charge pump. However other operating factors take priority when the charge pump size isselected. In these cases, therefore, the β-value must be varied.
However, if the β-value is increased (different filter material) without the structural dimensions of thefilter being increased, then the consequence is that:© Danfoss | Sept 2018

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessInitial questions and answers••

the differential pressure rises (applies for new, uncontaminated filter element)
the dirt absorption capacity drops (reduced service life)

Schematic of Series 90 variable pump with suction filter

M
A
rv
Se
o

B
L1
L2

S
2.65 x 106 particles > 10 mm/min

b35 = 75 (b10 = 2 equivalent 50%)
Q = 23 l/min
5.3 x 106 particles > 10 mm/min
230 particles > 10 mm/ml
P001 318E

Definition of β-ratio, efficiency, filter fineness
The β-ratio is defined in ISO 16 889-1999 Multi-pass test (old: ISO 4572-1982) as:

As a characteristic number the β10-ratio is to be specified.
Example:This number defines the ratio of the number of particles before and after the filter. This means from 500
particles larger than 10 µm before the filter, 490 particles are retained in the element and only 10 passthrough or from 50 particles before the filter, 49 are retained and only 1 passes through. This can be
expressed as a filter efficiency:

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessInitial questions and answers
Filter efficiency:

OR:

This efficiency makes the filter performance more understandable. The table below shows clearly the
relationship between β-ratio and efficiency. In practice the following term is often used: βX = 75 (=
98,67% efficiency)
The observant reader will notice that increasing the β-ratio by 50% (from 50 to 75) the efficiency only
increases by 0.67%, therefore βX-ratios above 75% are not reasonable.
In some filter manufacturers catalogues you sometimes find βX larger than 2000.
The real efficiency increase is shown in the table β-ratio vs. efficiency. The β10 = 75-ratio has been
established as a standard. This specifies the particle size (indicated as “X“) were the β-ratio is equal to 75.
This particle size is used to classify the filter fineness.
Example:β35 = 75 equals a filter fineness of 35 µm
The term absolute filter fineness may not be used for this. Together with the β-ratio the relateddifferential pressure at the filter element has to be specified. Unfortunately this is not always specified in
the filter manufacturers catalogue.
β-ratio versus filter efficiency
β-ratio versus efficiency© Danfoss | Sept 2018

β-ratio

Efficiency

β-ratio

Efficiency

1.0

0

6.4

84.4

1.1

9.1

6.8

85.3

1.2

16.7

7.0

85.7

1.3

23.1

7.2

86.1

1.4

28.6

7.4

86.5

1.5

33.3

7.6

86.8

1.6

37.5

7.8

87.2

1.7

41.1

8.0

87.5

1.8

44.5

8.2

87.8

1.9

47.3

8.4

88.1

2.0

50.0

8.6

88.4

2.1

52.4

8.8

88.6

2.2

54.4

9.0

88.9

2.3

56.5

9.2

89.1

2.4

58.3

9.4

89.4

2.5

60.0

9.6

89.6

2.6

61.5

9.8

89.8

2.7

62.9

10

90.0

2.8

64.3

11

90.9

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessInitial questions and answers
β-ratio versus efficiency (continued)

8 | © Danfoss | Sept 2018

β-ratio

Efficiency

β-ratio

Efficiency

2.9

65.5

12

91.6

3.0

66.6

13

92.3

3.1

67.7

14

92.9

3.2

68.8

15

93.3

3.3

69.7

16

93.8

3.4

70.6

17

94.1

3.5

71.4

18

94.4

3.6

72.2

19

94.5

3.7

72.9

20

95.0

3.8

73.7

30

96.7

3.9

74.4

40

97.5

4.0

75.0

50

98.0

4.2

76.2

60

98.3

4.4

77.3

70

98.6

4.6

78.3

75

98.67

4.8

79.2

80

98.7

5.0

80.0

90

98.9

5.2

80.8

100

99.0

5.4

81.5

200

99.5

5.6

82.1

500

99.8

5.8

82.8

1000

99.9

6.0

83.3

2000

99.95

6.2

83.8

-

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Fluid cleanliness
Definition of cleanliness levels per ISO 4406
The cleanliness level of a hydraulic fluid is determined by counting number and size of particles in thefluid. The number of particles is defined as a cleanliness level according to ISO 4406.
Definition of cleanliness levels per ISO 4406
Number of particles per 100 ml

Number of particles per 1 ml

Cleanliness levels per ISO 4406

1-20.01 - 0.02

1

2-4

0.02 - 0.04

2

4-8

0.04 - 0.08

3

8-16

0.08 - 0.16

4

16-32

0.16 - 0.32

5

32-64

0.32 - 0.64

6

etc.

etc.

etc.

4 x 103 - 8 x 103

40 - 8013

8 x 103 - 16 x 103

80 - 160

14

16 x 103 - 32 x 103

160 - 32015

32 x 103 - 64 x 103

320 - 64016

64 x 103 - 130 x 103

640 - 130017

130 x 103 - 250 x 103

1300 - 2500

18

250 x 103 - 500 x 103

2500 - 5000

19

The step to the next cleanliness level means double or half the number of particles.
The old ISO 4406-1987 defines the cleanliness level of particles larger than 5 µm and 15 µm. As an
example: if 1910 particles/ml larger than 5 µm and 71 particles/ml larger than 15 µm are counted, the ISO
4406-1987 code level is 18/13.
In 1999 both, the definition for particle counting and the definition of ISO code was changed. The
required cleanliness class definition is now determined by ISO 4406-1999. The allocated particle sizes are:Comparison of old and new standard ISO 4406
Old ISO 4406-1987

New ISO 4406-1999

not defined

4 μm (c)5 μm

6 μm (c)15 μm

14 μm (c)Please note, that “(c)” must be added to the new definition in order to identify that it is the new ISO 4406.
The old method for particle counting may still be used.
The ISO 4406-1999 cleanliness class 22/18/13 means:
22 specifies the number of particles larger than 4 µm (c),18 specifies the number of particles larger than 6 µm (c),13 specifies the number of particles larger than 14 μm (c) related to 1 ml respectively 100 ml of theinspected fluid.
Measurements with the same fluid sample will result in the same cleanliness class for both methods asshown in the table below.

© Danfoss | Sept 2018

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Design Guidelines for Hydraulic Fluid Cleanliness
Fluid cleanliness
Number of particles per milliliter, particle count comparison
Particle size

1 μm

Not standardized

4000

Old ISO 4406-1987-

New ISO 4406-1999-

ISO 4406 cleanliness class19

4 μm (c)5 μm

6 μm (c)

6 μm (c)

15 μm

-

-

-

-

-

-

2000

-

-

180

4000

-

2000

180

-

19

18

18

15

15

The new method counts more smaller particles and less larger particles.
For better understanding please see the graph beside. This graph demonstrates the effect of the change
to the new particle sizes 4 µm (c), 6 µm (c), and 14 µm (c).
Again, the actual number of particles of a sample is of course the same, only the counting method isdifferent. Although it may look like, the new method does not allow more particles.
ISO 4406-1999 versus prior cleanliness classes

Number of particles per ml

5000

4000

ISO 4406-1987
ISO 4406-1999

3000

2000

1000

0

0

10

5

15

Particle size (mm)17P001 948E

Together with this ISO 4406 change a new calibration standard ISO 11 171-1999 and a new Multi-pass
test ISO 16 889-1999 for filters have been developed.
Comparison between old and new standards
Comparison between old and new standards
Old standards

Test description

New standards

ISO 4402-1991

Automatic particle counter (APC) calibration

ISO 11 171-1999

ISO 4402-1991

Cleanliness code

ISO 4406-1999

ISO 4402-1991

Multipass test for filters

ISO 16 889-1999

Fluid cleanliness
The cleanliness of a fluid is one of the most important features to guarantee a satisfying performance ofthe hydraulic system. Contamination of a fluid with solid particles can lead to a failure of the complete
hydraulic system by locking of the pistons or blocking the valves. Different systems have different
sensitivity to solid contamination of the fluid, so different levels of fluid cleanliness are determined by ISO
4406. The determination of the cleanliness level is made by counting the particles, distinguishing theparticle size.
Further information to fluid cleanliness, filter compatibility in a system and the cleanliness levels can befound in the document Design Guidelines for Hydraulic Fluid Cleanliness, Technical Information,
BC00000095.
Fluid cleanliness requirements
To achieve the specified unit life a cleanliness level as shown below must be met. Fluid samples shall betaken either in the loop or at the entry to the pump, which is typically the suction line.
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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Fluid cleanliness
Fluid cleanliness requirements depends on the product and the products acceptable continuous or ratedpressure limits.
Fluid cleanliness requirements according to productProduct

Required cleanliness class
ISO 4406-1999

Curve in the diagram
Required fluid cleanliness

Steering components with open center

22/20/17

A

Orbital motors

22/20/16

B

Steering components with LS and closed center

21/19/16

C

22/18/13

D

18/16/13

E

Proportional spool valves
Axial & radial piston pumps and motors
Gear pumps and motors
Cartridge and electrohydraulic valves

These cleanliness levels can not be applied for hydraulic fluid residing in the component housing/case or
any other cavity after transport.
In general for fluid change and new fluid top up minimum cleanliness class 23/21/15 and for firstmachine start up at the factory minimum cleanliness 25/22/17 must be met if not otherwise specified.
Exceeding these levels may result in start-up damage.
The before mentioned requirements reflect the experience gained from a broad range of applications.
For very high lifetime requirements or contamination sensitive components (e. g. servo valves) better
cleanliness levels are necessary.

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Fluid cleanliness
Required fluid cleanliness diagram
ISO Solid Contaminant Code per ISO 4406-1999(Automatic Particle Counter (APC) calibration per ISO 11 171-1999)
1 000 0002625
24First machine start up
ISO 25/22/17

10 00023

Fluid change + top up
ISO 23/21/15

22

A = ISO 22/20/17

21

B = ISO 22/20/16

20

C = ISO 21/19/16

19

D = ISO 22/18/13

18

E = ISO 18/16/13
1000

ISO class number

Number of particles per ml > Indicated size

100 000

17
16
15
14

100

13
12

10

11
1

4

6

10

14

Particle size µm (c)12 | © Danfoss | Sept 2018

100
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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
Example:The design is explained below taking an SPV 9/075 with a charge pump volume of 17 cm3 by way of
example. A continuous pressure of 240 bar is assumed. Accordingly in section 3 the cleanliness class
18/13 to ISO 4406 results from curve A.
Closed Circuit
Design of a filter in the CC suction line
Examinations have revealed that a filter in the suction line with a β35 – β45 = 75 at a differential pressure of
0.25 bar achieves the required cleanliness level 18/13 under normal operating conditions. In some
applications even better cleanliness levels are achieved. In order to assure that the cleanliness class ismaintained we recommend that samples of the hydraulic fluid be drawn during the running-in time andthat the particles be counted. A certain, constant cleanliness level will emerge during the operating time(see the illustration below).
An analogous graph can be drawn up for any particle size. The following reference can serve as a further
guide.
A filter element with a β10 = 2.0 – 1.5 at 0.25 bar differential pressure (50% – 33% filtration efficiency forparticles > 10 µm) generally reaches the above mentioned β35 – β45 = 75 value.

Cleanliness level (ISO 4406)

Cleanliness level evolving as a function of operating hours for particles larger than 15 µm
16
15
14
13
12
11
10

Filter A
Filter B

1

10
Operating hours

100
P001 320E

Design of a filter in the CC charge circuit
We recommend using filter elements with β15 – β20 = 75 (β10 = 10 corresponding to 90% filtration
efficiency) at the differential pressure occurring in the application. A strainer with a mesh of 100 µm – 125µm has to be used to protect the charge pump against coarse contamination. However the actual
filtering work has to be performed by the filter in the charge circuit.
On the basis of what has been said so far the question arises: Why is a higher filter fineness (β-value)
necessary in a filter in the charge circuit?
The answer is: The Multi-pass test per ISO 16 889 determines the β-values at constant volume flow andalmost constant differential pressure increase. These optimal conditions do not exist in the charge circuitso that a reduced β-value sets in as a result of volume flow and pressure fluctuations. The filter industry
and research institutes are in the process of developing practice-oriented test methods to solve thisproblem. Furthermore, the possible level of pressure fluctuations is much lower in a suction filter.
Moreover in certain circuits the filter is arranged behind the charge circuit pressure limiting valve, so thatthe whole charge pump volume flow does not constantly pass the filter. Related to the whole charge
pump volume flow, an arithmetically lower β-value emerges here too. It is recommended that thecleanliness level actually existing or evolving in the course of time is determined here too.

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
Open Circuit
Design of a filter in the OC suction line
In the open circuit, the whole main volume flow is taken from the tank; therefore, the suction filters must
be adequately dimensioned in order to achieve appropriate differential pressures and service lives.
Alternatively, a lower filter fineness can be selected, since a higher volume flow is available. For further
information, please see Design of a filter in the return line.
Design of a filter in the OC return line
In the case of return line filters in open circuits, special attention must be paid to discontinuous volume
flows through working cylinders with differing area ratio. Under certain circumstances it might therefore
be advisable to select a larger filter than would be necessitated by the suction line alone. In any case astrainer with a mesh size of 100 – 125 µm should be provided in the suction line.
Filtration in OC with multiple pumps
A machine with several pumps using the same reservoir may use one filter only to save costs. Every circuit
must be protected by a suction strainer with 100 –125 µm mesh.
This mesh equals theoretically: β100-150 = 75
This term should not be used for a screen. The off-line filtration may feed other functions and circuits likethe steering equipment.
Recommendations for a new filter element
Recommendations for differential pressure: 0.1 bar (pressure drop) of a clean element
In OC or CC suction line

β-ratios

In OC return line

30 mm2/s [141.2 SUS]

Viscosity
Flow

In CC charge line

charge pump displacement x rated speed
β35-45 = 75 (β10 ≥ 2)

β15-20 = 75 (β10 ≥ 10)

By-pass filtration
As in charge line. There will and must be exceptions to these recommendations to ensure an economic
filtration system.
Dirt absorption capacity, maximum differential pressure
A further criterion for selecting the filter size is the dirt absorption capacity and differential pressure risein the case of increasing contamination, see the illustrations below.
When the Multi-pass test to ISO 16 889 is performed, the dirt absorption capacity is also determined. It is
important to distinguish between the apparent dirt absorption capacity, which is the amount of dirt
added during the test, and the real dirt absorption capacity, which is the amount of dirt actually retainedin the filter.
The following equation applies:
Added dirt quantity (apparent dirt absorption capacity) minus dirt quantity remaining in the oil isthe actual dirt absorption capacity.
The differential pressure starts in an approximately linear manner before rising exponentially.
Once this zone is reached, it is time to change the filter, since a little dirt means a large increase in the
differential pressure.
In the case of the above filter element, the contamination indicator should respond either optically orelectrically at approximately 2.2 bar and indicate that it is time to change the elements concerned.
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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
Rise of the differential pressure when dirt is added

Depending on the filter material, certain maximum differential pressures must not be exceeded since thefilter material may be damaged and dirt already retained will be released again, i. e. the dirt is pumped
through.
The curve of the β-value versus the differential pressure shows this (see the illustration Retention rate as afunction of the differential pressure).
If it is expected that the filter element will not be changed in time before the rupture point is achieved, abypass valve must be installed.
A bypass valve must also be provided if the differential pressure rises to an inadmissible level during cold
starts, which is usually the case.
Retention rate as a function of the differential pressure© Danfoss | Sept 2018

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
Why a bypass?
During operation the differential pressure at the filter element rises due to contamination. Without abypass, especially during cold starts, this would lead to demage to the filter element or collapse of the
support elements. This can be effectively prevented by the use of a bypass.
Although the effective filter efficiency is reduced by the short opening of the bypass during cold starts,
the hydrostatic unit is not immediately damaged as a result of this.
Function of a bypass

The cleanliness level simply deteriorates as a function of the time during which the bypass is in operation
and as a function of the newly generated particles. Working with an open bypass for several hours ordays should be avoided. This condition can be monitored reliably with a contamination indicator (seeContamination indicator on page 16). The system operator thus determines the service life of thehydrostatic units and the rest of the system by regularly checking the contamination level of the filter andchanging the filter elements in time.
It is important to understand that if used as explained above, a bypass is always better than the sudden
release of a particle or a dirt cloud due to damage to an element whereby the cloud is passed throughthe whole system (including the high-pressure circuit) and finally lands in the tank after irreparable
damage is sustained by the sliding parts. If there is no contamination indicator either, this damage is notnoticed and it may be that the system is operated for a long time with this unintentional bypass (after
the element has been destroyed!) until overheating caused by reduced efficiency of the hydrostatic units
is recognized. This then turns out to be much more expensive than the additional bypass and thecontamination indicator would have been (see Contamination indicator).
A bypass should be arranged as shown above or even further away from the element if the design allows
this. Under certain circumstances it may even be a design advantage if the filter elements are to bebolted directly on to the pump (charge circuit filtration).

C Caution
A bypass may never be situated in the base of the filter element since the dirt settles in this area and isflushed into the system again.
Contamination indicator
The contamination indicator responds when a predetermined differential pressure occurs as a result of
growing contamination of the filter element. An optical signal appears, or an electrical contact isenergized.

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
Function of the indicator (differential pressure)

1. Pressure before the filter element
2. Pressure behind the filter element
3. Position of the magnets in a clean element
4. Position of the magnets in a contaminated element
Air breather
Sufficient attention must also be devoted to air breathers, since a considerable portion of thecontamination makes its way into hydraulic systems via unsuitable ventilation systems. Design measures
such as pressurizing reservoirs are often not economically efficient by comparison with today’s air
breathers.
Under certain circumstances it may be necessary to observe the Pressurized Container Regulations if thepressure content product derived from tank volume times pressure exceeds a certain value.
Unfortunately there is no standard for air breathers corresponding to the Multi-pass test to ISO 16 889.
The filter fineness quoted by the manufacturer of the air breathers has to be relied on. This does not
permit comparisons between manufacturers A and B since, as already mentioned, there are nostandardized tests.
Generally speaking the fineness of the air breathers must be equivalent to or better than the “working
filters” present in the system.
Therefore only the βx = 75 values and the given filter fineness of the air breathers can be taken as
standard values.
What is to be Done if the Required Cleanliness Class is not Achieved?
A brief reminder
It was explained before that the cleanliness level is influenced by:• the β-value (filter fineness, filtration ratio)•

the volume flow through the filter

If the required cleanliness level is not achieved using the 17 cm3/U charge pump and the β10 = 75 filter, it
is not necessarily expedient to use a larger charge pump. This may necessitate a larger filter since thedifferential pressure more quickly reaches the limit of p = 0.25 bar (risk of cavitation) with a clean filter
element, which leads to insufficient service life of the element. The energy balance also deteriorates as aresult of the higher power loss.
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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessSelection of an appropriate filter and filtration system
In this case filters with a higher β-value must be used. However, since the higher β-values generally also
involve higher differential pressures, it is often also necessary to move the filter to the charge circuit. If
the filter elements are designed accordingly, higher differential pressures are admissible here so that theoverall dimensions of the filter can be reduced - representing an installation advantage. It must also be
clarified and checked to what extent new contamination can be reduced and prevented.
It was explained before that inadequate ventilation facilities are a cause of fresh contamination. An
improvement of the cleanliness level can often be achieved by using ventilation filters with better filter
fineness, especially for applications with working cylinders with differing area ratios.
For the design of the ventilation filter the differential pressure (caused by the differential air volume flow)
must be kept as low as possible in order to prevent cavitation in the suction area of the pumps. It should
also be checked whether unsuitable piston rod seals or leaks are the cause.
Dirt which does not enter the system or is not caused by wear need not be removed by filtration.

Why Loop Flushing?
Filtration is intended to remove contamination from the hydraulic fluid. For this, however, thecontamination must be passed to the filter element. In a closed circuit without circuit purging, existing
contamination can only be removed from the system with the oil leaking from the pistons, the control
valve etc. Since only particles smaller than the leak gap width can leave the closed circuit, the remaining
particles stay in the circuit and can lead to erosion damage in areas with high flow velocities.
This can be avoided by circuit purging, i. e. by forcing 5–6 l/min from the low pressure circuit via an spool
valve and a purge relief valve.
The contamination flushed out in this way (including particles larger than the leak gap width) can now bepassed to the filter element installed in the system and be removed.

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Taking of Fluid Samples
Fluid samples
Fluid samples must be drawn very carefully into appropriate bottles to prevent extraneous dirt from,falsifying the sample result.
The sample bottle should contain a label with the following information:• Sample number

•
•
•
•
•

Source of sample
Sampling method
Date and time of sampling
Nature of fluid
Comments/remarks if necessary

Sampling according to ISO 4021 from a system in operation
Sampling points should be provided at the design stage of the hydraulic installation. They should bearranged in the turbulent main flow.

C Caution
Take precautionary measures to protect personnel and equipment.
Sampling device

•
•
•
•
•
•

•
•

If turbulent flow conditions prevail in the main flow, a typical sampling device(as shown in Typical sampling device in practice on page 19).
A quick fastening coupling 6 is permanently attached to the opening through which the sample is tobe withdrawn
A dust protection cap 1 is provided for the part 6
The remaining part of the unit 2-5 is secured for sampling
The inner diameter and length of the capillary tube are selected in agreement with the desired
sample quantity
Capillary tubes with an inner diameter of less than 1.25 mm may not be used.
Other cross-sectional forms (e. g. rectangular) can be used, provided that the smallest internal
dimension is not less than 1 mm
The end of the capillary tube is sharpened and deburred in order to facilitate the subsequent
penetration of the film which covers the opening of the sample bottles
If no turbulence can be guaranteed in the flow, a device for generating turbulence must be used,
e. g. a turbulent sampler

Typical sampling device in practice

1. Dust protection cap
2. Valve without check device
3. Capillary tube for drawing off fluid© Danfoss | Sept 2018

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness
Taking of Fluid Samples
4. Cover cap with capillary tube
5. Ball valve
6. Check valve, outer part for quick fastening
Sampling method

•
•
•
•
•
•
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•

Ball valve 5 is opened
Allow at least 200 ml fluid to flow through the sampling device before collecting the fluid
Without closing the ball valve, place the sample bottle in the position for collecting the fluid
Pierce the protective film covering the bottle opening with the sharp end of the capillary tube
Draw a sample of not more than 90 % and not less than 50 % of the bottle volume.
When a sufficient sample quantity has been collected, remove the sample bottle before stopping theflow with the ball valve
Seal the sample bottle immediately after withdrawing the capillary tube
If a sampling device with quick-fastening coupling is used, the removable parts of the sampling
device are to be dismantled and all other fluid traces are to be removed by flushing with a suitable
solvent
Immediately after dismantling the dust protection cap is replaced on the permanently mounted partof the quick fastening coupling.

Sampling from a tank according to CETOP RP 95 H
Sampling from the tank should only be carried out if it is not possible to sample from the main flow.
Clean the outer surface of the tank around the places from which the sample is to be drawn.
Sampling:
The fluid in the tank should be mixed well in order to ensure that the sample is typical. To this end warm
up the system by running it under operating conditions. Then draw a sample (at least 150 ml) with theaid of a pipette or a cleaned disposable syringe. Pass the pipette about half way down into the fluid.
Ensure that the pipette does not touch the side walls of the tank or come too close to the bottom. Fill thecontents of the pipette into the sample bottle and seal this carefully. Cover the tank again or close it withclean covering film if further samples are required.

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessWorking hints
The frequency and intensity of the maintenance work to be performed depend on the burden generated
by environmental influences and on the workload.
Special attention must always be paid to the operational suitability and cleanness of the hydraulic fluid.
Scavenging and running in
Before a hydraulic system is commissioned, the assembly dirt must be removed. This is best done byflushing the whole installation with a portable filter unit. Mineral oil (or another medium compatible withthe hydraulic fluid to be used subsequently) is pumped through the whole system or parts of the systemat the highest possible flow velocity. The assembly dirt is filtered in the filter unit.
During this process the elements of the built-in filters are to be removed. Small or less sensitive systems
can also be scavenged with the built-in filters during the running-in process. It must be ensured that thesystem is run without load but with a displacement which is gradually increased up to maximum.
106

Particle count per unit volume

The illustration beside shows the relation between thedesign-specific, admissible cleanliness level and theactual cleanliness prior to commissioning.
Typical relation between the design-specific, admissible
cleanliness level A and the actual cleanliness level prior
to commissioning level D.
It is vital that the system be flushed and run at low
pressure until the required cleanliness level is achieved.

105

104

103

D
A

102

5

10 15
25
50
Particle size in m m (log 2)

100
P001 326E

Monitoring of contamination
Permanent
Every filter should be equipped with an optical/electrical indicator of the contamination level at the filter.
In this way it is possible to establish at any time whether there is still any dirt absorption capacity orwhether the elements have to be changed. The checks should be carried out daily once the operating
temperature is achieved.
If a contamination indicator with electrical signal device is used, the selected signal is emitted at
operating temperature when the filter element is contaminated. During the warming up phase a“contaminated” signal is nearly always emitted due to the higher differential pressure unless thecontamination indicator is equipped with a signal cut-out for the cold start phase.
CyclicalWith regular monitoring, filters are suitable as wear surveillance elements for the components of thehydraulic system. If the operator keeps a log of filter changes, it can be assumed in the event of shorterchanging intervals that the wear of the system components is increasing. The origin of the maincontamination can be ascertained by analysing a fluid sample and the contaminated element. A© Danfoss | Sept 2018

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Technical Information
Design Guidelines for Hydraulic Fluid CleanlinessWorking hints
comparison of the results with the materials used allows preventive maintenance before a complete
failure interrupts production or operation. Adherence to the required cleanliness level is checked bymeasuring the contamination and this ensures that no premature wear or failure occurs.
These samples must be drawn at specially designed sampling points as explained before.
Topping up hydraulic fluid
Any fluid used to top up losses should always be poured in via a fine filter in order to maintain thecleanliness class. Where appropriate facilities are available, the return flow filter can be used. It isadvisable to provide a permanent connection which should be included in considerations at the design
stage. Any opening of the tank/reservoir for maintenance purposes (topping up hydraulic fluid, sampling,changing filter elements in built-in tanks etc.) should always be avoided as far as possible by an
expedient design. Even though some ventilation filters have a so-called filling screen (mesh width >100 µm), this still does not afford any protection against the penetration of particles of the order ofmagnitude of 10 – 100 µm.
Changing the element
If the contamination indicator shows a contaminated element, this must be changed without delay due
to the high rate of increase of pressure drop as the element becomes more contaminated. Extreme caremust be taken when changing the element.
The operating instructions must be followed precisely.
The following standard values apply for filter maintenance intervals:
1. 24 hours after commissioning the system
2. After the running-in period (50 - 100 hours of service)
3. Normal maintenance after 300 - 500 hours of service

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Technical Information
Design Guidelines for Hydraulic Fluid Cleanliness© Danfoss | Sept 2018

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