Fire Resistant Hydraulic Fluids

The function of a hydraulic fluid is to provide a means of power transfer in a hydraulic circuit.  The fluid must lubricate the pumps and motors without impairing the function of the other components in the total hydraulic system. 

Basic Functions of Hydraulic Fluid

The basic functions of a hydraulic fluid are there to provide:

  • Rust Inhibition
  • Demulsibility
  • Filterability and compatibility
  • Hydrolytic stability
  • Corrosion Control
  • Oxidation Stability
  • Thermal Stability
  • Anti-wear

There are two classes of hydraulic Fluids:

  • Conventional Mineral
  • Fire Resistant 

Fire Resistant Hydraulic Fluids

There are three main types of fire resistant hydraulic fluids:

  • High Water Fluids
  • Water Glycols
  • Synthetics    

High Water Fluids

High water fluids are also known as high water base fluids (HWBF), high water content fluids (HWCF), 5-95 fluids or ISO HFA fluids. 

The original fluids were 5 percent ‘soluble oil’ emulsions (oil-in-water) having a milky appearance, or 5 percent solutions (synthetic chemical additives in water), which are clear in appearance.

Oil-in-water emulsions contain tiny droplets of specially refined oil dispersed in water. We say that water is the continuous phase and the fluid’s characteristics are more like water than oil.

Around 1980, micro-emulsions with considerably smaller droplet size were introduced. These emulsions have a translucent, almost transparent, appearance. The dispersed internal phase in micro-emulsions is synthetic in nature as opposed to mineral oil in ‘soluble oil’ emulsions.

High water fluids are highly fire resistant and have excellent cooling characteristics. When properly formulated, these fluids offer strong protection against rust and some degree of steel-on-steel lubricity. However, because the viscosities approach that of water, pumps are generally de-rated in order to obtain adequate service life.

Operating Temperatures

The operating temperatures should be limited to a maximum of 1200F (490C) in order to minimise evaporation and deterioration of the fluid. Often, this is not a problem as high water systems tend to run cooler than oil systems because they have higher specific heat and thermal conductivity characteristics. Temperatures below freezing (320F or 00C), however, may cause separation of the phases or otherwise affect the fluid additives.

Because of the relatively high density and low viscosity of the fluid, inlet conditions and fluid conductor sizing should be carefully controlled to keep the fluid at a relatively low velocity. Excessive turbulence produces a release of air and fluid vapour in the form of bubbles, which cause cavitation, noise, erosion and wear.

Compatibility with Seals, Metals and Protective Coatings

Because of the great diversity of fluids, the supplier of each specific fluid must be consulted for compatibility information. In general, the seals normally used with petroleum oils are satisfactory for these fluids. Asbestos, leather, paper or cork materials should not be used, because they tend to deteriorate in water.

Although high water fluids are formulated with corrosion inhibitors, galvanic action can occur with certain combinations of metal (such as copper-to-steel or unanodised aluminium-to-steel).

Petroleum compatible paints cannot be used because of the solvent effects of the fluids. Special epoxy-type paints have been found to be compatible with some of the fluids.

Water-Glycol Type Fluids

Water-glycol fluids (ISO HFC) are compounded of: 

  • 35 percent to 45 percent water to provide resistance to burning.
  • A glycol (a synthetic chemical of the same family as permanent anti-freeze ethylene or other glycols).
  • A water-soluble thickener to improve viscosity.

Water-glycol fluids also contain additives to prevent foaming, rust, corrosion and to improve lubrication. 

Water-glycol fluids generally have good wear resistance characteristics, provided that high speeds and loads are avoided. The fluid has a high specific gravity (it is heavier than oil), which can create a higher vacuum at pump inlets. Certain metals such as zinc, cadmium and magnesium react adversely with water-glycol fluids, generating gummy residues, which plug orifices and filters and cause valve spools to stick. It is recommended that parts, which are alloyed or plated with these metals not be used with water-glycol. Examples of such parts might be galvanised pipe, and zinc or cadmium plated strainers, fittings and reservoir accessories.

Many of the synthetic seal materials used with petroleum oils are also compatible with water-glycol fluid. Asbestos, leather and cork-impregnated materials should be avoided in rotating seals, since they tend to absorb water.


Some disadvantages of these fluids are as follows:

  • They must be measured continually for water content and evaporation must be made up continually to maintain required viscosity.
  • Evaporation may also cause loss of certain additives, thereby reducing the life of the fluid and of the hydraulic components.
  • Operating temperatures must be kept low.
  • The cost of water-glycol fluids (at the present time) is greater than that of conventional oils.

Synthetic Fire Resistant Fluids

Synthetic fire resistant fluids (ISO HFD) are laboratory synthesised chemicals, which are less flammable than petroleum oils. Typical of these are: 

  • Phosphate esters
  • Polyol esters
  • Halogenated (fluorinated and/or chlorinated) hydrocarbons
  • Mixtures of phosphate esters or polyol esters and petroleum oil.

Since the synthetics do not contain any water or other volatile material, they operate well at higher temperatures than water containing fluids.

They also are suitable for higher-pressure systems than the water containing fluids.

Synthetic fire resistant fluids do not operate best in low temperature systems. Auxiliary heating may be required in cold environments.

This information has been sourced, reviewed and adapted from materials provided by Castrol.

For more information on this source, please visit Castrol.


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