How many times have users of hydraulic oils observed—despite no apparent change in their processes—that the service life of industrial filters in the production circuit could vary widely? At times it dropped significantly, then increased again, and this in a completely random manner. This phenomenon becomes even more pronounced when fine filtration is required, i.e., with a filtration rating below 10 µm. In such cases, the first reflex is often to question the consistency of the filters’ performance. But have you ever looked into the filterability of the oil you are using?
What is filterability?
Several standardized protocols (AFNOR, BENSCH, DENISON) are used to define this concept for hydraulic fluids. The principle is to pass a given volume of fluid through a specified filter surface. Depending on the standard, either time or volume is measured to determine:
• a Filterability Index (FI), corresponding to the time ratio between the first 50 cm³ and the last 100 cm³ (out of a total volume of 300 cm³);
• a Filterability Factor, corresponding to the volume that passes through the filter before clogging, out of a total of 1000 cm³.
How can you determine the filterability of your oil?
Increasingly aware of the issue, some oil manufacturers have begun to mention it. Certain technical data sheets now include terms such as “Good filterability.” But the information remains limited, especially since this parameter can fluctuate significantly from one production batch to another. It is well known that, depending on the geographic origin of the crude oil, the resulting oil will not exhibit the same filterability characteristics at all.
We now know that this property depends mainly on the cleanliness of the base oils and additives, and above all on the hydrolytic stability of the functional additives—meaning each individual compound. As a result, for the exact same oil grade from the same manufacturer, the service life of the filters, independently of any external contamination, can vary greatly. This phenomenon becomes increasingly noticeable as we improve the efficiency of industrial filters, i.e., with ever finer filtration ratings.
For example, in applications requiring very fine filters (down to 1 µm absolute), such as hydraulic systems equipped with servovalves, the fluids used must exhibit excellent filterability characteristics, even in the presence of water (up to 2%). Otherwise, filter life will be significantly reduced, and not solely due to the retention of contaminants in the oil.
Practical example
We tested the filterability (AFNOR and BENSCH methods) of two hydraulic oils from two different manufacturers with identical properties, disregarding the effect of temperature.
Operationally, we had observed that the service life of the filters—strictly identical in both cases and used under exactly the same conditions—was markedly different.
1) AFNOR method: filterability (without water) according to ISO 13357-2 (formerly NF E48-690).
The methodology is simple and easy to implement: under a constant pressure of 1 bar, 300 cm³ of fluid pass through a 0.8 µm membrane. The time difference required to filter 50 cm³ is measured between the start and the end of the test.
| Sample | Filterability Factor | Comments | Photo |
|---|---|---|---|
| Oil A (tank outlet) | 2.29 | Thick clogging gel / Various particles | Photo PO46-1 |
| Oil A (truck outlet) | 5.26 | Thick clogging gel / Various particles | Photo PO46-2 |
| Oil B (drum) | 1.01 | No gel / Uniform, fine particles | Photo PW46-1 |
2) BENSCH method
The methodology is simple and easy to implement: under a constant pressure of 1 bar, 1000 cm³ of fluid pass through a 1.2 µm membrane. The volume that passes through is measured until the membrane becomes clogged (i.e., the liquid no longer flows through).
| Sample | Indice de Filtrabilité | Commentaires | Photo |
|---|---|---|---|
| Oil A (tank outlet) | 18.23 | Thick clogging gel / Numerous particles | Photo PO46-3 |
| Oil A (truck outlet) | 15.63 | Very thick clogging gel / Numerous particles | Photo PO46-4 |
| Oil B (drum) | 104 | No gel / Numerous particles | Photo PW46-2 |
First observation: visual inspection of the membranes alone does not allow one to evaluate—or even estimate—filterability.
Second observation: the AFNOR Filterability Factors are not good, and even quite poor, for oil A. The value for oil B is almost perfect, as it is close to 1.
Using oil A (compared with oil B) can only have a negative impact on the service life of the filter elements, regardless of their brand.
Third observation: the BENSCH method confirms the results obtained with the AFNOR method. Either method can therefore be used interchangeably to determine the filterability of an oil.
Conclusion
- When you need to use filter elements with a filtration rating below 10 µm absolute, it is advisable to ask your oil supplier whether information is available regarding its filterability.
- When the service life of filters varies significantly, even though no apparent change in the process has occurred, you should not automatically blame the quality of the filter elements, but instead take into account all parameters that may explain this variation.
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