The same filter element fitted on different batches of oil gives different differential pressures, and this is a common sight on site. Most people blame the oil first, but the real situation is usually two effects added together: the filter medium has itself captured contaminants, and the flow resistance differs because the oil temperatures differ. The second effect is easily overlooked, because it leaves no visible trace of dirt and shows up only in the numbers.
A pressure drop is the sum of two parts
The differential pressure across a filter element consists of two contributions. The first is the filtration resistance: the oil has to pass through the porous structure of the medium and the particles are held back on the fibres, and this resistance rises steadily as the dirt holding capacity is used up, returning to its initial level after an element change. The second is the flow resistance: the friction produced as the oil moves through the housing and the outer chamber of the element against the pipe wall, the cage and the sealing face, and this part is set by the viscosity of the oil, which changes with temperature.
The filtration accuracy of the ZYD-100 is ≤1 μm, with the fine filtration stage built from five 150-500-5 fine filter elements in parallel. The point of parallel arrangement is that the flow is shared between the five elements, each of them carries less flow, and the filtration resistance therefore falls. But that only reduces the first part; the second is still carried by every element.
Temperature changes one of the two parts
When the oil is cold the viscosity is higher and the differential pressure to be crossed at the same flow is greater; as the oil warms up, the viscosity falls and the differential pressure falls with it. That is why the differential pressure of the same batch of oil is clearly higher during a cold start and why the reading drops a little after the machine has run for a while. The temperature control range of the TYA series is 45~80 °C, with the upper limit pulled down to 70 °C when the water content of the oil is high, and that temperature span alone is enough to produce a visible difference in the differential pressure reading.
| Condition observed | Effect on the differential pressure | Effect on the actual flow | How to verify it |
|---|---|---|---|
| The oil temperature is near the lower end of the temperature control range | The flow resistance is high and the differential pressure reading is high | Less oil passes through the element at the same pump speed | Record the oil temperature and the differential pressure together, not the pressure alone |
| The oil temperature has just reached the set value | The differential pressure is at its lowest point for the batch | The flow is at its highest point for the batch | Compare against the previous run in the same oil temperature range |
| After an element change the differential pressure is higher than before the change | A new element has a low resistance, so the cause is more likely the end cap sealing or a bypass | The same, and the sealing face of the housing also has to be checked | Check how the end caps are seated and confirm that no bypass valve has been left open |
| The differential pressure rises steadily as the machine runs | The dirt holding capacity is being used up, which is normal decay | The flow falls at the same time | Plan the element changes against the replacement interval instead of waiting for the pressure alarm |
How to take the temperature out of the judgement
The most direct method is to record the oil temperature alongside the differential pressure and never read the pressure on its own. Only when the same machine, the same oil and the same elements are compared across readings taken at similar oil temperatures does the difference reflect the state of the element itself. The second method is to fix a reference point, for instance taking all readings once the oil has reached the set temperature, so that data from different batches can be compared.
- When the differential pressure rises, check the oil temperature first: if the rise is caused by low temperature it falls back once the oil is warmed up, and the element does not need changing.
- When the differential pressure rises, check the state of the element: only a steady rise at a normal temperature means the dirt holding capacity is the cause.
- Differential pressure readings taken during a cold start do not belong in the acceptance record, because they reflect the oil temperature and not the capability of the machine.
- If the temperature control runs close to its upper limit for long periods, the flow setting has to be reviewed to see whether it suits this oil.
Why the upper limit is pulled down to 70 °C
Pulling the upper limit of the temperature control down to 70 °C when the water content is high limits the rate at which water evaporates as the temperature rises. The higher the temperature, the more water vapour flashes out, the heavier the gas load in the pumping path, and the harder the condensing section and the pump have to work. The effect of that constraint on the filtration stage is direct: the permitted oil temperature range becomes narrower, and so does the margin available to offset the effect of viscosity.
There is no temperature column on the differential pressure gauge, so the temperature has to be added by the person reading it; without that information the differential pressure is only a reading and not a judgement.
