Sizing filters involves selecting the best combination of dirt-holding capacity, pressure drop, and economy to match the application at hand.
However, the circuit may contain just a gear pump and directional-control valve in the circuit. Because a gear pump and directional-control valve are relatively tolerant of dirt, a coarser, lower performance filtration option—such as paper or wire mesh nominal filtration—could be a suitable option. Knowing what hydraulic components will accompany the filter and understanding how they impact filter performance requirements can save unnecessary costs in the long run.
3. Know the Viscosity
One variable often underestimated when selecting a filter is fluid viscosity. In reality, viscosity has a significant impact on filter sizing. Fluid viscosity is sometimes overlooked in the early stages of sizing hydraulic filters. As a result, filters larger than otherwise necessary may be specified, resulting in unnecessarily high operating costs.
Not understanding the importance of fluid viscosity can lead to under sizing the filter, high pressure drop, and premature tripping of clogging indicators. Conversely, oversizing the filter can contribute to higher costs, a larger footprint, and higher replacement costs. Users should understand the hydraulic system enough to estimate the normal temperature range so that the most accurate minimum temperature is selected for sizing purposes. Properly selecting the correct operating temperature range will ensure that the filter doesn’t prematurely go into bypass mode and will avoid specifying a larger filter than needed.
4. Know the Clogging Indicator
Assuming a system’s filter has a clogging indicator (analog or digital), users should create an appropriate target for a low pressure drop when sizing filters. Generally, clogging indicators a differential pressure ranging from 1 to 8 bar. As a general rule, the ratio of the clogging indicator setting to the clean pressure drop should be at least 3:1.
For example, assume a filter has a 5-bar optical clogging indicator. Based on our ratio, the highest acceptable clean pressure drop target would be approximately 1.7 bar. By not understanding the impact of this principle, users are at risk of sizing their filters incorrectly, whether too large or too small. Although oversizing filters is a commonly-used practice to ensure the highest amount of protection, the consequence is that it adds cost to the circuit.
5. Know the Flow Rate
Careful consideration of the flow rate is extremely important when sizing filters. Rexroth recommends that the maximum flow rate be considered in this measurement. Also note that intermittent flows can far exceed the pump’s maximum flow. An example of this scenario is an accumulator circuit where the accumulator stores of volume of pressurized fluid and releases it to the system to supplement pump flow. The flow rate through a pressure filter immediately downstream of the accumulator should take this additional flow into account. Underestimating the flow rate through the filter increases pressure drop through the filter element, which reduces efficiency of the circuit and increases operating and maintenance costs.
