By Josh Cosford, Contributing Editor
In Part 4 of the hydraulic oil analysis series, we will tackle one of the most essential qualities of hydraulic oil you must test and understand. Particle contamination, for example, may take months or years to erode critical components to the point of failure, but water infiltrating your pumps, valves, and actuators could lead to catastrophic failure within hours or minutes.
Water reduces film strength, reducing oil’s ability to lubricate moving parts effectively, leading to metal-to-metal contact and accelerated wear. Water saturation also increases the vapor bubble creation rate, which can lead to cavitation damage in the form of pitted metal surfaces and erosion.
Free water doesn’t need to circulate throughout the circuit to harm the hydraulic circuit over time. Even excessively “humid” oil can lead to damage and eventual failure. Metal surfaces and internal components can corrode in the presence of water, leading to rush, pitting or weakening of ferrous metals.
The presence of excessive water also accelerates the degradation of the oil itself, increasing oxidation, sludge and microbial growth. In some cases, excessive water saturation also degrades the seals, increasing the likelihood of both internal and external leaks.
With water contamination being so detrimental to your hydraulic system, ensuring you perform regular testing on your hydraulic oil will prevent the above symptoms and problems from destroying your hydraulic components from within. Most labs provide water saturation results as a default test when you send them a sample, but if you’re unsure, ask and request it. The commonly performed Karl Fischer test uses a titration vessel and a special reagent to react with the water, and based on the amount of reagent used, the test accurately determines the water content.
Most labs will provide you with a water saturation number expressed in parts per million (ppm), which, as you guessed, is how many water molecules there are in every million molecules of hydraulic oil. Ppm can also be converted into percentages by dividing by 10,000, which seems odd until you remember that percentage is already 1/100 of the total. Regardless, 500 ppm equates to 0.05% water saturation.
A target “dryness” is hard to pinpoint because there are so many variables affecting water saturation. As well, the quality of oil and its additive package may help hydraulic oil retain more oil within saturation. You definitely want less than 500 ppm when the oil is tested at the lab at room temperature. But just like particle contamination, you can’t have a too low result. I recommend you stay under 200 ppm as performed in the lab.
If your oil is cloudy or milky, there is no need to get your oil tested. You absolutely have full water saturation hydraulic oil to the point it can no longer be held within saturation, and you must deal with it immediately. The easiest method to correct your water problem is to replace the oil. However, that can be an expensive affair, especially if you have more than a few gallons.
On the other hand, what are your options if you could keep all that precious oil and simply dry it out? Free water is easily removed with water-removing filter elements or by draining the settled water after the system is cool. However, highly saturated oil still needs humidity removed before it becomes clear again. A vacuum dehydrator is the most popular and effective method to dry hydraulic oil. The machine heats the oil and applies a vacuum to the evaporator chamber, which reduces the boiling point of the water, boiling it out of the oil and sucking it away. The machine is expensive but can often be rented.
Just as with all hydraulic oil analysis results, the lab can provide you with trends from each machine so you can monitor water saturation over time to see if you need to address a water contamination problem.
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