The condensate water is the final product of the dehydration. The water in the oil is vaporised and turns back into a liquid in the condensing section, and that part of the path is visible. The condensate water is therefore not only a harmful substance that is drained off, it is also a progress indicator that needs no instrument: the discharge rate, the colour, the acidity and the conductivity tell you roughly how far this batch of oil has been dehydrated and what condition the oil itself is in.
What the colour tells you
Clear or slightly pale yellow is the condensate water of a normal oil; the colour comes from the trace of dissolved oil molecules and it varies with the depth of the colour of the oil itself.
A distinctly darker, brownish or even black colour means that the condensate water has carried out the oxidation products or the carbonised particles of the oil, which means that the oil itself has clearly aged and the return on further dehydration is falling.
A cloudy white or emulsified appearance means that emulsified oil or polar impurities have got into the condensate water. Water carried out in the form of oil droplets usually means that there is emulsification in the system, and in such a case the efficiency of the dehydration is held back.
The acidity points to the degree of ageing of the oil
Normal condensate water is close to neutral. If the condensate water is acidic, it means that acidic degradation products such as organic acids have already formed in the oil and have been carried out together with the water vapour. The higher the acidity, the deeper the oxidation of the oil.
The value of this signal is that it turns "the oil has got old" from a qualitative impression into something that can be judged: samples of the condensate water can be taken at intervals during the treatment to follow the trend of the acidity, which is much faster than taking an oil sample, sending it for test and waiting for the result.
The conductivity points to the source of the impurities
The conductivity of pure water is very low. If the conductivity of the condensate water rises, ionic substances have appeared in the water, possibly from the additives of the oil, from metal corrosion products, from salts that got mixed in or from water-soluble degradation products.
If the conductivity rises while the colour stays normal, the source of the water should be suspected not to be the oil itself but an ingress from outside, for example a leak into the condensate line or the environmental water vapour drawn in through a leak in the vacuum system. These two causes need a completely different treatment.
Working back the progress of the dehydration from the discharge rate
The water comes out of the oil fast at first and slowly afterwards. At the beginning the discharge of the condensate water is large; as the water content in the oil falls to the ppm range, less and less water can be vaporised and the discharge falls with it.
A simple test can therefore be used: if the condensate water has practically stopped discharging and the water content in the oil is stable at the 3 ppm level, the dehydration is close to complete. In the other direction, if the water content is still high while the condensate water has already run dry, the conditions for the vaporisation are in all probability insufficient: the temperature is too low, the vacuum level is not deep enough, or the water in the oil is in a state that is hard to vaporise.
One exception has to be kept in mind: if the oil is already fully dehydrated and the condensate water is still discharging continuously and steadily, the moisture has to be suspected to have another source instead of increasing the vacuum further.
| Signal | Normal appearance | Abnormal appearance | What it points to |
|---|---|---|---|
| Colour | Clear or pale yellow | Darker, brownish or black | Oxidation or carbonisation of the oil |
| Turbidity | Clear and separated | Milky white and cloudy | An emulsified condition carrying out oil droplets |
| Acidity | Close to neutral | Acidic | Acidic degradation products have formed in the oil |
| Conductivity | Low | Clearly higher | Ionic impurities or water ingress from outside |
| Discharge rate | Large at first, then smaller until it stops | Still discharging after the oil is fully dehydrated | There is a source of water other than the oil |
- Fix an interval for the sampling, a trend is more useful than a single reading.
- Condensate water black and acidic: the oil has aged, the focus moves to assessing a change of oil instead of purifying further.
- Conductivity abnormally high: check the condensate line and the vacuum seals first, do not simply increase the vacuum.
- Compare the condition of the condensate water with the water content results from GB/T 7600 and build up an empirical baseline of your own.
I would put it this way: the condensate water is the only visible output of the dehydration, and reading it properly is more useful than looking at the vacuum gauge once more.