Replacing the filter element is the operation carried out most often on site, and it is also the one most easily done wrongly. An element fitted the wrong way round gives no sign of it for a short time: the machine starts, the pressure is normal, the oil flows, but the filtration accuracy has already been lost. By the time the cleanliness test fails, several batches have already gone through.
What happens when the element is fitted the wrong way round
The consequences fall into three main types. The first is a failure of the seal. The end face of the element is sealed by the pressure of the sealing ring, and the oil flows from the outside of the medium to the inside. If the element is fitted the wrong way round, the sealing face is compressed in the reverse direction, the load is uneven, and the sealing ring is pushed out of position or even squeezed out, so the oil passes straight through the gap between the end face and the housing, a path that bypasses the medium completely. The machine runs normally, the outlet flow is normal, and the accuracy has already been lost.
The second is an abnormal differential pressure reading. With the element reversed, its resistance characteristics change: the differential pressure may be markedly low, because much of the oil takes the bypass path and the flow inside the element is too small, or it may fluctuate up and down. Treating a low differential pressure as the signal that the element has reached the end of its life and replacing it wastes the consumable for nothing.
The third is oil seeping out. When the end face is not sealed tightly, oil seeps out along the gap; the quantity is small but continuous, and it leaves a trail of oil on the outside of the housing, where it is easily mistaken for a leaking connection.
How to determine the correct direction
- Look at the flow marking on the housing: the element or the filter housing normally carries a stamped or silk-printed arrow, which points in the direction of the oil outlet.
- Look at the end with the support skeleton: the end carrying the support skeleton, with the larger perforations, is the inlet side, and the dirty oil enters this end first.
- Look at the structure of the medium: the filtration runs from the outside in, the outer layer takes the large particles and the inner layer does the fine interception, so the side where the end face and the skeleton face each other is the outlet side.
- Check the marking on the part against the documentation: for the common specifications, coarse elements 150-250-50 and 110-250-50 and fine element 150-500-5, the installation direction of each model is defined by the marking supplied with the part and by the manual, and it must not be worked out from memory.
Whether a wrongly fitted element has to be removed and refitted
It has to be removed, and it must not be made to do. After the sealing ring has been loaded in the wrong direction it may already be deformed, and continued use will bring the seepage or the bypass back on the next run. New seals are to be fitted on reassembly and the old ones are not to be put back; if the sealing face carries light scratches or indentations, assess whether they affect the seating and replace the sealing face where necessary.
Six checks after every removal and refitting
- The direction marking agrees with the actual flow.
- The sealing ring sits completely in the groove and has not been pushed out of position or rolled over.
- The end face and the sealing face are clean, with no foreign matter and no residue of the old seal.
- The locking is secure and the fasteners are tightened as specified with a consistent torque.
- After reassembly, measure the differential pressure across the filtration section and compare it with the baseline value; if it is clearly low, check the direction and the seal first.
- Take an outlet oil sample, compare it with the sample kept previously, look at the appearance, and carry out the cleanliness test at the scheduled interval.
| Symptom | Possible sign of reversed fitting | How to confirm | Action |
|---|---|---|---|
| The differential pressure across the filtration section is clearly below the baseline | End face bypass, the actual flow through the element is too small | With the power disconnected, dismantle and inspect the end face seal and the flow marking | Remove it, refit it the right way round and replace the seals |
| The outlet oil still carries particles and the cleanliness stays below the limit | The oil bypasses through the gap at the end face | Sample the inlet and the outlet at the same time and compare the cleanliness | Dismantle, refit and re-test |
| Oil seeps out on the outside of the filter housing | The end face is not sealed tightly | Wipe it dry, reproduce the condition and observe where the oil seeps out | Replace the seals and check the end face for indentations |
| The differential pressure fluctuates up and down | The element is not seated, or its resistance characteristics have changed through reversed fitting | Stop the machine and check that the installation is properly seated | Reinstall and re-check the differential pressure |
Safety and hygiene requirements
Removing an element must be done with the machine stopped, the pressure released and the power disconnected. A removed element should not be put back at random, because the water and the particles in the oil will contaminate the new element and the sealing face. Keep the element that has been taken out until the end of the processing cycle, as the sample used to judge the actual load on the element; what the element has held back carries more information than the replacement date alone.
If the differential pressure is lower after an element change than it was before, the first thing to do is not to fit another element but to check the direction and the seal once more.