Abstract:
Dam members are provided with an upper discharge weir surface permitting flow of separated liquid thereover during normal or stable centrifuge operations, and a notch or the like interrupting said weir surface or disposed immediately therebelow to provide a lower discharge weir surface permitting flow of separated liquid therethrough to insure an absence of liquid being discharged at solid discharge ports during start-up.

Description:
STATEMENT OF THE INVENTION 
     This invention relates to centrifugal separators and more particularly to improved configurations of dam members employed therewith to insure an absence of liquid being discharged through the solids discharge ports during start-up operations. 
     BACKGROUND OF THE INVENTION 
     Centrifuges operating at very deep, or negative ponds, usually rely upon the solids which accumulate at the solids discharge end to hold back the liquid in the bowl to thereby prevent the liquid from pouring out through the solids discharge openings. During start-up however, when insufficient solids are present in the bowl, liquid very frequently pours out both the centrate, or liquid discharge end, and the solids discharge end. 
     As a consequence of liquid being discharged during start-up through the solids discharge ports, an unpleasant condition arises. More specifically, when a hauling truck, for example, is disposed below a solids discharge chute of the centrifuge in order to receive the solid discharge, but initially receives an outpouring of liquid, typically for several minutes, with a subsequent plopping of solids thereinto, the solids oftentimes comprising sewage, a modestly discomforting situation results. Additionally, the truck&#39;s contents are not considered &#34;truckable&#34;, i.e., the liquid seeps and leaks through cracks and openings in the hauling truck creating a general, or possibly even a serious nuisance along the highway. When solids are removed by belt conveyors, as oftentimes done, liquid flowing over the conveyor edges creates similar problems. 
     Past attempts to correct the abovementioned problem involved the use of stationary skimmers, or pieces of metal plate adjustably secured over the hub openings, or plates which could be adjustably rotated over the hub openings wherein the plates were provided with off-centered liquid discharge holes, and the like. Each of the above had drawbacks, i.e., careful and time-consuming adjustment of the dam or dam-like members was required; or the distance between the centrifuge axis of rotation and the inner surfaces of the solids discharge ports and liquid discharge ports were critical for satisfactory operation; or the device yet failed to adequately prevent liquid from discharging through the solids discharge ports. 
     Still other means and methods exist for controllably discharging separated liquids from the bowl through dams or dam-like members. 
     For example, in U.S. Pat. No. 3,172,851, assigned to the present assignee, a process of separating solids from a liquid-solids mixture is disclosed wherein the solids comprise particles of relatively small size and having a density only slightly different from the liquid. The solids are caused to automatically form a dam of settled solids adjacent the solids discharge ports to thereby substantially block the flow of liquids therethrough. 
     Additional patents disclosing means for controllably discharging liquids from separated solids-liquids mixtures from centrifuges wherein dam-like members are employed include the following U.S. Pat. Nos. 3,623,656; 3,228,594; and 3,795,361. 
     The present invention insures that truckable solids only are discharged at the solids discharge ports of the centrifuge. To that end, the centrifuge is provided with improved dam means which is unitary, of simple construction, and is readily adjustably mounted to the hub to partially cover the hub openings. The improved dam members permit a portion of the liquid to leave the bowl at a shallower level than that level normally required to initiate flow over the dam weir, and thus substantially, if not completely, eliminating the possibility of any liquid pouring out the cake or solids discharge end, while yet permitting and controlling the level of normal liquid flow over the weir after conditions have stabilized within the centrifuge. 
     The improved dam means is effected by providing a notch or cut-out portion in the typically curved weir surface of the dam member, the curve substantially coinciding with the curvature of the liquid level in the rotating bowl. The weir surface however may be substantially straight to approximate the shape of the liquid surface, later described. The notch or cut-out portion, in effect, serves to increase the radial distance between the dam&#39;s lower or outer weir and the axis of rotation of the bowl to thereby permit the liquid to be discharged over the notch during start-up. Absent the notch or cut-out portion, liquid will be discharged from the solids discharge ports of a typical centrifuge, notwithstanding the supply feed rate will be reduced during start-up. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, part in section, of existing centrifuge apparatus, the front hub member thereof having one of its openings cooperating with a conventional dam member. 
     FIG. 2 is a view of the front hub member of FIG. 1 (the hub shaft shown in section), looking in the direction of arrows 2--2, omitting the conventional dam member but including several dam members of the present invention mounted in operating position. 
     FIGS. 3A, 3B and 3C, 3D, 3E and 3F are embodiments of the dam members of the present invention. 
     FIGS. 4A and 4B diagrammatically illustrate pool surfaces during start-up operations (low flow) and normal operation (high or normal flow) respectively. 
     FIG. 5 is a fragmentary view of FIG. 2 illustrating a modification thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, centrifuge 10 comprises a frame 12 supporting bearings 14 into which are journaled the ends of a hollow, elongated centrifuge bowl 16 of circular cross-section. Bowl 16 rotates about its longitudinal axis within housing 18. Bowl 16 is typically belt driven by a motor (not shown), the belt extending around pulley 20. Bowl 16 rotates at speeds capable of generating a centrifugal force up to several thousand times greater than the force of gravity. Disposed within bowl 16, and mounted coaxially therewith, is a helical screw conveyor 22 adapted to rotate at a speed slightly different than that of bowl 16 by suitable means, typically gear box 24 having torque control means (not shown) and a spline shaft within the bowl shaft connected to conveyor 22. Helical screw conveyor 22 includes coiled screw flights 26, the distal edges thereof generally complementing the inside contour of bowl 16, but spaced a short distance therefrom to provide some clearance therebetween. 
     Bowl 16 is provided at its rearward end with a tapered or convergent portion 28, commonly referred to as the beach area. Symmetrically arranged around the beach area 26 is a group of solids discharge openings 30. 
     The front end of bowl 16 is provided with a hub member 32 supported within main bearing 14. Hub 32 may readily be separated from bowl 16 by simply removing bolts 34 adjacent periphery of the hub. A plurality of spaced openings 36 are provided symmetrically uniformly in hub 32 for discharge of the separated liquids therethrough into a liquids discharge duct or chamber (not shown). 
     Helical screw conveyor 22, being hollow, defines an area therewithin designated feed chamber 36. The process feed stream, or liquids-solids mixture to be separated, is introduced into feed chamber 36 through an axially extending feedpipe 38. The mixture is next delivered through a plurality of radial passages 40 disposed within helical screw conveyor 22 into a separation chamber 42 exteriorly conveyor 22 and interiorly bowl 16. By virtue of the controlled differential in the speeds of rotation of conveyor 22 and bowl 16, the solids are urged up the beach 28 for discharge through openings 30 while simultaneously therewith the liquids are discharged through hub openings 36. 
     During start-up however, as discussed above, the normal build-up of solids adjacent the solids discharge openings which tend to restrain the liquid phase from passing therethrough is absent due to an insufficient level of solids present in the bowl. To insure that liquid does not outpour from the solids discharge openings during start-up, hub openings 36 coact with improved dam members having variable height weir surfaces allowing lower levels of liquid to flow through hub openings 36 over lower weir surfaces during start-up while yet permitting and controlling separated liquid to flow in a normal manner over the dam&#39;s upper or normal weir surfaces under typical operating conditions or periods of high liquid flow. 
     Referring now to FIG. 2, bowl 16 and hub 32 rotate as a unit through hub shaft 44. Each opening 36 cooperates with a dam 46 (only two shown) screw mounted to hub 32, or the dam may be formed integrally with the hub. Dam 46 is provided with a rectangularly configured notch 48 (FIG. 3A) centrally disposed over typically curved weir surface 50 which substantially coincides with the level of liquid during rotation of the bowl. 
     Dam 52 (FIG. 3B) is provided with a V-notch 54, while dam 56 (FIG. 3C) includes a circular opening 58 immediately below weir surface 50, while dam 59 (FIG. 3D) includes an elliptical opening 59&#39;, although openings 58 and 59&#39; may be raised to break the continuity of weir surface 50. The centrally disposed cut-away portion may be semicircular 58&#39; (FIG. 3E), semi-elliptical 59&#34; (FIG. 3F), or of any suitable configuration, it being understood that the particular shape or pattern thereof is not intended to be limited to those shown and described. Further, the dam may be provided with opposed ears 60 to facilitate threaded attachment to the hub or the ears may be abandoned in favor of a continuous curved surface 50. Spaced bolt holes 62 in the respective dam members permit the dams to be readily secured to the hub 32 in desired cooperating relationship with openings 36. 
     In the practice of my invention, during start-up, the operator reduces the supply feed rate such that substantially all of the centrate liquid L (FIG. 4A) flows through notch 48 since its depth is at a lower liquid level (greater radial distance from the bowl axis of rotation) than the solids discharge ports 30. Within a few minutes of operation, sufficient quantities of solids accumulate against the wall of bowl 16 and up beach 28 to form the normal build-up at the solids discharge area. The feed rate may then be brought up to normal operating range and the pond level raised to the liquid L surfaces. Effluent then discharges over normal weir surface 50 (FIG. 4B). The variable height discharge weir dam members of my invention thus prevent liquid from outpouring through the solids discharge ports during start-up by allowing centrate to flow over a lower height discharge weir portion (notch) while simultaneously automatically controlling the liquid level within the bowl by permitting centrate to flow over the upper height discharge weir portion (typically curved weir surface) during normal centrifuge operations. 
     Optionally, hub 32 itself may be provided with a plurality of spaced holes 64 (FIG. 5) in lieu of the notches 48, 54, or openings 58, 59&#39;, or cut-away portions 58&#39;, 59&#34;, previously described, it being understood that holes 64 will be disposed at the same liquid level thereof. The dam members 66 will therefore require no notches, openings or cut-out portions. Dam member 66 includes a substantially straight weir surface 68, as may the other dam members of the present invention, to approximate the shape of the liquid surface.