The smell of an oil is normally stable, and a sudden change means that new volatile substances have appeared in the oil. The smell is the most sensitive online indicator: many measured figures are still inside their acceptable range while the nose has reacted first. But the smell can only tell you that something is wrong, not what it is, so two typical odours have to be separated.
A burnt smell: it points to local overheating
A burnt smell comes from the cracking of oil molecules at high temperature. There are two paths by which it forms. The first is a local temperature on the surface of the heating element far above the oil temperature, the classic dry run: the level inside the machine has fallen too low, the heating element comes out of the oil, the exposed part heats the air directly, and the film of oil on the element cracks and produces the burnt smell. The second is the heat generated by mechanical friction inside the pump: the oil at the friction point is heated to its decomposition temperature, and the burnt smell is carried back through the return line into the whole tank and comes out at the oil outlet.
The characteristics of a burnt smell are sharp, short and pungent. It sometimes appears together with the running noise of the machine, and once it appears it does not go away on its own. Its danger lies in the fact that local overheating changes the chemical properties of the oil at the same time, produces insoluble products and carries those products into the whole drum.
A sour smell: it points to oxidation products
A sour smell comes from the acidic substances and the low-molecular-weight cracking products formed when the oil oxidises, and it is typical of oil that has run at high temperature for a long time, that has been heated and cooled repeatedly, or that has been in contact with air and moisture for a long time. There is one aid to identifying it: this kind of oxidation normally changes the other indicators at the same time, with the water content rising, the cleanliness getting worse and the acid value increasing.
The characteristics of a sour smell are dull, irritating and strongly volatile but not sharp. It usually only becomes obvious after the machine has been running for a while and the oil temperature has come up, and it lessens after shutdown and cooling but does not disappear completely.
What to do after the smell has been noticed
First, reduce the feed rate immediately and stop the heating so that the temperature comes down. Second, stop the machine, bring the oil temperature down to a level that is not too hot to touch, and only then start the inspection. Do not smell the machine while it is running, and do not put your nose close to a machine that is still hot: hot oil vapour is in itself irritating and carries a risk of scalding, and the odour is amplified at high temperature, which makes the source harder to identify rather than easier.
Third, after cooling, take a sample for testing, covering at least the water content (GB/T 7600), the cleanliness (NAS 1638 class 5) and the breakdown voltage (GB/T 507), and keep a sample as the reference for later comparison. Fourth, work through the heating system and the pump on the machine side item by item.
| Odour characteristic | Most likely source | How to confirm | Action |
|---|---|---|---|
| A sharp burnt smell, with abnormal noise or a hot casing | Dry run of the heating element, the level too low | After cooling, check the level in the heating area and the surface of the heating tube | With the machine stopped and the power off, check the heating tube, confirm the level and replace it |
| The burnt smell appears or grows worse with the rotation of the pump | Mechanical friction inside the pump overheating | After stopping, listen to the pump and check the temperature at the coupling and the shaft seal | Stop the machine, open up the pump shaft and the bearings, and check the alignment of the coupling |
| A dull, irritating, sour smell | Oxidation of the oil producing acidic substances | Sample and test the water content and the breakdown voltage, and compare with the retained sample | Bring the upper temperature limit down, shorten the heating time, and degas and dehydrate again |
| Both odours are present | Local overheating combined with overall oxidation | Investigate the machine first along the burnt smell path, then look at the temperature control strategy | Solve the hot spot first, then reassess the condition of the oil |
Why the machine has to be stopped and cooled before the inspection
There are three reasons. The first is safety: oil above 80 °C causes scalding when it splashes onto the skin, and opening a flange while the machine is hot can release pressurised steam. The second is judgement: the odour is amplified by the vapour at high temperature, and the burnt smell from a local hot spot covers up the other signals, whereas after cooling the source of the smell is easier to separate. The third is data: only a water content and a particle count measured after the temperature has settled can be compared, because sampling while hot introduces bubbles and moisture.
When you smell a burnt odour, reduce the load; when you smell a sour odour, reduce the temperature. Neither is settled by running the oil through the machine once more, so find the source first.