The four most prominent indicators on the machine parameter table are the water content in oil of ≤3 ppm (GB/T 7600), the gas content in oil of ≤0.01% (GB/T 423), the breakdown voltage of ≥65 kV (GB/T 507) and the cleanliness of NAS 1638 class 5. Someone taking delivery of a vacuum oil purifier usually assumes that once these four are within limits, the oil has been treated properly. That judgement holds in most cases, but in oil testing there are two questions that these four do not answer.
What the four have in common is that they all measure the physical content of the oil: water, gas and particles, together with how well the oil withstands an electric field in the presence of particles and water. A vacuum oil purifier does the same kind of work at the physical level: the heating vaporises the water, the negative pressure lets the dissolved gas escape, and the filter element holds the particles back. The machine can remove the physical impurities, but it cannot take away the chemical products that have already formed.
What dielectric loss and acid value each tell you
Dielectric loss describes the extent to which the oil converts electrical energy into heat in an alternating electric field, and belongs to the electrical properties of the oil. In long-term operation of insulating oil, the dielectric loss deteriorates together with the ageing of the oil and the rise in water content; when the water content goes up, the water molecules take part in the polarisation and the loss rises with them, and this part of the effect can be removed by degassing.
The acid value says how far the oil has already deteriorated, and measures the extent to which the oil has been oxidised and has produced acidic products. The acidic products form slowly in the oil under high temperature and oxygen, and a purification machine does not turn the acidic substances that have already formed back into neutral ones; it is only responsible for making the remaining oil cleaner. Neither of these two indicators can be measured on the spot at site; a sample has to be taken and sent to a laboratory, which gives a figure according to the corresponding test method.
| Indicator | The question it answers | Basis | Whether the machine can give it directly |
|---|---|---|---|
| Water content in oil | How much water is in the oil | GB/T 7600 | ≤3 ppm after treatment |
| Gas content in oil | How much dissolved gas is in the oil | GB/T 423 | ≤0.01% after treatment |
| Breakdown voltage | How well the oil withstands the electric field | GB/T 507 | ≥65 kV after treatment |
| Cleanliness | To what extent the oil is contaminated with particles | NAS 1638 | NAS class 5 |
| Dielectric loss | How large the loss is in an alternating electric field | Laboratory testing | No, sampling and laboratory testing are required |
| Acid value | To what extent the oil has deteriorated | Laboratory testing | No, sampling and laboratory testing are required |
Why these two are not on the machine parameter table
Because they are not the working result of this machine. Water content, gas content, cleanliness and breakdown voltage are all states that can be measured directly on the oil after treatment, and they reflect whatever degree of treatment the machine has carried out. Dielectric loss and acid value, on the other hand, are the result of sampling and laboratory testing; they measure the chemical state of the oil, and a machine doing physical purification cannot produce these two figures however steadily it runs.
This also explains why some acceptance inspections get stuck on the test method: both sides say the result is conforming, but they are not talking about the same thing. Conforming on the machine side means the four purification indicators are within limits; conforming on the user side may also include dielectric loss and acid value. If the criteria of both sides are written into the clauses, the disagreement appears on paper rather than at the acceptance inspection.
How the acceptance clauses should be written
- List the purification indicators and the oil condition indicators separately. For the first four, state the standard number and the acceptance value; for the latter two, state the test method, the acceptance value and who is responsible for the sampling.
- State clearly who is responsible for the sampling and when it is taken. The sampling step itself can change the test result, so the container, the sealing method and the storage time are all written into the clauses, and both sides work to the same method.
- Allow time for re-treatment and re-testing. When dielectric loss or the acid value is out of limits, one more pass of degassing will usually not change it; the arrangement for rework and the division of responsibility have to be agreed in advance.
- Do not use the machine indicators as if they were the oil indicators. Bringing the four indicators within limits with a purification machine demonstrates the capability of that machine and that process; it does not mean that the whole state of this batch of oil is conforming.
The four standard indicators say what is in the oil; dielectric loss and acid value say what the oil has become. The two have to be accepted separately.
