Changing a filter element is not a complicated job: take the old one out, fit the new one, tighten the end cap, reset the pressure switch. The sequence is clear. But one step in it is often skipped, and the new element goes into its housing without being wetted first. The step looks optional, yet it decides how the new element behaves during the first period of a run.
The pores of a dry medium hold air, not nothing
During manufacture and packaging the medium is dry. The fibres are woven into a thin felt, and the pores in that felt hold air at this point rather than oil. When oil enters the housing at the normal flow rate, the liquid and the air compete for the pore entrances, and the liquid only takes the pores near the entrance; the rest still hold air.
This state brings three consequences. The first is a lower flow capacity: only part of the pore volume is available, so the flow through the same pressure drop is below the rated value. The second is a pressure drop reading that is too high: the same flow is pushed through fewer pores, so the difference across the element is higher than it will be once things settle, and an alarm on the pressure switch straight after an element change is easily mistaken for a reversed element or an end cap that was not tightened. The third is subtler, a change in the structure of the medium itself: fibres that are dry relax when the liquid wets them, and the pore size distribution behind the rated filtration accuracy is measured with the medium fully wetted. An element that takes the first oil while still dry shows a gap between the measured filtration performance and the rated value for the first period of the run.
The filtration accuracy of the ZYD-100 is ≤1 μm, the cleanliness corresponds to NAS 1638 class 5, and the standard configuration is five 150-500-5 fine filter elements in parallel. Both of these figures describe the result in a stable running state, not the moment when the element has just been fitted and the oil has not yet soaked into it.
What the wetting step actually does
The wetting step has one objective: to make sure the pores of the medium are already full of oil before the oil arrives. The method is not complicated, but the order of the steps matters.
- Use the same oil the machine is going to process as the wetting medium. Changing the medium is a common mistake, because the wetted state of the medium in one liquid is not the same as in another.
- Let the oil enter the housing at a speed below the normal flow rate. The low speed is necessary: if the flow is too fast, the oil forms a channel along the surface of the element and the inside of the element stays dry.
- Watch the vent of the housing until bubbles no longer come away in a continuous stream.
- Only switch to the normal flow rate once the bubbles have stopped, and let the oil level inside the housing settle.
- Before resetting, confirm that the pressure drop has fallen back into the normal range, so that the initial pressure drop reading of the new element means something.
Along the whole filtration chain it is not only the fine filtration section that needs this. The coarse filter elements 150-250-50 and 110-250-50 and the secondary filter element 110-400-20 need the same treatment when they are changed, and that matters most where the secondary element and the fine element work as a combination: if one of the two enters carrying air, its protective effect on the fine section downstream is reduced.
How to confirm the step has been done
| What is seen | Likely cause | How to confirm |
|---|---|---|
| Pressure drop clearly high after the element change | The element was not fully wetted | Feed the oil slowly in the order above and watch the vent |
| Bubbles continue at the vent | Air is still inside the housing | Extend the slow feed and confirm the bubbles fall away |
| Pressure drop falls back on its own | The wetting process has finished | Record the reading before and after the fall |
| Impurities in the oil at the start | Loose material on the medium is carried out | Confirm by sampling and drain the first section if needed |
When one step can be left out
If the element has been immersed in oil throughout transport and storage, the seal is intact and the medium is already in a wetted state, then fitting it directly on arrival is acceptable. This can be judged roughly from the appearance of the element, but the more reliable method is still to look at the state label supplied with the medium in the delivery documents. Where this cannot be confirmed, wet the element and fit it: the extra time is a few minutes, while skipping the step costs pressure drop, accuracy and element life all at once.
A new filter element is not yet a working filter element at the moment it goes into the machine. It becomes one once it is full of oil.
