The same filter element behaves differently in oil at 3 ppm than it does in oil with clearly more water in it. On many sites a rising pressure drop is put down to the element being clogged with dirt, and the water is left out of the picture. The way water is present in the oil differs, and so does the path by which it acts on the filter medium.
Water in oil is present in more than one form
The water in an oil falls into two forms according to its state. Dissolved water consists of molecules dispersed evenly through the oil phase at molecular scale, and its effect on the medium is mainly to change the physical properties of the medium rather than to form a blocking layer. Free water is dispersed in the oil as small droplets whose size may be far larger than the pores of the medium.
Free water follows the same path into the element as solid particles: it is caught first at the outer surface of the medium or at the entrance of the pores, and then forms a liquid film and liquid bridges on the outside of the medium. Once the bridges join up, the outer layer of the medium acts as an extra film of filter cloth, the flow resistance rises with it, and the visible result is a higher pressure drop reading.
The filtration accuracy of the ZYD-100 is ≤1 μm and the water content is limited to ≤3 ppm (GB/T 7600). Those two figures describe the capability limit of the same machine, and when the water content is over the limit the accuracy figure loses its meaning in practice, because the problem caused by the water has already happened before the oil reaches the filtration section.
Why the pressure drop reads high when there is water
Water is not the only source of a high pressure drop; it usually turns up together with particles and gum, so the symptom has to be broken apart before the cause can be named. The table below lists a few common combinations with the direction to take for each, so that the dominant factor can be identified first and only then the right part is touched.
| Symptom observed | Possible cause | Direction to take |
|---|---|---|
| The pressure drop rises slowly and the oil looks cloudy or milky to the eye | Free water enters the outer layer of the medium and forms a liquid film | Dehydrate first, do not write the element off on the spot |
| The pressure drop rises quickly and the upstream pressure clearly climbs | A large amount of free water and impurity reaches the filtration section together | Stop the machine and check, there is very likely a source of water upstream |
| The pressure drop is normal while the measured water content is still high | The water is present in the dissolved state and was not held back by the medium | The problem is not in the filtration section, go back to the degassing and heating sections |
| The pressure drop is still high after a new element is fitted | The source of the water was not removed and the new element meets the same conditions | Check the oil supply, the seals and the heating section instead of changing elements repeatedly |
Water droplets and dissolved water act on the fine elements differently
Free water can be held back by the medium. Its droplet size is usually far above the 1 μm filtration accuracy, so free water present as particles is partly caught in the filtration section. But as soon as the droplets spread out on the outside of the medium into a continuous film, they amount to an extra screen on the outside of the element, and since the pore distribution of that film is not controlled, the filtration accuracy loses its meaning with it.
Dissolved water is a different matter altogether. It is dispersed in the oil phase at molecular scale, the medium has no holding effect on it, and pressure is not the driving force for its removal. The oil that has been through the filtration section therefore does not necessarily have a lower water content. Water content is brought down by the vacuum degassing and the heating sections; changing the element will not do it.
This distinction is useful when troubleshooting: if the water content has not improved after the element was changed, do not assume the element was fitted the wrong way round, go back to the vacuum and the heating sections. Those two sections correspond to the working vacuum of -0.08 ~ -0.0999 MPa and the temperature control range of 45~80 ℃ respectively.
Points to note when changing and reassembling
- If the element that came out carries obvious water, record its condition before cleaning it and do not decide on the spot that it can still be used.
- Make sure the water in the filter housing has been drained before assembly, because free water left in the housing goes straight into the new element.
- Use the same oil the machine processes as the wetting medium and let the oil in slowly until no more continuous bubbles come out of the vent.
- Watch how the pressure drop builds up when the new element is put into service; only then does the initial reading mean anything.
- Where the site allows it, test the water content to GB/T 7600 instead of judging it by eye.
Two clues have to be read together
The pressure drop tells you what happened in front of the element; the water content tells you what state the oil itself is in. Reading either one on its own leads to a wrong conclusion: only the pressure drop and a water problem gets mistaken for a filter life problem, while only the water content leads to expectations of the filtration section that it cannot meet.
Put in operational terms the division of work is simple: the filtration section holds back particles and free water, the degassing and the heating sections bring the dissolved water down. Work out which section is meant to do what first, and only then talk about replacing consumables and adjusting the settings.
The pressure drop tells you what happened in front of the element, the water content tells you what state the oil is in, and the two clues have to be read together.
