Abstract:
A rotary drum for thickening pulp from a slurry, the drum including drainage channels delivering filtrate from a pulp mat on an outer surface of the rotary drum to at least one filtrate chamber at an end of the rotary drum, a flat face valve and plenum chamber assembly in the filtrate chamber. The flat face valve juxtaposed against drainage outlets of the drainage channels, wherein the flat face valve does not block the drainage outlets during a majority of the rotation of the rotary drum. At least one opening on the flat face valve extending through the front surface and aligned with the drainage outlets to provide a fluid passage through the flat face valve and to a plenum chamber behind the flat face valve such that air in the filtrate is collected in the upper portion of the plenum chamber and removed from the rotary drum.

Description:
RELATED APPLICATION 
     This invention claims the benefit of U.S. provisional patent application No. 61/708,259, filed on Oct. 1, 2012 and PCT international application no. PCT/US2013/062.856, filed on Oct. 1, 2013. The entirety of each of the above-identified priority patent applications is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention generally relates to rotary drum vacuum washer filters for washing pulp and increasing the consistency (dewatering) of pulp in the pulp and paper industry, and for washing and dewatering suspensions of solids and liquids on other industries (such as but not limited to wastewater treatment, food processing, cement production and mining). In particular, the invention relates to a filtrate suction control valve for rotary drum vacuum washer filters. 
     Pulp is typically formed of wood chips or other cellulosic material. The pulp has been processed to separate the fibers in the cellulosic material. The pulp flows in a slurry. The rotary drum vacuum washer filter separates the pulp from the liquor in the slurry. The rotary drum vacuum washer filter is also referred to as a filter because it removes liquor and wash liquid from the pulp. 
     The removed liquor and wash liquid drain into a filtrate chamber at the end of the rotary drum and flow down a drop leg conduit. The downward flow of liquor and wash liquid create a suction that draws the liquor and wash liquid through the pulp mat and porous surface of the rotary drum. 
     The suction may be disrupted if excessive air or other gases are in the drop leg conduit. Air and gases fill the drainage channels of the rotary drum during a portion of the rotation of the drum. To prevent the air and gases in the drainage channels from entering the filtrate chamber and drop leg conduit, a valve is typically used to block the drainage channels while they are filled with air and gases. One type of conventional valve is mounted on a stem extending through a trunnion supporting an end of the rotary drum, as is shown in U.S. Pat. No. 7,981,248. Another type of valve is the conventional flat-face valve plate (no openings in the plate) that blocks the outlets to the drain channels during the portion of their rotation when the drain channels have large amounts of air or other gases. 
     To maintain high levels of suction in the rotary drum, air and gases should not flow into, or at least not become excessive in the filtrate chamber, elbow or drop leg conduit. A difficulty with the conventional flat-face valve plate is that air and other gases captured in the drain channels are released into the filtrate chamber as the drain channels pass the flat-face valve plate. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A flat-face valve has been conceived for a rotary drum vacuum washer filter. The flat-face valve vents gas or air from drainage channels in the washer filter to a plenum chamber. The flat-face valve reduces the amount gas or air entering the filtrate chamber and drop leg conduit of the vacuum washer filer. 
     The flat-face includes a plate seat and a plenum chamber adjacent the back of the plate seat. Openings in the plate seat allow air or other gases and filtrate from the drainage channels to enter the plenum chamber. An upper gas vent connected the plenum chamber vents air and other gases from the plenum chamber. A lower liquid drain allows filtrate liquids to drain from the plenum chamber. 
     A rotary drum has been conceived that includes drainage channels delivering filtrate from a pulp mat on an outer surface of the rotary drum to a filtrate chamber at an end of the rotary drum vacuum washer filter. A flat-face valve is mounted to the housing for the rotary drum and is proximate to the filtrate chamber. The flat-face valve prevents air and other gasses from entering the filtrate chamber and the drop-leg conduit that drains the filtrate chamber. The flat-face valve also vents air and other gases captured in the drainage channels of the rotary drum before the captured air and gases can enter the filtrate chamber. 
     The stationary flat-face valve may include: a plate seat having a front surface juxtaposed against filtrate drainage outlets of the rotary drum, wherein the plate seat does not block the drainage outlets during a majority of the rotation of the rotary drum; a plenum chamber adjacent a backside of the plate seat; a mounting bracket extending towards a stationary housing of the rotary drum washer filter, wherein the mounting bracket is configured to be fixed to the stationary housing of the rotary drum washer filter and position the front surface of the plate seat in juxtaposition to the drainage outlets; at least one opening on the plate seat extending aligned with the drainage outlets, said drainage opening providing a passage through which air, gases and filtrate from the drainage channel enters the plenum chamber behind the plate seat; and a gas vent at an upper portion of the plenum chamber. The flat-face valve plate may be configured to be aligned with the 12:30 to 4:30 rotational positions of the drainage outlets. 
     The plenum chamber may be a crescent shaped box attached to the back of the plate seat. The gas vent extends from an upper region of the plenum chamber to an external outlet of the rotary drum, and a liquid drain extends from a lower region of the plenum chamber to a liquid collection device, such as a filtrate tank. 
     A rotary drum vacuum washer filter has been conceived comprising: a housing including a vat configured to receive a slurry of a liquid and a fibrous cellulosic material; a rotating drum including a porous outer surface and drainage channels extending from the outer surfaces to an end of the drum, wherein the rotating drum sits within the vat and the drainage channels are arranged in an annular array and include outlets forming an annular array proximate the end of the drum; a stationary filtrate chamber adjacent the end of the drum and facing the annular array of outlets, and a stationary flat face valve at least partially within the filtrate chamber, the flat face valve comprising: plate seat including a front surface configured to be adjacent the outlets of the drainage channels, wherein the plate seat covers the drainage outlets during a portion of the rotation of the outlets and does not cover the drainage outlets during a majority of their rotation; an opening on the plate seat extending through the front surface and aligned with the drainage outlets; a plenum chamber adjacent a back surface of the plate seat, wherein the opening provides a fluid passage from the drainage outlets, through the plate seat and into the plenum chamber, and a gas vent extending from an upper region of the plenum chamber to an external outlet. 
     A method of dewatering pulp has been conceived including: forming a pulp mat on a porous surface of a rotary drum having a lower drum portion in a vat of pulp slurry; drawing filtrate through the pulp mat and the porous surface, through drainage channels in an annular array of drainage channels within the drum, into a filtrate chamber at an end of the drum, through a drop leg conduit and down to a filtrate tank below the vat, wherein the flow of the filtrate through the drainage channels, the filtrate chamber and the drop leg creates a suction at the porous surfaces that draws the filtrate through the pulp mat; covering outlets to the drainage channels with a flat plate seat to occlude the flow of filtrate and gases from the drainage channels to the filtrate channel during a period of the rotation during which the drainage channels have substantial amounts of gases, while opening the outlets during a majority of the rotation; during the covering of the outlets, directing the filtrate and gases from the drainage channels into a plenum chamber on a side of the flat plate seat opposite to the outlets; venting gases in the plenum chamber, and draining liquids in the plenum chamber. 
     A method has been conceived for dewatering pulp including the formation of a pulp mat on a screened surface of a rotating drum having a lower drum portion in a vat of pulp slurry and drainage channels draining filtrate passing through the porous surface to a filtrate chamber at an end of the rotary drum, the method comprising: as the porous surface of the rotary drum rotates through vat, a pulp mat forms on the porous surface due to suction applied to the porous surface; draining the filtrate from the drainage channels into the filtrate chamber and to a drop leg conduit extending below the vat; applying a wash liquid to the pulp mat after the porous surface rotates up and out the vat, wherein the wash liquid flows through the pulp mat and porous surface into the drainage channels; draining the filtrate flowing through the drainage channels into a drop leg conduit; as the porous surface rotates down towards the vat, moving the pulp mat away from the application of the wash liquid; removing the pulp mat from the porous surface before the porous surface rotates down into the vat; for the drainage channels associated with a portion of the porous surface not receiving the application of the wash liquid, diverting the flow of filtrate and gases from the filtrate channels through openings in a plate seat and into a plenum chamber, and venting gases from the plenum chamber away from the filtrate chamber and the drop leg. 
     The diversion of the flow of the filtrate occurs in a range of the 2:00 to 5:30 rotational positions of the rotary drum. The plate seat and plenum chamber may be in a flat valve within the filtrate chamber or attached to a filtrate chamber. 
     These features, and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art when the following detailed description of the preferred embodiment is read in conjunction with the appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an end of a rotary vacuum drum washer filter wherein the end of the housing is open to expose the rotary drum, vat and other interior components of the rotary vacuum drum washer filter. 
         FIG. 2  is a schematic diagram of a drop leg conduit configuration for the rotary vacuum drum washer filter shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram showing a portion of an end of a rotary vacuum washer filter having a novel flat-face valve. 
         FIG. 4  is a front view of the plate seat of the flat-face valve plate. 
         FIG. 5  is a schematic diagram showing a top view of the flat-face valve plate mounted to the housing of the rotary vacuum drum washer filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing detailed description of the preferred embodiments is presented only for illustrative and is not intended to be exhaustive or to limit the scope and spirit of the invention. The embodiments were selected and described to best explain the principles of the invention and its practical applications. One of ordinary skill in the art will recognize many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention. 
       FIG. 1  shows a conventional rotary drum vacuum washer filter  10  that includes a rotary drum  12  in a vat (illustrated by an upper surface of the pulp slurry in the vat) of pulp slurry. The rotary drum  12  turns in a clock-wise direction at an exemplary rate of approximately 2 to 4 revolutions per minute (RPM), such as 3 RPM. The rotary drum  12  is partially submerged in the vat  14 , such as up to the horizontal centerline of the rotary drum  12 . As the rotary drum  12  rotates through the slurry (3:00 to 9:00 positions) in the vat  14 , a pulp mat  15 , also referred to as a “cake”, forms on the screened surface  16  of the rotary drum  12 . The screened surface is porous such that filtrate and other liquids can flow through the surface to the longitudinal drainage channels  18   
     The screened surface  16  and deck  17  form a cylindrical sheath over the longitudinal drainage channels  18  of the rotary drum  12 . The longitudinal drainage channels  18  may extend the length of the rotary drum  12  and are parallel to the axis of the rotary drum  12 . The screened surface  16  may be formed of a metal or plastic mesh material that covers a deck  17  of the rotary drum  12 . The deck  17  may be a wire structure, a perforated plate or corrugated plate. The pulp mat  15  and screened surface  16  form a filter that prevents the passage of pulp fibers and allows the passage of water, liquor and other liquids. The water, liquor and other liquids passing through the pulp mat  15  become filtrate which collects in longitudinal drainage channels  18  under the deck  17  and flow through an annular array of drainage channels  20  to a filtrate chamber  21  that may be at the end of the rotary drum  12 . 
     The pulp slurry in the vat  14  typically has a low consistency, such as 1.5% pulp and 98.5% liquid by weight. The pulp is drawn from the vat  14  to the pulp mat on the cylindrical screened surface  16 . As the pulp mat  15  and screened surface  16  rise out of the vat  14 , the pulp mat  15  has a consistency of about 10% pulp and 90% liquid. After rising out of the vat  14 , the pulp mat  15  is washed by water spray nozzles  22  that are typically spraying water on the pulp mat  15  as the pulp mat  15  and screened surface  16  rotate through the 9:00 to 12:00 positions of the rotary drum  12 . 
     Suction draws liquids through the pulp mat  15  and screened surface  16 . The suction causes the pulp mat to form as the screened surface rotates through the vat  14 . Similarly, suction draws the wash water from the spray nozzles  22  through the pulp mat and screened surface, and into longitudinal drainage channels  18  below the screened surface  16 . 
     As the screened surface  16  passes over the top rotational position (12:00), the pulp mat  15  moves beyond the wash water spray from the nozzles  22 . As the pulp mat and screened surface  16  rotate through the 1:00 to 2:00 position, a scraper  24  removes the pulp from the screened surface  16 . When removed, the pulp may have a consistency of 12% cellulosic material or greater. The removed pulp is collected in a pulp collection chamber  26  for further processing. 
     The suction is released as the screened surface rotates through the 12:00 to the 3:00 positions. The suction is released to allow the scraper to remove the pulp mat. The suction is also released to minimize the amount of air and other gases that are drawn into the drain channels and associated drainage channels while no wash water or other liquids are applied to the screened surface. 
     As the screened surface  16  rotates past the 3:00 position, the screened surface  16  re-enters the vat  14 . Suction is reapplied to the screened surface typically at about 155 degrees (about 5:15 position) past the top rotational position (12:00), to the longitudinal channels  18 . The process repeats as the screened surface  16  passes through the vat  14  as the rotary drum  12  rotates. 
       FIG. 2  schematically illustrates a conventional drop leg  28  for a rotary drum vacuum washer filter  10 . The motive force for the suction applied to the screened surface  16  is a vacuum created by the flow of liquid filtrate through the drainage channels  18 ,  20  and down through a drop leg conduit  28  and to a sealed filtrate collection tank  29  ( FIG. 2 ). The tank  29  may be approximately 30 feet (ft.) to 40 ft below the vat  14 . The drop leg  28  is a conduit for the filtrate discharged from the rotary drum vacuum washer filter  10 . Within the sealed filtrate collection tank  29  ( FIG. 2  shows one-half of the chamber) may be an anti-foaming device  30  to separate gas and liquid from the fluid from the drop leg  28 . 
     To maintain high levels of suction, excessive amounts of air and other gases should not flow into the drop leg  28 . The presence of air in the drop leg  28  creates a proportional decrease in the suction (vacuum) applied the drainage channels  18 ,  20 . If too much air or other gases enter the drop leg  28 , the flow of liquid filtrate into the drop leg conduit  18  may be interrupted and result in reduced suction applied through the drainage channels  18 ,  20  to draw filtrate liquid through the pulp mat  15  and screened surface  16 . 
     Air enters the longitudinal drainage channels  18  and drainage channels  20  as the screened surface  16  rotates from just over the top position (12:00) to the vat  14  (3:00 to 4:00). During this portion of the rotation, the pulp mat and screened surface are not covered with a liquid from the spray nozzles  22  or the vat  14 . To prevent air from entering the filtrate chamber and the drop leg  28 , a conventional flat-face valve plate  31  blocks the outlets  25  of the drainage channels  20 . For the portion of the rotation of the rotary drum  12  corresponding to the 12:30 to 5:30 positions, the conventional flat-face valve plate  31  blocks the outlets to the drainage channels  20  to air and other gases from entering the drop leg conduit  28 . As the outlets  25  rotate beyond the trailing edge  33  of the conventional flat-face valve plate  31 , air and other gases trapped in the drainage channels  18 ,  20  flow into the filtrate chamber  21  and the drop leg conduit  28 . 
       FIG. 3  is a cross-sectional view of a portion of the end of vacuum washer rotary drum  40  similar in most respects to the rotary drum vacuum washer filter  10  shown in  FIG. 1 , but including a flat-face valve  42  that includes a plate seat  44  and a plenum chamber  45 . 
     The flat-face valve  42  is stationary and may be mounted to the housing for drum. The front of the plate seat  44  is immediately adjacent and covers the outlets  25  to the drainage channels as the outlets rotate through, for example, the 12:30 to 5:30 positions. The plenum chamber  45  is on the back side of the plate seat  44 . The plenum chamber  45  receives filtrate, air and other gases that flow through openings  47  in the plate seat  44 . The openings  47  are shown in dotted lines because the openings are covered by the plenum chamber  45 . The plenum chamber  45  has a gas vent  48  and a liquid drain  49 . The gas vent  45  may discharge the gases to a cyclone or other device to remove liquid and particulates from the gases. The liquid drain may connect to a conduit to the filtrate collection tank ( FIG. 2 ). The conduit may be separate from the drop leg conduit. 
     The plate seat  44  may have a planar front surface and have a perimeter in the shape of an arc. The shape of the plate seat corresponds to the portion of the outlets  25  to be covered by the valve  42 . The plate seat  44  may be configured to cover the outlets  20  as they move through angular positions of the drum from 1:00 to 2:00 to 4:00 to 5:30, as is conventional for a flat face valve. For example, the plate seat may block the outlets  20  as they rotate from the 12:30 to 5:30 positions, which correspond to the rotary drum  12  turning from about 12 degrees past top dead center (TDC—the 12:00 position) to 155 degrees from TDC. 
     The outlets  25  of the drainage channels  20  ( FIG. 1 ) connect to a tube sheet  50  at the end of the vacuum washer rotary drum  40 . The tube sheet  50  may be a metal plate having a circular perimeter and a center opening for the center shaft  51  of the vacuum washer rotary drum. The shaft  51  may be supported by trunnion bearings in a conventional manner. The tube sheet  50  has openings for each of the outlets  25  and may support the ends of the drainage channels  20 . The tube sheet  50  may be in the same plane as the end surface  52  of the vacuum washer drum. Alternatively, the tube sheet  50  may be recessed or protruding from the end surface  52  of the vacuum washer drum. 
     The plenum chamber  45  may be an elongated chamber or an arc-shaped box formed between the back panel  54  and the plate seat  44 . The inlet to the plenum chamber is formed by the openings  47  in the plate seat  44 . Air and other gases enter the plenum chamber along with filtrate and other liquids. The air and other gases rise to the top of the plenum chamber and exhaust through the upper gas vent  48 . Filtrate and other liquids settle in the bottom of the plenum chamber  45  and drain through the lower liquid drain  49 . The gas vent  48  and liquid drain  49  may extend through the vat wall. A resilient annular coupling  78  may connect the vent or drain to conduits for the vented gases or the drained liquids. 
     The narrow filtrate channel  68  is formed between the tube sheet  50  (shown in dotted lines in  FIG. 3 ) and the wall of the housing, such as the vat wall  56 . The narrow filtrate channel  68  direct filtrate liquids from the outlets  25  of the drainage channels to a filtrate chamber  21  below the narrow filtrate channel  68 . The narrow filtrate channel may have a depth between the tube sheet and vat wall of ten to 12 (10-12) inches (254 mm to 308 mm). 
     The flat face valve  42  may be entirely contained within the narrow filtrate channel  68 . The flat face valve may be entirely in the narrow filtrate channel  68  to avoid having to create seals between the flat face valve and the vat wall  56  to prevent air leakage into the narrow filtrate channel  68 . A difficulty with containing the flat face valve within the narrow filtrate channel  68  is the depth of the plenum chamber  45  is constricted by the depth of the narrow filtrate channel  68 . The plenum chamber  45  may have an internal depth of four to five (4-5) inches (100 mm to 127 mm). 
     The plenum chamber should have sufficient interior volume to provide a passage for the air and other gases, and filtrate that flow through the openings  47  in the plate seat  44 . Further, the internal volume of the plenum chamber should be sufficient to allow air and gas to separate from the filtrate. To provide sufficient internal volume and in view of the depth constraint, the plenum chamber may have an elongated arc-shaped length and an extended width. For example, the width or length of the plenum chamber may be greater than that needed to cover the openings  25  on the tube sheet. 
       FIG. 4  is a front view of the plate seat  44  which shows the openings  47  for air and filtrate. The openings  47  may be arranged in arc-shaped rows to align with corresponding annular rows of the outlets  25  of the drainage channels. The upper gas vent  48  and lower liquid drain  49  are shown for purposes of illustration in  FIG. 4  and need not be physically attached to plate sheet. 
       FIG. 5  is a top down view of the flat face valve  42  mounted to a vat wall  56  of the vacuum washer drum  32 . The plate seat  44  is mounted to a back panel  54  of the plenum chamber  45 . The plenum chamber is formed by the back panel and the plate seat. Bolts  60  fasten the plate seat  44  to the back panel  54 . 
     A mounting bracket  62  extends from the plate seat  44  or the back panel  54 , and attaches the flat face valve  42  to the vat wall  56 . The vat wall  56  is part of the housing for the drum  40 . The mounting bracket  62  is fastened to a matching bracket  66  on the inside surface of the vat wall  56 . When bolted together, the brackets  62 ,  66 , securely hold the flat face valve  42  and align the plate seat  44  with the openings  25  in the tube sheet  50 . The brackets  62 ,  66  are configured, e.g. slidable with respect to each other, to adjust the distance between the front of the plate seat  44  and the tube sheet  50 . The brackets  62 ,  66  may also be configured to position the flat-face valve to be parallel to the tube sheet and aligned with the outlets  25 . 
     A narrow filtrate channel  68  is formed between the tube sheet  50  and the vat wall  56 . Filtrate liquid flows from the outlets  25  and into the narrow filtrate channel  68 . Filtrate from the filtrate channel  68  flows into the filtrate chamber  21  ( FIG. 1 ), to the drop leg conduit  28  and to the filtrate collection tank  29  ( FIG. 2 ). The flat face valve  42  may be housed within the narrow filtrate channel  68 . 
     A conventional circumferential seal  70  forms a barrier extending around the perimeter of the narrow filtrate channel  68 . The seal  70  prevents the filtrate liquid from leaking out of the filtrate channel  68  and prevents air from leaking into the channel  68 . The seal  70  may include a stationary metal ring  72  attached to the vat wall  56  and a rotating metal ring  74  attached to the vacuum washer drum  32 , such as to the outer perimeter of the tube sheet. A resilient ring seal  76  may fill an annular gap formed between the stationary and rotating metal rings  72 ,  74 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.