Once the oil has been purified to a certain level, the site occasionally asks for one step further: can this batch of oil be put under vacuum once more. The question usually stands on one specific indicator that has not met its target, and that indicator often points to the gas content rather than to the water content or the particles. Deciding whether this is worth doing depends on first separating what a second degassing pass can change from what it cannot, otherwise the effort is easily pointed in the wrong direction.
A second degassing pass changes only one class of component
Gas in oil exists in three forms: gas dissolved in the oil molecules, gas dispersed as fine bubbles, and gas present as vapour at high temperature. Vacuum degassing is directed at the first two, separating them from the oil phase and carrying them away. If a second degassing pass is carried out thoroughly enough, the benefit is concentrated on this part as well, and any expectation beyond this range has to be discounted.
Water content and particles are not on this line. Water vapourising under vacuum takes heat, and the working temperature range of the degassing stage already covers this process, so putting the oil under vacuum once more will not bring the water content down by another order of magnitude. Particles will certainly not become fewer because of an extra vacuum pass; they are determined entirely by the filtration stage, by the specification and the integrity of the element.
| Indicator | What mainly determines it | Whether a second degassing pass can improve it |
|---|---|---|
| Cleanliness NAS 1638 class 5 | Filtration accuracy and condition of the element | No |
| Water content within 3 ppm | Heating, vaporisation and vacuum separation | Very little marginal benefit |
| Gas content within 0.01% | Depth of the vacuum degassing | This is the only main field of a second degassing pass |
The cost has to be worked out first
A second degassing pass means the same batch of oil goes through heating and vacuum once more, consuming heating power and running time. The total power of the ZYD-100 is 72 + 5 kW, the vacuum pump set and the Roots blower set are the parts that work continuously, and the oil pump has to deliver the oil to the flash tank again. Running a second time, the cost in time multiplies, and the working hours of the pump set and the heating elements grow at the same time.
At the same time, the benefit of a second degassing pass diminishes. The machine itself has a two-stage vacuum structure working with three-dimensional flash evaporation, a working vacuum of -0.08 ~ -0.0999 MPa and an ultimate vacuum of not more than 5 Pa. During the first pass there is a lot of dissolved gas in the oil and the volume of gas drawn off is large; during the second pass the oil is already fairly clean and there is much less gas to draw off, so most of the pumping time goes into holding the vacuum rather than removing gas.
When it is worth doing
Once the conditions are listed, the decision becomes simple. A second degassing pass suits the following situations: the gas content is close to its limit while the other indicators all have margin, which means the gap lies only in the degassing step; the oil has just finished fine filtration, so the water content and the particle level are already stable and another vacuum pass will not undo the earlier steps; the site genuinely has a long enough working window to carry the cost of running it a second time.
- The gas content is close to its limit, the other indicators all have margin, and the gap lies only in the degassing step.
- The oil has finished fine filtration, the water content and the particle level are stable, and another vacuum pass will not overturn the earlier steps.
- The working window is long enough to carry the time for one more pass of heating and vacuum.
- The viscosity and flash point margin of the oil meet the requirements, so that the second heating brings no additional risk.
Conversely, if two or more of the water content, the cleanliness and the breakdown voltage are out of specification, putting the oil under vacuum once more will not solve it, and the cause should be looked for across the whole process. When the breakdown voltage is below 65 kV, it has to be established first whether water or gas is responsible, because the treatment path for the two is not the same, and once the direction is wrong no amount of additional time recovers it.
A second degassing pass is a path that is responsible for the gas content alone; using it to solve a water or particle problem points the wrong way.
