Abstract:
Means and method of straining process streams of liquid-suspended solid particulates includes a primary industrial strainer having means periodically to remove deposited solids by generating a backwash stream. A backwash retention device (BRD) is provided for straining the backwash stream from the primary strainer. Periodically, after a predetermined number of backwash cycles, solids deposited in the BRD are removed by blowing down the BRD, producing a by-product stream more concentrated in suspended solids than the backwash stream.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to apparatus and methods for cleaning the backwash stream of industrial strainers. 
     2. Description of Prior Art 
     The use of industrial strainers for removing solid particles from process fluids streams is quite common. Such strainers comprise a rotating drum having apertures in which strainer elements are disposed. Liquid, containing suspended solids, is introduced into the drum and passed through the strainer elements on which the suspended solid particles are deposited. Clarified liquid is passed through the drum for disposal or return to the liquid entering the strainer. For example, U.S. Pat. No. 5,312,544, incorporated herein by reference, discloses such a strainer. As noted in that patent, such strainers are not operated as a centrifuge, since rotation of the drum is at a relatively low rate, for example, about 8 to 14 rpm. 
     Periodically, the solids deposited in the strainer elements must be removed, in order to keep the strainer elements free for passage of liquid to be strained. Typically, this is accomplished by backwashing the strainer elements with the process liquid, that is, by reversing the direction of fluid flow through the strainer elements in a limited portion of the rotating strainer drum, whereby deposited solids are washed out of the strainer elements and carried away in a backwash fluid stream. In the past, such backwash fluid stream, containing a relatively high concentration of suspended solids, was simply disposed of to the sewer or other disposal site. Suck an operation also is shown in U.S. Pat. No. 5,312,544. 
     SUMMARY OF THE INVENTION 
     This present invention provides a backwash retention device (BRD), comprising a hollow filter element, for example in the form of a cylindrical drum, having a plurality of apertures for entrapping solid particles suspended in the backwash stream when the backwash fluid stream is introduced to one side, e.g. the interior, of the filter element and passed through the apertures therein to the exterior of the filter element. The interior of the BRD is connected to the backwash fluid stream outlet of a primary industrial strainer by a line containing a first isolation valve for establishing and disconnecting communication between the primary strainer backwash stream and the BRD. 
     In another embodiment of the invention, the BRD is connected to the fluid inlet to the primary strainer by a line having a second isolation valve therein, whereby when the primary strainer is completely isolated from fluid flow, fluid to be strained by-passes the primary strainer and is introduced directed into the BRD which thus serves as a primary strainer while the principal strainer itself is isolated for repair or maintenance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a strainer connected through a backwash check valve to the BRD; 
     FIG. 2A is a plan view of the BRD; 
     FIG. 2B is an end view of the BRD; 
     FIG. 3 is a cross-section view of the BRD, taken along line A--A of FIG. 2B; 
     FIG. 4 is a schematic diagram of an alternate embodiment of the invention, wherein there is provided an upper fluid circuit by means of which the strainer can be isolated from the process fluid stream and the back ash retention device serves as a primary strainer, and 
     FIG. 5 is a control diagram showing a means of control of operation of the inventive device and method. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 a primary strainer, generally denoted by the numeral 1, has a housing 5 provided with an inlet 2 for a fluid stream to be strained, and an outlet 3 for the strained fluid stream. Inlet valve 4 is provided in the inlet stream line 2, and outlet valve 6 is provided in the outlet stream line 3. Strainer 1 also is provided with a backwash fluid outlet line 7 and a first isolation valve 8. Line 7 connects the strainer 1, through valve 8, to a backwash retention device (BRD), denoted generally by the numeral 9. Device 9 is provided with an outlet fluid stream line 11 having a backwash valve 12 therein, and a blowdown line 13 having a blowdown valve 14 therein. Valve 8 serves, when closed, to isolate the BRD 9 for repair or maintenance. 
     In operation of this embodiment, periodically, as determined by a timer or by a pressure differential controller, backwash fluid enters line 7 from primary strainer 1, passes to the BRD 9 where the backwash fluid stream is strained and the cleaned fluid stream passes out through outlet line 11 and valve 12. The cleaned fluid stream may be returned to the outlet stream of the primary strainer, recycled for reuse, or otherwise disposed of. Periodically, blowdown valve 14 is opened and accumulated solids plus liquid is discharged from device 9 through blowdown line 13. The blowdown stream is highly concentrated in suspended solids so that the volume of material to be disposed of is reduced. 
     As more clearly shown in FIGS. 2A, 2B and 3, the BRD comprises an outer shell 16, mounted, by means of a flange fitting 17, about an inner shell 18 having a plurality of fine apertures extending therethrough and serving to filter larger particles out of the backwash fluid stream which is introduced into the interior of inner shell 18 through an inlet 19. The cleaned fluid stream exits the backwash retention device 9 through an outlet 21 to line 11 and thence through back ash valve 12. Periodic blow-down of the backwash retention device results in liquid-entrained solids being ejected through a blow-down outlet 22. 
     Another embodiment of the invention, particularly useful with industrial strainers having fluid inlet lines less than 8 inches in diameter, is shown in FIG. 4. In that Fig., a further fluid line 23 is connected to the primary strainer fluid inlet line 2 and is connected, through a second isolation valve 24, to backwash fluid stream line 7 and BRD 9. By closing valves 4, 6 and 8, strainer 1 can be isolated from the process fluid stream to be strained and that fluid stream then is directed through line 23 and open valve 24 to the BRD 9 which, in such configuration, serves as an alternate strainer, for example, during repair or maintenance of the primary strainer 1. 
     Turning next to FIG. 5, the numeral 25 denotes a switch by means of which operation of the backwash valve 12 can be switched to manual or automatic control and operated through a backwash valve controller 26. With switch 25 set in the manual or hand position, the backwash valve 12 opens and remains open when the controller 26 is moved to the &#34;open&#34; position and closes and remains closed when controller 26 is moved to the &#34;closed&#34; position. Similarly, a switch 30 controls manual operation of the blow down valve 14. The manual mode of operation of the backwash valve 12 and the blow down valve 14 is an override to an automatic mode of operation and, in the manual mode, the backwash valve 12 and the blowdown valve 14 are opened and closed manually by visual observation of pressure gages (not shown) measuring the actual pressure drop across the primary strainer 1 and BRD 9. 
     As indicated, the apparatus of the invention also is operable in an automatic mode. For such purpose, there is provided an automatic strainer repeat cycle timer 27, an automatic primary strainer differential pressure switch 28 which reads the pressure drop across the primary strainer inlet and outlet, a blow down valve counter (pulse and repeat) 29, a blow down valve single shot timer 31, and a second or blow down differential pressure switch 32 which reads the pressure drop across the BRD 9. 
     With the switch 25 in the automatic position, operation of the blow down valve 14 is controlled by counter 29, single shot timer 31 and differential pressure switch 32. The counter 29 is tied into the repeat cycle timer 27 and is set to a predetermined number of primary strainer backwash cycles. When this preset number of cycles is reached, the single shot timer 31 energizes and opens the blow down valve 14 and the retention device 9 is flushed out. After the single shot timer operates, the blow down valve is closed and the counter 29 goes back to zero. If, at any time, the automatic differential pressure switch 28 detects that the actual pressure drop across the primary strainer 1 has increased more than a predetermined extent, e.g. approximately 3 p.s.i.g., above the normal pressure drop, the backwash valve 12 is opened and remains open until the actual pressure drop across the primary strainer has decreased to the normal pressure drop. At that point, control of the backwash valve 12 returns to the repeat cycle timer 27. In essence, the straining media of the primary strainer 1 is backwashed on regular time intervals as prescribed by the repeat cycle timer and, if at any time the periodic backwashing is insufficient and unacceptably high pressure drop across the primary strainer occurs, the automatic strainer differential pressure switch 28 overrides the repeat cycle timer 27 and opens the backwash valve 12 and keeps it open until the actual pressure drop of the primary strainer equals the normal pressure drop. 
     If, at any time, differential pressure switch 32 detects that the actual pressure drop across the BRD 9 has increased a predetermined extent, e.g. approximately 3 p.s.i.g., above the normal pressure drop, the blow down valve 14 opens and remains open until the actual pressure drop across the BRD 9 decreases to the normal pressure drop, at which point, control of the blow down valve is returned to the counter 29 and the single shot timer 31. 
     The apparatus and methods of the invention can be used to treat many types of process fluid streams comprising particulate solids entrained in a liquid, including but not limited to such streams generated in the paper, steel and other industries. The invention is environmentally advantageous in reducing the total amount of solids-containing waste for disposal.