The same vacuum oil purifier does not work with one set of test criteria for insulating oil and hydraulic oil. Treating the breakdown voltage as the acceptance criterion for hydraulic oil, or watching only the cleanliness while ignoring the water content of insulating oil, both lead to a conclusion that means nothing. The test list follows the oil, and the settings on the machine follow the oil as well; these are two sides of the same judgement.
What the two kinds of oil care about
Insulating oil is used in transformers and similar equipment, and what it fears most is water, gas and particles. Each of the three damages the insulating performance, so insulating oil has a defined set of scales: water content measured to GB/T 7600, with a limit of ≤3 ppm for the ZYD-100; gas content measured to GB/T 423, limited to ≤0.01%; breakdown voltage measured to GB/T 507, at least ≥65 kV; and cleanliness classified to NAS 1638, which is class 5 on the ZYD-100. These four belong together, and leaving one out makes the set incomplete.
Hydraulic oil, gear oil and turbine oil are concerned about different things. They carry force and lubricate inside a system, and the damage that matters comes from particles entering a fitting surface, so cleanliness class, water content and wear debris in the oil are the usual data. Breakdown voltage has no meaning as a criterion for these oils, and gas content is normally not used as a factory figure. The additives and anti-wear agents in lubricating oil can also interfere with the result of some test methods.
The machine settings have to follow the oil as well
The temperature control range of the TYA series is 45~80 °C, and the upper end is pressed down to 70 °C when the water content is high. That upper limit is not an arbitrary rule of thumb, it is set against the degassing condition when there is a lot of water: a higher oil temperature gives a better degassing result, but the closer the temperature gets to the flash point the more oil vapour flashes off, and the load on the condensation section and the vacuum pump rises with it. Different oils do not react to temperature in the same way.
Pressure works the same way. Cleaning and element replacement become necessary once the pressure passes 0.3 MPa, and that limit is met more easily when the oil is more viscous, because the pressure drop at the same flow rate is then higher. Selection and the processing plan are not two separate tables: the higher the target figures are set, the tighter the filtration configuration and the settings have to be.
The test items of the two oils side by side
| Test item | Criterion for insulating oil | Criterion for hydraulic oil |
|---|---|---|
| Water content | GB/T 7600, ZYD-100 limit ≤3 ppm | Set by the equipment and the oil, still measured |
| Breakdown voltage | GB/T 507, limit ≥65 kV | No meaning as a criterion |
| Gas content | GB/T 423, limit ≤0.01% | Normally not a factory figure |
| Cleanliness | NAS 1638 class 5 | Cleanliness class together with particle counting |
| Filtration accuracy | ≤1 μm corresponds to NAS 1638 class 5 | Set by the system accuracy and its components |
The three mistakes most easily made on site
- Carrying over the test report of the previous batch. The test item, the oil type and the standard number have to be read together, because a single final figure on its own is easily misread.
- Treating a water content pass as a pass on the oil. A water content below 3 ppm only shows that one item is within limits, gas and particles remain separate judgements.
- Changing the oil without changing the machine settings. After the oil type changes, the temperature upper limit, the pressure criterion and the filtration configuration all have to be reviewed.
How to write the criteria clearly
A test plan that can be aligned has to state at least three things: the name and grade of the oil, the standard that was applied, and the threshold that decides the result. Checking the plan against these three items usually removes most of the disagreement. The criteria follow the oil and the test plan follows the criteria, and these two are two views of the same thing.
The scale of a criterion is set for one oil, and measuring a different oil with the old scale always gives the wrong answer.
