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Common Causes and Types of Hydraulic Fluid Contamination

Common Causes and Types of Hydraulic Fluid Contamination


If your hydraulic system experiences hydraulic fluid contamination, it can result in severe damage. The most effective way to deal with contamination is to be aware of the potential sources that could result in hydraulic cylinder repair. We’ve outlined five different types of hydraulic fluid contamination and how our team can help you make the necessary repairs. 

5 Types of Hydraulic Fluid Contamination

It’s normal for various types of hydraulic systems  to be potentially exposed to contamination while it runs. Here are some common types of contamination that could impact your machinery. 

Air Contamination

If your hydraulic fluid resembles a foamy substance, it could be the result of air contamination. As you can imagine, air does not have a positive impact on your hydraulic fluid or system components as it impacts its compressibility. If there is enough air contamination, it can also impact your machine’s efficiency such as the torque from your final drive and the cycle time for your attachments. 

The process of minimizing air contamination is simple: if you can avoid it, don’t expose your hydraulic system unnecessarily to air. You should also make sure that your hydraulic system reservoirs and suction lines are not adding air to the oil.

Water Hydraulic Contamination

Another type of hydraulic fluid contamination is from water as it can pose a threat to the health of your hydraulic fluid. In addition to impacting compressibility, it can also result in corrosion issues. Water can impair the hydraulic fluid’s ability to act as a lubricant and result in sludge formation. 

The effects of water in hydraulic oil can also include cavitation in your machine’s pump which can be expensive and tricky to repair (or replace). Looking at the fluid can give you insight into whether it’s been contaminated. If the fluid has a milky appearance, it’s likely contaminated by water. Make sure to avoid leaving your hydraulic fluid exposed to the atmosphere where moisture can be absorbed. 

Chemical Contamination of the Hydraulic Fluid

Another way you might see contamination in hydraulic systems results from chemicals. The main cause of chemical contamination stems from the natural degradation of the hydraulic fluid. The chemicals that make up your fluid break down into different components over time, and these are not ideal for your system. In cases of excessive heat, this process speeds up. In the case that two hydraulic fluids are mixed together and they don’t have compatible additives, it could result in a chemical reaction that results in chemical contamination. This is an essential reason why you shouldn’t mix different types of hydraulic fluid. 

Ingressed Contamination

Ingressed particulate contamination occurs when particles have traveled from the outside of your hydraulic system to the inside (as opposed to generated contamination). This type of contamination may include sand, mud, dirt, and dust. These particles typically enter your hydraulic system when it’s opened while it’s being repaired or maintained. It could also enter through leaking seals, another reason that we encourage machine owners to take care of leaking seals as soon as possible. 

In the case that fluid begins to leak out, contamination can enter. To avoid this, open your hydraulic in a clean environment, and clean the equipment off before you open it so nothing falls inside of the system. 

Particulate Hydraulic Contamination

If there are particulate contaminants that are circulating on the inside of your fluid power system, this debris can pollute the hydraulic fluid inside of your system. When there are abrasive particulate contaminants such as sand, dirt, or metal flakes, it can score precision-machined parts in your ground support equipment. This could result in your hydraulic pumps and motors failing. With particulate hydraulic contamination, there is an increased risk of cavitation or the formation of vapor-filled cavities. 

Common Sources of the Hydraulic Fluid Contamination

When it comes to hydraulic cylinders industries, this contamination is widespread. Here are some common sources of dirty hydraulic fluid. 

Contamination During Production

New hydraulic fluid has an average cleanliness code of ISO 4406 from 17/16/14 to 20/18/16. This average is higher than the typical recommendation from manufacturers for their equipment, however, experts agree that this high level of cleanliness isn’t enough for today’s hydraulic systems. 

So where does the contamination originate during hydraulic fluid production? Facilities that produce hydraulic fluid might accidentally expose the fluid to contamination during mixing or processing. These facilities don’t have strong control over how the fluid is stored, allowing for contamination in other ways. This is why there’s an emphasis on filtering the fluids that go into a hydraulic system. 

Hydraulic Contamination During the Fluid Transfer

Contamination in hydraulic systems could also occur during the fluid transfer process. There is also the risk that the fluid is exposed to dust or other particles that are naturally airborne. If the environment itself isn’t clean and the equipment in use isn’t clean, it’s nearly impossible to avoid contamination. 

If the system hasn’t been flushed before adding the new fluid, it can intensify the contamination, and mixing the fluids may result in chemical reactions. Try to always flush the system before you add fresh fluid and don’t leave the lid off longer than necessary. Any hydraulic fluid should be filtered before it’s added to the system. 

Contamination During the Storage

The storage period is another source of hydraulic oil contamination. It’s possible that moisture can impact the interior of sealed waterproof plastic containers, a process known as “breathing.” This becomes an issue if a container is exposed to varying temperatures throughout the day. 

Although it’s not possible to see moisture contamination, it can still be incredibly damaging to the hydraulic system’s performance and the system as a whole. Containers that aren’t properly sealed or they’re left open can potentially result in contamination through moisture, dust, and debris. Ideally, fluids will be stored at a controlled temperature with the container laying on its side. 

Built-In Hydraulic Fluid Contamination

It’s possible that the contamination is built into the hydraulic system itself during the manufacturing process. For example, a sand cast part in the motor of the hydraulic system has a few specks of casting sand left over on the inside. Or maybe there’s a small remnant of sealing tape left on the hydraulic port. Regardless of the source, there are scenarios that could result in system contamination. One way to combat built-in hydraulic fluid contamination is through filters. Change these filters out regularly and handle them carefully during installation. 

Contamination During Service

Hydraulic oil contamination can occur when the fluid is exposed to temperatures that are higher than normal. It’s also key to note that hydraulic fluid naturally degrades over time. This is why manufacturers always recommend that the fluid is changed regularly. The ingestion of air and moisture could also result in fluid contamination. 

Seals, even when they aren’t leaking, could be another contamination source. There are small particles that can get into the system on cylinder rods, something that the internal filters are supposed to capture. It’s possible that the components themselves can result in contamination. If you suspect contamination, make sure to flush the system. 

Hydraulic Fluid Contamination During the Maintenance

Another noteworthy type of hydraulic fluid contamination occurs during maintenance. As you can imagine, the possibilities of how contamination occurs during maintenance are seemingly endless, so it’s easiest to address how to prevent it. 

To start, the exterior of all the components should be cleaned, especially so if they’re used in a dirty environment such as construction equipment. Make sure to use lint-free cloths and cap hoses and plug ports upon opening. All fittings and hoses should be carefully handled to reduce the risk of contamination. If there are new components, they shouldn't be taken out of their packaging until they’re ready to be installed. 


Even though it takes more of a conscious effort, it’s still possible that hydraulic fluid contamination can be minimized with the right proactive measures. If you need a reliable hydraulic cylinder repair company, our team at Cylinders, Inc. is happy to help you. We are highly experienced in the effects of water in hydraulic oil and can help improve the functionality of your system. Contact us today to get started.

Cylinders inc. team



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