Should hydraulic oil be changed when it turns black?
author: Leila
2024-09-12
It is very common for hydraulic oil to turn black. The color of newly purchased hydraulic oil is amber or light yellow. After use, it will turn black, brown or even black. What is the matter with hydraulic oil turning black? Does the blackening of hydraulic oil mean that the performance is reduced and it cannot be used anymore? Some people think that the blackening of hydraulic oil is a natural phenomenon and there is no need to pay attention to it. So what is the matter with hydraulic oil turning black? What should I do if I find that hydraulic oil turns black?
If you find that hydraulic oil turns black, you should first find out why the hydraulic oil turns black.
There are two most common reasons for hydraulic oil to turn black: high temperature and oxidation. The first thing to explain is that no matter which of the two causes the hydraulic oil to turn black, it is not necessary to change the oil, but to extract some oil samples for oil testing (note: the extracted oil samples must be representative). Sometimes hydraulic oil Although it has turned black, the performance is still good and it can continue to be used. In some cases, although the hydraulic oil has not turned black, its performance is not up to standard and it cannot be used. Therefore, it is not possible to judge whether the hydraulic oil can continue to be used simply by looking at whether the hydraulic oil has turned black.
Although whether the hydraulic oil turns black does not mean anything, in actual use, the blackening of the color of the hydraulic oil must attract our attention. Because the blackening of the hydraulic oil may be normal or abnormal. Therefore, the blackening of the hydraulic oil often reminds us to find the cause to see if there is any abnormality, especially when the color of the hydraulic oil suddenly turns black and the transparency suddenly changes.
Sometimes, the oil temperature of the oil tank is normal, but there is a local high temperature inside the hydraulic system, which causes the local temperature to suddenly increase, causing the high-temperature oxidation of the hydraulic oil.
There are many reasons for the local high temperature of hydraulic oil. Sometimes there is air inside the hydraulic oil, and sometimes it is a hydraulic valve failure, which leads to abnormal local pressure and local high temperature. In this case, the overall oil temperature of the hydraulic oil seems normal, but the local high temperature causes the hydraulic oil to turn black.
If the hydraulic oil is taken for testing, the acid value and viscosity should be tested first. If both are normal, it means that the hydraulic oil turns black not due to oxidation, but it is likely due to high temperature. Using an infrared detector can quickly find the high temperature point of the hydraulic system. If it is a problem with the hydraulic valve, then the hydraulic valve must be replaced. In general, a paint film caused by high temperature can be observed on the hydraulic valve, which is generally a brown hard substance covering the inner surface of the hydraulic valve. In this case, the blackening of the hydraulic oil reminds us to replace it as soon as possible. Valve parts, to avoid the problem from expanding to affect the entire hydraulic system.
Oxidation is also a major cause of hydraulic oil deterioration. Oxidation does cause the color of the oil to gradually turn black, but it is not the only reason. Therefore, the degree of blackening of the hydraulic oil does not necessarily reflect the degree of oxidation. The color change of hydraulic oil during the oxidation process is also related to the composition of the oil, whether it is contaminated, etc. For example, contamination of hydraulic oil can also cause the oil to turn black, which is not necessarily oxidation. To know the degree of oxidation of hydraulic oil, it is necessary to determine it through oil testing. If the test finds that the acid value of the oil increases, it means that the hydraulic oil is oxidized, and the viscosity of the oil will also increase.
In addition to high temperature, the reasons for the oxidation of hydraulic oil include: water in the hydraulic oil, dust impurities mixed in, and metal powder such as copper, especially when copper and water are in contact with hydraulic oil. When the hydraulic oil is oxidized, the acid value increases, and it is easier to corrode metal parts. In addition, if the impurities mixed in the hydraulic oil are dissolved in the oil, the viscosity of the hydraulic oil will increase, forming harmful paint film and sludge inside the equipment. These substances form a layer of harmful attachments on the inner surface of the equipment, further accelerating the oxidation and deterioration of the hydraulic oil, and causing other hazards. Sludge will greatly increase the rate of deterioration of hydraulic oil. Under the premise of other conditions being the same, 1% sludge can halve the service life of hydraulic oil.
Through normal use and maintenance, we can minimize the rate of oxidation and deterioration of hydraulic oil. The oxidation process is a chemical change, and the rate of all chemical changes is affected by temperature. For every 10°C increase in temperature, the oxidation rate increases by about one time, and the service life of hydraulic oil is halved. Conventional hydraulic oil is generally mineral oil, and the maximum allowable operating temperature does not exceed 60°C. If it exceeds this temperature, the life of hydraulic oil is halved for every 5°C increase.
The pressure of the hydraulic system also has an impact. When the pressure increases, the viscosity of the hydraulic oil increases, which will cause increased friction and then heat. In addition, increased pressure is more likely to cause air stagnation. If bubbles are mixed in the hydraulic oil, when it enters the high-pressure area with the oil flow, the bubbles are instantly compressed, and the temperature will increase rapidly, causing the hydraulic oil to oxidize and deteriorate at high temperature or even partially carbonize.
Therefore, when the equipment is running, the pressure should be reduced as much as possible while ensuring normal operation.
Summary: When you find that the hydraulic oil has turned black, you don’t necessarily have to change the oil. You should first take some oil samples for oil testing, because the blackening of hydraulic oil does not necessarily mean that the hydraulic oil is unusable.
When collecting oil samples, remember that the oil samples collected should be representative. The most representative oil sample should be from the downstream of the oil pump (not behind the filter element). The second choice is the middle of the oil tank. It is best to sample while the equipment is running or immediately after shutting down.
If you have not done oil testing before, you can collect oil samples every 13 weeks at first, and then adjust them as appropriate based on the test results and the use of the equipment. If the test results are relatively stable, you can increase the sampling period as appropriate, and if problems are found during the test, you can sample more frequently. Archiving multiple test results for comparison will help us better understand and predict problems with hydraulic oil and hydraulic equipment.
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