There are two constraints on the upper limit of the heating temperature, and they are easily run together. One is a process constraint: the oil and the water should vaporise a little faster. The other is a safety constraint: the oil vapour must not accumulate in the equipment to a level where it can be ignited. The first corresponds to the flash evaporation temperature, the second to the flash point, and a margin has to be left between the two, and that margin is not a matter of guesswork.
What the flash evaporation temperature and the flash point are
The flash evaporation temperature is a process parameter: the operating temperature in the flash evaporation section of a vacuum oil purifier at which the oil surface evaporates and carries the light fractions and the water vapour away. It is determined by the target water content, the vacuum level of the equipment and the vapour pressure characteristics of the oil. It is a process value that can be adjusted, and that has to be adjusted.
The flash point is a safety property: the lowest temperature at which the oil vapour, at a certain concentration in air, ignites spontaneously when it meets an ignition source. It is an inherent property of the oil and it varies with the type of oil and the additive package, so the flash points of different batches of transformer oil, hydraulic oil and lubricating oil are not the same.
How the margin is set
At least four items have to be considered when the heating temperature is set: the flash point stated on the safety data sheet for the oil; the actual pressure in the flash evaporation chamber, which determines how low the absolute pressure has to be before the oil boils; whether the oil contains light fractions that volatilise easily; and whether the equipment has surfaces that can be heated to ignition or any open flame risk.
Once these four items have been confirmed, the margin left should keep the working temperature clearly below the flash point and retain an adequate safety boundary. The common problem on site is not that the margin is too small, but that the safety data sheet was never opened and a setting from another machine, or even from another oil, was carried over from memory.
Why the safety data sheet has to be checked
The oil processed by the same type of equipment may come from different suppliers, different grades and different additive systems, and the flash point differs with it. The difference between mineral oil and synthetic oil is particularly pronounced. Copying a value from experience is at its most dangerous when the batch of oil changes, because the setting on the machine has stayed the same while the properties of the oil have changed.
Another point that is easily overlooked is how representative the sample is. The bottom and the upper layer of a tank, oil that has stood for a long time and fresh oil that has just been made up can all differ in the distribution of the volatile fractions. Setting the process parameters for the whole batch from a sample taken at one moment carries a bias in itself.
Two typical consequences of a wrong temperature setting
The first is a setting that is too high. A large amount of the light fractions is drawn off, the flash point of the oil falls, and the dielectric and the lubricating performance are impaired at the same time; the flash evaporation efficiency does not necessarily improve either, because once a large amount of oil and water vapour enters the vacuum chamber the pumping speed is used up and the ultimate vacuum cannot be reached at all, which makes the setting not worth the trade.
The second is a setting that is too low. The water cannot vaporise effectively, the dehydration proceeds slowly, the amount of condensate drained off falls, the water content stays high for a long time and the acceptance figures are not reached; and then the temperature is turned up to catch up on the schedule, which ends up back at the first consequence.
What the two errors have in common is that they show up in the data first: one as a deterioration of the oil properties, the other as a dehydration efficiency that does not meet the target. The temperature setting should therefore not be adjusted on the spot by the operator, but determined in advance from the information on the oil and recorded.
| Item | Meaning | What determines it | Consequence of a wrong setting |
|---|---|---|---|
| Flash evaporation temperature | The operating temperature at which the light fractions and the water vaporise | A process setting, adjustable | Too low slows the dehydration, too high damages the oil |
| Flash point | The lowest temperature at which the oil vapour can be ignited | An inherent property of the oil | A safety boundary that must not be crossed |
| Pressure in the chamber | Determines the boiling point of the oil | The state of the vacuum system | A change in pressure moves the effective boiling point |
| Safety margin | The distance between the working temperature and the flash point | Set from the safety data sheet and the site conditions | An insufficient margin leaves the risk uncontrolled |
- When the batch of oil or the supplier changes: check the safety data sheet again and review the temperature setting.
- Confirm whether there is any open flame, hot surface or high-temperature pipework on site, and assess the risk of ignition.
- Write the temperature setting into the operating record instead of relying on a verbal handover.
- If the colour of the oil changes or an abnormal smell appears: lower the temperature first, then check whether the oil has overheated.
The flash point is your ceiling and the flash evaporation temperature is the step you choose; the margin between the two has to be left by you.