When to change a filter element is often settled on site by rule of thumb: every three months, or every six months. The problem is that the actual condition of the element bears no fixed relation to that interval. With a dirtier oil the element is blocked after two months; with a cleaner oil the same element still has capacity left after a year. The differential pressure is a more reliable criterion than elapsed time.
Two kinds of element, two ways of dealing with them
The difference between cleaning and replacement comes first of all from the structure of the medium. The stainless steel mesh elements used in the coarse filtration stage (such as the pre-filter element 150-250-50 of the ZYD-100 and the coarse filter element 110-250-50 of the TYA series) are essentially screens: the retained matter sits on the surface, and cleaning restores the flow capacity. The deep-medium elements are a different case. The fine filter elements 150-500-5 and 110-600-5 and the second-stage element 110-400-20 hold the contaminant inside the fibre structure of the medium, and they cannot be washed clean. Washing them hard damages the structure of the medium and lowers the accuracy, so they can only be replaced.
| Element type | Structure | Disposal method |
|---|---|---|
| Pre-filter / coarse filter | Stainless steel mesh | Strip down and clean it first |
| Second-stage element | Deep medium | Replace, do not clean |
| Fine filter element | Deep medium | Replace |
The criteria: the differential pressure and the accuracy
One of the operating points of the TYA series is an explicit rule: if the pressure exceeds 0.3 MPa during normal filtration, the filter should be cleaned or the element replaced. That figure is a reference given for the structure of the machine as a whole, not a general threshold for every model, but the logic of the judgement is the same: a differential pressure that keeps rising means the filtration area is gradually being filled up. If it goes further, the overpressure protection operates and the machine stops. What has to be watched is therefore the differential pressure curve, rather than waiting for the alarm.
A second criterion is easily overlooked: the differential pressure is normal but the accuracy is not met. This normally means that the element is damaged or that the bypass seal has failed, rather than that the element is blocked. An aged sealing ring, an element that is not seated correctly and a deformed element can all produce this appearance of a low differential pressure and poor filtration performance, and in that case changing the element does not solve the problem.
How to bring the consumable cost down
- Replace on the differential pressure, not on the calendar: tie the element change to the differential pressure reading, so that a dirty element is changed early and a clean one late, and the consumables are used where they are really needed.
- Stage the filtration and share out the load: the coarse stage at the front catches the large particles, so the fine filter element only has to face the fine ones, and the dirt holding capacity is distributed sensibly.
- Understand what the parallel arrangement means: the 5 parallel 150-500-5 fine filter elements of the ZYD-100 increase the filtration area and the flow capacity, but if any one of them is damaged the contamination reaches the downstream side, so the parallel arrangement improves the life and not the accuracy.
- Keep stock of the consumables in common use: the downtime often hinges on the delivery of the elements, so the sizes in regular use should be stocked in advance, and changing an element itself takes only a few minutes.
The real cost is not the price of the element. It is the downtime caused by a rising differential pressure when the element is not changed, and the rework caused by an accuracy that is not met when the differential pressure falls.