The Comprehensive Guide to Hydraulic Oil Cleaning

Hydraulic Oil Cleaning



Hydraulic Oil Cleaning

Hydraulic fluids are the lifeblood of the hydraulic system. Maintaining clean hydraulic oil is essential for the proper functioning and longevity of any hydraulic system. Contaminants such as dirt, paint, grime, metal particles, water, cutting fluids, and other impurities can cause damage to the system and reduce its efficiency. To ensure that your machinery continues to operate at peak performance, it is critical to regularly clean the hydraulic oil. Machinery life can be greatly extended through proper lubrication maintenance. This blog provides a greater understanding of the importance of cleaning hydraulic oil and the various cleaning methods and techniques.

    1. Understanding the Importance of Clean Hydraulic Oil
    2. Types of Contaminants in Hydraulic Oil
    3. The Importance of Regular Maintenance
    4. Best Practices for Maintaining Clean Hydraulic Oil
    5. Methods of Hydraulic Oil Cleaning


Understanding the Importance of Clean Hydraulic Oil

Clean-Hydraulic-OilClean hydraulic oil ensures that the system will run at optimal efficiency. Contaminants in hydraulic oil can restrict the flow of oil to the system’s components, causing them to work harder and use more energy. This increases the wear and tear on the parts. This causes increased energy consumption and higher operating costs. Clean hydraulic oil helps extend the life of the system. When hydraulic oil is contaminated, it can cause damage to the internal parts, leading to premature wear and the need for costly repairs or replacements. Keeping the hydraulic oil clean, the system can operate for more extended periods without requiring major repairs, saving both time and money.

When talking about hydraulic oil cleanliness, we often refer to the ISO particle count of the oil. According to the ISO 4406:99 standard, the ISO particle count measures the number of particles greater than 4, 6, and 14 microns in every milliliter of fluid. The particle count is then converted to what is referred to as the ISO Code or ISO Range Code. Results from an oil cleanliness test are typically reported in a three-number format such as 18/16/13, where 18 represents the range code representing the number of particles that are 4 microns and larger, 16 the range of particles that are 6 microns and larger, and 13 represents particles 14 microns and larger.

The goal is to prevent damage and downtime.

Using the correct hydraulic oil and ensuring they are in a suitable chemical condition is a prerequisite for success. Still, big changes in component service life are achieved by aggressive contamination management. In most situations, the amount of particle contamination in the hydraulic oil is the single biggest factor determining how long a lubricated component will last. The usual way in which most machines fail is because they “wear out,” but wear rates can be controlled, and the primary purpose of lubrication is to do just that. When particle contamination is reduced, machine wear rates decrease, and their component service life increases.

By the time you can see particle contaminants in your hydraulic oil, the oil cleanliness has well exceeded what the oil laboratories particle counter can count. This effectively means that your hydraulic oil is’ too dirty’ and will not meet the manufacturer’s recommended standards. Maintaining clean hydraulic oil is essential for any hydraulic system’s proper functioning and longevity. It ensures that the system runs at optimal efficiency and extends the life of the components. 


Types of Contaminants in Hydraulic Oil

types-contaminants-in-hydraulic-oilContamination is defined by the American National Standards Institute (ANSI) as “any material or substance which is unwanted or adversely affects fluid power systems or components, or both.” Contaminants can come from various sources, including the environment, the manufacturing process, and even the hydraulic oil itself. Although hydraulic oil systems are considered closed systems, they are not immune to contamination. Every form of contamination damages the hydraulic system in its own way and requires a different treatment. Understanding the types of contaminants present in the oil is crucial for selecting the appropriate cleaning method and ensuring the system is protected from damage.

Solid Contamination

Solid contamination is, most of the time, an insoluble contamination type that will not dissolve in the hydraulic oil. Solid contamination causes abrasion, adhesion, fatigue, erosion, and silting within the hydraulic system. Abrasion occurs when a large solid contamination particle gets stuck between two moving components and digs itself into one of them, scratching and damaging the surface of both moving components. Adhesion occurs when the oil film thickness decreases to the point that metal-to-metal contact starts occurring, resulting in surface weld and shear. Fatigue occurs when solid particles get stuck and repeatedly dent the component’s surface, resulting in surface cracking and damage. Due to fatigue, the particles will get released to start damaging other hydraulic system components. Erosion occurs when particles impact and damage the component surfaces by removing the component’s material. Silting occurs when solid particles build up in the hydraulic system.


A major reason for hydraulic oil degradation is oxidation. This is a chemical reaction between oxygen and hydrocarbon molecules in hydraulic oil. Hydraulic oxidative degradation is rated by the Acid Number (AN) or Total Acid Number (TAN). It displays the amount of potassium hydroxide (KOH) in milligrams needed to neutralize the acids in one gram of oil. The acid affects the fluid color and viscosity and causes the development of varnish and sludge.

Soft Contamination

Soft contaminants are another insoluble contamination type that causes problems for a wide range of hydraulic applications, especially turbines. Soft contaminants will cause sludge, a soft dark substance that moves around the hydraulic system until coming to rest at the bottom.

Soft contaminants can lead to the formation of varnish. Contaminated fluid containing metals and moisture particles will oxidize. This results in fluid degradation and a rising Total Acid Number (TAN) and eventually in the formation of varnish. This process is accelerated by temperature fluctuations (heat) which causes hot spots.

Varnish creates a sticky film on the components of the hydraulic system, catching all sorts of contaminated particles. This continues to build up, forming an abrasive and destructive surface file that results in sticky deposits adhering to the metal surfaces of the hydraulic oil loop, including piping, valves, heat exchangers, filters, and other sensitive components. Varnish is one of the most underestimated contaminants that will lead to a reduced service life of the hydraulic oil, filters, and system, causing unscheduled downtime and maintenance.

Water Contamination

Water contamination is present in hydraulic oil or lubricating oil as dissolved water or free water. Dissolved water develops when individual water molecules bind with oil molecules. Since these water molecules are too small to see with the naked eye, they will not change the fluid color. To minimize the damaging effects of water contamination, the water concentration in the hydraulic oil should be as low as possible and well below the oil saturation point. After the saturation point is reached and more water enters the hydraulic system, an emulsion will be created. This changes the hydraulic oil from clean to a cloudy, milky substance. Adding more water to this emulsion leads to free water. Free water is mostly found on the bottom of the oil reservoirs due to the higher specific density.

Hydraulic fluids hold more water at high temperatures.

Water contamination causes many problems, like corrosion and oil degradation by oxidation. Water contamination causes a change in hydraulic oil viscosity resulting in reduced lubrication thickness, causing metal-to-metal contact. The combination of free water with small copper or iron particles created by wear will have a catalytic effect on oxidation. The corrosion particles will spread through the system, causing the formation of new corrosion in the tank, and the cycle continues. Water contamination causes a depletion of additives which results in loss of dielectric strength, oxidation, accelerated metal surface fatigue, and oil breakdown. 

The water concentration in a hydraulic fluid is marked as RH percentage (relative humidity), ppm (parts per million), or % (percentage weight or volume).

Air Contamination

Air can exist inside a hydraulic system in a dissolved or free state. Dissolved air will not pose a problem. Free air becomes a problem when it passes through the system components. It causes pressure changes that compress the air and produce a large amount of heat in small air bubbles. This heat can destroy both the additives and the hydraulic fluid itself. Air contamination produces oxides that promote the formation of particles and form sludge. This is a potential oxidation source, accelerating metal parts’ corrosion. Wear and interference increase if oxidation debris is not prevented or removed.

Foam is a good indication of air contamination.

To help prevent air contamination, use flooded suction pumps, system air bleeds, proper reservoir design, return line diffusers, and desiccant breathers.


The Importance of Regular Maintenance

hydraulic-oil-regular-maintenanceRegular maintenance is crucial for preventing contamination and ensuring that the system runs at optimal efficiency. This is critical for maximizing uptime and reducing repair costs. Regularly checking and replacing filters is a key component of regular maintenance. As the hydraulic filter becomes clogged, it loses its effectiveness and can no longer remove contaminants from the hydraulic oil. Regularly replacing hydraulic filters ensures that the oil remains clean and the system continues to run efficiently.

Don’t wait for contaminants to destroy your machines. Keep it clean!

Regular monitoring and oil analysis provide an early warning of possible mechanical problems. By regularly monitoring and testing the oil, you can detect and address any issues before they cause damage to the hydraulic system. This includes visually inspecting the oil for contaminants, using oil analysis tests, and using a particle counting device.

Drive uptime and lower the total cost of ownership!

Regular maintenance is essential for maintaining clean hydraulic oil and ensuring that the system runs at optimal efficiency. This includes checking for leaks, replacing worn components, and ensuring the system is properly lubricated. By regularly checking and replacing filters, monitoring and testing the oil, and servicing and maintaining the entire system, you can prevent contamination and extend the life of the system.


Best Practices for Maintaining Clean Hydraulic Oil


To prevent contamination, it is essential to keep the system clean, use proper filtration, use the correct type of oil for the system, and store the oil in a clean, dry place. Properly handling the oil is also crucial. This includes being careful not to introduce contaminants when filling or changing the oil and properly disposing of used oil—using oil analysis to monitor and test the oil for contaminants. By regularly monitoring and oil analysis, you can detect and address any issues before they cause damage to the system.

Knowing the system’s cleanliness levels is the first step in contamination management.

Full-flow hydraulic filters (in-line filters) designed into the hydraulic system keep the fluid clean while in service. When the full-flow hydraulic filters are not serviced and max their dirt-holding capacity, the hydraulic filter will go into bypass mode. This allows dirty hydraulic oil to circulate within the hydraulic system. Offline centrifuge filtration systems (kidney loop system) allow the full-flow hydraulic filters to last longer and removes smaller micron contaminants that pass through the full-flow filters. Centrifuge filtration systems clean hydraulic oil of tiny abrasive particles, like sand and dust. 

Contamination in hydraulic systems is impossible to avoid completely. There are several sources of contamination. 
Contaminated new fluid – New oil is not clean oil and must be filtered before use.
Ingress contamination – Particles from the surrounding air enter the hydraulic oil through openings, including reservoir breathers, leaking cylinder rods seals, and when the system is opened for maintenance.
Contaminated components – Replacing and repairing components with contaminated components leads to contamination.
Internally generated contamination – For example, changes in temperature cause condensation that leads to water contamination and friction of components leading to solid contamination in the hydraulic oil.

The majority of hydraulic system failures are related to particulate matter contamination.

Protect your hydraulic fluid cleanliness with these five (5) steps:
– Filter new hydraulic oil before filling a reservoir or system.
– Match oil cleanliness to hydraulic system requirements.
– Consider hydraulic systems designed with easily accessible filter systems.
– Set up a maintenance schedule based on your operational requirements and environmental conditions.
– Clean the areas around the filter before you change it.

Maintaining clean hydraulic oil requires a combination of preventive measures, proper handling, and regular monitoring and testing. By following these best practices, you can ensure that your system runs at optimal efficiency and extends the life of the components.


Methods of Hydraulic Oil Cleaning 

Cleaning hydraulic oil from contamination and, therefore, from malfunctions, breakdowns, and expensive hydraulic oil changes. Hydraulic oil cleaning oil can take place in several different ways or combinations. Different hydraulic oil cleaning methods have different degrees of efficiency and cost.

Clean hydraulic oil is the key to an effective process and improved sustainability!

Sedimentation/Gravity Separation

Gravity separation is naturally occurring and environmentally friendly. Gravity separation separates the hydraulic oil and water based on their specific gravities. Over time, gravity separation will separate the solids from the liquids. Gravity separation will only separate the free water from the hydraulic oil.


Filtration is the physical or mechanical process of retaining particles in a fluid by passing the liquid through a filtration medium. The hydraulic oil is forced through the filtration medium, leaving contamination particles behind in the filter. 

Air filters clean the air before it enters into the hydraulic oil reservoir. This air can be highly contaminated due to a dusty environment. Only a few air filters are capable of removing both water and solid particles from the air, which significantly reduces the oxidation process.

Bypass filtration (bypass filters) acts as a kidney loop, filtering the hydraulic oil from the main system. After the hydraulic oil is filtered, it returns to the system’s oil reservoir. By using filter elements with different filter fineness, higher fluid cleanliness can be achieved. The best method of capturing and retaining fine particles is by installing a centrifugal bypass filtration system.

Offline filtration (offline filters) makes it possible to filter the hydraulic oil even when the main hydraulic system is not operating. These systems can be stationary or mobile. Changing offline filter elements is easy and can be done without interrupting the hydraulic system. An offline filtration system should operate continuously, circulating the hydraulic oil volume in the hydraulic system many times per day.

Pressure filtration has a typical filter fineness between 2-20 μm. Pressure filtration can be used to protect the whole hydraulic system, part of the hydraulic system, or just a component immediately downstream of the transfer pump.

Return filtration is the most common form of fluid filtration. Return filtration occurs on the return line, in or on the hydraulic reservoir, just before the hydraulic oil returns to the reservoir. 

Suction filtration prevents the ingress of large particles (150-200 μm) from the hydraulic oil reservoir into the system’s circuit. Suction filters are typically placed between the pump and the inlet pipe with flanges. This basic form of filtration doesn’t contribute to the cleanliness level.

Centrifugal Separation

Clarification-Purification-Centrifugal-SeparatorCentrifugal separators use centrifugal force to separate liquids and solids. A disc stack centrifuge is a type of centrifugal separator with a series of conical discs providing a parallel configuration of centrifugation spaces. The disc stack centrifuge removes solids (usually impurities) from liquids or separates two liquids phases from each other by centrifugal force. The denser solids or liquids that are subjected to these forces move outwards towards the rotating centrifuge bowl wall, while the less dense fluids move towards the center. The disc stack increases the surface settling area, which speeds up the separation process. The concentrated denser solid or liquid is then removed continuously, manually, or intermittently, depending on the design of the disc stack centrifuge bowl.

For high-speed disc stack centrifugal separators, the below can be taken to calculate the solid reduction (valid for non-oil soluble particles and solids with a density ≥ 2000 kg/m³):
– 100% of particles removed with size 10 μm and above
– 90% of particles removed with size 5 μm to 10 μm
– 70% of particles removed with size 3 μm to 5 μm

A centrifugal separator has a high degree of efficiency and operates continuously at high contamination levels. Entrained gases are not removed. A compact system includes the disc stack centrifuge, a feed pump, and a control system. 

Designs for different separation duties

A clarifier is a centrifugal separator for solid-liquid separation. This process removes solids such as contaminated particles, sediments, natural organic matter, and color from the hydraulic oil. This processed hydraulic oil becomes clear. This disc stack centrifuge is very suitable for clarifying liquids that have a small proportion of suspended solids.

A purifier is a centrifugal separator for liquid-liquid-solid separation. This process separates two (2) liquids of different densities and solids, such as water, oil, and fines, from each other. Purifiers are specifically designed to remove contaminant particles, and solid impurities, and free water from hydraulic oils. The advantage of a purifier is that it can remove continuous free water. A purifier can remove both contaminated particles and water in a single operation.

Vacuum Dehydrating

Vacuum dehydration systems are oil purification units that can be applied directly to various types of machine reservoirs. The systems dehydrate and clean most types of oils, such as lubricating, hydraulic, transformer, and switch oils, by removing contaminated particles, gasses, and water. Actively filtering moisture can prevent contamination-related problems such as oxidation, degradation, and corrosion. Vacuum dehydration systems remove large quantities of free, emulsified, and dissolved water, particulates, and gaseous contamination.

Looking for a quote? Get started below!