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As the name of the category suggests, fire‐resistant fluids are specifically used in applications where fire or explosion hazards must be limited. These fluids are in most cases water based. A basic classification of these fluids can be done according to ISO 7745 [10], summarized by the tree diagram of Figure 2.3.

HFA fluids contain at least 80% of water, and they can be further divided into HFAE, which are oil‐in‐water emulsions with anti‐wear additives and HFAS, which contains other chemical solutions in water. HFB are emulsions of water in oil, with a minimum of 40% of water content. HFC are glycol‐in‐water solutions (from 35% to 60% of water content), and they also include additives to improve fluid viscosity. HFD fluids are synthetic products without water. In particular, the HFDR fluids are based on phosphate esters; the HFDS fluids are based on chlorinated hydrocarbons; the HFDT fluids are a base of a mixture of HFDR–HFDS fluids; finally, HFDU are fluids with other synthetic products (not further specified).

For HF fluids, especially those containing water, the maintenance or replacement of the working fluid is essential. It is also important to periodically monitor the composition of the fluid to ensure it has not been altered. Hydraulic fluids are often referred to with their commercial name rather than with the correct ISO denomination. A significant example in the fire‐resistant fluid category is the Skydrol, which is a fire‐resistant fluid used in aviation. This fluid belongs to the HFDR category.

Figure 2.3 Classification of fire‐resistant fluids according to ISO 7745.

Source: Adapted from Zarotti [8].