It is important for a hydraulic fluid to neither initiate nor support fire. Most hydraulic fluids will however bum under certain conditions. There are many hazardous applications where concern for human safety demands the use of fire resistant fluids. Examples include coal mines, hot metal processing equipments, aircraft and marine fluid power systems.
A fire resistant fluid is one, which can be ignited but will not support a flame when the ignition source is removed. Flammability of a fluid is defined as the ease of ignition and the ability to propagate a flame.
In order to determine the flammability of a hydraulic fluid, the following characteristics are tested:
• Flash point: It is the temperature at which the fluid surface gives off vapors, which can ignite when a flame is passed over it.
• Fire point: It is the temperature of a fluid at which the fluid surface gives off vapors, which are sufficient to support combustion for a time of 5 s, when a flame is passed over it.
Various fire-resistant fluids have been developed in recent times, to reduce fire hazards. There are basically three types of fire-resistant fluids that are commonly used in hydraulic applications. They are discussed below.
This solution contains about 40% water and 60% glycol. These solutions have high viscosity index values, but the viscosity rises as the water evaporates. The operating temperature range of these fluids lies between -23 °C (-9.4 °F) and 83 °C (180 °F approx.). Most of the newer synthetic seal materials are compatible with water-glycol solutions. Metals such as zinc, cadmium and magnesium react with water-glycol solutions and hence should not be used.
This type of fluid contains about 40% water completely dispersed in a special oil base. It is characterized by small droplets of water completely surrounded by oil. Although water provides good coolant properties, it makes the fluid more corrosive. As a result a greater amount of corrosion inhibitor additives are necessary. The operating temperature range of this fluid lies between -28 °C (-18.4 °F) and 83 °C (180 °F approx.). Even in the case of this fluid, it is necessary to replenish the water to maintain proper viscosity of the fluid. These types of fluids are compatible with most rubber seal materials found in petroleum base hydraulic systems.
It is a chemically formulated fluid designed to inhibit combustion and generally has the highest fire-resistant temperature. Typical fluids belonging to this type are phosphate esters and chlorinated hydrocarbons.
The disadvantages of these types of fluids are their low viscosity index, incompatibility with most natural and synthetic rubber seals and high costs. In particular, the phosphate esters readily dissolve pipe thread compounds, paints and electrical insulation.