Hydraulic fluid is a medium used to transmit power within machinery, performing functions like lubrication, heat transfer, and corrosion prevention. Whether hydraulic fluid is flammable depends entirely on its chemical base and formulation. While many common fluids are combustible, specialized fluids are engineered to resist ignition and flame propagation. Understanding the base chemistry and technical metrics of flammability is necessary for assessing the fire risk in any hydraulic system.
Flammability Varies by Chemical Composition
The most widely used hydraulic fluids are derived from petroleum, classifying them as hydrocarbon oils. These mineral-based fluids are inherently flammable, possessing a relatively low flash point that allows them to ignite easily when exposed to sufficient heat or a spark. They pose a high risk when leaking under pressure, as the resulting fine aerosol mist can ignite instantly upon contact with a hot surface.
Water-based hydraulic fluids, such as water-glycol solutions or oil-in-water emulsions, offer a sharp contrast. These formulations contain a significant percentage of water (35% to 95%), which acts as a powerful flame suppressant. The water content prevents ready ignition and helps extinguish flames by cooling the source, making them preferred for high fire potential environments.
Synthetic fluids, including phosphate esters and polyol esters, offer fire resistance without relying on water. Phosphate esters (HFD-R) have exceptional fire-resistant properties due to their chemical structure but can be costly. Polyol esters (HFDU) provide a balance of high-performance lubrication and fire resistance, with high flash points that reduce the risk of flammable vapor release.
Defining Fire Resistant Hydraulic Fluids
Fire-resistant hydraulic fluids are designed to resist ignition and limit flame spread, though they are not completely fireproof and can still burn under extreme conditions. The International Organization for Standardization (ISO) 6743-4 classification system categorizes these specialized fluids based on their composition. The four main groups are HFA, HFB, HFC, and HFD.
The HFA and HFC classifications denote water-containing fluids. HFA is typically a high-water-content emulsion, and HFC is a water-glycol solution, usually containing 35% to 60% water. The presence of water provides the primary fire-suppressing mechanism. HFB fluids are water-in-oil emulsions, offering improved fire resistance over mineral oil but containing less water than HFA and HFC fluids.
The HFD category includes anhydrous, or water-free, synthetic fluids. These are divided into HFD-R (phosphate esters) and HFDU (synthetic esters, such as polyol esters). HFD fluids are chosen for systems that require high operating temperatures and pressures, where water-based fluids might not be suitable due to temperature limitations or compatibility concerns.
Key Safety Metrics for Flammability
The fire hazard of hydraulic fluid is quantified using specific technical measurements defining how the fluid behaves when heated or exposed to an ignition source.
The Flash Point is the lowest temperature at which a fluid produces enough vapor to briefly ignite, or “flash,” when an external flame is introduced. At this temperature, the fluid is not yet hot enough to sustain combustion.
The Fire Point is a higher temperature representing the minimum point at which the fluid generates sufficient vapor to sustain a continuous flame for at least five seconds after ignition. The fire point is typically 8 to 10 percent higher than the flash point and is used to assess a fluid’s volatility and resistance to sustained burning.
A third metric is the Autoignition Temperature. This is the temperature at which the fluid spontaneously ignites without the presence of any external spark or flame. Operating a hydraulic system above this temperature creates a severe fire risk, as contact with a hot surface alone can cause immediate combustion.
Safe Handling and Storage Practices
The safe management of hydraulic fluids centers on eliminating ignition sources and preventing the formation of flammable mists. All hydraulic systems should be regularly inspected to ensure system integrity. High-pressure leaks from a ruptured hose can atomize the fluid, dramatically increasing the fire risk even for less flammable fluids.
Maintaining a clean work environment is also important. Rags or absorbent materials soaked with hydraulic oil pose a spontaneous combustion risk and must be stored in approved, self-closing oily waste containers.
Proper storage requires keeping hydraulic fluids in sealed, compatible containers away from direct sunlight, heat sources, and ignition sources. For dispensing flammable liquids, metal containers should be bonded and grounded to prevent static electricity buildup, which can generate a spark capable of igniting vapors. Adequate ventilation must be ensured where fluids are used to prevent the concentration of flammable vapors.
In the event of a spill, prompt cleanup using absorbent materials is necessary to prevent slip hazards and reduce the potential for ignition. Firefighting media for hydraulic fluid fires typically involves dry chemical, foam, or carbon dioxide extinguishers, as using water can sometimes spread the burning liquid. Even fire-resistant fluids are only resistant, not fireproof, meaning safe operating procedures remain necessary.