Three conclusions are commonly drawn from oil testing: a water content in the oil of ≤3 ppm (GB/T 7600), a gas content in the oil of ≤0.01% (GB/T 423), and a breakdown voltage of ≥65 kV (GB/T 507). Each of the three has a clear test basis and a target value. When a test report arrives on site and only one of them is out of specification, what really takes the time is usually not the remedial work but the decision about which one to deal with first and which one to re-test first afterwards.
The three indicators point to different stages when they are out of specification
A water content out of specification points first to where the water came from. The degassing stage can deal with dissolved water and emulsified water, but if the incoming oil itself carries water, or if water in the tank and in the pipework is brought in with it, running the oil through the machine once does not solve the problem. A gas content out of specification points to whether degassing actually took place, and that involves both the state of the machine and deviations introduced by the sampling method. A low breakdown voltage cannot be judged on its own: it is affected at the same time by the water content, the particle content and the bubbles, and the results of the other two have to be read alongside it.
The relationship between the three is this: the water content and the gas content are two items that can be dealt with separately, while the breakdown voltage is closer to a combined result. So when the breakdown voltage is the only one out of specification, the correct move is not to stare at the voltage looking for a cause but to test the water content and the gas content once each and break the problem apart.
| Item out of specification | Target value and basis | Direction to investigate first | What to re-test first afterwards |
|---|---|---|---|
| Water content | ≤3 ppm (GB/T 7600) | Source of the incoming oil, water in the tank, whether the degassing temperature is reached | Water content |
| Gas content | ≤0.01% (GB/T 423) | Atomisation and residence time, sampling method | Gas content |
| Breakdown voltage | ≥65 kV (GB/T 507) | Test the water content and the gas content as well first, then look at the particles | Water content and breakdown voltage together |
| Cleanliness | NAS 1638 class 5 | State of the filtration section and the sealing | Cleanliness |
Order the response by the pattern of the exceedance
The first pattern is a single item out of specification with the others normal. Here the route is the clearest: if it is the water content, follow the source of the water; if it is the gas content, follow the degassing process; then deal with it, re-test the same item, and once it is confirmed as compliant, run one complete four-item test to confirm that no other indicator has been affected. A single exceedance usually means that one stage was not carried far enough, not that the machine has failed.
The second pattern is that the water content and the gas content are both out of specification. This class usually has to go back to the degassing stage itself: whether the oil temperature has reached the set value, whether the vacuum level is in its working range, and whether the oil feed and the running time were sufficient. When the water content is high the upper temperature limit is pulled down to 70 °C; that limit has to be observed, and the temperature should not be raised in order to speed up the degassing.
The third pattern is that the breakdown voltage alone is low while the water content and the gas content are both compliant. This class has to be investigated in two directions, towards particles and towards bubbles. The particle direction corresponds to the state of the filtration section, including whether the elements have taken up dirt, whether the seals are intact and whether anything is being bypassed; the bubble direction corresponds to whether the freshly degassed oil was allowed to settle in the tank and whether air was carried into the sample. The breakdown voltage is the result of these three kinds of factor acting together, and staring at the voltage on its own does not reveal the root cause.
How to confirm that the oil really is compliant after the work
Re-testing is not a matter of sending the same sample off once more and calling it done. What has to be confirmed is the state of this batch of oil after treatment, so the re-test sample should be taken at the treated oil outlet and under the same conditions as the earlier sample. If the conditions have changed, the difference between the two sets of data cannot be attributed to the performance of the machine. Once the water content and the gas content have each been re-tested and confirmed, test all four together to form a complete record on the same batch of oil; that record is what later judgements about the state of the machine and the state of the oil are based on.
- When you get a report that only says out of specification, first ask which item it is and what the other items came to.
- Carry out the three tests on the same batch of oil sample wherever possible; figures from different batches cannot simply be assembled into one conclusion.
- When the water content is out of specification, confirm the source of the incoming oil first, then confirm whether the temperature inside the machine has genuinely risen.
- When the breakdown voltage is low, test the other two as well instead of adjusting the machine repeatedly.
- After the work, run one complete four-item re-test to form a record that can be compared.
The three indicators are not three gates in parallel; each has its own part to play. Once it is clear which item is out of specification, the order of the work that follows does not go round in circles.
