Hydraulic oil is a fluid medium used to transfer power within machinery. Most standard hydraulic fluids are combustible under specific conditions. The risk of fire depends significantly on the fluid’s chemical composition and the operational environment. Understanding the fluid’s properties and the mechanics of ignition provides the context for mitigating fire hazards.
Understanding Flash Point and Fire Point
The combustion risk of hydraulic fluid is defined by two thermal measurements: the flash point and the fire point. The flash point is the lowest temperature at which the fluid produces enough flammable vapor to ignite briefly when an external ignition source is applied. This flash is momentary because the heat generated is insufficient to sustain the combustion of the bulk liquid.
Standard petroleum-based hydraulic oil typically has a flash point ranging from 150°C to 320°C (300°F to 600°F). The fluid itself does not burn at this temperature; only the vapor mixture above the liquid surface ignites. The fire point is a slightly higher temperature, often 50 degrees or more above the flash point, where the fluid produces enough vapor to sustain a continuous flame after ignition. This property is a more relevant metric for assessing the sustained fire hazard of a hydraulic fluid.
How Fluid Composition Affects Fire Risk
The chemical makeup of hydraulic fluid dictates its inherent fire resistance and international classification. Standard petroleum-based mineral oils (ISO 6743-4 classifications H, HL, or HM) are effective for power transmission but present the highest fire risk. These hydrocarbon oils readily combust when exposed to an ignition source. Their widespread use makes them responsible for many industrial fires.
A second category achieves lower flammability through the inclusion of water, which acts as a fire suppressant.
Water-Containing Fluids
- Oil-in-water emulsions (HFA)
- Water-in-oil emulsions (HFB)
- Water-glycol solutions (HFC)
The water content, which can be over 35% in water-glycol fluids, dramatically increases the heat required for ignition by forcing the water to boil off first, delaying the fluid’s temperature rise.
The highest level of fire resistance is achieved with synthetic fluids (HFD), which contain no water and are chemically engineered for low flammability. This group includes phosphate esters (HFDR) and synthetic anhydrous liquids (HFDU). These fluids are designed for high-risk environments like steel mills and foundries where extremely hot surfaces are present. The low-flammability properties of these synthetics require different maintenance considerations, as they can sometimes be corrosive to standard seals, paints, and insulation materials.
High-Risk Ignition Scenarios
The most dangerous scenario for hydraulic oil ignition involves a high-pressure leak. When fluid escapes through a small crack or pinhole, the high system pressure causes the fluid to atomize into a fine, flammable mist. This mist significantly increases the fluid’s surface area, making it easier to ignite than the bulk liquid.
This atomized spray can travel a considerable distance, sometimes up to 15 meters, creating a severe torch-like fire if ignited. The spray can easily find an ignition source, such as an open flame, a welding arc, or a surface operating above the oil’s auto-ignition temperature. Standard petroleum oils have auto-ignition temperatures ranging from 260°C to 400°C, meaning they can ignite spontaneously without a spark when contacting a hot surface. The fine mist can also create an explosion risk if it contacts a surface above 200°C, vaporizes, and concentrates in the air. The combination of high pressure, atomization, and a nearby heat source bypasses the fluid’s published flash and fire points, creating an immediate and intense fire hazard in industrial and mobile machinery.
Prevention and Fire Response
Preventing hydraulic fluid fires begins with rigorous maintenance to contain the fluid within the system.
Prevention Measures
- Regularly inspect and replace worn seals, hoses, and connections to prevent high-pressure leaks and atomization.
- Install shielding or barriers around high-pressure lines to contain spray and protect it from hot surfaces or ignition sources.
- Maintain proper housekeeping by promptly cleaning up spills and preventing the accumulation of combustible materials near equipment.
- In high-risk areas, use fire-resistant fluids like water-glycols or phosphate esters.
- Incorporate automatic shut-down mechanisms that isolate fluid flow upon detecting a pressure drop or leak.
If a fire occurs, the appropriate fire extinguisher must be used to combat the Class B fire caused by flammable liquids. Dry chemical (ABC or BC) and carbon dioxide (CO2) extinguishers are effective on standard oil fires, as they smother the flame by removing oxygen. Applying a stream of water directly to burning petroleum oil can spread the fire by floating the burning oil. For water-based fire-resistant fluids, a water mist extinguisher may be suitable. The flow of hydraulic fluid must be shut off at the source to extinguish a persistent torch-like fire caused by a high-pressure leak.