Is Hydraulic Fluid Flammable？

Hydraulic fluid is the blood of the hydraulic system. Its safe use is related to production stability and personnel and property safety. The fire risk caused by hydraulic fluid is a major hazard that needs to be strictly prevented and controlled during work.

So, how flammable is the hydraulic oil exactly? What should I do to reduce the risk of hydraulic fluid combustion? Reading this article, you will find the answer.

Combustion Indicators of Hydraulic Fluid

When understanding the flammability of hydraulic fluid, you need to first clarify three key temperature indicators: flash point, ignition point and autoignition temperature. They define the combustion characteristics of hydraulic fluid from different dimensions and are the key basis for evaluating its safety level.

Flash Point

Flash point refers to the lowest temperature at which, when hydraulic fluid is heated, the vapor on its surface mixes with air and, upon encountering a flame, can cause a brief flash of fire. This flash fire is a combustion that goes out in a flash.

The reason why the flame goes out immediately is that at the flash point temperature, the rate at which the hydraulic fluid evaporates and generates steam due to heating is slower than the rate at which the combustion consumes the steam. The concentration of combustible vapor is insufficient to sustain a continuous chemical combustion reaction.

Flash point reflects the content of low-boiling-point combustible components in the fluid. The higher the flash point is, the fewer the volatile and flammable components in the fluid are, and the stronger the safety of the fluid in high-temperature environments will be. It also represents the lowest temperature at which the fluid begins to become unstable.

Ignition Point

The ignition point is also known as the burning point. Its temperature is higher than the flash point. The ignition point refers to the lowest temperature at which a mixture of steam and air in hydraulic fluid can continue to burn for no less than 5s when exposed to an ignition source after reaching the flash point.

When the fluid temperature reaches this value, the rate at which the hydraulic fluid surface evaporates and generates steam is already equal to or greater than the rate of combustion consumption.

At this point, once an external fire source is ignited, even if you move the fire source away, the combustion reaction can continue autonomously for at least 5s without the need for an external heat source.

The ignition point indicates that the hydraulic fluid can maintain the spread of fire. In a fire accident, once the ambient temperature exceeds the ignition point, simply removing the ignition source is no longer enough to prevent the fire. You must take active measures to put out the fire.

Autoignition Temperature

The autoignition temperature refers to the lowest temperature at which hydraulic fluid can ignite on its own without an external fire source, relying solely on its own heat accumulation or environmental heating. The autoignition temperature is usually much higher than the flash point and the ignition point.

The autoignition temperature can measure the tendency of hydraulic fluid to self ignite in a specific thermal environment. Due to the absence of an external fire source, the autoignition temperature is very important when designing high-temperature equipment and assessing thermal environmental risks.

These three indicators usually follow this order: flash point < ignition point < autoignition temperature. You can quickly understand it by referring to the table below.

Combustibility and Flammability of the Hydraulic Fluid

The concepts of flammable liquids and combustible liquids have strict boundaries in industrial safety standards such as OSHA and NFPA. It is mainly determined by the flash point.

Flammable Liquid

Flammable liquids refer to substances with a flash point lower than 37.8°C (100°F). This type of liquid has extremely high volatility at room temperature. It can continuously release steam that can be instantly ignited by tiny sparks. Common flammable liquids such as gasoline, alcohol and acetone.

Hydraulic fluid is generally not considered a flammable liquid.

Combustible Liquid

Combustible liquids refer to substances with a flash point higher than 37.8°C (100°F). Hydraulic fluid is a combustible liquid. Under normal temperature and pressure storage conditions, hydraulic fluid is very stable and is difficult to ignite.

Practical Industrial Application

The standards for flammability and flammability are based on the static storage of hydraulic fluid. When hydraulic fluid is subjected to extreme working conditions of high temperature and high pressure or when the fluid performance deteriorates, its flammability risk will increase. You need to pay more attention.

Why does Hydraulic Fluid Catch Fire?

High-Pressure Leakage Forms a Fluid Mist

This is a unique and extremely risky fire-causing factor specific to the hydraulic system. The working pressure of hydraulic systems is usually high. When the high-pressure pipeline or seal of the system ruptures, the hydraulic fluid will spray out at a very high speed.

This high-pressure spray will atomize the fluid into extremely small droplets, forming a fluid mist zone that can be suspended in the air. The huge surface area of fluid mist makes its contact with oxygen extremely sufficient. The combustion reaction of this mixture of combustible steam and air is more rapid.

Even a weak electrostatic spark or a heat source far below the conventional ignition point of hydraulic fluid can cause a violent instantaneous explosion and spread rapidly.

External Ignition Source

The hydraulic fluid itself will not spontaneously combust at the normal operating temperature. However, if the sprayed fluid mist comes into contact with a heat source that is higher than its flash point or ignition point, it may be ignited.

In actual industrial settings, such ignition sources are widely present and come in various forms.

• During maintenance operations or production processes, open flames such as welding and cutting sparks may be generated.
• Electrical equipment failure, short circuit or electrostatic discharge can generate electrical sparks.
• The hydraulic system may overheat during long-term operation or poor design. For example, overheated surfaces of mechanical components caused by prolonged friction, bearing damage, or lack of lubrication.

When hydraulic fluid vapor or leaked fluid comes into contact with these high-temperature heat sources or sparks, it will be quickly ignited.

Combustion Performance of Different Types of Hydraulic Fluids

Mineral Oil-Based Hydraulic Fluid

Mineral-based hydraulic fluid is the most traditional type of hydraulic medium and is also the most widely used category of hydraulic fluid at present. It is mainly composed of refined petroleum hydrocarbons.

The flash point of the mineral oil-based hydraulic fluid is relatively low, typically between 150°C and 220°C. The corresponding ignition point is between 200°C and 300°C or higher.

However, you should be aware that once mineral oil-based hydraulic fluid is ignited, it will release an enormous amount of energy. The flame has a high temperature and continues to burn, accompanied by a large amount of thick smoke.

The mineral oil-based hydraulic fluid has good lubrication properties, low cost and moderate thermal stability. You can apply it in regular industrial conditions without high temperatures or risks of open flames, such as machine tools, injection molding machines, excavators and cranes.

Synthetic Hydraulic Fluid

Synthetic hydraulic fluid is made from chemically synthesized base oils and mainly includes HFDR hydraulic fluid and HFDU hydraulic fluid. The flash point of synthetic hydraulic fluid is usually between 230°C and 280°C. Their ignition point is also higher, usually exceeding 300°C.

HFDR hydraulic fluid is a type of fire-resistant hydraulic fluid. It has self-extinguishing properties. The chemical composition of HFDR hydraulic fluid, phosphate ester, is very stable and difficult to oxidize and decompose. Even in extreme forced ignition tests, once you remove the ignition source, the flame will immediately extinguish and cannot sustain combustion.

HFDU hydraulic fluid is also a type of fire-resistant hydraulic fluid. It has incorporated special additives inside. HFDU hydraulic fluid can exhibit good flame-retardant properties under high temperature conditions.

Synthetic hydraulic fluid is mainly used in high-temperature or open flame environments, such as aerospace hydraulic systems and high-pressure equipment in steel metallurgy.

Water-Based Hydraulic Fluid

Water-based hydraulic fluid reduces its flammability by mixing water with other components. Water-glycol hydraulic fluid is the most typical water-based hydraulic fluid. Its water content ranges from 35% to 50%.

Strictly speaking, water-based hydraulic fluid has no flash point. Because when water evaporates, it takes away heat and also isolates oxygen. However, if water-based hydraulic fluid is exposed to extreme heat sources for a long time and the water evaporates, the remaining substances may still catch fire.

The lubricity of water-based hydraulic fluid is poor, and its operating temperature should not exceed 60℃. It is very common in situations close to high-temperature surfaces or in welding heat environments, such as in mining machinery, the steel industry, and heavy industrial equipment.

Biodegradable Hydraulic Fluid

Biodegradable hydraulic fluid is manufactured using vegetable oil or synthetic ester as the base oil. Its flash point usually ranges from 200°C to 300°C.

The advantage of biodegradable hydraulic fluid lies in its environmental friendliness. After being leaked into the soil or water sources, it can naturally degrade by 90% within 28 days. And it has very low toxicity to the environment. However, its antioxidant activity is not very good. It is prone to deterioration when used at high temperatures for a long time.

Biodegradable hydraulic fluid is highly suitable for environmentally sensitive areas. You can use it in agricultural machinery, forestry logging equipment, water conservancy engineering equipment, marine engineering equipment, etc.

How to Reduce the Risk of Hydraulic Fluid Fire？

Select the Appropriate Hydraulic Fluid

Fire prevention begins in the planning and design stages. You need to choose the appropriate hydraulic fluid based on the working condition of the equipment.

For example, in high-risk conditions with high temperature and open flame risks, such as hot rolling equipment in steel plants and underground hydraulic systems in mines, you should prioritize using flame-retardant hydraulic fluid, such as water ethylene glycol hydraulic fluid and HFDR hydraulic fluid.

This type of hydraulic fluid has a high flash point and is not easy to sustain combustion even when in contact with an ignition source.

Strictly Maintain the Hydraulic System

Leakage and overheating in the hydraulic system are two major direct causes of fires. You need to promptly resolve any issues through regular maintenance.

Strictly Control the Risk of Leakage

Regularly inspect the wear and aging of pipelines, joints, and seals. For the high-pressure section of the pipeline, you need to use materials with a higher pressure resistance rating. The seals should be of a type that is compatible with the hydraulic oil.

We suggest that you install leakage monitoring devices, such as pressure sensors and oil leakage alarms. Once a leak is detected, you can immediately stop the machine for handling to prevent the high-pressure fluid from atomizing and forming a combustible mixture.

Prevent Uncontrolled Oil Temperature

You should make sure that the cooling system is operating properly. Regularly clean the dust from the radiator and check the coolant level. Avoid oil temperature exceeding the flash point due to cooling failure.

You also need to set the system pressure reasonably to reduce the additional heat generated due to excessive pressure.

In addition, you can install an oil temperature monitoring instrument in the fuel tank and set a high-temperature warning threshold. When the oil temperature exceeds the threshold, it will automatically trigger an alarm or shut down the engine.

Regularly Check the Performance of the Hydraulic Fluid

You can take regular samples to test the flash point, moisture content, acid value and other indicators of hydraulic fluid according to the running time of the equipment. When the flash point drops by more than 20% or the fluid deteriorates, replace it with new hydraulic fluid immediately.

Standard Operation

Improper operational behavior can easily trigger a fire. You need to establish strict operating and maintenance standards to prevent human errors.

Daily Operation Standards

You need to make sure that the operators receive professional training before they start working. Avoid overloading equipment to prevent abnormal high temperatures in the system.

Meanwhile, it is strictly prohibited to stack flammable and explosive materials near the hydraulic system. Maintain good ventilation around the equipment to reduce the accumulation of flammable vapors.

Safety Requirements for Maintenance Operations

Before performing welding, gas cutting, or other hot work on the hydraulic system, you should thoroughly drain the hydraulic oil from the pipeline and blow the pipeline with inert gas. Be sure to disconnect the power supply of the equipment to prevent electrical short circuits and sparks during disassembly

Emergency Response Process

You need to develop an emergency plan for oil leakage. Once a leak occurs, immediately cut off the power source of the system. Use tools such as oil-absorbing felt to quickly clean the leaked oil, avoiding the oil from spreading to high-temperature components or near ignition sources.

Install Protective Measures

You can further reduce the probability of fire by installing protective devices and fire-fighting equipment.

Install Safety Protection Devices

You can install explosion-proof safety valves and relief valves at key nodes of the hydraulic pump outlet and high-pressure pipeline to prevent abnormal pressure rise in the system from causing pipeline rupture.

Install temperature monitoring and fire suppression linkage devices near the fuel tank and pipelines. When the temperature reaches the dangerous threshold, the fire suppression system will automatically activate.

Equip the Firefighting Equipment

You need to equip suitable fire extinguishing equipment around the hydraulic system according to the equipment type and scenario, such as dry powder fire extinguishers and carbon dioxide fire extinguishers.

In high-risk conditions, you can also install a fixed fire extinguishing system, which can quickly control the fire when it occurs.

Optimize Equipment Layout

You need to isolate and protect the high-temperature components and pipelines of the hydraulic system. For example, using a heat shield to wrap around high-temperature areas such as exhaust pipes to avoid direct contact with leaked oil.

Maintain sufficient spacing during equipment installation to facilitate ventilation, heat dissipation, and emergency operations.

FAQs

Will Flame-Retardant Hydraulic Fluid not Catch Fire at all?

No. Flame-retardant hydraulic fluid only has a much higher ignition threshold than ordinary hydraulic fluid. The key advantage of this type of oil is its extremely high flash point and its self-extinguishing properties.

However, if it is exposed to a prolonged environment of high-temperature open flames, or if the fluid deteriorates due to water evaporation or formula failure, there is still a risk of combustion.

Will Mixing Hydraulic Fluid Increase the Risk of Fire?

Yes. Different types and brands of hydraulic fluid have significant differences in their base oil components and additive formulations.

For example, for certain types of fire-resistant oil, if 2% to 5% of mineral oil is added, their originally excellent fire resistance properties will be completely lost. You should strictly follow the requirements of the equipment manual when using hydraulic fluid.

Will the Risk of Fire be Reduced when Using Hydraulic Fluid in a Low-Temperature Environment?

Yes. But this does not mean that the fire risk has completely disappeared.

If the equipment starts with excessive load due to low temperature, it results in an abnormal increase in system pressure. When the pipeline leaks due to low-temperature brittleness and the oil comes into contact with high-temperature components such as the engine exhaust pipe. It may still cause combustion.

Can Hydraulic Fluid Fires be Directly Extinguished with Water?

Absolutely not. Because the density of oil is lower than that of water, if the burning oil is directly sprayed with a water flow, the burning oil will float on the water surface and spread in all directions with the water flow, causing the fire to expand suddenly.

What’s even more dangerous is that when water encounters extremely hot oil, it will instantly vaporize and expand, causing the burning oil droplets to splash around.

You must use a dry powder extinguisher, carbon dioxide extinguisher or special foam extinguishing system to extinguish the fire by isolating oxygen.

Final Thoughts

In conclusion, although hydraulic fluid does not ignite as easily as gasoline at normal operating temperatures. As a combustible liquid, it still poses a certain risk of fire in practical applications.

If you have any further questions about the flammability of hydraulic fluid, please feel free to communicate with us. Let’s work together to keep the safety of your hydraulic system.