Design Guideline for Hydraulic Fluid Cleanliness Applications_Rev-F_03-2003

8/8/2019 Design Guideline for Hydraulic Fluid Cleanliness Applications_Rev-F_03-2003

1/24

x = ?

x = ?

Design Guidelinefor Hydraulic FluidCleanliness

TechnicalInformation

10075

50

10

10 20

5

0.5 1 5

10

-value

Differential pressure (bar)E

x = ?

Back


2/24

2 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationPurpose of this Leaet, Contents

2003, Sauer-Danfoss

Sauer-Danfoss can accept no responsibility for possible errors in catalogues, brochures and other printed material. Sauer -Danfoss reserves theright to alter its products without prior notice. This also applies to products already ordered provided that such alterations can be made withoutsubsequent changes being necessary in specications already agreed. All trademarks in this material are properties of the respective companies.Sauer-Danfoss and the Sauer-Danfoss logotype are trademarks of the Sauer-Danfoss Group. All rights reserved.

PURPOSE OF THIS

LEAFLET

This leaet is intended to assist the designer of an installation, a set or a hydrostatic driveto ensure that the requirement for a specic minimum cleanliness of the hydraulic uid

is met by means of design measures such as the selection of an optimal lter, or prefer-ably of an economically efcient ltration concept. This includes start-up, operation andtopping up of hydraulic uid.

Why is ltration necessary? .......................................................................................................................3Where does the dirt come from?.............................................................................................................3

Assembly dirt ............................................................................................................................................3Operating dirt ...........................................................................................................................................4

How can the required cleanliness level be achieved? .....................................................................4Denition of: -ratio, efciency, lter neness ..................................................................................6

Cleanliness Features.....................................................................................................................................8Denition of cleanliness levels per ISO 4406 .................................................................................8New particle size denition .............................................................................................................. 10Recommendation for lter neness / retaining rates (Beta-ratios) ....................................10Technical Requirements of Hydraulic Fluids............................................................................... 11Required uid cleanliness diagram................................................................................................ 12

Closed circuit ............................................................................................................................................... 13Design of a lter in the suction line ............................................................................................... 13Design of a lter in the charge circuit ........................................................................................... 13

Open circuit.................................................................................................................................................. 14Design of a lter in the suction line ............................................................................................... 14Design of a lter in the return line ................................................................................................. 14Filtration in circuits with multiple pumps.................................................................................... 14Dirt absorption capacity, maximum differential pressure ..................................................... 15

Why a bypass? ............................................................................................................................................. 16Contamination indicator ......................................................................................................................... 17

Air breather............................................................................................................................................. 17What is to be done if the required cleanliness class is not achieved?............................... 18Why loop ushing? .............................................................................................................................. 18

Sampling according to ISO 4021 from a system in operation ................................................... 19

Sampling device ................................................................................................................................... 19Sampling method ................................................................................................................................ 19Sampling from a tank according to CETOP RP 95 H................................................................ 20

Scavenging and running in............................................................................................................... 21Monitoring of contamination................................................................................................................ 22

Topping up hydraulic uid................................................................................................................ 22Changing the element........................................................................................................................ 23

CONTENTS

INITIAL QUESTIONS AND

ANSWERS

REQUIRED FLUID

CLEANLINESS

SELECTION OF AN

APPROPRIATE FILTER AND

FILTRATION SYSTEM

TAKING OF FLUID SAMPLES

WORKING HINTS

Back


3/24

3DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical Information

In hydrostatic systems a series of sliding surfaces act as hydrostatic-hydrodynamic bear-ings with gap heights in the range of 10 m. That is why dirt is the greatest enemy of

hydraulic systems, since depending on its nature and composition it generates wear andthus shortens service lives.

This is true for all elds of mechanical engineering and it cannot be repeated oftenenough. At the present time it is not possible to predict the length of the service life of ahydrostatic unit as a function of the cleanliness of the hydraulic uid. The fact that theseconstraints are not known for roller bearings either, even though very many parametershave been researched for these parts in particular, shows just how complicated thesewear mechanisms are.

Although more effort is currently being concentrated on trying to measure dirt sensitivityof hydrostatic units in short-term contamination tests, such experiments are unsuccessfulbecause contamination sensitivity cannot be measured like pressures or speeds. Theleading manufacturers of hydrostatic equipment have therefore decided to give priorityto investigating the fundamentals of wear caused by contamination in hydrostatic unitswithin the framework of a joint research project.

It is uncertain to what extent service life prognoses will be possible at the end of theresearch project, if at all. It can, however, be stated that the cleaner a system, the higherits service life expectancy.

A satisfactory service life is achieved if the cleanliness level as required below ismaintained.

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, mouldingsand, core residues, cleaning-rag lint, welding beads, scale etc. Products supplied bySauer-Danfoss are therefore cleaned in modern cleaning installations after completion ofmachining operations on the individual parts. Careful attention is also paid to cleanlinesswhen these clean individual parts are assembled to form highgrade hydrostatic units.

Initial Questions and Answers

WHY IS FILTRATION

NECESSARY?

WHERE DOES THE DIRTCOME FROM?

Back


4/24

4 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationInitial Questions and Answers

WHERE DOES THE DIRT

COME FROM?

(continued)

HOW CAN THE REQUIRED

CLEANLINESS LEVEL BE

ACHIEVED?

However, since dirt occurs during the nal assembly in a vehicle, of a set etc., especiallyduring the piping work, it is advisable to ush the whole system prior to commissioning.

The basic contamination of the clean hydraulic uid supplied must also be added to theassembly dirt. As investigations have shown, new hydraulic uid can contain basic dirtlevels in excess of the cleanliness level admissible for optimum operation (see Cleanlinessrequirements section). That is why a system should always be lled up via a lter assembly.Particles of the same order of magnitude as the gap widths are to be considered asespecially critical.

Operating dirt

Fine dirt from the surrounding environment is drawn into the hydraulic system duringoperation via piston rods or other moving seals. Abrasive particles from the componentsare also pumped through the system with the uid. A frequently underestimated sourceof contamination is from unsuitable venting facilities of uid tanks. Fluctuations involume cause ne dust to be drawn into the tanks, from where it causes abrasion of thesliding combinations in the system.

A ltration system must be designed in such a way that it is able to retain the new dirtentering the overall system in the lter in order to maintain the required cleanliness levelthroughout the whole operating life.

An exampleof this is described below:If a lter is used in the suction line or the charge circuit, the charge pump size selected -in our example 17 cm3 - determines the volume ow available for ltering as a function ofthe speed. The following theoretical calculation serves to illustrate this (see an illustration

P001 318):

Assuming that the pump runs at a nominal speed of 1500 min-1, then at an assumedvolumetric efciency of 90 %, a charge pump volume ow of approx. 23 l/min results. Acontamination level of 230 particles larger than 10 m/ml has developed in the oil tankand a lter with 35 = 75,10 = 2 (= 50 % ltration efciency, see table below) is tted in thesuction line. The contamination is also distributed uniformly in the uid. Approximately5.3 x 106 particles larger than 10 m per minute are now drawn from the tank with thecharge pump ow. The lter element holds back 50 % of the particles larger than 10 m,so that 2.65 x 106 particles larger than 10 m reach the charge pump each minute.

If 2.65 x 106 particles larger than 10 m are also passed to the system per minute (via

ventilation lters, piston rods or abrasion), there is no change in the cleanliness level. Iffewer than 2.65 x106 particles larger than 10 m are passed to the system, a lower i.e. abetter cleanliness level is achieved. However if more than 2.65 x106 particles larger than10 m are passed to the system, a higher i.e. a worse cleanliness level is achieved.

As mentioned at the beginning, this calculation is very close to conditions encounteredin practice, but these will never be so exact because particles will settle at the bottom ofthe tank in areas with a low ow velocity. Nor are the particles so uniformly distributedwhen they pass through the lter. This example is merely intended to illustrate theprinciple and it can be established that:

Back


5/24

5DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


HOW CAN THE REQUIRED

CLEANLINESS LEVEL BE

ACHIEVED?(continued)

The cleanliness level is improved if:

the lter neness is improved

(higher -value for a certain particle size, i.e.10 = 2 becomes 10 = 2.5). the volume ow* via the lter is increased.

The cleanliness level deteriorates if:

the lter neness deteriorates(lower -value for a certain particle size, i.e.10 = 2 becomes 10 = 1.8).

the volume ow* via the lter is reduced.

The ow volume cannot generally be selected freely since it is determined in a closedcircuit by the size of the charge pump. However other operating factors take prioritywhen the charge pump size is selected. In these cases, therefore, the -value must bevaried. However, if the -value is increased (different lter material) without the structuraldimensions of the lter being increased, then the consequence is that: the differential pressure rises (applies for new, uncontaminated lter element) the dirt absorption capacity drops (reduced service life).

* Note the change of the -value shown on page 14, diagram P001 332E. The alteration of the volume ow alsochanges the differential pressure and hence the value too. However the volume ow has more inuenceon the cleanliness level.

Schematic Series 90 variable pump with suction lter

P001 318E

2.65 x 106 particles > 10 m/min

35 = 75 (10 = 2 equivalent 50 %)

Q = 23 l/min

5.3 x 106 particles > 10 m/min

230 particles > 10 m/ml

L2 S

Servo

M

A

B

L1

Back


6/24

6 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


The -ratio is dened and determined in ISO 16 889-1999 Multi-pass test(old: ISO 4572-1982) as:

Number of particles > x m upstream of the lterX = Number of particles > x m downstream of the lter

As a characteristic number the 10-ratio is to be specied.

Example: 500 particles > 10 mm upstream of the lter10 = = 50 10 particles > 10 mm downstream of the lter

This number denes the ratio of the number of particles before and after the lter. Thismeans from 500 particles larger than 10 m before the lter, 490 particles are retainedin the element and only 10 pass through or from 50 particles before the lter, 49 areretained and only 1 passes through. This can be expressed as lter efciency:

500 - 10 50 - 1Filter efciency = = = 98 % 500 50

or: 1 1Filter efciency = 1 - = 1 - = 98 % 10 50

This efciency makes the lter performance more understandable. Table below showsclearly the relationship between -ratio and efciency. In practice the following term isoften used:

X = 75 (= 98,67 % efciency)

The observant reader will notice that increasing the -ratio by 50 % (from 50 to 75) theefciency only increases by 0.67 %. Thereforex-ratios above 75 are not reasonable.In some lter manufacturers catalogues you sometimes nd X larger than 2000. A look inthe table below shows the real efciency increase. The 10 = 75-ratio has been establishedas a standard. This species the particle size (indicated as x) were the -ratio is equal to75. This particle size is used to classify the lter neness.

Example: 35 = 75 equals a lter neness of 35 m

The term absolute lter neness may not be used for this.

Together with the -ratio the related differential pressure at the lter element has to bespecied. Unfortunately this is not always specied in the lter manufacturers catalogue.

DEFINITION OF:

-RATIO, EFFICIENCY,

FILTER FINENESS

Back


7/24

7DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


DEFINITION OF:

-RATIO, EFFICIENCY,

FILTER FINENESS(continued)

nosirapmoC susrevoitar- i

oitar- ycneiciffe oitar- ycneiciffe

1 0.0 4.6 4.48

1.1 1.9 8.6 3.58

2.1 7.61 0.7 7.58

3.1 1.32 2.7 1.68

4.1 6.82 4.7 5.68

5.1 3.33 6.7 8.68

6.1 5.73 8.7 2.78

7.1 1.14 0.8 5.78

8.1 4.44 2.8 8.78

9.1 3.74 4.8 1.88

0.2 0.05 6.8 4.88

1.2 4.25 8.8 6.88

2.2 5.45 0.9 9.88

3.2 5.65 2.9 1.98

4.2 3.85 4.9 4.98

5.2 0.06 6.9 6.98

6.2 5.16 8.9 8.98

7.2 9.26 0.01 0.09

8.2 3.46 0.11 9.09

9.2 5.56 0.21 6.19

0.3 6.66 0.31 3.29

1.3 7.76 0.41 9.29

2.3 8.86 0.51 3.39

3.3 7.96 0.61 8.39

4.3 6.07 0.71 1.49

5.3 4.17 0.81 4.496.3 2.27 0.91 5.49

7.3 9.27 0.02 0.59

8.3 7.37 0.03 7.69

9.3 4.47 0.04 5.79

0.4 0.57 0.05 0.89

2.4 2.67 0.06 3.89

4.4 3.77 0.07 6.89

6.4 3.87 0.57 76.89

8.4 2.97 0.08 7.89

0.5 0.08 0.09 9.89

2.5 8.08 0.001 0.99

4.5 5.18 0.002 5.99

6.5 1.28 0.005 8.99

8.5 8.28 0.0001 9.99

0.6 3.38 0.0002 59.99

2.6 8.38

"efficiency"

Back


8/24

8 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


The old ISO 4406-1987 denes the cleanliness level of particles larger than 5 m and15 m. As an example: if 1910 particles/ml larger than 5 m and 71 particles/ml largerthan 15 m are counted, the ISO 4406-1987 code level is 18/13.In 1999 both,the denition for particle counting and the denition of ISO code was

changed. The required cleanliness class denition is now determined by ISO 4406-1999.The allocated particle sizes are:

Please note, that (c) must be added to the new denition in order to identify that it isthe new ISO 4406. The old method for particle counting may still be used.

The ISO 4406-1999 cleanliness class 22/18/13 means:

22 species the number of particles larger than 4 m (c),18 species the number of particles larger than 6 m (c),and13 species the number of particles larger than 14 m (c) related to 1 ml respectively100 ml of the inspected uid.

CLEANLINESS FEATURES Denition of cleanliness levels per ISO 4406

The cleanliness level of a hydraulic uid is determined by counting number and size of

particles in the uid. The number of particles is dened as a cleanliness level according toISO 4406.

selcitrapforebmuN

lm001rep

selcitrapforebmuN

lm1rep

levelssenilnaelC

6044OSIrep

2-1 20.0-10.0 1

4-2 40.0-20.0 2

8-4 80.0-40.0 3

61-8 61.0-80.0 4

23-61 23.0-61.0 5

46-23 46.0-23.0 6

.cte .cte .cte

01x4 3 01x8- 3 08-04 31

01x8 3 01x61- 3 061-08 41

01x61 3 01x23- 3 023-061 51

01x23 3 01x46- 3 046-023 61

01x46 3 01x031- 3 0031-046 71

01x031 3 01x052- 3 0052-0031 81

01x052 3 01x005- 3 0005-0052 91

.selcitrapforebmunehtflahroelbuodsnaemlevelssenilnaelctxenehtotpetsehT

Definition of cleanliness levels per ISO 4406

denifedton )c(m4

m5 )c(m6

m51 )c(m41

Old ISO 4406-1987 New ISO 4406-1999

Required Fluid Cleanliness

Back


9/24

9DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


The new method counts more smallerparticles and less larger particles.For better understanding please see thegraph beside. This graph demonstratesthe effect of the change to the newparticle sizes 4 m (c), 6 m (c), and14 m (c).Again, the actual number of par ticles ofa sample is of course the same, only thecounting method is different. Althoughit may look like, the new method doesnot allow more particles!

CLEANLINESS FEATURES

(continued)

Measurements with the same uid sample will result in the same cleanliness class forboth methods as shown in the table below.

Together with this ISO 4406 change a new calibration standard ISO 11 171-1999 and anew Multipass test ISO 16 889-1999 for lters have been developed.

ISO 4406-1999 versus prior cleanliness classes

00

5 10

1000

2000

3000

4000

5000

15 17

Particle size (m)

Numberofparticlesp

erml

ISO 4406-1987ISO 4406-1999

P001948E

Number of particles per milliliter, particle count comparison

eziselcitraP m1 )c(m4 m5 )c(m6 )c(m41 m51

dezidradnatstoN 0004 - - - - -

7891-6044OSIdlO - - 0002 - - 081

9991-6044OSIweN - 0004 - 0002 081 -

ssalcssenilnaelc6044OSI 91 91 81 81 51 51

sdradnatsdlO noitpircsedtseT sdradnatsweN

1991-2044OSI noitarbilac)CPA(retnuocelcitrapcitamotuA 9991-17111OSI

7891-6044OSI edocssenilnaelC 9991-6044OSI

2891-2745OSI sretlifroftsetssapitluM 9991-98861OSI

Comparison between old and new standards

Required Fluid Cleanliness

Back


10/24

10 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationRequired Fluid Cleanliness

For charge pressure and return line ltration a suction screen with a mesh width of100 - 125 m must be used in the suction line to prevent sudden damage due to largeparticles.Please see Design Guideline for Hydraulic Fluid CleanlinessTechnical Information for furtherinformation on how the cleanliness requirements can be achieved.

Recommendation for lter neness / retaining rates (Beta-ratios)

CLEANLINESS FEATURES

(continued)

13 m

d = 13 m

Square 78.5 m2

d = 10 m

Old: New:

P001 935E

New particle size denition

The particle size denition has been changed also. The old standard dened the largest

particle extension as the particle size. The new standard uses the projected square areaand converts this to an equivalent diameter. Please see the picture below.

78.5 m2 4d = = 10 m

ISO 4407 (under revision) species particle counting with a microscope. Only particleslarger 5 m and 15 m are manually counted and specied as /18/13. The is used inplace of the rst scale number, while 18 is allocated to 5 m and 13 to 15 m.

)tiucricnepo+desolc(noitartlifnoitcuS 54-53

(57= 01> )2

)tiucricdesolc(noitartliferusserpegrahC 02-51 (57= 01 > )01

)tiucricnepo(noitartlifenilnruteRlareneg

54-53(57=

01> )2

srotomdnaspmupraegrof 02-51

(57= 01> )01

Recommended-ratios

Back


11/24

11DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationRequired Fluid Cleanliness

Fluid cleanliness requirements

To achieve the specied unit life a cleanliness level as shown below must be met. Fluid

samples shall be taken either in the loop or at the entry to the pump which is typicallythe suction line.Fluid cleanliness requirements depends on the product and the products acceptablecontinuous or rated pressure limits.

TECHNICAL

REQUIREMENTS OF

HYDRAULIC FLUIDS

In general for uid change and new uid top up minimum cleanliness class 23/21/15 andfor rst machine start up at the factory minimum cleanliness 25/22/16 must be met ifnot otherwise specied. Exceeding these levels may result in start-up damage.

The before mentioned requirements reect the experience gained from a broad range ofapplications. For very high lifetime requirements or contamination sensitive components(e.g. servo valves) better cleanliness levels are necessary.

tcudorPssalcssenilnaelcderiuqeR

9991-6044OSI

margaidnievruC

woleb

ertnecnepohtiwstnenopmocgnireetS 71/02/22 A

srotomlatibrO 61/02/22 B

ertnecdesolcdnaSLhtiwstnenopmocgnireetS61/91/12 C

evlavloopslanoitroporP

s, ASC-valverotomdnaspmupnotsiplaidar+laixA31/81/22 D

srotomdnaspmupraeG

evlavciluardyhortcelednaegdirtraC 31/61/81 E

Fluid cleanliness requirements

Back


12/24


13/24

13DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


The design is explained below taking an SPV 9/075 with a charge pump volume of17 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.

Design of a lter in the suction line

Examinations have revealed that a lter in the suction line with a 35 - 45 = 75 at a dif-ferential pressure of 0.25 bar achieves the required cleanliness level18/13 under normaloperating conditions. In some applications even better cleanliness levels are achieved.In order to assure that the cleanliness class is maintained we recommend that samplesof the hydraulic uid be drawn during the running-in time and that the particles becounted. A certain, constant cleanliness level will emerge during the operating time (seethe illustration below).

Cleanliness level evolving as a function of operating hours for particles larger than 15 m

Selection of an Appropriate Filter and Filtration System

EXAMPLE

CLOSED CIRCUIT

Filter A

Filter B

16

15

14

13

12

11

10

Cleanlinesslevel(ISO4406)

1 10 100

P001 320EOperating hours

An analogous graph can be drawn up forany particle size.The following reference can serve as afurther guide.

A lter element with a 10 = 2.0 - 1.5 at0.25 bar differential pressure (50 % - 33 %ltration efciency for particles > 10 m)generally reaches the above mentioned35 - 45 = 75 value.

Design of a lter in the charge circuit

We recommend using lter elements with 15 - 20 = 75 (10 = 10 corresponding to 90 %ltration efciency) at the differential pressure occurring in the application. A strainerwith a mesh of 100 m - 125 m has to be used to protect the charge pump againstcoarse contamination. However the actual ltering work has to be performed by thelter in the charge circuit.

On the basis of what has been said so far the question arises:Why is a higher lter neness (-value) necessary in a lter in the charge circuit?

Answer:

The Multi-pass-test per ISO 16 889 determines the -values at constant volume ow andalmost constant differential pressure increase. These optimal conditions do not existin the charge circuit so that a reduced -value sets in as a result of volume ow andpressure uctuations. The lter industry and research institutes are in the process ofdeveloping practice-oriented test methods to solve this problem. Furthermore, thepossible level of pressure uctuations is much lower in a suction lter. Moreover incertain circuits the lter is arranged behind the charge circuit pressure limiting valve, sothat the whole charge pump volume ow does not constantly pass the lter. Related tothe whole charge pump volume ow, an arithmetically lower -value emerges here too.

As already said above for the lter in the suction line, it is recommended that the clean-liness level actually existing or evolving in the course of time be determined here too.

Back


14/24

14 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


Design of a lter in the suction line

Unlike the situation in the closed circuit, the whole main volume ow is taken from

the tank. That is why these suction lters must be adequately dimensioned in order toachieve appropriate differential pressures and service lives. On the other hand a lowerlter neness can be selected, since a higher volume ow is available.(For further information please see next page.

Design of a lter in the return line

In the case of return line lters in open circuits, special attention must be paid to discon-tinuous volume ows through working cylinders with differing area ratio. Under certaincircumstances it might therefore be advisable to select a larger lter than would benecessitated by the suction line alone. In any case a strainer with a mesh size of 100 - 125m should be provided in the suction line.

Filtration in circuits with multiple pumps

A machine with several pumps using the same reservoir may use one lter only to savecosts. Every circuit must be protected by a suction strainer with 100 -125 m mesh.

This mesh equals theoretically: 100-150 = 75This term should not be used for a screen.The off-line ltration may feed other functions and circuits like the steering equipment.

Recommendations for differential pressure (pressure drop) of a new lter element

and -ratios for closed and open circuit

In suction line (closed and open circuit): Differential pressure: 0,1 bar (clean element) at: Viscosity: 30 mm2/s Flow: closed circuit: charge pump displacement x rated speed

open circuit: pump displacement x rated speed -ratio: 35 - 45 = 75 (10> 2)

In charge line(for closed circuit): Differential pressure: 0,7 bar (clean element) at: Viscosity: 30 mm2/s Flow: charge pump displacement x rated speed -ratio: 15 - 20 = 75 (10> 10)

In return line (only open circuit): Differential pressure: 0,5 bar (clean element) at: Viscosity: 30 mm2/s Flow: charge pump displacement x rated speed -ratio: general: 35 - 45 = 75 (10> 2)

for gear pumps and motors: 15 - 20 = 75 (10> 10)

By-pass ltration: as in Charge line

There will and must be exceptions to these recommendations to ensure an economicltration system.


OPEN CIRCUIT

Back


15/24

15DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


Dirt absorption capacity, maximum differential pressure

A further criterion for selecting the lter size is the dirt absorption capacity and differen-

tial pressure rise in the case of increasing contamination, see illustration below.

When the Multi-pass-test to ISO 16 889 is performed, the dirt absorption capacity is alsodetermined. 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 retained in the lter.

The following equation applies:

Added dirt quantity (apparent dirt absorption capacity) minus dirt quantity

remaining in the oil is the actual dirt absorption capacity.


OPEN CIRCUIT

(continued)

Change filter!

6

4

3

2

Differentialpressure

bar

10 20 30

P001 321E

Dirt Capacity (g)

5

1

In the illustration beside the rise inthe differential pressure starts in anapproximately linear manner beforerising exponentially.

Once this zone is reached it is timeto change the lter, since a little dirtmeans a large increase in the differentialpressure. In the case of the above lterelement, the contamination indicatorshould respond either optically orelectrically at approximately 2.2 bar and

indicate that it is time to change theelements concerned.

Rise of the differential pressure when dirt is

added

Depending on the lter material, certainmaximum differential pressures must notbe exceeded since the lter material maybe damaged and dirt already retainedwill be released again, i.e. the dirtis pumped through.

The curve of the -value versus the

differential pressure shows this(see illustration besides).

If it is expected that the lter elementwill not be changed in time beforethe rupture point is achieved, a bypassvalve (see below) must be installed.A bypass valve must also be providedif the differential pressure rises to aninadmissible level during cold starts,which is usually the case.

Retention rate as a function of the differential

pressure

10075

50

10

10 20

5

0.5 1 5

10

-value

Differential pressure (bar)P001322E

Back


16/24

16 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationSelection of an Appropriate Filter and Filtration System

Although the effective lter efciency is reduced by the short opening of the bypassduring cold starts, the hydrostatic unit is not immediately damaged as a result of this. Thecleanliness level simply deteriorates as a function of the time during which the bypass isin operation and as a function of the newly generated particles. Working with an openbypass for several hours or days should be avoided. This condition can be monitoredreliably with a contamination indicator (see below). The system operator thus determinesthe service life of the hydrostatic units and the rest of the system by regularly checkingthe contamination level of the lter and changing the lter elements in time.

It is important to understand that if used as explained above, a bypass is always betterthan the sudden release of a particle or a dirt cloud due to damage to an elementwhereby the cloud is passed through the whole system (including the high-pressure

circuit) and nally lands in the tank after irreparable damage is sustained by the slidingparts. If there is no contamination indicator either, this damage is not noticed and it maybe that the system is operated for a long time with this unintentional bypass (afterthe element has been destroyed!) until overheating caused by reduced efciency of thehydrostatic units is recognized. This then turns out to be much more expensive than theadditional bypass and the contamination indicator would have been (see section on page15 for contamination indicator).

Note: A bypass should be arranged as shown above or even further away from the element if the design allowsthis. Under certain circumstances it may even be a design advantage if the lter elements are to be bolteddirectly on to the pump (charge circuit ltration). A bypass may never be situated in the base of the lterelement since the dirt settles in this area and is ushed into the system again.

WHY A BYPASS? Function of a bypassDuring operation the differential pressureat the lter element rises due to contami-

nation. Without a bypass, especiallyduring cold starts, this would lead todemage to the lter element or collapseof the support elements. This can beeffectively prevented by the use of abypass.

P001 324E

Back


17/24

17DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


CONTAMINATION

INDICATOR

AIR BREATHER

The contamination indicator responds when a predetermined differential pressureoccurs as a result of growing contamination of the lter element. An optical signal

appears, or an electrical contact is energized.

Function of the contamination indicator (differential pressure)

1. Pressure before the lter element2. Pressure behind the lter element3. Position of the magnets in a clean

element4. Position of the magnets in a contami-

nated element.

P001 323

Sufcient attention must also be devoted to air breathers, since a considerable portionof the contamination makes its way into hydraulic systems via unsuitable ventilationsystems. Design measures such as pressurizing reservoirs are often not economicallyefcient by comparison with todays air breathers. Under certain circumstances it maybe necessary to observe the Pressurized Container Regulations if the pressure 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 testto ISO 16 889. The lter neness quoted by the manufacturer of the air breathers has tobe relied on. This does not permit comparisons between manufacturers A and B since,as already mentioned, there are no standardized tests. Generally speaking the neness ofthe air breathers must be equivalent to or better than the working lters present in thesystem. Therefore only the x = 75 values and the given lter neness of the air breatherscan be taken as standard values.

Back


18/24

18 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


As a brief reminder:It was explained before that the cleanliness level is inuenced by:

the -value (lter neness, ltration ratio) the volume ow through the lter.

If the required cleanliness level is not achieved using the 17 cm3/U charge pump andthe 10 = 75 lter, it is not necessarily expedient to use a larger charge pump. This maynecessitate a larger lter since the differential pressure more quickly reaches the limitof p = 0.25 bar (risk of cavitation) with a clean lter element, which leads to insufcientservice life of the element. The energy balance also deteriorates as a result of the higherpower loss.

In this case lters with a higher -value must be used. However, since the higher -valuesgenerally also involve higher differential pressures, it is often also necessary to movethe lter to the charge circuit. If the lter elements are designed accordingly, higherdifferential pressures are admissible here so that the overall dimensions of the lter canbe reduced - representing an installation advantage. It must also be claried and checkedto what extent new contamination can be reduced and prevented.

It was explained before that inadequate ventilation facilities are a cause of freshcontamination. An improvement of the cleanliness level can often be achieved by usingventilation lters with better lter neness, especially for applications with workingcylinders with differing area ratios.

For the design of the ventilation lter the differential pressure (caused by the differentialair volume ow) 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 sealsor leaks are the cause.

Remember: Dirt which does not enter the system or is not caused by wear need not beremoved by ltration.

Filtration is intended to remove contamination from the hydraulic uid. For this, however,the contamination must be passed to the lter element. In a closed circuit withoutcircuit purging, existing contamination can only be removed from the system with the oilleaking from the pistons, the control valve etc. Since only particles smaller than the leakgap width can leave the closed circuit, the remaining particles stay in the circuit and can

lead to erosion damage in areas with high ow velocities.

This can be avoided by circuit purging, i.e. by forcing 5 - 6 l/min from the low pressurecircuit via an spool valve and a purge relief valve.

The contamination ushed out in this way (including particles larger than the leak gapwidth) can now be passed to the lter element installed in the system and be removed.

WHAT IS TO BE DONE

IF THE REQUIRED

CLEANLINESS CLASSIS NOT ACHIEVED?

WHY LOOP FLUSHING?

Back


19/24

19DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003

Design Guideline for Hydraulic Fluid CleanlinessTechnical InformationTaking of Fluid Samples

Fluid samples must be drawn very carefully into appropriate bottles to preventextraneous 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 uid Comments/remarks if necessary

Sampling points should be provided at the design stage of the hydraulic installation.They should be arranged in the turbulent main ow.

Sampling device

Important: Take precautionary measures to protect personnel and equipment. If turbulent ow conditions prevail in the main ow, a typical sampling device as

illustrated in the illustration P001 325 A quick fastening coupling 6 is permanently attached to the opening through which

the sample is to be 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 lm which covers the opening of the sample bottles. If no turbulence can be guaranteed in the ow, a device for generating turbulence

must be used, e.g. a turbulent sampler.

Sampling method

Ball valve 5 is opened. Allow at least 200 ml uid to ow through the sampling device before collecting the

uid. Without closing the ball valve, place the sample bottle in the position for collecting

the uid. Pierce the protective lm covering the bottle opening with the sharp end of thecapillary tube.

Draw a sample of not more than 90 % and not less than 50 % of the bottle volume. When a sufcient sample quantity has been collected, remove the sample bottle

before stopping the ow 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 uid traces are to be removed byushing with a suitable solvent.

Immediately after dismantling the dust protection cap is replaced on the permanentlymounted part of the quick fastening coupling.

TAKING OF FLUID

SAMPLES

SAMPLING ACCORDING

TO ISO 4021 FROM

A SYSTEM IN OPERATION

Back


20/24


21/24

21DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


The frequency and intensity of the maintenance work to be performed depend on theburden generated by environmental inuences and on the workload.

Special attention must always be paid to the operational suitability and cleanness of thehydraulic uid.

Scavenging and running in

Before a hydraulic system is commissioned, the assembly dirt must be removed. Thisis best done by ushing the whole installation with a portable lter unit. Mineral oil(or another medium compatible with the hydraulic uid to be used subsequently) ispumped through the whole system or parts of the system at the highest possible owvelocity. The assembly dirt is ltered in the lter unit.

During this process the elements of the built-in lters are to be removed. Small or lesssensitive systems can also be scavenged with the built-in lters during the running-inprocess. It must be ensured that the system is run without load but with a displacementwhich is gradually increased up to maximum. The illustration belowshows the relationbetween the design-specic, admissible cleanliness level and the actual cleanliness priorto commissioning.

Working Hints

WORKING HINTS

Typical relation between the design-specic, admissible cleanliness level Aand the actual cleanliness level prior tocommissioning D.

It is vital that the system be ushed andrun at low pressure until the requiredcleanliness level is achieved.

Particlecountperunitvolume

Particle size in m (log2)

102

10 15 5025 100

P001 326E

5

103

104

105

106

D

A

Back


22/24

22 DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


Permanent

Every lter should be equipped with an optical/electrical indicator of the contamination

level at the lter. In this way it is possible to establish at any time whether there is still anydirt absorption capacity or whether the elements have to be changed. The checks shouldbe carried out daily once the operating temperature is achieved.

If a contamination indicator with electrical signal device is used, the selected signal isemitted at operating temperature when the lter element is contaminated. During thewarming up phase a contaminated signal is nearly always emitted due to the higherdifferential pressure unless the contamination indicator is equipped with a signal cut-outfor the cold start phase.

Cyclical

With regular monitoring, lters are suitable as wear surveillance elements for thecomponents of the hydraulic system. If the operator keeps a log of lter changes, itcan be assumed in the event of shorter changing intervals that the wear of the systemcomponents is increasing. The origin of the main contamination can be ascertained byanalysing a uid sample and the contaminated element. A comparison of the results withthe materials used allows preventive maintenance before a complete failure interruptsproduction 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 uid

Any uid used to top up losses should always be poured in via a ne lter in order to

maintain the cleanliness class. Where appropriate facilities are available, the returnow lter can be used. It is advisable to provide a permanent connection which shouldbe included in considerations at the design stage. Any opening of the tank/reservoirfor maintenance purposes (topping up hydraulic uid, sampling, changing lter ele-ments in built-in tanks etc.) should always be avoided as far as possible by an expedientdesign. Even though some ventilation lters have a so-called lling screen (mesh width> 100 m), this still does not afford any protection against the penetration of particles ofthe order of magnitude of 10 - 100 m.

MONITORING

OF CONTAMINATION

Working Hints

Back


23/24

23DKMH.PN.980.D1.02 520L0467 Rev. F 03/2003


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 becomesmore contaminated.Extreme care must be taken when changing the element. The operating instructionsmust be followed precisely.

The following standard values apply for lter maintenance intervals:

1. 24 hours after commissioning the system2. After the running-in period (50 - 100 hours of service)3 Normal maintenance after 300 - 500 hours of service

Literature:

1. Guidelines to Contamination Control in Hydraulic Fluid Power Systems (1985),AHEM (The Association of Hydraulic Equipment Manufacturers, London)

2. Filterbel fr Hydraulikssigkeiten und Schmierstoffe,MAHLE Industrielter, hringen

3. Leitfaden zur Optimalen Auswahl von Argo Filtertypen fr Hydrauliksysteme,ARGO Filter, Kraichtal-Menzingen.

Figure and text source: 9, 17, 18, 20, 21, 22, 23 MAHLE Industrielter

MONITORING

OF CONTAMINATION

(continued)

Working Hints

Back


24/24

Sauer-Danfoss Hydraulic Power Systems

Market Leaders Worldwide

Sauer-Danfoss is a comprehensive supplier providing completesystems to the global mobile market.

Sauer-Danfoss serves markets such as agriculture, construction, roadbuilding, material handling, municipal, forestry, turf care, and manyothers.

We offer our customers optimum solutions for their needs anddevelop new products and systems in close cooperation andpartnership with them.

Sauer-Danfoss specializes in integrating a full range of systemcomponents to provide vehicle designers with the most advancedtotal system design.

Sauer-Danfoss provides comprehensive worldwide service for its

products through an extensive network of Authorized ServiceCenters strategically located in all parts of the world.

OUR PRODUCTS

Hydrostatic transmissions

Hydraulic power steering

Electro-hydraulic power steering

Electric power steering

Closed and open circuit axial piston

pumps and motors

Gear pumps and motors

Bent axis motors

Orbital motors

Transit mixer drives

Planetary compact gears

Proportional valves

Directional spool valves

Cartridge valves

Hydraulic integrated circuits

Hydrostatic transaxles

Integrated systems

Fan drive systemsElectrohydraulic controls

Digital electronics and software

Electric motors and inverters

Sensors

Sauer-Danfoss (US) Company2800 East 13th StreetAmes, IA 50010, USAPhone: +1 515 239-6000, Fax: +1 515 239 6618

Sauer-Danfoss (Neumnster) GmbH & Co. OHGPostfach 2460, D-24531 NeumnsterKrokamp 35, D-24539 Neumnster, GermanyPhone +49 4321 871-0, Fax: +49 4321 871 122

Sauer-Danfoss (Nordborg) A/SDK-6430 Nordborg, DenmarkPhone: +45 7488 4444, Fax: +45 7488 4400

Back

Documents

Design Guideline for Hydraulic Fluid Cleanliness Applications_Rev-F_03-2003