On the day new oil arrives, the drums carry a certificate and the sample has been tested against the standard. But between leaving the factory and being poured into the transformer, there is a whole stretch of road: filling the drums, transport, unloading, temporary storage and injection. Everything that happens along that road changes the state of the oil. What this article sets out is what actually happens on that road, and how to stop it at the gate before it reaches the machine.
Filling and transport: the drum mouth and the temperature
If there is still air in the drum when the oil is filled, a gas layer is left above the liquid. When the temperature changes during transport, the solubility of the gas and the thermal expansion of the liquid differ, and water vapour in the gas layer separates out and forms condensation on the inside of the lid. The quantity is small, but it is a real part of a water content figure expressed in ppm.
A drum that is not sealed properly, or that is turned on its side or knocked during transport, is an entry point for particles. What this stage brings in is mainly rust and dust from the packaging.
Open storage: condensation between day and night
When a drum stands outdoors, the drum is heated during the day and cools at night. When the wall of the drum falls below the dew point of the air, water vapour condenses into droplets on the wall and runs back down into the oil. This is what is usually called the drum taking up moisture, and it behaves differently from standing in water: it is the temperature difference that draws the water vapour of the air back into the oil.
The storage conditions for a settling tank matter in the same way. The breather valve is what keeps the pressure inside the tank balanced with the outside while holding back the water and the dust in the air; if the valve fails or the lid does not close properly, that route opens again.
Unloading and injection: the stage that is easiest to overlook
The hose and couplings used for unloading, drum bottoms and tank bottoms whose insides have not been cleaned, and the air left behind when a line has not been emptied properly during injection all carry particles and air into the system together. The injection stage deserves particular attention: if air is left in a line and not purged, it goes into the transformer along with the oil.
| Stage | What it can carry in | How it is spotted on arrival |
|---|---|---|
| Filling the drums | Residual gas in the drum, rust on the drum wall | Water droplets or rust marks inside the lid, sediment at the bottom of the drum |
| Open storage | Water vapour from the air, rain, dust | Water marks on the outside of the drum, cloudiness near the liquid level inside |
| Unloading from the tanker | Residue in the hose lining, particles at the connections | The first part of the oil as it is unloaded is visibly worse |
| Settling tank | Sediment at the tank bottom, damp air carried in when the breather valve fails | A water layer or sediment at the bottom of the tank |
| Injection into the transformer | Air and particles left in the line | A deviation in the sample drawn from the lower part of the transformer after injection |
Run a re-check on arrival
The point of a re-check on arrival is to keep the responsibility outside the machine. The sample is taken as specified, water content is judged under GB/T 7600, breakdown voltage under GB/T 507 and cleanliness under NAS 1638, and if any one of them fails, that stretch of road is examined first instead of running the oil through the machine and settling the account afterwards.
Oil that fails the re-check should not go straight into the transformer, and it should not be dealt with on the side by running it through the machine once more. It should first be read as a signal: the source of this batch or the road it travelled is faulty, and once that is clear it can be decided whether the batch is returned, treated separately, or kept apart from the other batches.
Conforming at the factory and conforming on arrival are two things
The supplier is responsible for the oil as it leaves the factory and the user is responsible for the oil as it arrives, and where the responsibility for the stretch in between falls depends on the contract and on the records. Set the points down clearly: the factory test report, the re-check on arrival and the sampling records before and after injection. With those three in place, any later dispute can be traced to a position on the timeline.
The clock on how clean the oil is starts again the moment it leaves the drum, and the machine is only one station along that road.
