Abstract:
A method of treating pulp on a porous surface of a rotating drum cylinder having a lower drum portion in a vat of pulp slurry and a radial array of filtrate conduits including: as the porous surface of the drum rotates through the vat, drawing filtrate from the slurry through the porous surface by the application of a suction to the filtrate conduits vacuum; draining the filtrate from the filtrate conduits into the filtrate chamber and to a filtrate suction conduit extending to an elevation below the vat; forming a pulp mat on the porous surface which passes filtrate and substantially blocks fibers in the pulp slurry; removing the pulp mat on the porous surface from the vat as the drum rotates; draining filtrate from the filtrate suction conduit, and before excessive gases passing through the porous surface enter the filtrate conduits, switching a fluid flow downstream of the filtrate conduits to a gas vent passage offset and below from a drum rotational axis.

Description:
CROSS RELATED APPLICATION 
       [0001]    This application is a divisional of application Ser. No. 11/762,111 filed Jun. 13, 2007 and claims the benefit of application Ser. No. 60/829,313 filed Oct. 13, 2006, both of which are incorporated in their entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The field of the invention is rotary drum vacuum filters used in the pulp and papermaking industry to form a mat of wood pulp and separate the mat from its filtrate. In particular, the invention relates to gas vent on the drum suction control valve in the discharge elbow assembly of the filter. 
         [0003]      FIG. 1  shows a rotary drum vacuum filter  10  that includes a rotary drum  12  in a vat  14  of pulp slurry. The drum is partially submerged in a pulp slurry vat vessel, such as up to the horizontal centerline of the drum. The drum turns in a clock-wise direction at a preferred rate of approximately 2 to 4 revolutions per minute (RPM) and most preferably at 3 RPM. As the outer drum surface rotates through the slurry (3:00 to 9:00 positions), a pulp mat  16  forms on the outer face  17  of the drum. 
         [0004]    To promote mat formation, suction is applied to the drum porous outer surface  17 , e.g. a screened or wire surface. The porosity of the surface  17  is sufficiently fine to retain fibers on the surface and pass primarily filtrate into the channels  18  behind the porous surface. The channels  18  are arranged in a longitudinal array behind the screen and extending the length of the drum. 
         [0005]    The channels drain into radial channel  20 , or tubes, that lead to a central filtrate chamber  28 . 
         [0006]    As the surface  17  of the drum travels up and out of the vat (corresponding to the 9:00 to 12:00 rotational positions of the drum), the pulp mat  16  on the surface is washed with a liquid spray  22 , e.g., wash water, that cleans the pulp mat of chemical liquor. The suction draws the water and liquor from the pulp mat into the channels  18  behind the drum surface  17 . The channels continue to drain into the channels  20  which drain into the filtrate chamber that is typically at one end of the drum and coaxial to the drum. 
         [0007]    As the drum surface passes over the top rotational position (12:00 to 1:00), the wash water spray is stopped. As the drum rotates towards the 2:00 position, the suction stops, but water continues to drain through the pulp and into the channels and ribs. Air also starts to enter the channels and ribs because of the stoppage of wash water. 
         [0008]    The concentrated pulp is generally referred to as a pulp cake. As the drum rotates through to the 2:00 to 3:00 position, a scraper  24  removes the pulp mat from the drum surface. The pulp cake is collected in a chamber  26  for further processing. 
         [0009]    Vacuum washers typically receive a low consistency pulp slurry (1.5% pulp by weight) in the vat vessel. The pulp is thicken as the drum surface rises on the drum surface out of the vat to about a 10% consistency. The pulp is further thickened to a discharge consistency from the drum of 12% or greater. 
         [0010]    After the cake is removed, the channels and ribs are typically filled with air. As the drum surface (now scraped clean of the pulp mat) rotates past the 3:00 position, the surface renters the vat  14 . Suction is reapplied to the channels and ribs after the surface is submerged into the vat. A pulp mat  16  begins to form again on the drum surface  17 . The formation of a pulp mat, water cleaning of the mat, and scraping of the map off the drum is a continuous process that occurs as the drum rotates. 
         [0011]    The motive force for the suction on the drum surface is the vacuum created as the extracted filtrate drops approximately 30 feet (ft.) to 40 ft. (10 to 13 meters) from the rotary drum vacuum washer  10  to a filtrate tank (below the washer). The pipe through which the filtrate passes is known as a drop leg  32  ( FIG. 2 ). 
         [0012]      FIG. 2  shows a conventional end of a rotary vacuum filter having a drum  12  and a drainage path for liquor and wash water (collectively filtrate) that flows from the longitudinal channels  18  ( FIG. 1 ) and radial channels to a filtrate chamber  28  typically at one end of the filter  10  and coaxial to the drum. Suction to the drum surface  17  is generally provided through the channels  18  that extend behind the screen on the drum face  17 . Liquor and water (collectively “filtrate”) enter the channels and are drawn by suction into rib conduits that extend radially and partially axially from the channels near the drum face to an filtrate chamber  28  typically at one end of the drum. 
         [0013]    The axial filtrate chamber  28  provides a drainage path for the flow of filtrate from the ribs and channel in the drum. The filtrate chamber  28  is traditionally coupled, (through a trunnion conduit  34  and an elbow joint  30 ), to a drop leg conduit  32  that drains the filtrate flow down below the vat  14  to a filtrate collection vessel (not shown). 
         [0014]    The drainage of the filtrate into the drop leg  32  creates a suction that draws the filtrate through the filtrate chamber  28 , ribs  20  and channels  18 . To maintain high levels of suction, gas, e.g., air, should not flow into (or at least not become excessive) in the chamber  28 , elbow  30  or drop leg  32 . If too much air enters the drop leg, the suction level (sub-atmospheric pressure) lessens, the flow of liquid filtrate into the drop leg may be interrupted such that reduced suction will be applied to the filtrate chamber  28 , ribs and channels and air enters the filtrate flowing through the drop leg which may cause the filtrate to foam and require downstream processing to remove the air. Accordingly, there is along felt need to prevent gas from entering the elbow joint  30  and drop leg  32 . 
         [0015]      FIG. 2  shows an exemplary prior art approach to preventing gas from entering the elbow joint  30  and drop leg  32 . The filtrate chamber  28  in the drum  12  is coupled to a trunnion conduit  34  that rotates with the drum. The trunnion conduit  34  is driven through a worm gear  36  and a matching drive worm gear collar  37  to rotate the drum. The elbow  30  and down leg  32  conduits are stationary. An inlet end of the elbow is coupled to the outlet of the rotating trunnion conduit.  FIG. 2  is an exploded view of the trunnion conduit and elbow and down leg. In practice, the outlet of the trunnion conduit is rotatably coupled to the inlet to the elbow conduit  30  and the elbow and down leg  32  conduits are connected. 
         [0016]    A center shaft  38  extends from the elbow into the trunnion conduit  34 . The center shaft is of a relatively small diameter as compared to the inner diameter of the filtrate passage in the elbow and down leg. The center shaft  38  is hollow to allow gases in the filtrate to vent into the shaft and avoid entering the filtrate passage in the elbow  30  and down leg  32 . 
         [0017]    The center shaft supports a valve segment  40  that includes a generally arc shaped section that extends from about the 1:00 position to the 5:00 position relative to the rotation of the drum. The outer face of the valve segment is positioned in the filtrate chamber and juxtaposed against the drainage outlets for the ribs  20  (as the ribs pass through the 1:00 position to the 5:00 position). The drainage outlets of the ribs open to the filtrate chamber  28 . 
         [0018]    The valve segment blocks the outlets of the ribs in the drum as the ribs rotate through the 1:00 to 5:00 positions. The arc width of a conventional valve segment is typically about 130 degrees which corresponds to rotating the drum through the 1:00 to 5:00 positions. The ribs are prevented by the valve segment from draining to the filtrate chamber  28  and into the trunnion conduit. As the ribs rotate from 1:00 to 5:00, filtrate and gases, e.g., air, in the ribs are intended to remain in the ribs. The valve segment  40  prevents most of the gases in the ribs from flowing into the filtrate chamber  28  and to the trunnion conduit  34 , elbow conduit  30  and down leg conduit  32 . 
         [0019]    The valve segment  40  also prevents suction from being applied to the ribs as the ribs pass from the 1:00 to 5:00 positions. Suction is neither needed nor desired as the surface  17  of the drum passes from the 1:00 to 5:00 positions because gravity holds the pulp mat  16  on the surface until the scraper  24  ( FIG. 1 ) removes the pulp cake  16  at about the 2:00 to 3:00 position. Suction if applied from the 1:00 to 5:00 positions would draw air into the channels and ribs and impede removal of the pulp mat. 
         [0020]    The valve segment  40  does not block the application of suction to the ribs or the drainage of filtrate from the ribs as the ribs rotate from the 5:00 position to the 1:00 position. As the ribs move through the vat, suction (applied through the ribs by the down leg) draws a pulp slurry onto the drum face screen and pulls filtrate through the screen and into the channels, ribs and to the filtrate chamber  28 . Similarly, as the ribs move up out of the vat to the top drum position (3:00 to 12:00), the suction draws filtrate, including the wash water, through the screen and into the channels, ribs and filtrate chamber. The flow of filtrate into the ribs moving from the 5:00 position to the 1:00 position is sufficient to create a substantial suction as the filtrate flows into the elbow conduit  30  and down leg conduit  32 . Substantial amounts of air are prevented from entering the elbow and down leg because the channels and ribs are substantially filled with liquid filtrate as the channels are submerged in the vat and pass under the water spray, which occurs as the drum moves from the 5:00 position to the 1:00 position. After the channels rotate past the water spray (at about the 12:00 to 1:00 position), the outlets to the ribs are block by the valve stem to prevent gas from entering the filtrate chamber and trunnion conduit. 
         [0021]    The valve segment  40  does not prevent all gases from entering the elbow and down leg. Air enters the ribs as the liquid filtrate drains from the ribs rotating from the 1:00 position until the channels for the ribs enter the vat. The air remains in the rib as the rib rotates down into the drum. As the drum is submerged and filtrate fills the ribs, a filtrate air mixture, e.g., foam, occurs in the ribs and can flow into the filtrate chamber  28 . The residual air and foam in the ribs should not be drawn into the filtrate chamber, trunnion conduit, elbow conduit and down leg conduit as suction is applied to the ribs. However, when suction is reapplied as the outlet of the ribs rotate past the 5:00 position, the residual air and foam in the ribs flow into the filtrate chamber. This air and foam may be sufficient to reduce the suction created by the drop leg, and create air bubbles in the trunnion. 
         [0022]    Air in a washer is detrimental because: (i) when the air is in the filtrate and the cake, it creates resistance to the flow of filtrate through the cake; (ii) air entrained in the filtrate and cake creates foam that is very stable and the foam must typically be eradicated with a costly defoaming agent, and (iii) air in the drop leg results in a lower vacuum created by the drop leg thereby reducing the motive force by which the washer operates. 
         [0023]    Prior attempts to vent gases from the filtrate have included adding a gas vent slot in the valve segment that is in fluid communication with the inner conduit formed by the hollow center shaft  38 . See e.g., U.S. Pat. No. 5,264,138. The slot may be aligned with the 3:00 to 5:00 position on the drum such that as the channels and ribs rotate down into the vat, the filtrate entering the ribs forces air into the slot and out through the center shaft (rather than into the filtrate chamber and trunnion conduit). The center shaft has a gas vent and a filtrate drain that extends externally of the elbow. The center shaft removes gases in the ribs that would have otherwise entered the elbow. The filtrate drain on the center shaft removes liquid filtrate that enters the hollow shaft with the gases. The gas vent removes gases from the filtrate that are directed into the center shaft. A difficulty with this approach to venting gases is that the center shaft is elevated at or above the liquid level of the vat such some of the air and foam remain in the ribs. The vat fills the ribs with filtrate liquid only to a level in the ribs that is no higher than the vat level. The gap in the ribs between the vat liquid level and center shaft  38  remains filled with air. Another difficulty with the slot open to the center shaft is that the slot is relatively narrow, e.g., 16 degrees, and the center shaft is narrow. The narrow slot and center shaft may not be sufficient to allow gas and foam to vent from the ribs, especially if the drum rotates relatively fast, e.g., above 3 RPM. Another approaches to providing a gas vent for a rotary drum filter include the LaVally valve shown in, for example, U.S. Pat. No. 4,683,059, and the air inflow restrictors shown in U.S. Pat. Nos. 5,683,582 and 5,503,737. However, there remains a long felt need for improved devices and methods for venting gases before they enter the elbow and down leg conduits of a rotary drum filter. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0024]    A gas vent has been developed for a valve segment of a rotary vacuum drum filter for condensing and washing pulp from a slurry to a pulp cake. The gas vent exhausts air in the filtrate piping, e.g., channels and ribs, of the drum before the air flows into a down leg where it could interrupt the suction needed for the drum. 
         [0025]    The gas vent is offset from the drum axis and has a large area inlet to vent all gases in the drum piping, even for fast rotating drums. 
         [0026]    In a rotary drum for condensing pulp from a pulp slurry vat, the drum including drainage pipes delivering filtrate from a pulp mat on an outer surface of the drum to a filtrate conduit coaxial with a drum rotational axis, a valve segment has been developed for the filtrate conduit comprising: an outer surface juxtaposed against drainage outlets of the pipes as the pipes pass air received as the pulp mat is removed from the drum (e.g., angular positions of substantially 1:00 to 5:00, wherein the valve segment does not block the drainage outlets during a majority of the rotation of the drum while filtrate is discharged from the outlets; an inlet aperture on the outer surface of the valve segment aligned with the drainage outlets of the pipes, said inlet aperture extending at least a majority of a arc of the valve segment; a closed passage extending from the inlet aperture to a gas vent external to the filtrate conduit, wherein the closed passage is offset from and extends above and below the horizontal centerline of the filtrate conduit. 
         [0027]    The valve segment may include a lower edge of the inlet aperture at an elevation no higher than a liquid level of the slurry vat and a lower portion of the closed passage at an elevation that is no higher than a liquid level of the slurry vat. The valve segment may comprise an outer plate supported on a support plate. 
         [0028]    The support plate may have an arc shape and conform substantially to an inner wall of the filtrate conduit and an inner support plate attached to the outer plate, wherein the closed passage is formed between the support plates. 
         [0029]    A rotary drum filter has been developed for removing filtrate from paper pulp comprising: a housing including a chamber to receive a vat of a pulp slurry; a rotatable drum cylinder mounted in the housing wherein a portion of the drum cylinder extends down into the vat, the drum cylinder including a screen surface to receive a mat of pulp as the drum rotates through the vat; an array of filtrate conduits in the drum in fluid communication with the screen surface and having outlets at a filtrate chamber coaxial with a rotational axis of the drum; a stationary suction conduit coupled to the filtrate chamber receiving the filtrate flowing through the screen surface, filtrate conduit and filtrate chamber, wherein the suction conduit extends to an elevation below (e.g., 30 feet or 10 meters below) the vat to create a suction in the filtrate chamber, filtrate conduit and at the screen surface; a stationary valve segment in the filtrate conduit including an outer surface juxtaposed to block the outlets of the filtrate conduits only while the filtrate conduits are rotated from an elevated position down into the vat and while the conduits fill with air; said stationary valve segment includes an inlet aperture aligned with the outlets of the filtrate conduits and a passage extending from the inlet aperture to a gas vent external to the filtrate conduit, wherein the closed passage is offset from and below a centerline of the filtrate conduit. 
         [0030]    The rotary drum filter may include a lower edge of the inlet aperture at an elevation no higher than a liquid level of the slurry vat and a lower portion of the closed passage at an elevation no higher than a liquid level of the slurry vat. The valve segment may be attached to a support outer plate having an arc shape and conforming substantially to an inner wall of the filtrate conduit. The outer support plate may be attached to an inner plate, wherein the closed passage is formed between the support plates. The valve segment may include an aperture plate including the inlet aperture and the aperture plate mounted on the outer surface. 
         [0031]    A method has been developed for treating pulp including the formation of a pulp web on a porous surface of a rotating drum cylinder having a lower portion in a vat of a pulp slurry and a radial array of filtrate conduits for draining filtrate passing through the porous surface to an axial filtrate chamber, the method comprising: as the porous surface of the drum rotates through the vat, drawing filtrate from the slurry through the porous surface by the application of a suction to the filtrate conduits vacuum; draining the filtrate from the filtrate conduits into the filtrate chamber and to a filtrate suction conduit extending to an elevation below the vat; forming a pulp mat on the porous surface which passes filtrate and substantially blocks fibers; removing the pulp mat from the vat by rotating the porous surface of the drum rotates up and out of the vat; continuing the draining of filtrate from the filtrate suction conduit as the filtrate conduits are rotated through angular positions at which fluid applied to the surface of the pulp is sufficient to fill the conduits; after fluid is no longer applied to the pulp mat and before excessive gases passing through porous surface enter the filtrate conduits, switching the fluid flow from the filtrate conduits from a liquid fluid path directed to the filtrate suction conduit and to a gas vent passage, wherein the gas vent passage is offset and below from a drum rotational axis. 
         [0032]    The method may further including switching the liquid fluid path directed to the filtrate suction conduit and to the gas vent passage at substantially a 1:00 rotational position of the drum. The method may further comprise switching the fluid flow from the gas vent passage to the filtrate suction conduit as the drum rotates through substantially past a 5:00 position. The method may further comprise switching the fluid flow from the gas vent passage to the filtrate suction conduit as the filtrate conduits become substantially filled with filtrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a side view of a conventional rotary vacuum drum filter wherein the housing is shown in cross-section to expose the drum, vat and other interior components of the drum filter. 
           [0034]      FIG. 2  is a side, perspective view of a conventional rotary drum filter with the trunnion conduit, elbow and drop leg conduits shown in exploded view. 
           [0035]      FIG. 3  is a side view of a front side of a valve segment and support mounted on an elbow conduit. 
           [0036]      FIG. 4  is a perspective view of a front side of a valve segment and support mounted on the elbow conduit shown in  FIG. 3 . 
           [0037]      FIG. 5  is a perspective view of a rear side of the valve segment and segment support mounted on the elbow conduit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]      FIG. 3  is a perspective view of a front side of a valve segment  50  mounted on a cantilevered support  51  which extends from the inlet to an elbow conduit  52 . The elbow and conduit are stationary and coupled to a trunnion conduit  34 , such as is shown in  FIG. 2 . The elbow has a mounting bracket  53  that couples to the stationary drive and bearing unit, in a conventional manner. The discharge of the elbow is connected to a down leg conduit  32  that extends to a filtrate collection vessel that is preferably at least 30 feet (10 meters) below the drum filter washer. 
         [0039]    The valve segment  50  may be a plate having an arc shaped in cross section. The valve segment  50  forms an arc of preferably about 130 degrees and extends preferably from the 1:00 to 5:00 positions with respect to the rotation of the vacuum drum. The valve segment is juxtaposed with the drainage outlets of the ribs  20  and extends into the filtrate chamber  28  in the drum  12 . The valve segment is off-set from the centerline  54  of the trunnion conduit  34 . The plate that forms the valve segment  50  includes one or more gas inlet apertures  58  arranged to be in alignment with the discharge of the ribs  20  in the drum. In the arrangement shown in  FIG. 3 , the gas aperture  58  is positioned at or near a distal end (opposite to the elbow) of the valve segment support  51 . 
         [0040]    The cantilever support  51  for the valve segment has a closed passageway  56  that extends from the valve segment plate  50  through the trunnion conduit  34  and into the elbow conduit  52 . The passageway  56  allows gas and foam collected from the ribs  20  to be exhausted out of the filtrate drum and out of the elbow. The outlet of the passageway  56  includes an upper gas vent  68  and a liquid filtrate drain  70 . 
         [0041]    The gas aperture(s)  58  of the valve segment preferably extend collective a majority of the arc of the valve segment  50 , as is shown in  FIG. 3 . In the embodiment shown here, the gas aperture(s)  58  collectively form an opening that extends up to about 100 degrees of the 130 degree arc formed by the valve segment. It is preferred that the aperture(s)  58  extend collectively at least 65 degrees. 
         [0042]    Further, the gas aperture(s)  58  may extend from a near top drum position of the valve segment  50  to a lower position  60  on the valve segment that corresponds to where the ribs have been fully vented of gas and foam, and are entirely filled with liquid filtrate. The elevation of the liquid level in the vat typically corresponds to the centerline  54  of the drum  12 . As the drum surface moves further into vat, liquid filtrate fills the ribs  20  and forces air and foam out of the ribs and into the aperture  58  of the valve segment. 
         [0043]    Preferably, the lower edge  60  of the aperture( 2 )  58  is at or below the angular drum position at which the ribs have been purged of air and foam. As shown in  FIG. 3 , the lower edge of the aperture  60  is at about the 4:00 position, plus or minus 5 degrees. The lower edge  60  may be determined for each drum based on the rotational drum position at which the ribs are filled with filtrate and no longer exhausting gas and foam. 
         [0044]    The large cross-sectional area of the gas aperture(s)  58  in the valve segment  50  ensures that substantially all gases vented from the ribs enter the gas passage  56  in the valve segment even for relatively fast rotating drums. The aperture(s)  58  are relatively long (AW) in the direction of drum rotation. This length facilitates the venting of gases from the ribs  20  into the passage  56  as the ribs move across the length (AW) of the aperture  50 . The low position, e.g., 4:00 to 5:00 position, of the lower edge  60  of the aperture  58  ensures that all air and foam are discharged from the ribs and into the passage  56 . 
         [0045]    The plate of the valve segment  50  may be mounted on an outer plate  64  of the valve segment support  51 . The outer plate may have an arc cross-sectional shape that faces and conforms to the inside wall surface of the trunnion conduit. The valve segment  50  may be a plate that has an arc cross-sectional shape that conforms to the outer plate  64 . The valve segment  50  is mounted, e.g., bolted, to the outer plate  64  and fits over an opening (not shown) in the outer plate  64 . 
         [0046]    The position of the valve segment  50  on the outer plate  64  may be adjustable, such as thorough the use of oval or race-track slots  66  in the plate that receive the bolts that attach the plate  62  to the outer plate  64  of the valve segment support  51 . Alternatively, the valve segment  50  may be welded to the outer plate  64  once the valve segment has been properly positioned with respect to the outlets to the ribs  20  in the drum. 
         [0047]    By adjusting the position of the valve segment on the outer plate  64 , the apertures  58  can be optimally positioned with respect to the angular movement of the drum and the outlet of the ribs  20 . The ribs pass filtrate from the drum surface to a filtrate chamber  28 . The ribs serve as drainage pipes for the drum. For example, the valve segment  50  may be moved slightly up or down on the support plate  64  to align the lower edge  60  of the aperture  58  to be sufficiently below the elevation at which the ribs  20  have fully discharged air and foam, and are discharging liquid filtrate. The valve segment may also be positioned laterally, e.g., parallel to the axis  54  of the drum axis, to be aligned with the discharge of the ribs  20 . 
         [0048]    The valve segment  50  may include a plurality of openings that define the gas aperture  58 . Between the openings may be a support bar  66  integral with the plate of the valve segment and bisecting the plate. The support bar  66  provides structural stiffness for the valve segment and the apertures  58 . The solid portions  65  of the valve segment (including the support bar) are relatively narrow (in the direction of AW) and have a relatively small cross-sectional area. Reducing the solid areas  65 ,  66  of the valve segments avoids unduly reducing the area of the aperture  58  or adversely disrupt the flow of gases into the gas vent passage  56 . 
         [0049]    The internal passage  56  in the valve segment support  51  vents gases that pass through the aperture(s) of the valve segment and are from the ribs and filtrate chambers. The passage  56  is offset from and extends above and below the centerline  54  of the trunnion conduit and drum axis. The lower portion of the passage is preferably at or just below the bottom edge  60  of the apertures  58 . Similarly, the lower portion of the passage  56  should be at or just below the angular position of the drum in which the ribs are filled with filtrate and gases and foam have been exhausted from the ribs. 
         [0050]    The internal passage  56  may extend from the inlet aperture(s)  58  of the valve segment  50  and to the elbow  52 . The passage  56  may have a gas vent  68  at an upper end of the passage and elbow, e.g., above the centerline  54 . The passage  56  also has a filtrate drain extending out of the passage and through the elbow. 
         [0051]    The filtrate drain is at a lower portion of the passage  56  and below the centerline  54  of the trunnion conduit and drum axis and preferably below the elevation of the lower edge  60  of the aperature(s)  58 . A substantial amount of filtrate may pass through the passage  56  as air and foam are discharged from the ribs into the passage. 
         [0052]    Further, liquid filtrate in the ribs may serve a purging action to push out air and foam from the ribs and the pushing liquid filtrate may flow into the passage  56 . 
         [0053]    Alternatively, the valve segment  50  may be integrated into the valve segment support such that the distal end of the outer plate constitutes the valve segment and openings in the outer plate constitute the gas apertures leading to the gas passage  56 . Further, the outer plate  64  and valve segment support  51  may be formed by a sturdy tube having a relatively large cross-sectional area and offset from and lower than the axis  54  of the drum. The tube may have an oval or kidney shaped cross-section to reduce the blockage to fluid flow in the trunnion conduit and conform to the inside wall surface of the trunnion conduit. 
         [0054]      FIG. 4  is a perspective view of a valve segment  50  supported by a valve segment support  51 . The valve segment is mounted on an outer plate  64  of the support. The support  51  is attached to the elbow conduit  52  and extends as a cantilever to the segment. A mounting bracket  53  provides a coupling for the elbow to the stationary drive and bearing unit. 
         [0055]      FIG. 5  is a perspective view of a rear plate of the valve segment support  51 . The valve segment support may be formed by welding together the pair of plates  64 ,  72  along their respective upper and lower edges. The outer plate  64  forms the front surface of the valve segment support and may have an arc shape that generally conforms to the inner wall of the trunnion conduit. The rear plate  72  may be an arc, flat or bent inward along a crease line (as shown in  FIG. 5 ). 
         [0056]    The rear plate  72  and outer plate  64  of the valve segment support  51  form a sturdy support and the gas vent passage  56 . The valve segment support may extend as a cantilever from the inlet of the elbow  52  into the trunnion conduit. A cylindrical post  78  on the distal end of the valve segment support may fit into a bushing ( FIG. 3 ) in the drum axel and inward of the axial filtrate chamber. Further, a triangular brace  80  may be welded to an inside surface of the rear plate  72  to provide additional support for the valve segment support. 
         [0057]    The internal gap between the front and rear plates of the valve segment support defines the gas passage  56 . End caps  82  welded to opposite longitudinal ends of the plates seal the ends of the passage. The passage  56  may alternatively be a tube extending along a back surface of the outer front plate and thereby render the rear plate optional. 
         [0058]    The valve segment  50  provides a means for removing the air from filter drum before the air enters the drop leg. The valve segment allows the ribs to vent gases into the passage  56  for substantially the entire rotational period during which the suction is not applied to the ribs. Further, the valve segment allows gas and foam from the ribs to vent entirely into the passage  56  (along with a substantial amount of liquid filtrate) to minimize air entering the elbow and down leg conduits. These features are contrary to the conventional approach of blocking liquid fluid flow through the ribs during most of the portion of the rotational in which suction is not applied to the ribs. 
         [0059]    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.