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5.2 BAR, BAND, AND DRUM SCREENS 97
FIGURE 5.1 Vertical intake tower with coarse screen.
The on-shore screens (Fig. 5.2) are usually equipped with automated raking mechanism, which periodically removes the accumulated debris and allows maintaining the open space between the bars at approximately the same flow-through velocity over time. Therefore, the design flow-thorough velocity of these screens is higher than that of the coarse screens of open intake towers, that is, 0.12e0.15 m/s (0.40e0.50 fps). It is important to maintain the intake through-screen velocity below 0.15 m/s (0.5 fps) at all times to minimize impingement of aquatic organisms on the screens.
5.2.2 Fine Screens
5.2.2.1 Rotating Screens
Fine self-cleaning mechanical bar screens typically have 3e10mm openings. They are installed vertically in water intake channels downstream of the coarse screens and are equipped with rotating cleaning equipment often combined with water-spraying nozzles to remove the debris from the screen surface. These nozzles periodically spray wash water supplied by pumps sized for 45e68 m3/h (200e300 gpm) with pressures of 4e7 bars (60e100 psi). Because one of the main functions of the fine screens is to protect the intake pumps from damage, the actual screen openings should be selected to be smaller than the distance between the intake pump impellers.