System for minimizing backwash water usage on self-cleaning strainers

Claim 1

In an automatic backwashing strainer system including a backwashing strainer with a backwash outlet; a backwash valve for controlling the flow of backwash water through the backwash outlet; an actuator operating the backwash valve, and control means for automatically activating the actuation, the improvement in said control means comprising: (a) a first timer which periodically produces a first control signal for a variable duration and with a variable cycle time; (b) a second timer which periodically produces a second control signal for a variable duration and with a variable cycle time, wherein the duration of the second control signal is shorter than the duration of the first control signal and the cycle time of the second control signal is longer than the cycle time of the first control signal; (c) a differential pressure switch which produces a differential pressure signal whenever the pressure drop across the strainer exceeds a predetermined upper limit and maintains said differential pressure signal until said pressure drop reaches a predetermined lower limit; (d) a latch responsive to the differential pressure signal which produces a third control signal while the differential pressure switch is activated and which maintains the third control signal for a variable time delay after the pressure drop across the strainer reaches the predetermined lower limit; (e) means for varying the duration and cycle time of the first and second control signals; (f) means for varying the time delay of the third control signal; and (g) means for selecting, in response to operating conditions of the backwashing strainer system, one or more of said control signals to activate the actuator, such that the strainer is cleaned with a minimum usage of backwash water.