Keeping it clean

Published:  14 October, 2014

The British Fluid Power Association publication ‘Fluid Power Engineer’s Data Book’ is a valuable source of information covering a wide range of technical topics related to hydraulic and pneumatic installation and maintenance. One critical theme covered is that of cleanliness control. This article outlines some of the key points covered within the book.

The presence of particulate contamination (dirt) is the single most important factor governing the life and reliability of fluid power systems. Operating with clean fluids is essential in order to achieve modern performance and reliability requirements. All of the aspects within this article are comprehensive described in be BFPA/P5 – Guidelines to Contamination Control in Fluid Power Systems. It also provides information for designing and maintaining clean systems.

TCL for a hydraulic system

The Target Cleanliness Level (TCL) is the operational cleanliness of the system and the level that should be achieved and maintained by the cleanliness control measures designed for that system. The TCL should be selected at the design stage and used to define the cleanliness through the production and commissioning processes. The method for selecting the TCL described in BFPA/P5 is based upon both the sensitivity of the system to particulate contamination and the life and reliability required by the user.

Design the filtration to achieve the required system TCL

Filtration Standards: A wide range of standards is available to test a filter’s capability to perform under various system conditions, namely:

Parameter –

Collapse/burst resistance BS ISO 2941

Fabrication integrity BS ISO 2942

Fluid compatibility BS ISO 2943

End load strength BS ISO 3723

Flow fatigue test BS ISO 3724

Flow/pressure loss BS ISO 3968

Pressure fatigue (housings) BS ISO 10771-1

Filter qualification programme BS ISO 11170

Testing differential pressure devices BS ISO 16860

Filtration performance BS ISO 16889

Low fatigue using high viscosity fluid BS ISO 23181

Degree of Filtration – BS ISO 16889

This ISO standard describes the ‘Multi-pass’ method for evaluating the filtration performance of a hydraulic filter element. The element is subject to constant circulation oil during which fresh contaminant (ISO Medium Test Dust) is injected into the rig. The contaminant that is not removed by the element under test is recirculated, thereby simulating service conditions. The test continues until the element is ‘blocked’.

The measure of the filter’s ability to remove contaminant is determined by the analysis of fluid samples extracted from upstream and downstream of the filter is expressed as the Filtration Ratio βx(c), thus:

βx(c) = number of particles larger than x μm(c) upstream of the filter number of particles larger than x μm(c) downstream of filter for the sizes measured.

BS ISO 16889 specifies the number of ratings to define the element’s performance over a wide size range and gives the μm(c) rating at β(c) values of 2, 10, 75, 100, 200 and 1000.

The test also gives a measure of the element’s ability to retain quantities of ISO Medium Test Dust.

Component cleanliness

Components should be cleaned to a level that is commensurate with the system TCL. Guidelines on how to achieve and measure component cleanliness are provided in BS ISO/TR 10949 and BS ISO 18413.


Flushing is a process designed to remove dirt introduced into the system during manufacture, assembly and initial operation. It is also used when significant maintenance is undertaken. The requirements are summarised below:

• Turbulent flow regime to pick up the particles from the walls of components and transport them to the flushing filter.

• The Reynolds Number (Re) defines the flow condition and should be greater than 4000, thus:

Re = 21,200 x Q/(v x d)


Q > 0.189 x v x d (l/min) to achieve Re > 4000.


Q = Flow rate (l/min)

v = Viscosity (mm/s)

d = pipe diameter (mm)

• A ‘fine’ filter to capture transported particles quickly and effectively.

Taking fluid samples

Fluid samples are extracted from the hydraulic system to determine the operating cleanliness level and whether the TCL is being achieved. To ensure that data is representative, care must be taken with this process and pre-cleaned sample bottles are essential. Relevant documents are BFPA/P5, BS ISO 3722, ISO 4021 and BS ISO 21018-1.

Cleanliness management

The requirements are defined in BFPA/P5 and the essential points are detailed below:

The system should be correctly designed to achieve and maintain the TCL.

Inspect filters regularly for signs of blockage and replace when indicating blockage.

Filter oil into the system.

Monitor the fluid cleanliness on a regular basis.

Promptly implement corrective actions if the TCL is exceeded to limit damage to components.

Have specifications for both fluid cleanliness (TCL) and filters.

Educate personnel involved with the process on the need and benefits of cleanliness.

More information

For more information on fluid and hydraulic cleanliness, or other hydraulic and pneumatic guidance, the BFPA publication ‘Fluid Power Engineer’s Data Book’ is available by contacting the Association at, or Tel: 01608 647900.

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