Fire resistant hydraulic fluids play a crucial role in protecting people, equipment, and facilities from the catastrophic risks of hydraulic system fires.
According to ISO 5598:2008, a fire-resistant hydraulic fluid is defined as one that is difficult to ignite and that shows little tendency to propagate flame.
These fluids are essential in high-risk environments where hydraulic systems operate under pressure and in close proximity to ignition sources.
Why Traditional Mineral-Based Hydraulic Fluids Pose Risks
Mineral-based hydraulic oils have long been the standard in hydraulic equipment due to their excellent lubricity, availability across a wide range of viscosities, and relatively low cost.
However, these petroleum-derived fluids are highly flammable. Under pressure, a ruptured hose, failed seal, or leaking fitting can release a fine mist of oil—essentially creating a combustible aerosol. When this mist encounters a hot surface, electrical discharge, friction, or open flame, there is a high-risk of ignition and catastrophic loss. In fact, more than half of all hydraulic fire losses are caused by failures in pipe or hose fittings that release pressurized, flammable spray.
These hazards are especially concerning in industrial environments where ignition sources are constant or unavoidable. This includes processes like plastic molding, die casting, metal forging, welding, and heat treating, as well as operations involving milling machines, boring mills, CNC tools, and forklifts.
In such applications, switching to a fire-resistant hydraulic fluid is a cost-effective way to mitigate risk and meet operational safety standards.
Where Fire-Resistant Fluids Are Used
Fire-resistant hydraulic fluids are used in a wide range of industries and equipment where the risk of ignition is elevated.
Common applications include metal production and processing, plastics manufacturing, construction, mining, and material handling.
They’re also used in power generation facilities, marine and offshore equipment, and transportation systems. In mobile equipment operating in enclosed or sensitive environments, fire-resistant fluids help reduce the risk of catastrophic failure while maintaining reliable performance under demanding conditions.
Evaluating Performance and Safety
Choosing the right fire-resistant hydraulic fluid isn’t simply about minimizing flammability; it’s about maintaining or even enhancing system performance.
Several critical properties must be considered when evaluating a fluid for your application. These include viscosity and viscosity index, thermal and oxidative stability, and hydrolytic resistance. Good fire-resistant fluids must also offer effective lubrication, resist foaming and air entrainment, and remain stable under pressure and shear.
Material compatibility is another key factor.
Some fire resistant fluids can degrade seals, hoses, and system components if not properly matched. You’ll also want to assess the fluid’s behavior in low-temperature environments, its tendency to promote corrosion, and whether it introduces risks like cavitation or poor air release.
Understanding ISO Fluid Categories
The International Organization for Standardization (ISO) classifies fire-resistant hydraulic fluids under ISO 6743-4, dividing them into six main categories based on composition and properties:
- HFAE – Oil-in-water emulsions: These fluids contain over 95% water. The high water content imparts strong fire resistance, but poor lubricity and a higher risk of corrosion. These solutions are suitable for specialty low-lubrication applications within a temperature range of +5°C to +50°C.
- HFAS – Chemical solutions in water: Typically, they contain at least 40% water by mass, providing good fire resistance, but also require corrosion and wear inhibitors. Also limited to +5°C to +50°C operating range.
- HFB – Water-in-oil emulsions: HFB solutions usually contain a higher oil concentration (around 60%) with a lower water concentration (around 40%). These fluids offer better lubrication than HFA types but still need additives for adequate performance. They appear milky and can generate steam at high temperatures.
- HFC – Water-polymer (glycol) solutions: Often about 35–60% water mixed with glycols and polyglycols. These fluids resist freezing and offer moderate lubricity. However, they may present compatibility issues with system materials and can shear thin under high stress. Effective within −20°C to +50°C.
- HFDR – Synthetic fluids based on phosphate esters: Provide excellent fire resistance and oxidative stability. However, they have a low viscosity index, are susceptible to hydrolysis, and can produce noxious fumes when burned. Spills are difficult to clean due to the fluid’s density.
- HFDU – Synthetic, non-phosphate ester fluids: Typically formulated with polyol esters, HFDU fluids offer excellent lubricity, high flash points, and broad temperature stability. They resist cavitation erosion, have low vapor pressure, and are better suited to high-performance, high-pressure applications. These fluids operate effectively from −20°C to +150°C.
Spotlight on HFDU Fluids: Performance Without Compromise
Among the available fire-resistant hydraulic fluids, HFDU stands out for its ability to deliver high performance without sacrificing safety. HFDU fluids can be formulated using synthetic polyol esters and are entirely water-free. As a result, they provide better lubricity and wear protection than water-based fluids, while still offering a high degree of fire resistance.
HFDU fluids exhibit excellent oxidative and thermal stability, low volatility, and high flash points, reducing the risk of flammable vapor release.
Because they don’t contain water, these fluids are less prone to cavitation erosion and do not suffer from the corrosion or foaming issues common with aqueous types.
Their wide operating temperature range and compatibility with standard hydraulic materials make them suitable for the most demanding applications.
What is FM Approval?
FM Approval is an important benchmark when evaluating the fire safety of hydraulic fluids.
FM Global evaluates hydraulic fluids under its Approval Standard 6930, which measures two key factors: the heat release rate during spray fires and the critical heat flux required for ignition. These parameters are combined into a Spray Flammability Parameter (SFP). Fluids with an SFP under 5 × 10⁴ qualify for FM Approval, indicating a high level of safety in fire-prone environments.
Choosing an FM Approved hydraulic fluid offers peace of mind and often supports insurance compliance and risk reduction programs in industrial settings.
Featured Product: BioFlo™ Synthetic HFDU
At J R Hess Company, we supply BioFlo HFDU and BioFlo Synthetic HFDU fluid. Both are premium options for organizations seeking a combination of fire resistance, environmental responsibility, and uncompromising hydraulic performance.
BioFlo HFDU is an FM Approved hydraulic fluid formulated using premium natural ester readily biodegradable base oils. Available in ISO grades 46 and 68, BioFlo HFDU is compatible with most mineral oils and polyolester hydraulic fluids but not with water-containing fluids.
BioFlo Synthetic HFDU is a high-performance, FM Approved fire-resistant hydraulic fluid that prioritizes environmental responsibility. Syntehtic HFDU is formulated with synthetic polyol esters (POE), which are highly biobased and include ashless additives to extend both equipment and fluid life. BioFlo Synthetic HFDU is designed to provide exceptional wear and corrosion protection, making it ideal for high-pressure hydraulic systems where potential fire hazards exist.
Contact us for more information about FM Approved, fire-resistant hydraulic fluids.