When a production line changes oil type, the most common practice on site is to top up the new oil straight into the equipment and carry on running once the oil level is reached. Nothing appears to be wrong with this in the short term; the problems usually show up a few weeks later, all at once: the viscosity has changed, the noise has increased, the seals are weeping and the breakdown voltage has fallen. By the time these symptoms appear the old and the new oil are thoroughly mixed, nobody can say what proportion there was, and the cost of putting it right is far higher than handling the changeover period properly from the start.
Which indicators change after the oils are mixed
Viscosity usually changes first. When two oils of different base oil grades are mixed, the viscosity falls proportionally between the two, and as long as the proportion does not deviate far from the design requirement this change on its own can usually be accepted. The trouble is that a change of type usually means a change of formulation system; the additive systems of oils from different manufacturers differ, polar additives can be incompatible, and the result shows as flocculation or precipitation of the additives, with a clear increase in the sediment after mixing.
The other change is in the additive concentration. The new oil carries a full additive package and is diluted when it is mixed into the old oil; the additives already consumed in the old oil have not been replenished. The viscosity index, the anti-wear performance and the oxidation resistance all lean towards the older oil. In other words, the mixing stage produces a third medium that is neither the new oil nor the old one, and it is not covered by the performance data of either side.
What filtration and degassing can and cannot solve
The boundary of the capability has to be set out clearly here. What the equipment can do is bring the particles down to NAS 1638 class 5, bring the water content down to 3 ppm by the GB/T 7600 method, bring the gas content down to 0.01% by the GB/T 423 method, and achieve a filtration accuracy of 1 μm. These are all treatments aimed at contaminants: the objective is clear, the method is fixed, each item can be checked against a certificate of analysis, and every one of them can be tied to a definite value. What lies outside that boundary is not within the capability of the equipment.
What it cannot do is restore the proportions. Filtration does not put the diluted additives back, degassing does not change the base oil proportions, and molecular-level compatibility is not covered by the filtration accuracy. So when an abnormal indicator appears during a changeover, the first step is to judge whether it is a contamination problem or a problem with the medium itself. In the latter case another round of purification is wasted effort, and the only way back is to keep adding new oil to pull the proportions right.
| Phenomenon during the transition | Which kind of problem it is more likely | Direction of treatment |
|---|---|---|
| Breakdown voltage falls (GB/T 507) | Moisture, gas or impurities rising again | Degas and filter first, then assess |
| Viscosity deviates from the nominal value of the new oil | The mixing proportion of the old and the new oil is off | Top up with new oil in proportion, or replace the oil outright |
| Flocculent material appears or the sediment increases | Incompatible additives | Filtration can remove it, but whether to continue mixing has to be assessed |
| Oil weeping at the seals, change in noise | A change in the characteristics of the medium and in the sealing conditions | Check the seals and the installation condition |
The treatment rhythm to use during the transition
The core principle is not to let the two oils mix freely inside the equipment for long. The safer approach is a staged displacement: drain off part of the oil held in the equipment, add new oil and run for an observation period, then drain and top up again, replacing only a part each time so that the additive concentration always stays within an acceptable range. The whole process is combined with batch sampling, with the result of each round recorded so that a trend is formed.
On sampling frequency, the rhythm during the transition should clearly be closer than in steady operation. The usual practice is to take a sample after each top-up and compare it with the result of the previous round. What matters is not the single reading but the direction of change: if the indicators keep improving after the top-up, the direction of the displacement is right; if they get worse after the top-up, stop and find out why instead of adding more new oil.
- Before the changeover, confirm whether the grade, the viscosity class and the additive system of the oil in use and of the target oil are compatible.
- Before each top-up, record the oil quantity at that moment and the total volume already processed, so that the mixing proportion can be calculated.
- Keep the sampling point fixed, so that differences in reading caused by different sampling points are not mistaken for a change in the oil.
- Do not skip the degassing step to keep to the schedule during the transition; the mixing stage is when moisture is most easily carried in.
- Prepare a plan for dealing with an abnormal result: continue the displacement, replace the oil completely, or stop and wait for the analysis.
The key during a changeover is not changing the oil completely, but keeping the change in the medium within a controllable range throughout the process.