Hydraulic Oil Cleanliness

Good quality hydraulic oil is essential in any hydraulic circuit. A key parameter in determining the quality of your hydraulic oil is its cleanliness, or as it is sometimes refered to, the oil's particle count.

Why is Hydraulic Oil Cleanliness Important?

Hydraulic fluid is used as the motive power for many actuated valves, often in subsea applications. Integral to the actuator are components with small bores that can become blocked if the fluid has particles of debris within it. Clearing a blockage can be difficult, time consuming and very expensive, especially in subsea applications. Therefore it is common to specify that the hydraulic fluid used in the system is cleaned to a measurable degree of cleanliness e.g. NAS 6.

What are the Standards for Hydraulic cleanliness?

Measuring hydraulic cleanliness is not straight forward, consequently various standards exist on this subject. The two most likely to be encountered in the oil & gas, and process industries are The ISO Cleanliness Code (ISO 4406), and The NAS 1638 cleanliness standard.

An introduction to each of these allowable particle contamination standards is provided below.

The ISO Cleanliness Code, ISO 4406

The ISO Cleanliness Code, ISO 4406, 1987 is perhaps the most widely used International standard for representing the particle contamination level of industrial fluid power systems. Under ISO 4406 cleanliness is classified by a two number code, e.g. 16/13, based on the number of particles greater than 5 µm and 15 µm respectively in a known volume of fluid. However some manufacturers have expanded the code to three numbers by the addition of a code number representing the number of particles greater than 2 µm, e.g. 18/16/13. Using the ISO particle count table below, we can see that an oil with a cleanliness rating of 18/16/13 would mean that it contained
1300 - 2500 particles greater than 2 micron in size
320 - 640 particles greater than 5 micron in size, and
40 - 80 particles greater than 15 microns in size.

The full table of particle count ranges for the ISO cleanliness code is shown in the ISO 4406 chart below:

ISO4406 Cleanliness chart



The NAS 1638 Cleanliness Standard

The NAS 1638 cleanliness standard was originally developed for aerospace components in the US but is still widely used for industrial and aerospace fluid power applications. It is used widely in the UK North Sea industries. NAS1638 is comprised of fluid cleanliness classes, each class defined in terms of maximum allowed particle counts per 100ml for designated particle size ranges. See the NAS 1638 chart below for the various cleanliness levels:

NAS1638 Cleanliness chart

Converting Oil Cleanliness Standards - ISO to NAS to ISO

Inevitably, there is a requirement to be able to compare these cleanliness classification codes. See the NAS vs ISO cleanliness chart below for a good approximation.

Note - The comparisons relate to particle count data only. To conform to any particular standard reference should be made to the recommended experimental procedure for hydraulic oil analysis and particle count test.

iso to nas chart

SAE AS4059 / ISO 11218

The SAE Aerospace panel developed AS4059 specifically for aircraft hydraulic systems. This subsequently became ISO 11218. It is unlikely you will encounter this standard in the process industries.


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