Читать книгу Automation of Water Resource Recovery Facilities - Water Environment Federation - Страница 239

Rake frequency for each rake is typically controlled by both time and differential level across the screen. Time between rakes is typically the cause of raking the bar screen. For example, a rake strategy may provide for a raking cycle every 10 minutes under normal operation. A rake action by differential level provides protection from screen blinding. For example, a rake cycle may be initiated if a differential level greater than 0.15 m (6 in.) for more than 25 seconds occurs. If the differential level exceeds normal, it is a signal that the rake is likely beginning to blind and more frequent rake action is needed.

During high facility flows, it is common to greatly increase the frequency of rake operation. This is attributable to flushing of the facility’s collection system, which tends to present a much higher concentration of screenings material. Often, when facility flow is much greater than normal, all the rake action is set for continuous operation. Each rake cycle typically is stopped when the rake reaches a safe position.