Lube-Tech
Due to the variability of components, precautions have to be taken while mixing fluids together. It is important to check the compatibility between hydraulic fluids.
For some companies, inadvertent or unintentional mixing of fluids is a frequently encountered problem. Mixing different fluids may be the result of improper markings on containers, similar product descriptions, lack of knowledge or information given to operators, and even carelessness.
The degree of incompatibility can vary from complete to slight depending on the miscibility, the base oils and additives chemistries and the loss of fluid performances.
2. Miscibility and compatibility Differences must be made between miscibility and compatibility. Lubricants use a combination of several base oils and additives in a precise ratio to obtain the desired performances; when mixing, reactions can occur to form insoluble materials, flocculation or stratification.
2.1 Miscibility
The miscibility is the ability of two substances to completely mix in any concentration to form a homogeneous solution.
Separation phase, flocculation, stratification, insoluble materials (precipitation) characterise immiscible fluids.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.137 page 2 2.2 Compatibility
The term compatibility is a general term which can be used for metals, seals, paints, materials, ...
Hydraulic fluid compatibility is the ability of hydraulic fluids to mix without degradation of properties or performances.
When hydraulic fluids are mixed, interactions between fluid’s ingredients may occur and lead to a lack of additives solubility, a change in composition or a deterioration of performances. Although new hydraulic fluids may be compatible, in-service fluid of the same type may be degraded or contaminated to such an extent that the new fluid added may not be compatible with the system fluid. In-service fluid compatibility with new fluid additions should also be evaluated on a case-by-case basis to check if the mixtures still have the required properties.
3. Hydraulic Fluids categories The categories of hydraulic fluids within this classification ISO 6743-4 include: - mineral oil which refers to specification ISO 11158 for categories HH, HL, HM, HR, HV, and HG; the difference between categories is based upon additive content. From HH non-inhibited, HL contains rust and oxidation inhibitors, while HR additionally include VI improver. HM fluids contain antiwear in addition to rust and oxidation inhibitors. HG fluids contain similar anti-wear additives to HM fluids plus additives for improved stick/slip performance and friction modifiers. The basic chemistry of HV fluids is similar to HM fluids but includes a VI improver.
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environmentally acceptable fluids which refer to specification ISO 15380 for categories HETG, HEES, HEPG, and HEPR; (base oil chemistry in table below)
- Figure 1. Examples of immiscible fluids It is obvious that immiscible fluids are incompatible. 24 LUBE MAGAZINE NO.166 DECEMBER 2021
fire-resistant fluids which refer to specification ISO 12922 for categories HFAE, HFAS, HFB, HFC, HFDR, and HFDU;
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