The same filter element can last twice as long on one machine as on another, and after a pump is replaced on the same machine the rate at which the differential pressure rises changes with it. That is not a fluctuation in the quality of the filter medium; the working conditions of the element simply are not the same. To judge how long a filter element will still last, you first have to know how much flow passes it per second, how many particles have collected on its surface, and what the oil temperature has done to the viscosity.
The life of a filter element is the result of three things together
Three variables sit between fitting an element and the differential pressure reaching its limit: the flow through it per unit time, the total number of particles entering it, and the oil temperature. The first two set the rate at which the dirt holding capacity is used up and the third sets how much resistance the collected dirt creates. The ZYD-100 has a flow rate of 6000 L/h, the ZYD-200 12000 L/h and the ZYD-300 18000 L/h, and the fine filter stage of the ZYD-100 is 5 elements of 150-500-5 in parallel. Put the flow rate and the number of parallel elements side by side and you get the flow that each element actually carries.
Flow: it is the flow velocity across the filter surface that sets the differential pressure
With the same element, doubling the flow roughly doubles the rate at which the differential pressure rises, because the pressure drop across a filter is a function of both viscosity and flow velocity. Adding parallel elements lowers the flow each one carries and flattens the differential pressure curve, but it does not increase how much dirt a single element can hold: parallel elements raise the available flow and the total dirt holding capacity, not the life of the individual element. For the site this means that once the number of parallel elements is fixed, the higher the treatment rate, the shorter the effective life of each element.
The oil: the amount of particles and the nature of the particles are different things
The total amount of particles decides how much an element can hold, and the nature of the particles decides how difficult it is for the flow to continue through the element once they are in it. Hard particles with sharp edges form a bridging layer on the surface of the medium, and the resistance rises much faster than it does with soft round particles; for the same volume of oil, a batch with a higher particle concentration brings the element to its differential pressure limit in a shorter time. Water takes part indirectly as well: the target after purification is ≤3 ppm (GB/T 7600) while the incoming oil is far above that level, and the water carries oil sludge and polar products into the medium with it. Such mixed dirt is harder to separate off when the element is cleaned than particles on their own.
Temperature: the change in viscosity rewrites the differential pressure reading
The same element shows a higher differential pressure reading when the oil is hotter, because the viscosity of the oil changes with temperature and the flow resistance follows it. The temperature control range of the TYA series is 45–80 °C, and the upper limit is pulled down to 70 °C when the water content is high. The same batch of oil passing through the same element at 45 °C and at 75 °C does not give the same reading. Comparing the filter condition of two machines therefore means nothing unless the oil temperature is the same, and the temperature has to be reported together with the data.
| Variable | How it changes on site | Effect on the element life | Can it be adjusted afterwards |
|---|---|---|---|
| Flow carried by each element | Varies with the treatment rate and the number of parallel elements | Sets the rate at which the differential pressure rises | The number of elements is fixed, the flow can be trimmed |
| Particle concentration in the oil | Varies from batch to batch of incoming oil | Sets the rate at which the dirt holding capacity is used up | It can be improved by upstream filtration |
| Oil temperature | Set by the temperature control and the ambient temperature | Changes the viscosity and the differential pressure reading | It can be adjusted, but it depends on the oil type |
| Hardness and shape of the particles | Varies with the source of the oil and the wear on the machine | Sets the rate at which a bridging layer forms | Essentially cannot be adjusted |
- Record the differential pressure value and the reason for each element change, not just a note that it was dirty.
- Compare the filter condition before and after on the same machine at the same oil temperature, otherwise the figures mean nothing.
- Do not mix new and old elements in the same set when comparing, the dirt baseline is different.
- Always state the treatment rate when comparing element lives; a comparison across flow rates favours the lower flow.
The life of a filter element is not a property of the filter medium alone. Write the three variables down in the operating record and the life becomes a number that can be compared.
