Abstract:
The clarifier cover has a shape defined by a low profile circular segment of revolution around the central column of the clarifier, for maintaining the volume of gas thereunder minimum. In another aspect, the support structure of the cover is made of a plurality of ring trusses disposed in a circular array and defining a circle, concentric with the circular wall of the clarifier, and a radial array of outer trusses each having an outside end mounted to the wall of the clarifier and an inside end mounted to the plurality of ring trusses. A series of inner trusses are individually affixed in an overhung mode to one of the outer trusses, such that the loading applied to the central column of a clarifier by the cover structure is minimal or negligible. The trusses define an array of triangular frame sectors. A flexible sheet sector affixed to each frame sector, and at least one of the flexible sheet sectors has a retractable section which is removably affixed to one of the triangular frame sectors and to the clarifier wall.

Description:
FIELD OF THE INVENTION 
     This invention pertains to covered clarifiers, and more particularly, the present invention pertains to a low profile retractable cover which is supported largely on the outside wall of a circular clarifier. 
     BACKGROUND OF THE INVENTION 
     The clarification of industrial effluent is normally effected by alternatively agitating and letting the effluent settle, and lifting floating scum from the surface of the effluent or scraping sediments at the bottom of the reservoir. The clarification process is often accompanied by a fermenting action and a generation of odorous bio-gases, and/or the release of volatile organic carbons. For environmental reasons, these gases must be collected and treated. Therefore, a clarification reservoir, or clarifier, is preferably covered and sealed to contain the off-gases. Also, a clarifier preferably has a piping system to transport the off-gases to a gas treatment plant. 
     An industrial clarifier is often circular in shape. The reservoir typically has a central column supporting a motor, a gearbox, and a bearing assembly carrying a surface-skimming boom, a bottom rake or both. These equipment must be accessible for inspection, repair or preventive maintenance. Therefore a first preferred feature of a clarifier cover consists in its ability to be opened, to inspect, repair or maintain the boom, the rake, the circumferential launder or other equipment inside the reservoir. A second preferred feature is that the cover must be adaptable to the integration of a catwalk to the central column, to allow access to the machinery on the central column at all times. 
     The covering of a clarifier is often associated with the implementation of environmental regulations. Therefore a cover is often installed on an existing clarifier which was not designed to support a cover structure. Therefore, the retrofit installation of a cover over an existing clarifier must be done in such a way that the cover does not apply a substantial load or side stress on the central column inside the clarifier. 
     Another preferred feature in a clarifier cover is that the enclosed volume above the level of the clarifier must be kept as small as possible to maintain the ventilation of the clarifier as efficient and as economically as possible. 
     Examples of various systems available for covering a reservoir are described in the following documents: 
     U.S. Pat. No. 3,130,488 issued on Apr. 28, 1964 to G. Lindström; 
     U.S. Pat. No. 3,683,427 issued on Aug. 15, 1972 to H. C. Burkholz et al.; 
     U.S. Pat. No. 4,136,408 issued on Jan. 30, 1979 to E. L. Dahlbeck et al.; 
     U.S. Pat. No. 4,400,927 issued on Aug. 30, 1983 to A. M. Wolde-Tinase; 
     U.S. Pat. No. 4,951,327 issued on Aug. 28, 1990 to V. J. Del Gorio, Sr.; 
     U.S. Pat. No. 5,381,634 issued on Jan. 17, 1995 to S. Pietrogrande et al; 
     U.S. Pat. No. 5,943,709 issued on Aug. 31, 1999 to H. Y. Chiu; 
     Although the cover structures of the prior art deserve undeniable merits, it is believed that a need still exists in the industry for a clarifier cover which has a low profile, which does not apply substantial load on the central column of a clarifier, which is easily openable for inspection, repair or maintenance of the equipment inside the clarifier and which is strong and durable and can accommodate a catwalk to the central column. 
     SUMMARY OF THE INVENTION 
     In the present invention, there is provided a retractable clarifier cover which has a low profile, which is particularly appropriate for a retrofit installation over an existing clarifier, and which has all the other aforesaid advantages. 
     In a first aspect of the present invention, there is provided a clarifier cover having a plurality of saddle brackets mounted on the circular outside wall of the clarifier and a central ring plate mounted on the central column of the clarifier. A support structure is affixed to the saddle brackets and to the central ring plate, and a flexible sheet cover is affixed to the support structure. The support structure has a shape defined by a low profile circular segment of revolution around the central ring plate. 
     The shape of the support structure is particularly advantageous for defining a relatively small volume of gas under the cover, whereby the ventilation of the clarifier is doable economically. The shape of the support structure is also advantageous for isolating the mechanical and electrical equipment that may be present atop the central column of the clarifier from the corrosive or inflammable gases which may be generated inside the clarifier by the content of the clarifier. 
     In accordance with another aspect of the present invention, there is provided a support structure for supporting a flexible sheet cover over a circular clarifier. The support structure comprises a plurality of spaced-apart saddle brackets disposed in a first circular array defining a first circle. There is also provided a plurality of ring trusses disposed in a second circular array and defining a second circle, concentric with and inside the first circle. The support structure also comprises a radial array of outer trusses each having an outside end mounted to one of the saddle brackets and an inside end mounted to the plurality of ring trusses. A series of inner trusses are individually affixed to and extend from the inside end of one of the outer trusses, toward the centre of the first circle. 
     In this structural arrangement, the outer trusses and the ring trusses constitute a self-supporting structure wherein the weight thereof rests on the saddle brackets. The inner trusses are affixed to the outer trusses in an overhung mode such that the loading applied to the central column of a clarifier by the cover structure is minimal or negligible. 
     In yet another aspect of the present invention, there is provided a retractable cover mounted over a circular clarifier. The retractable cover comprises a central ring plate mounted on the central column of the clarifier; a plurality of saddle brackets affixed to the outside wall of the clarifier, and a support structure affixed to the support brackets and to the central ring plate. The support structure is made of an array of triangular frame sectors, each having an apex over the central column and a base over the outside wall of the clarifier, and a flexible sheet sector affixed thereto. At least one of the flexible sheet sectors has a retractable section near the base of the corresponding triangular frame sector. This retractable section is removably affixed to one of the triangular frame sector and to the clarifier wall. The clarifier cover according to the present invention is thereby selectively openable for inspection, repair or maintenance of equipment inside the clarifier. 
     Other advantages and novel features of the present invention will become apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Four embodiments of this invention are illustrated in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which: 
     FIG. 1 is a perspective view of a circular clarifier having a retractable cover according to the first preferred embodiment of the present invention mounted thereon; 
     FIG. 2 is a cross-section view of the support structure of the retractable cover according to the first preferred embodiment, as seen along a radius thereof; 
     FIG. 3 is a perspective view of one of the triangular frame sectors of the support structure; 
     FIG. 4 is an enlarged partial perspective view of an outer truss and a saddle bracket supporting an outer truss on the clarifier wall; 
     FIG. 5 is a partial perspective view of a connection between an outer truss and an inner truss and of a connection between an outer truss and a ring truss; 
     FIG. 6 is a perspective view of an alternate connection between an inner truss, an outer truss and ring trusses; 
     FIG. 7 is a perspective view of a ring plate for retaining the inner trusses of a support structure to the central column of a clarifier; 
     FIG. 8 a cross-section view through the ring plate, showing a preferred attachment of a flexible cover thereto; 
     FIG. 9 a top view of a clarifier with the outer trusses and the ring trusses installed thereon; 
     FIG. 10 is a cross-section view through the clarifier and the outer truss and ring truss assembly as seen along line  10 — 10  in FIG.  9 . 
     FIG. 11 is a top view of the support structure as optionally assembled on the ground, and in a condition to be hoisted over a clarifier; 
     FIG. 12 is a partial top view of a support structure having a catwalk incorporated therein; 
     FIG. 13 is an enlarged cross-section view of a slotted rail mounted over the trusses of the support structure for retaining the sides of a flexible sheet sector to the trusses, as seen along line  13 — 13  in FIG. 16; 
     FIG. 14 is an enlarged partial view of a flexible joint between adjacent winding roll segments on a flexible sheet sector; 
     FIG. 15 is an enlarged partial view of an end of a winding roll on a flexible sheet sector; 
     FIG. 16 is a partial perspective view of a clarifier and a retractable cover according to the first preferred embodiment of the present invention; 
     FIG. 17 is a partial cross-section view of a clarifier having a cover structure according to the first preferred embodiment mounted thereon, as seen along line  17 — 17  in FIG. 16; 
     FIG. 18 is an enlarged partial view of a typical keyhole slot incorporated in a winding roll and in a sheet-stretching pipe; 
     FIG. 19 is a perspective view of a preferred puller used for stretching the flexible sheet sectors over the support structure; 
     FIG. 20 is a partial cross-section view of a clarifier having a removable cover according to a second preferred embodiment mounted thereon; 
     FIG. 21 is a partial cross-section view of a clarifier having a removable cover according to a third preferred embodiment mounted thereon; 
     FIG. 22 is a cross-section view of an alternative arrangement for retaining the flexible sheet cover to the lower cord of a truss, as seen along line  22  in FIG. 21; 
     FIG. 23 is a cross-section of a slotted pipe affixed to one of the trusses of the claifier cover, illustrating a mounting arrangement using self-tapping screws; 
     FIG. 24 is a side view of the slotted pipe looking inside the slot thereof; 
     FIG. 25 is a partial cross-section view of a claifier cover according to a fourth preferred embodiment; 
     FIG. 26 is an enlarged partial view of the cornice and soffit of the clarifier cover according to the fourth preferred embodiment, as seen in the detail circle  26  in FIG.  25 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will be described in details herein four specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the embodiments illustrated and described. The four embodiments do not differ substantially from one another but are nonetheless enclosed herein to better illustrate various manners of construction, installation and operation of the present invention 
     The first preferred retractable cover  30  has the shape of a circular segment of revolution or of a half-bagel. The retractable cover structure  30  is made of trusses and flexible sheet sectors stretched over or under the trusses. The trusses are supported principally on the outside wall  32  of the clarifier  34 , and are lightly anchored to the central column  36  inside the clarifier. Each flexible sheet sector  38  is made of a stretch-resistant nylon-based pliable sheet material. Each flexible sheet sector  38  is partially held to slotted rails  40 , mounted over the trusses. In the first preferred installation, each flexible sheet sector has along each side edge thereof a hem and a rod or oblong nodules enclosed in the hem (not shown) and engaged in the slotted rail  40  as it is customary with tarpaulin structures. The slotted rails  40  do not extend to the edge of the clarifier, such that one or more sheet sectors  38  are retractable away from the clarifier wall  32  as shown in FIG. 1 for the purpose of inspecting the launder  42  or other equipment inside the clarifier. The retractable section of a sheet sector  38  is openable by winding it on a winding roll  44  as illustrated in FIGS. 1 and 2. In FIG. 1, two sheet sectors have been omitted from the drawing for the purpose of illustrating the support structure of the cover. 
     When the cover is in a closed and sealed mode, each sheet sector  38  is held tight over the edge  46  of the clarifier  34  by a series of pullers  48  mounted in a structural angle  50  which is affixed to the clarifier wall  32 . A group of pullers  48  act upon the winding roll  44  of each sheet sector  38 . 
     The retractable cover  30  according to the first preferred embodiment is advantageous for its low height, shown by label  52 , between the maximum effluent level inside the clarifier and the inside surface of the flexible sheet  38 . The space inside the clarifier is therefore relatively small as compared to a conical or a dome-shaped structure for example. The air changes required to ensure a good ventilation of the clarifier is therefore also relatively small, and the equipment required to do this is relatively simple and inexpensive. 
     The circular segment of revolution or the half-bagel shape of the cover is also advantageous for isolating the equipment  54  mounted over the central column  36  of a circular clarifier, from the corrosive or inflammable gases which may be generated by the content of the clarifier. 
     Referring now specifically to FIG. 3, the support structure comprises outer trusses  60 , inner trusses  62  connected to the outer trusses, and ring trusses  64  extending laterally between the outer trusses  60 . There are also provided intermediate trusses  66  extending radially outwardly relative to the centre of the support structure, from a mid span of each ring truss  66 . The outer trusses  60  are supported on a series of saddle brackets  68  affixed to the outside wall  32  of the clarifier. The inner trusses  62  are supported primarily on the outer trusses  60  and are lightly anchored to a ring plate  70  affixed to the central column  36  of the clarifier. 
     FIGS. 4-8 illustrate various preferred connections between the trusses and the clarifier structure. The preferred saddle bracket  68  consists of an angle member with a pair of clevis plates  72  affixed to the upper portion thereof. Each outer truss  60  has a holed stem  74  adapted to connect to the clevis plates  72  with a bolt or a pin. This connection is referred to herein as the first clevis and stem connection  76 . The saddle bracket  68  is anchored to the clarifier wall through holes  78  in the upper portion thereof. 
     The ring plate  70  is affixed to the central column  36  by conventional means, and has a number of clevis brackets  80  to make respectively a second clevis and stem connection  82  with one of the inner trusses  62 , as shown in FIGS. 7 and 8. 
     Referring back to FIG. 5, each inner truss  62  is connected to an outer truss  60  by means of a pair of third clevis and stem connections  84 , only the upper one of the pair is illustrated in FIG.  5 . Each ring truss  64  is connected to an outer truss  60  by a pair of a fourth type of clevis and stem connections  86 . Alternatively, the trusses may be connected to each other by bolted connections  88 , as illustrated in FIG. 6, according to the preference of the manufacturer. 
     Some of the clevis brackets  80  may have a modified shape  90 , as illustrated in FIG. 7, to retain parallel trusses supporting a catwalk for example. The ring plate  70  also has a series of holes  92  therein to retain a clamping ring  94 , for holding the flexible sheet sectors  38  to the ring plate  70 , as illustrated in FIG.  8 . 
     Referring now to FIGS. 9 and 10, one of the most important feature of the present invention will be described. It will be appreciated that the assembly of the saddle brackets  68 , the outer trusses  60 , the intermediate trusses  66  and the ring trusses  64  constitute a self-supporting structure bearing entirely on the clarifier wall  32 . In this structural assembly, the ring trusses  64  constitutes a compression ring that holds the outer trusses  60  and the intermediate trusses  66  above the clarifier wall. The inner trusses  62  as illustrated in FIGS. 1-3 are supported to the outer trusses  60  in an overhung mode. Therefore, it will be appreciated that the inner trusses  62  apply a minimum load on the central column  36  of the clarifier. 
     During the installation of the support structure, the saddle brackets  68  are preferably shimmed up or down as needed to reduce any stresses that may be applied to the central column  36 . It is believed that the only load applied to the central column are generated by the deflection of the support structure under its own weight, under a snow load, a wind load, or by one or more imprecise connections between the trusses or along the side wall  32 . A proper design and installation of the support structure can be done to take these factors into consideration such that the actual loading on the central column of the clarifier is considered minimal or negligible. This feature is particularly appreciable to accommodate the retrofit installation of a cover structure over an existing clarifier wherein the strength of the central column is not known precisely. 
     Another important feature of the support structure according to the first preferred embodiment is that it can be assembled on the ground and lifted and transported over an existing clarifier with a crane. In these circumstances, an array of tie members  100 , as illustrated in FIG. 11, are installed between the outer trusses  60 , the intermediate trusses  66  and the ring trusses  64 . The entire support structure  102  can then be lifted up with hoisting cables attached at three or more locations to the connections of the outer trusses  60  with the ring trusses  64  for example. 
     When a catwalk  104  is installed in the cover structure  30 , the ring trusses  64  on both sides of the catwalk  104  are linked together by a stiffening beam  105  extending below or across the floor of the catwalk as illustrated in FIG. 12, in order to maintain the integrity of the structural ring defined by the ring trusses  64 . 
     Referring now to FIGS. 13-19 the attachment of a flexible sheet sector  38  over the support structure will be described. As mentioned earlier, each flexible sheet sector  38  is held to the outer trusses  60  and to the inner trusses  62  by means of slotted rails  40  affixed to the upper cord of the trusses. In a preferred installation over a clarifier having a launder portion  42 , the slotted rails  40  do not extend to the edge of the clarifier. The retractable section  106  of a flexible sheet sector  38 , is preferably immediately above the launder portion  42  and is held to the outer trusses  60  by rope lashings  108  through grommets (not shown) along both side edges of the retractable section  106 . The rope lashings  108  are tied to a pair of edge support plates  110 , affixed to the outer trusses  60  over the launder portion  42 . 
     The retractable section  106  is held in a closed mode over the edge  46  of the clarifier wall  32  by a series of pullers  48  mounted in a structural angle  50  affixed to the clarifier wall  32 , and pulling on a flexible winding roll  44 . For opening the retractable portion  106  of a flexible sheet sector  38 , the pullers  48  are disengaged from the winding roll  44 , and the winding roll  44  is turned upon itself for winding the flexible sheet  38  thereon as illustrated in FIGS. 1 and 2. For this purpose, one end of the winding roll  44  has a drive stem  112  for connection to a rotating brace tool or to other socket drive equipment. 
     In the preferred embodiments, the winding roll  44  is made in two or more segments  114 ,  116  which are linked to each other by flexible torque-transmitting joints  118 . The flexible winding roll  44  is thereby workable between a straight mode for rolling a sheet sector  38  thereon, and a curved mode to better seal the cover sector  38  against the edge  46  of the circular clarifier wall  32  as illustrated in FIG.  16 . Although the preferred flexible roll  44  is illustrated herein with flexible torque-transmitting joints  118  it will be appreciated that it may also be made of a single section of flexible plastic pipe material for example. 
     Referring now specifically to FIGS. 18 and 19, the preferred puller  48  has a faceted stem  120  for engagement with a socket drive tool. The stem is affixed to a winding shaft  122  to which is also affixed a ratchet wheel  124  on which a pawl  126  is engaged. The puller  48  further has a cable  130  affixed to the winding shaft, and a knob  132  crimped on the end of that cable. In use, the knob  132  is inserted into a keyhole slot  134  in a winding roll  44  and pulled toward the puller to stretch the flexible sheet sector over the edge  46  of the clarifier wall. 
     The structural angle  50  retaining the pullers  48  to the clarifier wall constitutes a guard rail for preventing damage to the retractable cover  30  or to the winding roll  44  by machinery moving near the clarifier wall  32 . 
     Referring back particularly to FIGS. 16 and 17, another feature of the retractable cover  30  according to the first preferred embodiment is illustrated. The central portion of each flexible sheet sector  38  is held tightbetween adjacent inner trusses  62  and adjacent outer trusses  60 , and toward the clarifier wall  32  by winches, in a similar manner as described for the retractable section  106 . A pair of pipes  140  are held inside hems  142  which are affixed to the sheet sector  38  by stitches  144  or the like, adjacent the retractable section  106 . The pipes  140  are pulled toward the clarifier wall  32  by winches  146 , two of which are illustrated in FIGS. 16 and 17. The winches  146  are similar to the pullers  48  mounted on the clarifier wall  32 . The winches  146  are affixed to the upper cord  148  of the outer trusses  60  and to the upper cord of the intermediate trusses  66 . The winches  146  are detachably engaged with the pipes  140  by means of knobbed cables, as the one shown in FIG. 19 
     In FIG. 20, there is illustrated a partial cross-section view of a retractable cover structure  150  according to the second preferred embodiment of the present invention. In this second preferred embodiment, have a curved lower cord  154 . The slotted rails  40  are mounted under the lower cords  154  of the trusses, for holding the flexible sheet sectors  38  under the trusses  152 . An edge plate  110  and rope lashing  108  are also used to retain each side edge of a retractable section of a sheet sector  38  to the trusses  152  over the launder portion  42  of a clarifier. In this embodiment, the space  156  between the maximum level of the clarifier and the cover is relatively small and therefore also relatively easy to ventilate. Moreover, the trusses  152  are kept outside the corrosive fumes that may be generated in some installations by the content of the clarifier. 
     It will be appreciated that the clarifier cover may comprise an inner flexible sheet as illustrated in FIG. 20, as well as an outer flexible sheet as described in the first preferred embodiment. This arrangement constitutes a third preferred embodiment  160  of the present invention, and is partly illustrated in FIG.  21 . This arrangement is advantageous for combining the characteristics of the first and second preferred embodiments. The space  162  between the inner sheet and the outer sheet can be used as an air space to insulate the content of the clarifier against heat losses for example, or by circulating fresh air therein to prevent over-heating of the clarifier content from the sun&#39;s rays. 
     Where a flexible sheet sector  38  is hung under the trusses of a support structure, such as illustrated in FIGS. 20 and 21, and the structure of the clarifier cover is compatible to the joining of two or more flexible sheet sectors into a single sheet panel, an alternative to the slotted rail  40  may be used. Referring to FIG. 22, two or more flexible sheet sectors  38  may be joined to form a single sheet panel  164  that is hung to a slotted pipe  166  affixed to the lower cord  154  of a truss. In this mounting, a strip of fabric  168  is wrapped and sewn over a rope, a flexible cord, a rubber hose  170  or the like, and bent outwardly to form opposite flaps  172 . The flaps are bonded or otherwise affixed to the sheet panel  164 . The covered rope, cord or hose  170  is inserted in the slotted pipe  166 . The flexible sheet panel is thereby removable from the trusses if the need arises. 
     Where the material of the slotted pipe is different from the material of the truss and welding is not appropriate to affix the slotted pipe to the truss, the slotted pipe  166  may be affixed to the truss with self-tapping screws  174 , rivets or other similar fasteners, as shown in FIGS. 23 and 24. When a power tool is used to install the fasteners, the slot  176  may have an enlarged region  178  at each fastener to accommodate for the tip of the power tool. 
     As can be seen from the illustration in FIG. 23, the slotted pipe  166  can be affixed to the truss in any position, such as alongside one of the cord  154  of the truss. This arrangement is a preferred alternative to the slotted rail  40 , to retain the flexible sheet sectors to both handrails or to the floor joists of a catwalk  104  for example. 
     In a fourth preferred embodiment  180  of the present invention, the retractable section  182  of the cover is disposed along a vertical wall of a raised-roof clarifier cover. This arrangement is advantageous for obtaining access to large equipment near the perimeter of a clarifier, or to let an excavator or other machinery reach inside the cover support structure. 
     Referring now to FIGS. 25 and 26, the principal features of the clarifier cover according to the fourth preferred embodiment are illustrated therein. The flexible sheet sectors extending over the roof trusses  184  are held to the trusses in slotted rails  40  as previously explained. The retractable section  182  of the cover is detachably held to the wall columns  186  by rope lashings  108  extending through edge plates  110  also as previously described. The retractable section  182  is held in a closed mode by pullers  48  and is rollable upwardly over a winding roll  44 . 
     In this embodiment, a rounded cornice  190  is mounted to the roof trusses  184  and defines an eave  192  under the roof trusses  184 . In this embodiment, the roof portion  188  of the cover is not continuous with the retractable section  182 . The retractable section  182  is anchored to the rounded cornice  190  and hangs down along the wall columns  186 . The roof portion  188  extends down just enough to wrap around the rounded cornice  190 , and carries at its lower edge one or more lengths of roof-stretching pipe  194 . These lengths of roof-stretching pipe  194  and the roof portion  188  are pulled around the rounded cornice  190  by a series of pullers  146  mounted to the cover support structure. The pullers&#39; cables  130  extend through the retractable section  182  and are attached to the lengths of pipe  194 , in keyhole slots through the pipe walls as previously explained. 
     The working of the pullers  146  causes the pipes  194  to simultaneously stretch the roof portion  188  and pull the upper portion of the retractable section  182  inside the eave  192 , to define a soft and a drip edge of that eave. This arrangement is advantageous for allowing rainwater  196  to drip away from the edge plates  110 , and falling snow to accumulate away from the wall columns  186 . 
     A removable cover according to one of the preferred embodiments mounted over a clarifier, has been found to be resistant, durable and sufficiently strong to permit one of more workers to walk thereon for the purpose of fixing it if the need arises. 
     As to other manners of construction, installation and operation of the present invention, the same should be apparent from the above description and accompanying drawings and accordingly further discussion relative to these aspects would be considered redundant and is not provided. It will also be appreciated that many changes and modifications may be made to the illustrated and described embodiments without departing from the essence of this invention. Therefore, the above description and illustrations should not be construed as limiting the scope of the invention which is defined by the appended claims.