Is Hydraulic Fluid Flammable?

Fluids play a key role in hydraulic systems. In fact, any operation requires hydraulic fluid within the hydraulic system.

These fluids can be affected by environmental changes such as variations in temperatures. Therefore, the whole hydraulic system can be impacted.

As temperatures increase, hydraulic fluid can begin to evaporate. As temperatures decrease, the fluid can freeze. This is one of the major concerns when we consider hydraulic fluid and climatic changes.

Hydraulic systems can operate small objects such as toys to large applications such as aircraft or mechanical robotics.

In many of these cases, there will be times when the applications come into contact with ignition sources and very hot surfaces.

Some examples of applications that come into contact with higher temperatures and ignition sources include hot metal shears, coke furnace door openers, and furnace chargers, and dischargers.

Such applications leave the question: what will happen if the hydraulic fluid becomes exposed to high-temperature situations and is hydraulic fluid flammable? Well, this generally depends on the type of hydraulic fluid that is used.

Hydraulic fluids are considered serious fire hazards due to their high ignition temperatures. If hydraulic oil is sprayed around an area, it can burn just as fiercely as other hydrocarbons.

Today, we will be discussing the flammability of hydraulic fluid and how safe it is around certain applications. Read on to find out more information when using such liquids.

What is a hydraulic system?

A hydraulic drive system consists of hydraulic fluid as well as three important mechanical components. These are considered pressure generators or hydraulic pumps and can be driven by an electric motor or a manual pump/engine.

The system consists of valves, pipes, filters with the motor can be hydraulic, a hydraulic cylinder, or a hydraulic actuator.

Just about all aircraft utilize some form of a hydraulic system. The hydraulic components of a general aviation aircraft are usually limited to its wheel brakes.

In larger aircraft, the hydraulic system can provide the motive power for other systems such as the landing gear retraction/extension, nose wheel steering, flight control actuation, emergency electric generation, and much more.

As you can guess, the risk of fires from hydraulic fluids is a serious issue. A hydraulic fluid fire can lead to the loss of an aircraft or piece of machinery and in the worst-case scenario, the loss of lives.

Categories of hydraulic fluids

As we mentioned above, there are different categories of hydraulic fluids. These are:

  • Mineral oils
  • Fire-resistant fluids
  • water/oil emulsions
  • Water glycol
  • Phosphate esters

Petroleum-based hydraulic fluids, such as mineral oils, have a flashpoint that ranges from 300 to 600 degrees Fahrenheit. The flashpoint is the lowest temperature at which vapors can ignite.

Water-based hydraulic fluids are inflammable. However, an issue with corrosion can arise when using these types of fluids. To test how flammable hydraulic fluids are, research is undertaken depending on the ISO 12922 specification.

Fire-resistant hydraulic fluids tend to be classified as oil and water emulsion, Anhydrous Synthetics, and Water-Polymer solutions.

Fire-resistant hydraulic fluids that contain water will have more than 35% of water content. ISO 12922:2012 classify these into:

  • HFAE
  • HFAS
  • HFB
  • HFC
  • HFDR
  • HFDU

HFAE – Hydraulic fluids in this category are considered oil in water emulsion. They have a milky to translucent emulsion appearance. These fluids contain more than 80% of water and are resistant to aging. HFAE has applications in the field of mine support, hydraulic strut extensions below ground, and hydrostatic drives.

HFAS – These are synthetic aqueous fluids that have a transparent appearance. These fluids are free from mineral oils and contain more than 80% of water content. The main applications of HFAS are Mine support, press hydraulics, foundry technology, and much more.

HFB – HFB fluid is a water in oil emulsion that contains more than 40% of water. An application of this is coal mining. The minimum ignition temperature (flashpoint) of HFB is 1,202 degrees Fahrenheit or 650 degrees Celsius according to ISO standards.

HFC – This is known as glycol solutions, polyalkylene glycol solutions, or water glycols. The flashpoint of this fluid is 1,112  degrees Fahrenheit or 600 degrees Celsius. It is a water-polymer solution that contains more than 35% of water.

It has application fields where water-free hydraulics are not used. These include the steel industry, foundries, coking plants, hardening plants, injection molding, forming presses, and more.

HFDR – This uses phosphoric acid ester. It tends to have poor viscosity as well as poor temperature characteristics. HFDR is considered a hazardous working material as it can develop toxic gases in the case of a fire. One application that uses HFDR is the turbine control system.

HFDU – HFDU can be categorized into ester-based and glycol-based. Glycol-based HFDU generally has good viscosity, good temperature characteristics, shear stability, and is age resistant. It is a water-soluble fluid with excellent wear protection.

Mobile systems that have high thermal loading properties use glycol-based HFDU while Ester-based HFDU has a higher dirt dissolving capacity.

So, is hydraulic fluid flammable?

Hydraulic fluid is extremely flammable. It has been a factor in many fires over the years and, when it does set alight, it can increase the extent of fire damage significantly.

Hydraulic systems are highly pressurized. This means flames from a hydraulic oil fire can spread over a great distance.

In factory settings, these flames can easily reach cables and other combustible materials which will catch fire quickly and rapidly get destroyed.

Therefore, it is essential that you assess the risks that are related to hydraulic oils so you and others are protected at all times.

The risk of a hydraulic oil fire can actually be eliminated by shifting non-combustible hydraulic fluids or by using electrically or pneumatically operated equipment instead.

In Summary

Hydraulic fluid can be extremely flammable due to its high flashpoint. Some of these fluids are more prone to catching fire than others due to their properties and water content.

However, sufficient and proper care should be taken whenever hydraulic fluids are in use or nearby. With preventative measures, catastrophic fires can be avoided.


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