In general, fire-resistant hydraulic fluids can be divided into two categories: hydrous and anhydrous fluids. Classification is shown in the table below.

Hydrous fire-resistant fluids HFA – these fluids contain an extremely high water content of over 80%. HFA-E are mineral or synthetic oil based oil-in-water emulsions. HFA-S belong to synthetic solutions that are generally polyglycol based. These usually contain more water than the emulsions.

HFA fluids are often used at temperatures between +5°C and +55°C and at relatively moderate pressures for underground mining. As they are very thin-bodied like water, leakage losses are a frequent occurrence. Their useful life may be impaired by bacteria and fungal infestation.

HFB – this category also includes water-in-oil emulsions. These contain about 40% water; the remainder is usually mineral oil. These are also fire-resistant, but not approved for use in Germany due to their poor fire properties.

HFC – these are the most common hydrous, fire-resistant hydraulic fluids as they have the best fire resistance and hydraulic properties. Fluids of this type are water-glycol based, aqueous polymer solutions and have a water content of about 50% or about 20% as HFC-E.

HFC fluids are prescribed for underground mining. They are also used wherever hydraulic fluid escaping under high pressure can ignite on contact with hot materials. At temperatures above 600°C, these fluids should not ignite or continue to burn. These fluids can be used at ambient temperatures of -20°C to +65°C and up to working pressures of 250 bar. When using these fluids, a stable ratio between water and glycol must be ensured as changes in viscosity may result due to the evaporation of water at high temperatures.

Anhydrous fire-resistant fluids HFD – these fluids have the best hydraulic properties of all fire-resistant hydraulic fluids. They are preferably used when working with a naked flame or red-hot materials. These fluids are also used as a control fluid in steam turbines or aircraft engines. HFD-R is phosphoric acid ester based, HFD-S is anhydrous chlorinated hydrocarbon based. Carboxylic acid ester is used for the production of HFD-U, which is particularly suitable for very high pressures or as an environmentally-friendly fire-resistant fluid, e.g. in tunnel drilling machines. The operating temperature range of HFD fluids is between -20°C and +150°C.

An installation to be changed from an HFC fluid to an HFD fluid must be thoroughly flushed beforehand. Vice versa, HFD residues in the system must be thoroughly removed and bound with special chemicals if necessary.

Oil analyses have been firmly established in the field of hydraulics for monitoring oil condition and for pro-active maintenance. The aim is to increase operational reliability and availability and reduce costs at the same time by:

• safely adjusting the oil changing intervals to suit the prevailing conditions by monitoring the condition of the oil;
• monitoring the oil purity as the most frequent cause of failure;
• ensuring that operators effectively optimise oil maintenance measures;
• recognising irregularities in good time to avoid unplanned failures.

Due to their special composition and the underlying conditions resulting from their use, fire-resistant fluids require the use of additional special investigation methods. Similar to rapidly biodegradable hydraulic fluids, anhydrous HFD fluids can be checked with „Biosets“ using the OELCHECK analysis kit 3. Owing to their high water content, certain aspects must be taken into account for hydrous HFC fluids. OELCHECK offers an investigation programme that has been specially adapted for hydrous HFC fluids.

For the routine testing of HFC fluids in medium-sized installations, OELCHECK offers the analysis kit 3. The following parameters are monitored:

• Element analysis
  Metallic particles, dust, corrosion, additives, tramp oils, salts, e.g. from the water.
• PQ-Index
  Magnetic metal particles or contaminants.
• Kinematic viscosity at 40°C
  Lubricity and fluidity, oil aging, water loss.
• Water content in %
  Monitoring of the application concentration, evaporation compensation.
• pH/reserve alkalinity
  Monitoring of chemical additives: residually active additive reserve for neutralisation and corrosion protection.
• Visual check
  Coarse contaminants, turbidity, discolouration, tramp oil.

For hydraulic installations with capacities of several hundred litres or installations with increased availability or additionally required longevity of components or oil filling, OELCHECK recommends the analysis kit 5, which also tests the following parameters:

• Solid contaminants
  Contamination control, filtration and indication of the solid contaminant content in per cent by weight.
• Density
  Control parameters for application concentration and tramp oil content
• Purity class
  Particle counting (fine and very fine particles), quality of filtration