Filtration accuracy is the figure the machine is sold on, and whether it is delivered depends on whether the oil really crosses the medium when it passes through the filtration section. Most of the deviations seen in the filtration result on site come back to one plain question: did all of the oil go through the filter element?
Which route the oil takes when the sealing is not right
The filter element is held by an end cap inside the filter housing. When the cap is tightened properly, the sealing face closes the outer circumference of the element, and the oil can only cross the surface of the medium into the centre tube. When it is not tightened properly, a gap is left between the sealing faces, and under the differential pressure the oil takes the path of least resistance, which is that gap.
The point is that this path produces no signal at all. It does not trigger the audible and visual over-pressure alarm, it does not shift the differential pressure reading out of its range, and every status on the control panel stays normal. The oil went past on the side, the machine does not know, and the site only finds out when the test report says the result is wrong.
Filtration accuracy ≤1 μm and cleanliness NAS 1638 class 5 describe the oil after it has crossed the medium. The oil that bypasses the medium carries with it the particle state the oil had on the way in.
Three contradictory signals point at the sealing
A sealing problem rarely shows up as a leak. It appears far more often as a set of figures that contradict each other, and reading them together is what separates it from a dirty filter element.
- The differential pressure is clearly low. The filtration resistance has not been built up, the pump runs at a light load, and the current reading is below what the normal duty gives.
- The element is not dirty. Taken out, the surface of the medium is clean and the dirt holding capacity is far below what it should be.
- The cleanliness does not reach the requirement over time. Fitting a new element changes nothing, which means the problem is not on the medium.
| Combination of readings | The more likely cause | Next action |
|---|---|---|
| Low differential pressure, low current, normal outlet flow | The end cap is not tightened home and a bypass path exists | Stop the machine and check the cap and element mating face on each branch |
| High differential pressure, dirty medium surface | The element is holding dirt as it should | Arrange cleaning or element replacement following the differential pressure procedure |
| Normal differential pressure, cleanliness still short of the requirement | Particles picked up at sampling or carried in from upstream | First check the sampling against the oil quality test procedure |
| Differential pressure higher after a new element is fitted | The new element is fitted the wrong way or has not seated | Check the flow direction marking and the seated state of the element |
The places most likely to be loose during assembly
Tightening the end cap is not finished by a single turn; a few details most easily leave a gap. Whether the sealing ring between the element and the housing has been pushed out of line, whether old sealing grease or debris is left on the ring, whether the bolts on the flange face of the cap have been tightened evenly in a diagonal sequence, and whether the element has been pushed over in the chamber so that it is loaded on one side.
The standard configuration of the ZYD-100 is 5 150-500-5 fine filter elements in parallel. The advantage of a parallel arrangement is that each element carries less of the flow and lasts longer, but it also means that 10 sealing faces are carrying the pressure. Any one element that is not tightened home creates a bypass on its own branch.
Parallel operation has a side effect worth noting: the total flow of five elements is decided by the tightest of them, while the bypassed oil leaves through the loosest. One element that is not tightened home therefore does more than fail on its own; it drags down the capacity of the whole set, and the difference between the branches is very hard to see on site.
How to confirm the sealing after the element is fitted
After an element change, do not stop at the fact that the machine turns. Two actions confirm the state of the sealing: watch the differential pressure build up during start-up, which should rise smoothly into the working range, and if it stays low or is clearly below that, stop the machine and check the end caps again; and have a sample tested, to confirm the filtration accuracy and the cleanliness instead of judging by the sound of the run.
The differential pressure alarm value of the 150-500-5 fine element is set at 0.35 MPa. That figure is an upper limit, not a pass mark. A differential pressure that runs low for a long time without ever reaching the alarm value is exactly what a sealing problem that has not been tightened home looks like.
Filtration accuracy is a promise written into the specification; whether it is delivered depends on every end cap actually being tightened home.
