Avoiding free water in a lubrication system - VAISALA - #2

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Effects of oil temperature on the solubility In lubrication systems, the oil is typically rather warm e.g. +40…+60 °C. In situations when temperature decreases considerable, like when engines are turned off, there is a risk of free water formation, because the oil cannot anymore hold the same amount of water. While the absolute water content (ppm) has remained unchanged, the saturation point has been reached and the risk of, for example, corrosion becomes likely (see Figure 2). In case free water has formed during a shutdown, the oil should be dried first or the system has to be started carefully, running only the lubrication system until normal operating temperature is reached and the water dissolves back into the oil. This phenomenon can be monitored much more easily with water activity (Aw, see p. 3). One should also consider that the hot lubrication oil in the reservoir can easily absorb more water if it is in contact with ambient air. This kind of long term "leak" will increase water contamination in oil slowly. Effects of oil ageing on the solubility The used oil's ability to hold water is remarkably higher than a new oil's. This is caused by the chemical reactions that take place over the life of the fluid changing the water solubility. This is a very important point to consider when oil is changed or when new oil is added to existing oil in the lubrication system. Figure 3 demonstrates how the oil ageing affects the water solubility (saturation level 2000 vs 6000 ppm). Figure 2 The effect of temperature on free water formation. The risk increases when engines are switch off and temperature decreases. When the water content reaches the saturation point free water starts to form. Figure 3 The effect of oil ageing on its water solubility (water content in ppm versus water activity reading). The test was at 40-47 °C with a new lubrication oil and the same oil after 30 000 hours in service.