The same vacuum oil purifier behaves differently on oil from different sources. The difference is often not in the machine but in the oil itself: whether it is mineral or synthetic based, which additives it carries, what equipment it has run in and for how long. These changes rewrite the viscosity, the solubility and the rate at which oil sludge forms, while the temperature control range, the filter section configuration and the vacuum system of the machine are all designed for common working conditions. Finding out what this batch of oil is before changing it saves a great deal more than adjusting the settings afterwards.
Mineral oil and synthetic oil: the viscosity changes by a different amount with temperature
The two kinds of base oil differ in how far the viscosity moves with temperature. Synthetic oil normally has a higher viscosity index, which means the viscosity changes less when the oil temperature changes; mineral oil is the other way round. Viscosity determines the differential pressure across the filter directly: with the same filter element the reading is higher when the oil is hot, because the flow resistance follows the viscosity; and when the oil itself is more viscous, the differential pressure is higher at the same flow rate. The temperature control range of the TYA series is 45–80 °C, with the upper limit pulled down to 70 °C when the water content is high, and this range does not behave the same way on the two kinds of oil. If the settings used for another oil are copied over on site, the temperature may already be in position while the differential pressure stays high.
Additives: the source of the cleanliness figures and of the oil sludge
Antioxidants, anti-wear additives, flame retardants, rust inhibitors — the way these additives are present decides what the oil produces after long operation. Antioxidants themselves are consumed: once they are used up, the oxidation chain reaction speeds up and the rate of oil sludge formation rises. Oil sludge is a polar product, and it affects two figures at the same time — the solid particles it carries lower the cleanliness, and its high polarity lets water adhere more easily to metal and filter medium surfaces, so the water content and the breakdown voltage follow as well.
Looking at the water content alone when changing oil is therefore not enough. A batch whose water content passes while its cleanliness is deteriorating usually means the antioxidants have been used up to a certain stage. The cleanliness target is NAS 1638 class 5, the filtration accuracy ≤1 μm, the breakdown voltage ≥65 kV (GB/T 507); only when the three are read together does it become clear which section is responsible.
Used oil: the contamination history is part of the oil
Oil drawn from a machine often carries the history of the machine it was in before: it may have been mixed with a different grade of hydraulic oil, it may have run at high temperature for a long time, it may already have taken in water and become emulsified. These traces show in two visible places: the amount of oil sludge and the shape of the particles, and the form in which the water is present in the oil — dissolved water needs a longer vacuum time to be drawn off, while emulsified water may be atomised and condensed again and again inside the machine.
For insulating oil the target is clear: water content ≤3 ppm (GB/T 7600) and gas content ≤0.01% (GB/T 423). If a batch enters the machine with a high water content and a lot of oil sludge, the running time and the number of vacuum stages needed to reach the figures are greater than for clean oil. That is determined by the state of the oil, not by the capability of the machine.
| How the oil has changed | What it affects on the machine | How to check it on site |
|---|---|---|
| Mineral oil replaced by synthetic oil | The viscosity changes by a different amount with temperature, so the differential pressure reading differs | Record the differential pressure before and after the change at the same oil temperature |
| The antioxidants are close to being used up | Oil sludge forms faster, and the cleanliness and the breakdown voltage both deteriorate | Compare the cleanliness and the breakdown voltage from two tests |
| The oil has been mixed with another grade of oil | The solubility and the additive system change, and the degassing time changes with them | Check the oil change records and the source of the batch |
| The oil contains emulsified water | The vacuum stage needs more time and water collects at the bottom of the tank | Check whether water comes out when the tank is drained |
| The oil has run at high temperature for a long time | More oil sludge and polar products, and the cleanliness drops | Include the cleanliness in the comparison made before and after every oil change |
One thing to do before changing the oil
- Get the test data for the incoming oil first, measuring it once each to GB/T 7600, GB/T 423, GB/T 507 and NAS 1638, because there is nothing to compare against until the baseline is clear.
- Check whether this batch is the same kind of oil as the one the machine handled last time, because that single point decides whether the configuration and the temperature control setting can be carried over or have to be changed.
- Put the test data from before and after treatment side by side, because the difference between the two is the result of the work.
- Do not record the data of different batches together, and mark the day the oil was changed separately.
- For oil with visible oil sludge, plan the cleaning of the tank bottom and the upstream filter sections in advance instead of waiting for the differential pressure alarm.
The machine is fixed, the oil is not. Finding out what the oil is before changing it saves a great deal more than adjusting the settings afterwards.
