On site the division of work between treating the oil and testing it is often collapsed into one: the machine has run, the indicators of the oil are within limits, so treating and testing are taken to be the same thing. In fact they are two entirely different systems. The purifier takes the unwanted material out of the oil; the testing equipment tells you whether it was taken out and whether the oil came out clean. The equipment, the method, the acceptance criteria and the person responsible all differ.
Which part of the treatment chain each indicator belongs to
A vacuum oil purifier works on three kinds of contaminant at the same time: water, gas and solid particles. The filtration accuracy of ≤1 μm applies to the particles, the separation section and the vacuum system act on the gas and on the water dissolved in it, and the heating section makes the water easier to release from the oil. All of that is the work of the machine, and the machine does not issue any report.
| Indicator | Limit and basis | The part of the machine it corresponds to |
|---|---|---|
| Water content in oil | ≤3 ppm (GB/T 7600) | The heating section and the vacuum separation section |
| Gas content in oil | ≤0.01% (GB/T 423) | The vacuum separation section and the spray atomisation |
| Breakdown voltage | ≥65 kV (GB/T 507) | The combined result of the other three |
| Cleanliness | NAS class 5 (NAS 1638) | The two-stage filtration section |
Read the same table in the other direction and the position of the testing equipment becomes clear: it measures the result, not the process. Water content, gas content, breakdown voltage and cleanliness each have their own test basis, and one oil sample can be sent once and returned with several results. A purifier, on the other hand, has no need to know what the report says; it runs on the process parameters.
Two common confusions on site
The first confusion is to take a machine that is running normally as proof that the oil is within limits. A machine running normally means the pumps are turning, the vacuum is being pulled and the pressure alarm has not acted. None of that is evidence about the quality of the oil. The fact that a machine runs steadily for a long time says that its working conditions are normal, which is a different question from whether this batch of oil meets its figures.
The second confusion is to take a test report as proof of the purification result. A report describes the state of the one sample that was drawn. It can show that the oil was within limits at the moment of sampling, but it does not automatically show that the whole treatment process was steady. A fault in the middle of the run that is then masked by the later treatment cannot be seen from a single report.
Joining the two lines
- Purify first, then sample and send for test. The order cannot be reversed, and testing untreated oil means nothing.
- Record the sampling point, the time, the batch number and the machine number together, so that the report can be traced back to the batch it belongs to.
- Only a sequence of test results from consecutive batches on the same machine shows whether the process is steady.
- Breakdown voltage is a combined indicator; it passing does not replace a separate judgement on the other three.
- When a test shows an indicator out of limits, first identify which one it is and map it back to the corresponding section of the treatment chain.
Treating the purifier and the testing equipment as separate things also has a direct use: when something goes wrong, both lines have their own records, and the cause can be found accurately. An indicator that is out of limits maps to one section of the chain, and the operating parameters and the condition of the machine in that section are checked, instead of taking the whole machine apart.
The machine is responsible for treating the oil clean, the report for stating whether it came out clean. Both need someone answerable.
