Yes. Under normal operating conditions, the hydraulic fluid will gradually lose its performance due to long-term repeated use. For maintenance engineers and factory managers, understanding the causes, signs and prevention methods of hydraulic fluid deterioration can help make sure the stable operation of your equipment and reduce maintenance costs.
Why does Hydraulic Fluid Go Bad?
High Temperature and Air Cause Oxidation
When the hydraulic fluid circulates in the hydraulic system, it will continuously come into contact with the air inside the hydraulic tank. The heat generated by the operation of the equipment will continuously increase the temperature of the hydraulic fluid. For every 10 ℃ increase in the temperature of the hydraulic fluid, the oxidation reaction rate will nearly double.
High temperatures will damage the molecular structure of the base oil in hydraulic fluid, causing the base oil to undergo continuous chemical reactions with oxygen, gradually generating substances such as organic acids, gums and sludge. These substances can darken the color and cause abnormal viscosity of hydraulic fluid.
Acidic substances can also corrode metal components such as hydraulic pumps and valves. At the same time, this will quickly consume the antioxidant additives in the hydraulic fluid, causing the fluid to lose its stability and ultimately deteriorate.
Water Enters
Moisture is a common cause of hydraulic fluid deterioration. The alternation of cold and hot in the operation of hydraulic systems can result in condensation water, aging and leakage of seals, improper operation when adding hydraulic fluid, and high environmental humidity, all of which can allow moisture to enter the hydraulic fluid and cause emulsification.
Excessive moisture in hydraulic fluid can cause it to turn milky and turbid, damaging the thickness and strength of the lubricating oil film and causing hydraulic components to lose lubrication protection.
Water will cause the hydrolysis reactions of rust-preventing, anti-wear and demulsifying additives, thereby losing their original functions. At the same time, moisture can accelerate the rusting of metals inside the system. The impurities generated by rusting will further deteriorate the hydraulic fluid, creating a vicious cycle.
Solid Impurities
During the operation of the hydraulic system, metal debris generated by internal component wear, rubber particles that fall off from aging seals, dust from the outside, and residual welding slag impurities during maintenance can easily enter the hydraulic fluid.
These tiny solid particles belong to hard pollutants. They are easy to scratch components such as hydraulic pumps, valves and hydraulic cylinders, thereby generating more metal debris.
At the same time, these solid impurities will block the oil circuits and filter elements, resulting in abnormal pressure in certain areas of the hydraulic system and poor heat dissipation. The combined effect of high temperature and impurities will cause the hydraulic fluid to deteriorate rapidly, shortening the lifespan of the oil.
Additive Consumption
The hydraulic fluid contains various functional additives such as anti-wear agents, antioxidants, preservatives, demulsifiers, rust inhibitors, and defoamers. They can slow down the aging of hydraulic fluid and protect the system components. But all the additives are consumable substances.
Over a long period of use, these additives will continuously be decomposed, consumed and lose their effectiveness. When the concentration of additives is lower than the effective value, the anti-wear and heat dissipation ability of hydraulic fluid will rapidly decrease, which cannot meet the working requirements of the hydraulic system.
Even if there is no obvious change in the appearance of the hydraulic fluid, it has deteriorated and cannot be used normally.
Mixing Different Types of Hydraulic Fluid
Hydraulic fluids of different brands, different models and different formulations have obvious differences in their base oils and additives. If hydraulic fluid is mixed, there may be a problem with incompatible components.
The base oils may separate and precipitate. The additives may undergo chemical reactions and lose their activity. This will disrupt the stability of the hydraulic fluid and cause it to deteriorate rapidly.
Improper Storage and Usage
Even if the hydraulic fluid has not been opened, if it is stored improperly, the hydraulic fluid will still deteriorate.
Long term exposure to direct sunlight, high temperature, humidity, and poor ventilation can accelerate the oxidation of hydraulic fluid and the decomposition of additives. If the storage container is not sealed properly, dust and moisture will enter the hydraulic fluid.
During use, the hydraulic system is constantly subjected to high pressure and heavy load conditions, and insufficient heat dissipation can accelerate the deterioration of the hydraulic fluid.
Signs Your Hydraulic Fluid Has Gone Bad
Color Change
Normal hydraulic fluid usually appears clear and transparent, with a uniform texture and no impurities. Color change is the most direct indication of hydraulic fluid deterioration.
When hydraulic fluid is exposed to high temperatures for a long time, the oxidation reaction will gradually darken the color of the hydraulic fluid, changing from light yellow to dark brown and finally appearing black.
This darkening of the color is due to the decomposition of hydrocarbons in the oil at high temperatures, which generates dark substances such as resins and asphaltenes. If water gets into the hydraulic fluid, the oil will turn milky and cloudy, and exhibit the phenomenon of emulsification and stratification.
Abnormal Smell
The normal hydraulic fluid has only a slight smell of mineral oil or no smell at all. When the hydraulic fluid goes bad due to oxidation and contamination, it will produce an abnormal odor.
Excessive oxidation can cause the hydraulic fluid to emit a burnt and pungent smell of oil fumes. Moisture intrusion and impurities can cause the hydraulic fluid to have an acidic and putrid odor. After the decomposition and deterioration of internal additives in the hydraulic fluid, there may also be irritating odors.
You can quickly determine whether the oil is functioning properly by its smell.
Sediments Appear
When the hydraulic fluid goes bad, it will produce viscous deposits such as gums, varnish films and oil sludge. These substances will adhere to the inner walls of the fuel tank, the oil pipe walls, the hydraulic valve gaps, and the filter surface.
When you open the fuel tank, you can clearly see a sticky brown or black substance adhering to the inner wall. When you disassemble the filter element, you will find that the filter screen is covered with a large amount of oil sludge and impurities.
This type of sediment can exacerbate oil pollution, block oil circuits, and cause valve cores to become stuck.
The Hydraulic Fluid Frequently Foams Up.
Special anti-foaming agents are usually added to the hydraulic fluid. During normal operation, the hydraulic fluid generates very few bubbles and these bubbles dissipate quickly.
When the hydraulic fluid goes bad, the anti-foaming additive will be consumed and lose its effectiveness. The bubbles fail to burst in time and will accumulate in the fuel tank, forming a large amount of persistent foam. In serious cases, it may even overflow from the fuel tank’s ventilation opening.
When the bubbles enter the high-pressure oil circuit, it will cause instability in the system pressure. At the same time, the impact caused by the bursting of the bubbles will further accelerate the oxidation of the oil, creating a vicious cycle.
Abnormal Increase in System Operating Temperature
When the hydraulic system is operating normally, the system temperature will stabilize within the standard working range. When the hydraulic fluid deteriorates, its viscosity, lubricity and heat dissipation performance will decline greatly.
The internal friction resistance of hydraulic fluid increases, and additional heat is generated during the flow process. At the same time, the gum and sludge generated by the deterioration of hydraulic fluid will block the oil circuit and heat dissipation channels, resulting in an obvious decrease in the system’s heat dissipation efficiency.
Increased Running Noise
The deteriorated hydraulic fluid cannot form a complete and stable lubricating film on the surface of the metal components. Components such as hydraulic pumps, valve bodies, and cylinders may experience direct metal friction, resulting in abnormal noises.
Furthermore, the bubbles in the hydraulic fluid will cause cavitation during the high-pressure circulation, resulting in vibrations and abnormal noises in the equipment. Impurities in hydraulic fluid can cause blockages in the oil circuit, resulting in harsh buzzing and knocking sounds when the hydraulic pump operates.
How to Prevent the Hydraulic Fluid from Going Bad Too Quickly？
Control System Operating Temperature
Excessive temperature will accelerate the deterioration of the hydraulic fluid. You can install plate heat exchangers or air-cooling systems. At the same time, you need to regularly clean the dust and oil stains on the surface of the heat dissipation components.
Avoid having your device operate continuously under excessive load and high pressure for an extended period of time, as this can cause the system to overheat.
Prevent Moisture from Entering
You need to regularly check the sealing performance of the seals and the connections of the pipelines. Replace the aging and damaged seals in time to prevent external moisture from entering.
For situations where there are frequent changes between hot and cold conditions, you can install a dry type breather cap to reduce the formation of condensate water in the oil tank. When adding or replacing hydraulic fluid, you need to operate in a dry environment. It is strictly prohibited to use tools with water in contact with the hydraulic fluid.
At the same time, you need to regularly check whether there is water accumulation in the hydraulic system and handle it in time.
Prevent Solid Impurities from Entering
You need to use a filtering device with appropriate precision and regularly clean or replace the filter element. When adding new hydraulic fluid, it must pass through a filtering device to prevent impurities from entering the system in the oil drum.
When disassembling and repairing the equipment, you need to remove the remaining impurities in the pipelines and the oil tank, and clean the components thoroughly before reassembling them.
Properly Use Hydraulic Fluid
You need to use the appropriate hydraulic fluid according to the manufacturer’s requirements, temperature, load, environment, and other working conditions.
When replenishing or replacing, you need to use hydraulic fluid of the same specification. If you need to use different types of hydraulic fluid, you need to empty the old oil from the system, clean the oil tank and pipelines, and then add the new hydraulic fluid.
In harsh working conditions such as heavy load, high temperature, and high dust, you need to shorten the oil change time appropriately. Don’t wait until the hydraulic fluid has completely gone bad before replacing it.
Proper Storage and Preservation
The hydraulic fluid should be stored in a cool, dry and well-ventilated indoor area. It should be kept away from direct sunlight, high-temperature heat sources, humid environments, and avoid outdoor storage. The container for storing hydraulic fluid needs to be well sealed to prevent dust and moisture from entering.
The opened oil drums should be sealed promptly and used first to avoid long-term exposure. At the same time, you need to make proper markings to prevent the use of inappropriate hydraulic fluid.
Regularly Inspect the Condition of the Hydraulic Fluid
You can take regular samples to observe the color, transparency, or presence of impurities in the hydraulic fluid. You can also analyze the viscosity, acid value, additive content and other indicators of hydraulic fluid through professional testing to determine whether the oil has gone bad.
FAQs
What is the Shelf Life of Unopened Hydraulic Fluid?
For the majority of mineral-based hydraulic fluids, when the sealing is good and the environment is suitable, their shelf life is usually around 5 years. However, hydraulic fluid containing a large amount of additives such as anti-wear agents or antioxidants may have an effective period of around 2 to 3 years.
Hydraulic fluid that has exceeded its shelf life, even if it doesn’t show any obvious abnormalities externally, will lose its functional additives such as anti-wear and anti-oxidation properties, which cannot guarantee the performance during use. It is not recommended to be used in precision hydraulic equipment.
How Long can the Unused Hydraulic Fluid be Stored after being Opened?
After being opened, the hydraulic fluid will come into direct contact with air, moisture and dust, and its deterioration process will accelerate. Under the premise of proper sealing and a good storage environment, it is recommended to use up the hydraulic fluid within 6-12 months after opening.
If the container is not sealed properly during storage, gets damp or is contaminated with impurities, you need to check the oil condition in advance. You cannot simply rely on the storage time to determine whether it can be used.
Can the Deteriorated Hydraulic Fluid be Reused after Filtration?
No. The filtering operation can only remove solid impurities from the hydraulic fluid. It cannot remove acidic oxidation products that have already dissolved in the oil. It is also impossible to recover depleted additives. You must replace and use new hydraulic fluid.
Can the Hydraulic Fluid Continue to be Used with Only Slight Discoloration and No Other Abnormalities?
The slight discoloration of hydraulic fluid indicates that the oil has begun to undergo an oxidation reaction, but it has not yet deteriorated severely. It is not recommended for long-term use, especially in heavy-duty and high-precision hydraulic equipment.
A slight color change means that the performance of hydraulic fluid has begun to decline, and continuous use will gradually increase component wear. It is recommended that you conduct oil testing in time and replace the oil when necessary.
Will a Low Temperature Environment Cause the Hydraulic Fluid to Go Bad Prematurely?
Usually, low temperature environments do not cause hydraulic fluid oxidation and deterioration. However, if the temperature is too low, the viscosity of the oil will increase and its fluidity will deteriorate. This will exacerbate component wear during equipment startup.
At the same time, low temperatures may cause the additives in the hydraulic fluid to precipitate, thereby affecting the stability of the oil. For long-term low-temperature working conditions, you should use special low-temperature hydraulic fluid.
Final Thoughts
The deterioration of hydraulic fluid is not caused by a single factor. By using and maintaining it properly, you can extend the lifespan of the hydraulic fluid and ensure the long-term stable operation of the equipment. If you need the appropriate hydraulic fluid, please feel free to contact us.
