Dehydration takes the water out of the oil, while oxidation changes the oil itself, little by little. The two take place at the same time and they promote each other. A question that comes up often on site is why the water content comes down and the oil still fails the limits. The reason is usually not the equipment, but the fact that this batch of oil has already aged.
How oxidation develops step by step
Oxygen reacts first with the unsaturated hydrocarbons and the aromatic compounds in the oil and forms peroxides; the peroxides break down further into intermediate products such as aldehydes and ketones; these then give rise to organic acids and resinous high-molecular products. These products polymerise and deposit gradually, forming a varnish film on the inner walls and on the heating surfaces of the equipment and sludge at the bottom of the oil.
Visually the process runs from light to dark: the colour deepens, the transparency falls, turbidity and sediment appear. The build-up of organic acids can also be measured as acid value, which is earlier and more quantitative than a visual judgement.
Why acid value and water content cause each other
One direction is that a rising acid value increases hydrophilicity. Organic acids and resinous products are strongly polar; they enclose the water droplets and stabilise the interfacial film, which makes the oil emulsify more readily. Emulsified water cannot migrate freely and cannot be drawn off under vacuum, so the difficulty of the dehydration rises with it.
The other direction is that moisture accelerates oxidation. Water present at high temperature speeds up the oxidation of some oils, and the oxidation products in turn increase the polar substances further, which forms a self-reinforcing cycle. This is why oil that has stood for a long time has both a high water content and a high acid value, instead of two results that have nothing to do with each other.
How oxidised oil behaves during vacuum degassing
What the oxidation products change is the surface properties of the oil. The surface tension falls, the oil foams more readily when it is drawn around in the tank, the foam is more stable than in fresh oil and the load on the defoaming increases. As the foam and the entrainment rise, the load on the vacuum pump goes up, the pump oil emulsifies faster and the running stability of the equipment falls with it.
At the same time, a higher acid value means more polar deposit, so the filter elements block faster and the differential pressure rises more noticeably. What is seen on site is a drop in the treatment efficiency, a faster consumption of the consumables and less and less improvement in the oil parameters.
When to change the oil and when to keep purifying
The first principle of the judgement is whether the purification can change this parameter at all. Water and gas can be removed under vacuum, the oxidation products in the oil cannot be taken out, they can only be filtered out in part. So when the acid value is high and goes with a clear colour difference and deposits, the purification is improving some of the parameters at the price of continuing to process a batch of oil that has already aged, at high temperature.
Whether the oil is changed in the end has to be decided against the limits set by the oil specification or by the equipment manual. This article gives no specific figures, because the acid value limits differ widely between the oil types.
| Symptom | What it points to | Can purification solve it | Follow-up action |
|---|---|---|---|
| Acid value high, colour clearly darker | The oil has aged | Cannot be reversed | Assess changing the oil, the purification is only a transition |
| Varnish film, sludge and deposits clearly present | Deposition of the oxidation products | Only part of it can be filtered out | Clean the equipment and assess changing the oil at the same time |
| Water content within the limits but the breakdown voltage still low | Polar products and residual gas | Partly improvable | Increase the degassing intensity and check the acid value at the same time |
| Defoaming difficult, the pump oil emulsifies faster | The surface properties have changed | Cannot be reversed | Shorten the batches and assess changing the oil |
| Only the water content and the particles are high | Contamination rather than ageing | Can be solved | Continue purifying according to the process |
- Include the acid value in the routine monitoring, do not watch only the water content and the particles.
- The acid value keeps rising: shorten the treatment batches and reduce the time the oil stays in the high temperature zone.
- Varnish film and sludge appear: clean the equipment and the pipework and start the decision on changing the oil at the same time.
- Water content within the limits but the breakdown voltage below the limit: check the acid value and the gas content together.
- Before the oil is changed, keep the test data of the old oil on file, it serves as the baseline for the next round.
Water can be taken out, ageing cannot be taken out; only when you know which of the two is getting worse do you know whether to keep purifying or to change the oil.