In several areas of hydraulic systems, the flammability of the mineral oil constitutes a high risk.
The group of flame-resistant hydraulic fluids is comprised of water-bearing emulsions (HFA and HFB fluids) of aqueous solutions (HFC pressure fluids) and water-free, synthetic fluids in the HFD group.
With the exception of the HFA group –– which is practically incombustible –– all of the others are only "flame resistant", i.e. they do burn, but they need a higher temperature than mineral oil to ignite (approx. 350–400°C). These groups are very similar to mineral oil in terms of their viscosity characteristics, while HFA is water-thin (Table S 1). In most cases, the HFA pressure fluids (DIN 24 320) require a special structure, while HFC and HFD pressure fluids can be used to operate all construction elements in hydraulic systems under certain constraints (VDMA 24 317).
Figure Table S 1: Properties of flame-resistant hydraulic fluid