Total pressure drop is the sum of the pressure drops of all stages
The pressure difference between the inlet and the outlet of the unit is not the reading of any single filter element, but the sum of the pressure drops of all resistance elements along the flow path. The oil first passes through the coarse filtration stage, then through the second stage element, and finally enters the parallel fine filter element group. The pressure drops of these three stages exist simultaneously at the same flow rate, and the total pressure drop reading adds them together. A rise in total pressure drop therefore only means that the resistance along this path has increased; it cannot directly indicate which filter element has changed state.
Treating the total pressure drop as the criterion for one filter element is the most common misjudgement in the field. If the pressure drop falls only slightly after the fine element has been replaced, the resistance lies mainly upstream; if the pressure drop hardly moves after the coarse element has been replaced, the problem lies in the fine filtration stage. If the judgement is made in the wrong order, the dismantling and reassembly work has been wasted, and the stage that really needs treatment is delayed instead, so the treatment cycle is prolonged as well.
Each stage intercepts something different, so the pressure drop profile differs
The three stages use elements of different specifications and intercept different objects, which determines that they behave differently when their pressure drop rises. The coarse stage uses elements such as 150-250-50 and 110-250-50; what these intercept are solid particles carried in from upstream. These particles are retained at the surface of the filter medium, so the pressure drop often rises quickly and sharply, with little relation to the permeability of the medium itself: the more firmly it is blocked, the faster the reading rises.
| Filtration stage | Common specifications | Behaviour when the pressure drop rises |
|---|---|---|
| Coarse stage | 150-250-50, 110-250-50 | Rises quickly and sharply, mostly related to solids carried in from upstream |
| Second stage element | 110-400-20 | Rises relatively gently, between the stage before and the stage after |
| Fine stage | 110-600-5, 150-500-5 | Rises slowly, affecting the cleanliness at the end first |
The second stage element 110-400-20 occupies the transitional position: its medium is finer than that of the coarse stage and coarser than that of the fine stage, and it intercepts the particles that remain after coarse filtration and are of a size between the two. The fine stage elements 110-600-5 and 150-500-5 face particles that are already very fine. The standard configuration of the ZYD-100 is 5 parallel elements of 150-500-5, with a filtration accuracy of ≤1 μm and a cleanliness of NAS 1638 class 5; these two figures are held by this stage. Once the intercepted objects are matched against the pressure drop profile, the observed symptoms can roughly narrow down the range.
Use stage-by-stage isolation to assign the pressure drop to a specific stage
The judgement that can be made on site requires no additional instrument. The key is to change the flow path of the oil so that the total pressure drop no longer includes the contribution of the other stages. The method is isolation by stage: let the oil flow only as far as one stage, read the pressure drop at that point, and then release the stages one by one. Subtracting the readings separates the pressure drop of each stage. If the pressure drop jumps up suddenly, the incoming oil from upstream has probably become dirtier; if it climbs slowly, the element at the end is probably approaching natural saturation.
For the sequence, isolate from the end backwards: disconnect the fine element group first so that the oil reaches only the second stage, take a reading, then go back to the coarse stage and take another reading. Each stage released adds one more difference value, so the direction of the diagnosis stays clear. Temperature has to be watched during the readings: the temperature control range of the TYA series is 45–80 °C, and the upper limit is lowered to 70 °C when the water content is high. When the oil temperature deviates from this range the viscosity changes, so the pressure drop readings cannot be compared directly with those from other periods, and the oil temperature should be stabilised before the isolation is started.
There is one easily overlooked detail: the pressure drop of the parallel branches is taken as the minimum value, and what it reflects is the worst branch. So when the total pressure drop is higher than expected while the reading of a single branch appears normal, the problem is likely concentrated in one heavily blocked element. Only by confirming the branches separately during the isolation can this part of the pressure drop be seen on its own; otherwise the difference value calculated is still ambiguous.
Thresholds are action lines, not judgement lines
In the TYA series, cleaning and element replacement are required when the pressure exceeds 0.3 MPa; the pressure drop alarm value for the fine elements is 0.35 MPa, and an over-pressure condition produces an audible and visual alarm and a shutdown. These three figures are not the same thing on the timeline: 0.3 MPa is the signal that prompts a person to act, and 0.35 MPa is the signal for the unit to stop itself. The distance between the two is the margin left for the response time, not two different cleaning standards, and the alarm action itself does not mean that the element has failed completely.
Likewise, the alarm value is not the cleaning value, and the cleaning value is not the element replacement value. When the pressure drop reaches 0.3 MPa, the first step is to determine which stage is blocked and how severely, and only then to decide whether to clean or to replace directly. The precondition for cleaning is that the deposits can be washed off; if the filter medium itself is damaged or deformed, cleaning can only restore part of its capacity, and after repeated cleaning the pressure drop baseline of that stage becomes higher each time.
A sequence of actions that can be followed
- First confirm which measuring points the total pressure drop reading comes from; when comparing across units, different reference bases mean there is nothing to compare.
- Carry out stage-by-stage isolation in the order from the end backwards, splitting the total pressure drop into stage-level readings and forming difference values.
- When the coarse stage pressure drop rises quickly, check the upstream source and the element installation first, and only then suspect the medium itself.
- When the second stage is abnormal, check whether it is the consequential load brought by the failure of the previous stage, instead of rushing to replace the second stage element.
- Arrange the treatment when the fine stage pressure drop approaches 0.3 MPa, and leave the margin up to the 0.35 MPa alarm shutdown.
- Record the pressure drop of this stage after each treatment, and gradually establish a baseline specific to this unit and this oil source.
A rise in total pressure drop is only a signal. Break it down to a specific stage first, then decide what to work on.
