A spent filter element is the cheapest operating record the machine produces. How much dirt the oil carried, what kind it was and which stage trapped it are all written on the surface of the element. The problem is that a spent element is normally thrown straight into the bin, while it can still answer several questions that are hard to answer on site: why the differential pressure on this machine rises so quickly, what exactly this batch of oil brought in, and whether an upstream filter stage has been bypassed.
The first step is to confirm which stage it was fitted in
The same element is read quite differently depending on the stage it is fitted in. The filter stages of the ZYD-100 are arranged in grades: the coarse filter elements are commonly 150-250-50 and 110-250-50, the second stage elements are 110-400-20, and the fine filter elements come in two sizes, 110-600-5 and 150-500-5; the standard configuration of the ZYD-100 is 5 elements of 150-500-5 in parallel. The coarse stage traps the large particles and flocs, the second stage handles the medium particles, and the fine stage faces the finest grade, which corresponds to the filtration accuracy of ≤1 μm and the cleanliness of NAS 1638 class 5 for the machine as a whole. Before reading an element, therefore, match its specification and its installed position first: the specification is marked on the housing or on the element end cap, and the position is confirmed from the removal and installation record.
If a fine filter element has caught large hard objects, the problem is most probably not in the fine stage but in the pipework in front of it, in the second stage or on the oil inlet side. Treating what is found on the fine element as evidence that the machine lacks filtration accuracy points in the wrong direction.
Colour: three tones point to three sources
The first type is a deposit that is dark brown to nearly black with a fine, even texture, usually a mixed dirt formed from oil sludge and polar products; it is common in oil that has consumed a lot of antioxidant, in equipment that has run for a long time, or in batches with a high water content at the inlet. The second type is a grey-yellow, thin deposit that forms a soft layer on the surface of the medium, most often impurities in an emulsified state with water involved; once water takes part, this dirt is harder to wash off than plain particles. The third type is fine metallic-bright particles and bright spots. These are not what the filtration is meant to stop, and they point instead to a wear source upstream that has to be found — the mechanical seal of the pump, the valve stem or the flanges of the upstream pipework.
Form: caking, a cake or loose particles
Form says more than colour does. A continuous layer on the surface of the medium with obvious scoring along the edge means this stage has been exposed to hard particles for a long time; an element with a cake forming only on the inlet face means the particles are concentrated upstream; a clean inlet face with a thin layer of fine powder on the back means the dirt was very fine, that is, dissolved material or very fine dispersed matter that has been trapped and then dropped off; an element that is swollen as a whole and has lost its elasticity is usually the result of water soaking the medium over a long period.
| What is seen on the spent element | The more likely explanation | How to confirm it |
|---|---|---|
| A dark brown deposit of fine, even texture over the surface | Mixed dirt formed from oil sludge and polar products | Check the antioxidant condition of this batch of oil and the accumulated running hours of the machine |
| Grey-yellow, thin and soft dirt that forms a loose layer | Emulsified dirt formed with water involved | Sample and test the water content against the ≤3 ppm (GB/T 7600) target |
| Metallic-bright particles or bright spots | A wear source exists upstream | Check the mechanical seal of the pump, the valve stem and the flanges of the upstream pipework |
| A hard cake on the inlet face with scoring along the edge | Long-term exposure to the abrasion of hard particles | Check whether large particles can enter directly through the pipework on the oil inlet side |
| The medium is swollen, has lost its elasticity and feels soft | Long-term exposure to moisture, the material has failed | Compare the hardness with that of a new filter element and replace the whole set if necessary |
Distribution: which side is dirty decides where the problem is
If the two sides of an element read differently, the problem is not in the element itself but on its upstream or downstream side. If the inlet face of this element is almost entirely black while the back is clean, the dirt fed in from upstream is exactly what this stage is there to remove and the element itself is sound; the other way round, if the back is dirtier than the inlet face, suspect that the sealing face has not been compressed properly, or that fine particles have bypassed the medium through the gap between the end cap and the element.
There is another case: the upper half of the element is dirty and the lower half is clean. This is usually not a problem with the element but with an uneven oil distribution at the inlet. The oil entered the filter stage before it had settled evenly in the tank, so one part of the flow arrived carrying heavy sediment while another part was relatively clean.
- At every element replacement, write the installed position and the date on the element housing with a marker pen, so that six months later the record still matches.
- Do not discard a spent element immediately; keep it at least until the oil from that run has passed acceptance.
- Take one photo of the inlet face and one of the back and file them; the two photos side by side show the distribution differences more easily than a written description.
- Record the oil temperature and the flow rate at the time as well; the behaviour of the same element under different operating conditions cannot be compared directly.
- If within one batch of elements only one is clearly dirtier than the rest, check where it was fitted and the operating record for that run first, and do not rush to replace the whole set.
The layer on the surface of a filter element is the bill left behind by the previous section of the oil path. Making sense of it is worth more than fitting one more element.
