The gas content on a test report is a total figure, but the gas in the oil is not one thing. Dissolved gas in the oil, free bubbles carried in with the oil flow, and the water vapour vaporised by the heating have different sources, different effects and different ways of being removed. Treating them as the same is the easiest place to go wrong when judging the degassing result on site.
The first kind: dissolved gas
The gas is dissolved in the oil in the molecular state. Its sources include the long-term oxidation of the oil, the small amount of air dissolved in it, the impregnation and the ageing of the insulating materials, and the contact of the oil with the air in the tank. Its characteristic is that it cannot be seen but is always present, and that it is held relatively firmly by the oil molecules. The effective means of removing it is to lower the partial pressure of the gas: drawing a vacuum pulls the pressure in the chamber down, the equilibrium of the gas in the oil is broken, and only then does the gas leave the oil phase and get carried away. This is the fundamental difference between vacuum degassing and ordinary filtration; filtration only separates the phases, and only a vacuum can change the dissolution equilibrium.
The second kind: free bubbles
Bubbles are a gas phase that can be seen with the naked eye, dispersed in the oil. They usually come from the air drawn in at the return line, from a failed seal, from air drawn in through the tank opening under negative pressure, or from the upstream agitation beating air into the oil. They are large and their interfacial film is thin; most of them merge and rise near the oil surface, but once the oil pump has carried them into the vacuum chamber they expand sharply at the moment the pressure drops.
The harm of the free bubbles is concentrated in two points. First, they occupy the effective volume of the vacuum chamber, the pumping speed is spent without result and the ultimate vacuum will not go deeper; second, the gas-liquid entrainment produced when a bubble bursts carries oil mist into the vacuum pump, and the pump oil is diluted by the oil and the water vapour and then emulsifies and carbonises. The consequences further down that chain are more troublesome than the bubbles themselves.
The third kind: water vapour
The water vapour is produced by the heating. The higher the oil temperature, the higher the saturated vapour pressure and the larger the share of the water vapour partial pressure in the gas phase. This part is not a gas contamination but the carrier of the dehydration: vaporising the water and then collecting it back as a liquid in the condensation section and draining it away is the normal route of the vacuum dehydration. The temperature control range of the TYA series is 45–80 °C, and it is within that range that the water is carried out of the oil by vaporisation.
Why drawing a vacuum deals with the first two kinds at the same time
The removal mechanisms for the dissolved gas and the free bubbles are in fact the same in origin: drawing a vacuum lowers the absolute pressure of the whole system. The gas inside a bubble overcomes the interfacial tension, expands and bursts, and the volume of the gas phase grows accordingly; the gas dissolved in the oil escapes because the equilibrium has been broken. The lower the pressure is taken, the more completely both of these are done, and that is what the ultimate vacuum figure exists to express.
The water vapour, on the other hand, has to be combined with the condensation. If the temperature is only lowered to a certain point, the water vapour condenses again into droplets in the pipework and the elements and sends the water that has already been removed back into the oil. The heat-absorbing condensation system for the medium, the oil-water separation structure and the automatic condensate separation stage together ensure that the water vapour is recovered instead of flowing back.
| Type of gas | Source | Typical effect | How it is removed | Signal on site |
|---|---|---|---|---|
| Dissolved gas | Oxidation, dissolved air, extractables from the materials | Reduces the dielectric withstand capability | Drawing a vacuum to lower the partial pressure | The gas content stays out of limits for a long time |
| Free bubbles | Air drawn in at the return line, a failed seal, air drawn in at the tank opening | They occupy the pumping speed and the oil mist enters the vacuum pump | Removed before they enter the chamber, or expanded and burst by the vacuum | The vacuum gauge becomes shallower, the pump oil behaves abnormally |
| Water vapour | Vaporisation by the heating | Condensation and flow-back, which obstructs the dehydration | Recovered by condensation and drained | A change in the amount of condensate drained off |
- The gas content is high but the amount of condensate is normal: the problem is most probably in the dissolved gas, and a deeper vacuum and a longer circulation are needed.
- The vacuum gauge fluctuates frequently and the oil pump makes an unusual noise: look at the entrainment caused by the free bubbles.
- The amount of condensate suddenly increases while the water content does not fall: suspect that the water vapour is condensing and flowing back into the oil.
The gas content figure only tells you what to do when it is split into its three parts: whether to adjust the vacuum, to adjust the temperature, or to go back to the return oil line.