Patent GB-779443-A (GB)
Title: Liquid filter backwash condition measurement
IPC: B01D35/06
Filed: 1956-01-17
Abstract: 779,443. Cleaning sand &c. filters. FOXBORO CO. Jan. 17, 1956 [Jan. 20, 1955], No. 1631/56. Class 46. In an open granular filter system comprising a filter bed and a backwash system for the filter bed, the backwash supply pipe delivers an adjustable and increasing flow whereby the pressure drop across the filter bed increases until a specific condition of granule suspension is accomplished whereupon the pressure drop essentially stops increasing, a pressure sensing means, responsive to the pressure drop detects the point at which the pressure drop stops increasing and as a result the increase in the backwash flow is terminated and the specific condition of the filter bed granule suspension is thereafter maintained in a filter bed washing action. The system is particularly applicable to sand filters. The filter bed contains a layer of earth 15, Fig. IV, supporting a screen 14 on which rests gravel 17 and above it filter sand 18. Water fills the bed to a level above a backwash outlet 20. Water to be filtered enters by a pipe 22 and filtrate escapes through a pipe 16 below the screen 14, the pipe also serving for the entrance of backwash water. Upper and lower pipes 27, 25, the latter at the level of the junction of the gravel and sand, lead to a pressure difference indicator or transmitter unit 30. In the simplest form, when the indicator records a pressure difference corresponding to complete suspension of the sand particles, the backwash flow which has been steadily increasing is checked manually to prevent any further rise and the same pressure maintained manually during the backwash operation. This pressure control may be effected automatically. The pressure difference between the pipes 25, 27 actuates the differential pressure transmitter unit 30 constructed as described in Specification 672,879 [Group XXIX] and this pneumatically operates an electric switch 31 to excite the solenoids which operate a valve 32 to close the supply of air from a pressure source 35 through a pipe 34 to a valve 37 in the backwash supply pipe 38, this valve having been pneumatically operated by pressure-air delivered through an adjustable pressure restrictor 36 to open gradually. Closure of the valve 32 traps pressure air in the pipe 34 to keep the valve opening constant until a bleed valve 43 is opened manually at the end of the backwash operation to disperse the pressure and permit the valve 37 to close. In a modification, the point at which the pressure differential ceases to change is sensed. The differential pressure transmitter 30 actuates a bellows 48, Fig. VI, which rocks a balance arm 46 against the pressure of a bellows 51 in a secondary system 50, the arm acting as a baffle for a nozzle 52 forming part of a pueumatic relay 53, as described in Specification 670,427 [Group XXIX] to upset the balance of the system which includes a switch 45 operated by difference of pressure between the sides of a pneumatic device which when this pressure is zero actuates the solenoid valve 32. There may be a manually controlled by-pass for this valve to facilitate starting up the system. The change in the pneumatic supply may be made linear by employing a variable restrictor relay 63. The nozzle 66 of the relay is baffled by an arm 67 displaced by the pressure difference between two bellows 68, 69, the former being in open connection with the relay and the latter through a pneumatic constrictor 70. The valve 37 is controlled by the flow control instrument described in Specification 670,419 [Group XXIX].