Prefilter sizing
Some types of trash and larger organics can be efectively filtered with relatively large openings.
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The table below lists the minimum prefilter surface area required to achieve two different velocities through a clean screen having a 50 percent open area. The table is intended to be a guide, rather than the solution. For example, it would be better to use a design velocity of 0.5 feet per second (half of the rule of thumb) for sites with heavy debris loading or that are otherwise visited/cleaned infrequently. In these cases, the prefilter can be partially plugged about half the time.
Minimum surface area required for two different target water velocities through a clean prefilter screen/grate with 50% open area
System flow rate, including backflush flow (GPM)
Required submerged prefilter area (ft2) to achieve 0.5 ft/s
Required submerged prefilter area (ft2) to achieve 1 ft/s
250 500 750 1,000 1,250 1,500 1,750 2,000 2.2
4.5 1.1 2.2 6.7 3.3 8.9 11.1 13.4 15.6 17.8 4.5 5.6 6.7 7.8 8.9
Another design decision is the selection of the prefilter opening size. Typical prefilter opening sizes range from 3/8 inch to 1 inch, but the decision is largely dependent on the type of debris present. Some types of trash and larger organics can be effectively filtered with relatively large openings. On the other hand, debris like stringy algae can slip through all but the smallest of openings.
Kyle Feist, MS, PE, CID, CAIS, is an engineer at the Cal Poly Irrigation Training and Research Center. His work involves technical consulting, training and research for both irrigation districts and on-farm irrigation systems.
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16 Irrigation TODAY | January 2019
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