Both are called water in the oil, but the difficulty of treating them can differ by an order of magnitude. 3 ppm of dissolved water in an insulating oil and the visibly emulsified water in the same oil have completely different effects on the filtration, on the vacuum degassing and on the performance of the oil. If the two kinds of water cannot be told apart, the result on site is easily a long degassing that still leaves the oil wet, or an oil whose figures simply do not meet the requirements after treatment.
The two forms in which the water exists
Dissolved water is the state in which the water is dispersed uniformly in the oil at the molecular level. The polar water molecules interact with the polar components of the oil, and at a low content the oil looks completely clear and transparent, with no separation and no sediment, and ordinary settling or heating will not drive it out.
Emulsified water is dispersed in the oil as fine droplets. The surface of each droplet is coated by the surface-active components, the oxidation products or the solid particles in the oil, which form an interfacial film that keeps the water droplets apart, so the oil does not separate over a long period. The oil looks cloudy and whitish, it appears hazy when the light is held up behind it, and small bubbles appear briefly on the surface when it is shaken.
The difference in how they are formed
Dissolved water comes mainly from the water in the oil itself, from the damp air drawn in by the container breathing, and from the extractables of the insulating materials; the quantity is small but it is present continuously. Emulsified water depends mainly on how the water got in: the water enters the oil in larger droplets and is broken up into fine droplets by shearing, by pumping agitation or by a local temperature drop, and is then stabilised by the interfacial film.
The effects on the figures and on the equipment are different
For the breakdown voltage, a trace of dissolved water is enough to pull the reading down noticeably, because the water molecules form a polarisation bridge in the electric field. Emulsified water acts through a different mechanism: it produces a distortion of the electric field at the droplet interface and at the same time carries the polar products on the interfacial film into the field region. With the two effects combined, the breakdown voltage of the oil in the emulsified state is normally lower at the same water content.
For the equipment, the emulsified water brings about several other things directly. The fine droplets form a dense wet film on the surface of the element very quickly, and the differential pressure rises faster; the oil in the emulsified state foams more easily in the vacuum, and the foam occupies the volume of the vacuum chamber; and the particles carried in the emulsified oil can pass through a filter layer designed to the nominal accuracy, so the cleanliness figure is hard to hold at NAS 1638 class 5.
Why the dehydration stage finds emulsified water harder
The essence of the dehydration is to move the water from the oil phase into another phase, and that only holds when the water exists as free water. Emulsified water is enclosed in the interfacial film; it can neither migrate freely nor be drawn off by the vacuum chamber. The first step is therefore always the breaking of the emulsion: the interfacial film is broken, the droplets merge and grow, and they settle out or are separated out, and only afterwards can the free water enter the normal condensation and recovery route.
Breaking the emulsion is not free: it needs a higher temperature or a longer settling time, it needs additional settling time, and sometimes it needs a demulsifying agent, and all of that means a longer treatment cycle and a higher energy consumption. The more emulsified the oil is, the larger this fixed overhead becomes.
Break the emulsion first and dehydrate afterwards; the order cannot be reversed. In the wrong order the equipment spends all its effort clearing the obstruction before it does any useful work.
| Item of comparison | Dissolved water | Emulsified water |
|---|---|---|
| Form of existence | Uniformly dispersed at the molecular level | Fine droplets, stabilised by the interfacial film |
| Appearance | Clear and transparent, no difference to be seen | Whitish and cloudy, hazy when the light is held up behind it |
| Main source | The water in the oil itself, damp air, extractables from the insulating materials | Water that entered in large droplets and was broken up by shearing |
| Effect on the breakdown voltage | A trace is enough to pull it down noticeably | The interface distortion adds on, so it is normally lower |
| Effect on the filtration | Relatively mild | A wet film on the element surface, the differential pressure rises faster |
| Route of removal | Heated to lower the solubility, then vaporised off | Break the emulsion and merge the droplets first, then go through the dehydration route |
- Settling and separation: oil on top, water below and the oil layer clear means that the water is mainly present as free water.
- Foaming when shaken: fine bubbles that take a long time to disappear point to the emulsified state.
- Looking at the light through it: a marked haze and a milky appearance are typical signs of emulsification.
- Clear separation but the water content is still high: there is still dissolved water in the oil phase, and separation does not mean the oil is clear of water.