Prefilter design recommendations By Kyle Feist, MS, PE, CID, CAIS e
More advanced commercial self-cleaning screen packages are also available. Self-cleaning screen products typically fit into one of the following categories: rotating cylinder screens, traveling screens and brushed screens. An example of a traveling screen is shown in figure 3.
With extreme debris loading, it might be a good idea to consider redundant prefilters — installing two prefilters in series as shown in figure 4. Additional prefilters can increase the level of overall prefiltration performance by increasing prefilter surface area, with the potential to use different-sized openings for improved debris control. Typically the first prefilter is fitted with larger openings to handle the largest debris, with smaller openings in the more downstream prefilter.
The prefilter assembly shown in figure 4 is clearly effective at removing organic material, as indicated by the large adjacent trash pile. The trash grate handles large debris, leaving only smaller debris to be handled by the rotating cylinder screen.
In many cases, prefilter selection depends on local site conditions, the availability of power (although solar-powered options are available), budget and convenience.
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1. Mount prefilters at an angle from vertical. The angle makes manual cleaning easier and automatic cleaning more efective. The angle also increases the submerged filter surface area compared to a vertically mounted prefilter with the same water depth.
2. Additional ways to achieve more prefilter surface area include:
• installing the prefilter in a deep box, where the depth of water can be artificially increased.
• a shallow but wide prefilter as shown in figure 5.
3. The water velocity of an upstream supply canal can be used to provide some passive self-cleaning functions. When fast- moving water is directed across the screen, trash can be picked up naturally and continue flowing downstream.
4. Special attention should be paid to make cleaning the prefilter easy for field staf, including an area to pile collected debris that is nearby but out of the way.
5. A common rule of thumb is to size the prefilter to achieve a maximum velocity of about 1 foot per second through the openings at the design flow rate. Maintaining lower velocities minimizes the head loss across the screen and can make cleaning easier.
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Figure 3. An automatic, self-cleaning traveling screen installed at a large canal turnout | Figure 4. Redundant prefiltration in a canal near Polson, Montana, featuring a trash grate upstream of a rotating cylinder screen | Figure 5. Relatively long and angled perforated plate prefilter in Colorado. Photo credit: Irrigation Training and Research Center
Photos credit: Cal Poly ITRC
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