Is Hydraulic Fluid Flammable? Yes and No…

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Hydraulic fluid or hydraulic oil is a very commonly used substance in many workshops, homes, and industrial environments. It can take different forms depending on what exact application it is being used for. But is it a fire hazard?

Water-based hydraulic fluids will generally not ignite. However, petroleum-based hydraulic fluids can catch fire at 300-600 degrees Fahrenheit (148-315 Celsius) and auto-ignite at 500-750 degrees Fahrenheit (260-398 Celsius), which, technically, makes them combustible and not flammable.

In fact, it’s a complicated substance that you really need to know more about before you start using it. Here’s what you need to know.

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Also read: What Makes Something Flammable?

What Is Hydraulic Fluid?

Hydraulic fluid is not a single item.

The term refers to a range of fluids that are used in machinery in order to affect power transfer. They can be made of mineral oils and they can even be pure water. 

You’ll find different hydraulic fluids in backhoes, power steering, flight control systems, industrial machinery, etc. 

The objective when choosing a hydraulic fluid for use in anything is to find one which has as close to zero compressibility (that is it doesn’t change volume very much when placed under the pressure of the system) as possible. 

The first hydraulic fluid to be used, that we know of, was in Ancient Egypt and, of course, they used water.

However, in the modern era, most hydraulic fluids tend to use a mineral oil base stock, but these are slowly being replaced, where possible, with natural oils which are renewable and biodegradable. 

Most hydraulic fluids contain a bunch of other chemicals and compounds to help “tune” their usefulness in a given application. 

What Is Hydraulic Fluid Used For?

Hydraulic Fluids have many properties and while their main use is for power transfer in systems, they can also be used as a heat transfer medium, sealing medium, lubricant, for ensuring the efficiency of pumps, for fire-resistance, modifying friction in a system, and radiation resistance too. 

One of the useful things about most hydraulic fluids is that they tend to have very low levels of toxicity and they are biodegradable. 

The expected life span of hydraulic fluid varies depending on the exact makeup of the fluid and the application it’s used for. 

Also read: Is Brake Fluid Flammable?

Is Hydraulic Fluid Flammable?

It depends.

In general terms, hydraulic fluids are not particularly flammable, even when made from petroleum products, and they certainly won’t catch fire from a spark at ambient temperatures. 

This is because they have high ignition temperatures.

If the hydraulic fluid is water-based, the answer is categorical “no”, because water is not flammable, it’s used to extinguish fires (and yes, you can use a water-based hydraulic fluid to extinguish a fire).

Some natural hydraulic fluids might have some flammability, but they should still be relatively safe to store on-site without too many precautions being required. 

Can Hydraulic Fluid Catch Fire?

is hydraulic fluid flammable

Yes. Though hydraulic fluid is not technically flammable, it absolutely will burn. In fact, nearly all materials will burn if you can get them hot enough in the first place. 

Once these fluids are burning, as you might expect from mineral oil products, they are hard to extinguish, and they put out a lot of heat. It’s a bit like burning oil or gasoline. 

This means that in factory premises, they try to keep stores of hydraulic fluid under certain levels to try and reduce the problems they may cause if the factory were to catch fire. 

Also read: Is Diesel Exhaust Fluid (DEF) Flammable?

At What Temperature Does Hydraulic Fluid Catch Fire?

It is very important to note in this section that there is no definitive answer to this question and if you need to know the temperature at which hydraulic fluid that you are using catches fire, you must refer to the documentation or the manufacturer. 

This is because there’s no such thing as standard hydraulic fluid.

Water-based fluids don’t catch fire, as we’ve already established. 

Other standard petroleum-based hydraulic fluids will tend to catch fire between 300- and 600-degrees Fahrenheit. 

They will also “auto-ignite” (that is burst into flame without a spark or flame) at a temperature of 500 to 750 degrees Fahrenheit! Yes, hydraulic fluid can spontaneously combust. 

Other hydraulic oils might have higher (or lower) flashpoints and auto-ignition temperatures. 

Is Hydraulic Oil A Combustible Liquid?

Yes. Hydraulic oil is a combustible liquid (unless water-based) and this is particularly true when the oil is being used in the equipment it is designed to work for.

That’s because it will be under pressure and pressure, as you probably know from your school chemistry and physics classes, tend to change the properties of substances.

In this case, it means that the hydraulic oil will be carrying a large amount of heat already (caused by the pressure) and thus it takes less additional heat to get the oil to combust. 

Thus, it’s important to ensure that any hydraulic equipment is properly maintained as a compromised system can (along with sparks, heat, or flame) result in a conflagration. 

Also read: Is Power Steering Fluid Flammable?

What Is Fire-Resistant Hydraulic Fluid?

You’ll find that the most common fire-resistant hydraulic fluids are those that contain large amounts of water such as water glycols and water/oil emulsions.

There are also some synthetic fluids that can be used as fire-resistant hydraulic fluid. 

These synthetic compounds are usually developed with the purpose of replacing a petroleum-based fluid in a very specific application.

You can see how much less-flammable these types of hydraulics fluids are, take a look:

Does Hydraulic Fluid Get Hot?

Yes, though technically speaking, ideally it wouldn’t.

Hydraulic fluid gets hot due to mechanical inefficiency in a given system these inefficiencies are not completely avoidable, and all systems are inefficient to a greater or large extent.

It’s worth noting that if the volume of heat generated is greater than the heat that can dissipate from the system, eventually the system overheats. 

Also read: Is Transmission Fluid Flammable? Yes and No…

Does Hydraulic Fluid Expand With Heat?

Yes. While hydraulic fluids are chosen because they can, to some extent, resist expansion when under heat or pressure, as with almost all fluids, they do expand as they get warmer. 

Can Hydraulic Oil Boil?

Yes and no. Again, much depends on the nature of the oil itself. A water-based oil will boil and usually at a temperature higher than standard water boils.

However, a mineral-based oil may not boil, instead, it will reach auto-ignition temperature and catch fire. 

How Do You Cool Hydraulic Oil?

We assume you mean how do you cool hydraulic oil within a given system?

As otherwise, you could just pop it in the refrigerator if you had a need for cool oil. 

Within a system, they use a large cooling reservoir, but they may also use a fan or a water-cooled heat exchanger if the volumes of heat are large enough to justify the expense. 

Also read: How Long Is Spilled Gasoline Flammable For?

What Happens To Hydraulic Fluid When It Is Overheated?

Yes, it matters if the hydraulic fluid becomes overheated.

It will begin to damage the system that it is within because it will break down the compounds within the system – it’s particularly damaging to sealant compounds at high temperatures.

Again, as you’d expect the exact nature of the problem is likely to depend on the application and the exact makeup of the hydraulic fluid. 

Is Hydraulic Fluid Hazardous?

Again, it’s impossible to be completely definitive when it comes to answering this question.

If you need to better understand a particular hydraulic fluid’s risks, read the material safety data sheet with it or contact the manufacturer.

However, most hydraulic fluids are relatively safe to use. That doesn’t mean that you should touch them with bare skin, swallow them or snort them. They certainly aren’t designed for human consumption or inhalation either. 

Waste hydraulic fluid can be more dangerous, however, as it may contain particles from the machinery or application that it has been used for. 

Also read: Is Diesel Fuel Flammable?

What Happens If You Get Hydraulic Oil In A Cut?


While it might be harmless, it might also make you very sick. If you do get hydraulic fluid in a cut, your eyes, on your skin, in your mouth, etc. consult a doctor immediately. 

Can I Use Hydraulic Oil On My Face?

No. Do not confuse hydraulic oil with hyaluronic acid in the face treatment. It won’t end well for you. 

What Can You Do With Old Hydraulic Oil?

Hydraulic oil must be disposed of properly at a recycling facility.

Don’t pour it down the drain or throw it in the trash.

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