Lube-Tech
The filterability test conditions are summarised hereafter:
Figure 2. Description of wet and dry filterability test ISO 13357-1&2
The Stage I determination is based on a comparison of the mean flow rate of a fluid through a test membrane with its initial flow rate The Stage II determination is based upon the ratio between the initial flow rate of fluid and the flow rate at the end of the test. It is considered that the Stage II result is a more severe test and is more sensitive to the presence of gels and fine silts in the oil. When a mixture passes the Stage II of filterability ISO 13357-1, it is considered compatible.
4.4 Step 2: surface properties The surface properties, including the air release value, the foaming characteristics and water separability are often sensitive to composition changes and oil degradation; the by-products are not only specific to the composition of the base oil and its source but will also vary depending on the degradation conditions; they are alcohols, aldehydes, ketones, esters, acids. One of the most important properties is the air release, which must remain acceptable during the fluid life. The degradation associated with the presence of air in the fluid are oxidation, fluid compressibility, Microdieseling, cavitation and wear.
A survey based on dry filterability and surface
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.137 page 5
properties carried out on more than 50 miscible oils showed than more than 25% of the tested oils were not compatible.
4.5 Step 3: loss of performance The last step is often not realised when the HF compatibility is examined. It is however important because, if the hydraulic fluids mixtures are slightly incompatible, the effect, like seals deterioration, deposit formation, oxidation, corrosion or hydrolysis won’t be seen rapidly.
In any case, vigilant monitoring and inspection must be done to detect signs of incompatibility at an early stage; unfortunately, once detected, damage control and remediation as well as fluid replacement may be the only practical alternatives.
This is where product performances evaluation through oil analyses can play an important role.
The last part of this article focuses on some examples of possible incompatibility, taking into account the types of HF mixed together, the components and the material.
The most relevant test that may put into evidence the incompatibility is described.
4.5.1.1. HM or HV mixed with HL or HR in a hydraulic system which contains copper- based alloys
These fluids are all mineral oil-based. The HL and HR contain rust and oxidation inhibitors (and VI improver for HR) and are known as R&O oils. Some hydraulic fluids which contain antiwear (HM or HV) can be aggressive to yellow metal (brass and bronze) as well as silver alloyed components in piston pumps.
4.5.1.2. Top up with HE fluids (HEES) In high pressure hydraulic system, the use of biodegradable fluids has come under scrutiny due
LUBE MAGAZINE NO.166 DECEMBER 2021 27
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