The water separability after steam treatment according to DIN 51589 is based on the behavior of water in steam turbine oils. In steam turbines, rapid separation of oil and water is critical to the survival of the sliding bearings of large power plant turbines. For oils from hydraulic systems and industrial gearboxes, this method, which is presented on our homepage under "Test methods", is only of limited use. The question here is how much condensate separates from the oil. In contrast to steaming the oil, DIN ISO 6614 uses the so-called Herschel test to specify how quickly oil separates from the water finely dispersed by high stirring speed. Dissolved water in the form of condensate is always present in small quantities in the fresh oil (20 to 600 ppm). In used oils it can be found in the form of condensate in almost any industrial or hydraulic oil in small quantities (usually less than 300 ppm). Intensive intermittent operation of equipment causes oil-filled systems to breathe.
Rule of thumb: 200 liters of oil expand by 7 liters when heated by 50°C or decrease in volume by about 7 liters when cooled. Volume compensation takes place via breathing nozzles or "breathers". Condensate is formed due to the moist air that has penetrated during breathing.
However, larger quantities of water can also enter the system in the event of leaks, e.g. in the cooling circuit or during machine cleaning with high-pressure cleaners.
In the case of oils in hydraulic systems or oil circulation systems, the excess water should separate quickly from the oil so that it can be drawn off at the lowest point of the tank. The standard for hydraulic oils DIN 51524 of TYPE HLP specifies a water separability of max. 30 minutes. At the latest after this time, complete 2-phase separation should be detected. Oils in circulation systems of paper machines and calenders must also separate the water as quickly as possible so that it can be completely removed from the oil in a short time by separators. The water must not be kept finely dispersed in suspension, as this can lead to corrosion (rust), cavitation or galling.