Two kinds of rework keep appearing when filter elements are changed on parallel filter housings: one is that a fine filter element has only just been fitted, the pump is started and the differential pressure reading turns out to be wrong, and tracing it back shows that sediment from the bottom of the tank has been drawn into the new element; the other is that the work area is smeared with residual oil, the sealing face cannot be cleaned, and however hard the new sealing ring is pressed down it will not seat. Both kinds of rework have the same root: there is still oil in the tank when the element change begins.
What draining the tank resolves
The oil held in the tank is not only the material waiting to be treated, it is also the carrier that moves the sediment around. After the machine has run for a long time, heavy particles, water and oxides settle down and build up a layer at the bottom of the tank. Dismantling the filter housings with a full tank of oil means the oil is standing in the sealing cavity and on the element end faces, and the removed elements and the cleaned-off sediment are mixed together, so it is no longer possible to tell which part was retained by the element and which was already in the tank, and the judgement loses its basis.
After draining, the sediment layer at the bottom, the oil sludge that has built up around the sealing faces by every possible means, and the residual film clinging to the inner wall of the housing can all be seen and dealt with separately. With the basis for the judgement clean, the two steps that follow follow naturally.
The parallel arrangement makes element-by-element replacement possible
The standard configuration of the ZYD-100 is 5 × 150-500-5 fine filter elements in parallel. The direct consequence of the parallel arrangement is that each element carries a share of the total flow, so any single element can be taken out and replaced without taking the other four out of service. That gives the site a more prudent way of working: sort the elements by differential pressure reading, replace the one with the highest reading first, observe how the differential pressure of the remaining elements changes after it is refitted, and only then decide whether the next one needs to come out, rather than renewing all five at once.
Element-by-element replacement only works if the valve of that one branch has been closed and the other branches are still passing oil. Closing the valve, releasing the pressure and confirming the isolation therefore cannot be skipped, and especially not on the branch that will not shut off properly — it means the housing is opened while it is still under pressure, and that is the step most easily skipped during the draining work and most easily the cause of an accident.
The two pressure figures cannot replace each other
The differential pressure alarm value for the 150-500-5 fine filter element is 0.35 MPa, and that is the electrical alarm setting matched to the machine; the TYA series has a pressure judgement line of 0.3 MPa, above which the element has to be cleaned and replaced. One is the threshold at which the machine reacts automatically, the other is the line at which a person decides whether to act, and they point in opposite directions: one acts when it is high, the other calls for action when it is high. The two figures differ, and neither substitutes for the other.
Mixing the two lines up has practical consequences: an alarm value that only operates at 0.35 MPa means the element has already been running for a long time with a distinctly elevated differential pressure; whereas the TYA figure of 0.3 MPa calls for cleaning and replacement to leave the element some margin and avoid the differential pressure continuing to climb and affecting the flow rate and the pump load. Each line stands on its own, and mixing them leads to cleaning and replacement being deferred to a point where it should not be.
| Step | Changing without draining | Changing after draining |
|---|---|---|
| Tank bottom sediment | Mixed into the element, the origin cannot be established | Cleaned separately, the origin can be judged |
| Sealing face | Soaked in residual oil, cannot be cleaned | Dry and inspectable, the sealing ring groove is visible |
| Differential pressure after refitting | The new element immediately takes in sediment | The reading reflects the state of the element itself |
| Work area | Residual oil spreads everywhere, the next element is even harder to change | The work can proceed one element at a time |
The sequence to follow on site
- Stop the pump, close the valve and confirm that the branch in question is isolated
- Drain the tank and deal with the sediment layer and the residual oil on the sealing faces separately
- After the element change, reset the parallel branches one at a time and observe the differential pressure reading before deciding on the next
- Record the condition of the removed elements as a reference for the next judgement
Draining is not done to make the element change convenient, it is done so that what comes out can explain what happened; if it cannot be established whether something was retained by the element or was already in the tank, the next judgement is still a guess.