Certain applications must operate near a heat source with elevated temperatures or even open flames or electrical heating units. Mineral oil is very flammable. It not only catches fire easily but will continue to burn even after removing the heat source. This fire hazard situation can be eliminated by several different choices of fluids. These fluids are not fireproof, only fire-resistant, which means they will burn if heated past a certain temperature but they will not continue to burn after removing them from the heat source.

Generally, the fire-resistant fluids do not have the same specifications as mineral oil-based fluids. Pumps often must be down rated because the fluids lubricity or specific gravity is different and would shorten the pumps service life drastically at elevated pressures or high rotary speeds. Some fire-resistant fluids are not compatible with standard seal materials so seals must be changed. Always check with the pump manufacturer and fluid supplier before using or changing to a fire-resistant fluid.

Water
Originally, hydraulic circuits used water to transmit energy (hence the word hydraulics). The main problem with water-filled circuits was either low-pressure operation or very expensive pumps and valves to operate with this low viscosity fluid above 500 to 600 psi. When huge oil deposits were discovered, mineral oil replaced water because of its additional benefits. Water made a brief comeback during an oil shortage crisis but quickly succumbed when oil flowed freely again.

In the late 90s, water again made inroads into oil-hydraulic systems. Several companies have developed reliable pumps and valves for water that operate at 1500 to 2000 psi. There are still limitations (such as freezing) to using water, but in certain applications it has many benefits. One big advantage is that there are fewer environmental problems during operation or in disposing of the fluid. Price also is a factor because water costs so little and is readily available almost anywhere.

Some suppliers are making equipment that operates on seawater to eliminate possible contamination of the earths potable water sources. These systems operate at elevated pressures without performance loss.

High water-content fluids

Some types of manufacturing still use water as a base and add some soluble oil for lubrication. This type of fluid is known as high water-content fluid (or HWCF). The common mixture is 95% water and 5% soluble oil. This mixture takes care of most of the lubricity problems but does not address low viscosity concerns. Therefore, systems using HWCF still need expensive pumps and valves to make them efficient and extend their life.

Rolling mills and other applications with molten metals are one area where HWCF is prevalent. Often the soluble oil is the same compound used for coolant in the metal-rolling process. This eliminates concerns about cross-contamination of fluids and the problems it can cause.

Water-in-oil emulsions
Some systems use around 40% water for fire resistance and 60% oil for lubrication and viscosity considerations. Again, these are not common fluids because they require special oil and continuous maintenance to keep them mixed well and their ratio within limits. Most manufacturers do not want the problems associated with water-in-oil emulsions so their use is very limited.

Water glycol
A very common fire-resistant fluid is water glycol. This fluid uses water for fire resistance and a product like ethylene glycol (permanent anti-freeze) for lubricity, along with thickeners to enhance viscosity. Ethylene glycol will burn, but the energy it takes to vaporize the water present quickly quells the fire once it leaves the heat source. This means a fire would not spread to other parts of the plant. Always remember fire-resistant not fireproof.

Water glycol fluids are heavier than mineral oil and do not have its lubricating qualities, so most pump manufacturers specify reduced rpm and lower operating pressures for water glycol. In addition, the water in this fluid can evaporate, especially at elevated temperatures, so it must be tested regularly for the correct mixture.

Cost is also a consideration. Water glycol is more expensive than oil and requires most of the same considerations when disposing of it.

Always check with the pump manufacturer before specifying water glycol fluid to see what changes are necessary to run the pump with this fluid. Seal compatibility is usually not a problem, but always check each manufacturers specifications before implementing this fluid. In addition, it is imperative to completely flush a system of any other fluids before refilling with water glycol.