Oil cleanliness classification has one purpose only: to describe the quantity and the size distribution of the particles in the oil. It does not describe water, it does not describe gas, and it does not describe the degree of oxidation. So when the phrase cleanliness NAS 1638 class 5 is read, the correct interpretation is that the particle control has reached that level, not that the oil as a whole is compliant.
How to read the class
NAS 1638 describes cleanliness with a set of codes, and the lower the code number the cleaner the oil. It is the continuation of the system in which the particle count above a given particle size in a given volume of oil is assessed, and each class corresponds to one limit for the particle count. Two directions are enough when reading the codes: within the same particle size range, the higher the code the dirtier the oil; for the same cleanliness code, the finer the particle size band the stricter the requirement.
It is not the same thing as the filtration accuracy
The most common misuse is to equate NAS 1638 class 5 with a filtration accuracy of 5 μm directly. The filtration accuracy describes the retention capability of the medium, while the cleanliness class describes how many particles are left in the oil sample after treatment; the two are related but not equivalent. The more sensible question is to which class this medium can purify the oil, rather than the argument that a 5 μm element is wanted and the oil will therefore be class 5.
Graded filtration: every stage of elements does one job
To reach a high cleanliness class reliably, what is needed is a graded filtration chain, not the finest element fitted and left to carry the load. Two commonly used configurations illustrate this:
- Insulating oil line (ZYD-100): the primary element 150-250-50 holds back the large particles above 50 μm, and 5 fine elements 150-500-5 carry the final ≤1 μm filtration.
- Hydraulic oil line (TYA series): coarse element 110-250-50, second-stage element 110-400-20 and fine element 110-600-5 are arranged in sequence, stripping the particles from coarse to fine stage by stage.
The direct benefit is that the fine elements do not have to carry the whole particle load on their own. Once the dirt holding capacity has been taken up by the elements ahead of them, the differential pressure across the fine stage rises markedly more slowly and the element replacement interval lengthens accordingly.
The two configurations have different objectives: the insulating oil line answers to the strict particle requirements of equipment at high voltage levels, while the hydraulic oil line puts more weight on the effect of the oil on the wear of the system valves. The choice of medium and the replacement interval are therefore different as well.
Do not neglect the sources of contamination
Particles do not arise out of nothing. Filling and storage of new oil, wear on the seals, residues in the pipework and the time the system stands open during maintenance are all sources of contamination. Seals that harden with age, a damaged strainer and a drum mouth that is not sealed tightly will all let the cleanliness of the treated oil deteriorate again after it has stood for a few weeks. Cleanliness is therefore an indicator that has to be reviewed at regular intervals, not a property that one treatment settles for good.
Before choosing the medium, set the target class, then work back to the accuracy of the stage you need and the number of elements; that is cheaper than adding the finest element on the strength of experience.