Backwash control for constant volume-pressure filtration system

In a system having a fluid filter that is automatically backwashed each time the differential pressure thereacross reaches a predetermined value, the desired base rate at which the drop across the filter should increase during its initial filtering sequence (i.e., before backwash) is determined and stored in digital format in a process controller along with a preset maximum differential pressure at which backwash is set to occur. During use of the filter the actual rate of change of the pressure drop thereacross is monitored continuously and compared with the rate that is stored in the controller. Whenever the actual rate of change of the pressure drop across the filter differs from the base rate, which is the usual case, the present backwashing pressure is automatically adjusted-usually downwardly. This reduction continues until the actual drop across the filter finally reaches the adjusted value of the preset backwashing pressure, at which time backwashing of the filter is automatically commenced. In a preferred embodiment, the adjustment is based upon the time differential at a given instant between the actual filter time and the desired or base curve time, and the difference is used to reduce the preset pressure in accordance with the inverse of the base curve rate. In another embodiment the difference between the actual differential pressure across a filter and the desired differential pressure at any instant is determined and subtracted from the preset backwashing pressure.