Changing the filter element as soon as the pressure drop rises is what many sites do. It is not necessarily the cheapest way and it is not necessarily the safest way either. A rising pressure drop has two kinds of cause, one reversible and one that is not. Washing only deals with the first one, and the second one stays no matter how often the element is washed, so what has to be told apart is the cause and not the pressure reading itself.
Two kinds of cause behind a rising pressure drop
The reversible kind is what attaches to the surface. Soluble material in the oil, oxidation products, emulsified water and fine impurities form a cake on the outside of the medium and block the entrances of the pores, and the flow resistance rises with it. This kind of blockage can be washed off so that the element returns close to its initial flow capacity, and the medium itself has taken no structural damage.
The irreversible kind is a change in the medium itself. Glass fibre media under a high pressure drop for a long time get compacted and their porosity falls, so the same flow needs a bigger pressure drop; a medium that has been knocked or fitted badly gets cracked or creased, and once the edge seal opens the oil passes straight around the medium; an ageing sealing ring that has deformed leaves a bypass path between the element and the housing. None of that can be washed away, and even if the pressure drop falls back for a while, the accuracy no longer holds.
How to read the pressure drop on a parallel arrangement
The standard configuration of the ZYD-100 is five 150-500-5 fine filter elements in parallel. The feature of this arrangement is that the total flow is carried by all five together, and the total pressure drop is the highest of the individual branches. That produces a situation that is easy to misread: one element can already be close to its limit while the other four are still clean, the total pressure drop has only risen by part of what it should, and the total reading does not show which element is the problem.
To judge a single element the branch pressure drop has to be measured, or the elements are checked one by one. The usual method on site is to record the total pressure drop and the running flow, then isolate one branch at a time and watch how far the total falls. The element whose removal brings a clear drop is the one that needs attention.
What washing can restore and what it cannot
| Source of the pressure drop | Effect of washing | Effect on the accuracy |
|---|---|---|
| Oil sludge and soluble deposits on the outside of the medium | Flow capacity clearly recovered, pressure drop close to the initial value | Accuracy essentially unchanged, the element can stay in service |
| Media compacted, fibre structure deformed | Cannot be washed off, the pressure drop only falls a little | The filtration accuracy is already affected, replace the element |
| Media cracked, creased or with an open edge seal | The pressure drop may even fall lower, because the oil bypasses the medium | Accuracy has failed, the element must be replaced |
| Sealing ring aged, end cover not pressed tight | Washing has no effect, the problem is on the sealing face | Accuracy has failed, replace the element and deal with the seal |
The order to judge things in on site
- Look at how fast the pressure drop rises. A slow rise points to surface deposits, dirty incoming oil or degraded oil; a sudden jump points to damage, a bypass or a fitting mistake.
- Take the element out and look at it before anything else. An even layer of dirt on the outside means the dirt came out of the oil; dark, glossy patches and creases mean the medium itself is damaged.
- Check the sealing ring and the contact face of the end cover, and deal with any indentation, foreign matter or distortion instead of changing the element alone.
- Wash with a medium that is compatible with the filter material, do not use a solvent that attacks it, and let the element dry fully before it goes back in.
- Record the pressure drop and the flow again after fitting it back and compare the figures with the ones taken before the element came out, so that the treatment can be judged.
Past the pressure limit it is no longer only a cleaning question
Cleaning and element replacement become necessary once the pressure passes 0.3 MPa, and that is a definite limit: at this pressure the element is already in a state where it should not be working. The filtration accuracy of the ZYD-100 is ≤1 μm and the cleanliness corresponds to NAS 1638 class 5, and these two machine figures rest on the filtration section. Once an element has suffered structural damage the figures no longer hold, and running on with a failed element produces oil that looks the same but measures outside the limits.
The pressure drop is only the symptom, the state of the medium is the evidence; clean does not mean still usable.
