Patent Publication Number: US-7721903-B2

Title: Overflow drainage system for floating roof storage tank

Description:
FIELD OF THE INVENTION 
   The present invention relates to a drainage device for use on a floating roof on a storage tank for liquid products. 
   BACKGROUND OF THE INVENTION 
   Storage tanks for oil and other liquid hydrocarbon products are typically provided with a floating roof. The floating roof moves vertically on the surface of the liquid product to prevent the escape of harmful vapors. A typical floating roof for covering a liquid product in a storage tank includes a deck that substantially conforms to the horizontal cross-sectional shape of the storage tank and has a vertical thickness. The roof is provided with one or more seals extending between the outer periphery of the floating roof and the inner wall of the tank. The roof floats on top of the stored liquid product and rises and falls as the amount of the liquid product increases and decreases. It is known to provide a drain pipe extending vertically through the top surface of deck to drain accumulated water and provide a passage or conduit for the water. The water drained from the surface of the roof passes through the stored liquid and accumulates at the bottom of the tank where it is eventually drained from the tank. 
   During seasonal or other periods of heavy rains, the amount of water that accumulates over the area of the tank cover can be substantially greater than that which can be removed by the centrally positioned drain. The water will therefore rise and can inundate the drain sealing mechanism. The presence of debris carried by the accumulating water can interfere with the proper operation of complex roof drain apparatus known to the prior art. 
   A further problem can occur when rainwater accumulates on the roof and its effective weight increases to a point where it can overcome the buoyancy of the roof, eventually forcing it into the liquid in the tank. It is therefore important to provide an effective and reliable means for draining water from the roof so that it does not exceed a pre-determined depth and weight. 
   Various approaches have been proposed for draining water from storage tank floating roofs. For example, U.S. Pat. No. 2,560,586 to Michaels discloses a floating roof drain which drains water collected over a valve of the floating roof which closes and opens a drain passing. The weight of the water collected over the valve pushes down a cover against the buoyancy force of a float connected to the valve to allow the water collected to flow from the roof. The valve is again closed when the depth of the water on the roof decreases until it is no longer sufficient to hold the valve open against the buoyancy force of the float. 
   U.S. Pat. No. 2,913,138 to Swick describes floating covers for tanks in which a drainage device is located at a low point of the roof structure in a deck. The drainage device comprises a cylindrical sump, a sump bottom formed with a shallow depression which constitutes a downward flow passage, a cylindrical neck extending from the bottom of the sump and an annular float member loosely surrounding the sidewall of the neck. A mercury seal is provided on the bottom and the float member rests on the bottom of the pool of mercury. Accumulated water in the sump buoys up the float member to interrupt the mercury seal and thereby provide a conduit for water to drain across the surface of the pool. 
   U.S. Pat. No. 3,883,032 to Fisher discloses an automatic drain valve for a floating roof which includes an orifice and a larger disk located under the orifice. A float attached to the disk allows the valve to be biased closed and water gathered on top of the roof will open the valve which permits the water to drain through the roof into the tank beneath to join the body of liquid product with the tank. 
   Japanese Patent No. JP5077883 to Kunio discloses an emergency drain device for a floating roof in which a drain pipe runs through a deck up to stored liquid in a tank and a float, which floats on accumulated liquid on the deck and is on an upper end of the drain pipe, prevents vapor from flowing out of the drain pipe. A weight-attached guide member makes the float return to the upper end of the drain pipe. 
   The above systems for draining water from floating roofs do not overcome the problems of reliability, performance under extreme conditions and secure sealing of the drain opening associated with floating roofs. That is, unfavourable weather conditions, such as high winds and a significant accumulation of rainwater, may cause the roof to sink. Another problem to be considered is evaporation losses of liquid product stored in the tank. 
   It is, therefore, an object of the present invention to provide an apparatus for effectively and reliably draining accumulated rain water from a floating roof on a liquid product tank by a self-opening and closing mechanism for a floating roof drain. 
   Another object of the invention is to provide an apparatus for draining accumulated rain water from the top of a roof on a liquid product tank which reduces product evaporation losses to the atmosphere. 
   A further object of the invention is to provide an automatic roof drain apparatus that is of simple and rugged construction and inexpensive to manufacture and install. 
   SUMMARY OF THE INVENTION 
   The above objects, as well as other advantages described herein, are achieved by a buoyant drain cover assembly for releasably sealing a drain pipe projecting from the surface of the floating roof assembly that comprises a drain pipe sealing cover secured to a plurality of depending buoyant members spaced apart from each other and configured to conform to the exterior of the projecting drain pipe and that permit vertical movement of the assembly relative to the projecting pipe. The assembly moves from a sealed position in which the assembly is supported by the top rim of the drain pipe, to an open position in which the buoyant members floating in water accumulated on the roof lift the cover from the rim of the drain. As the water is drained and the level subsides, the assembly is lowered relative to the drain pipe and the cover contacts and reseals the supporting rim of the drain pipe. 
   In a preferred embodiment, the portion of the underside of the cover resting on the rim of the drain pipe is provided with a sealing surface, which can be a polymeric material, such as PTFE, an engineering grade of nylon, HDPE or other wear and abrasion-resistant material. The seal is configured to engage the drain pipe rim, taking into account that the cover assembly must be sufficiently spaced from the exterior of the projecting pipe to permit free vertical movement as the water level rises and falls. In a further preferred embodiment, the projecting drain pipe terminates in a flanged contact surface, which contact surface can also be provided with a coating or layer of material to form a seal. 
   The buoyant members are preferably formed from a molded polymer foam having an exterior skin that is relatively smooth and resistant to abrasion and tearing. Two or more members are secured to the cover with a corresponding number of vertical spaces or flow paths between them to allow the water to flow into the drain pipe. In a preferred embodiment, two, three or four arcuate members are equidistantly positioned around a circular drain pipe with about a one-half inch or 13 mm clearance between the outer surface of the pipe and the corresponding contour of the buoyant member. Circumferential gaps between the buoyant members are provided to allow water to flow from the roof into the top of the open drain pipe. 
   The bottom surface of the buoyant members can be flat and when in position the closed supported position with the cover sealing the rim of the drain pipe the bottom is positioned at least one-half inch or 13 mm above the roof surface. As will be apparent to one of ordinary skill in the art, the precise dimensions, configuration, materials of construction and positioning of the assembly can be varied to provide the desired combination of a secure seal when the water level drops the cover on the rim of the drain pipe. 
   The improved cover assembly of the invention lifts the cover in order to drain accumulated water from the top of the roof reliably and automatically. The drainpipe and cover are preferably circular and at least two buoyant members, sometimes referred to as “pontoons”, are secured to the underside of the cover at the rim for imparting an upward buoyant or lifting force to the cover when immersed in a sufficient level of water on the deck. The pontoons are spaced apart from each other to allow water to pass between them and into the drain pipe. 
   The drain pipe preferably extends into, and terminates below the surface of the liquid in the tank in order to minimize the amount of vapors that might be displaced from the tank into the environment. 
   The seal is secured to the underside of the drain cover and provides a vapor-tight closure with the drain pipe against passage of vapors from the interior of the tank below. The improved cover of the present invention is configured to sealingly mate with the upper portion or rim of the drain pipe that extends above the top surface of the roof deck. Drain pipes are customarily round, but can be of any shape, and the underside of the cover is provided with a mating seal. For reasons of convenience and economy, a round drain pipe and mating cover and seal are preferred. 
   The underside of the cover is provided with a seal fabricated from a weather and wear-resistant material. The seal can be applied in the shape of a ring, a circular panel or can cover the entire underside of the cover. Other configurations and materials known in the art can be utilized. 
   The cover can be planar or provided with a depending rim, or flange. It is preferably provided with a handle or other projecting element(s) to facilitate lifting it into position on the end of the drainpipe. The cover is preferably fabricated from steel plate or other metal that is of sufficient thickness to assure that its weight will resist displacement, e.g., by wind, once it is installed on the drain pipe. Auxiliary retaining means can be provided to secure the cover against the risk of displacement during unexpectedly high wind conditions and the like. Selection of such safety devices will be apparent to those of ordinary skill in the art and do not form a part of the present invention. The size of the drain pipe and floatation members are selected to assure that water does not accumulate to a level that would permit the cover assembly to float out of alignment with the drain pipe. 
   The cover is secured to a plurality of buoyant members of a size and configuration that permit the cover to rest on, and seal the top of the drain pipe when there is less than a predetermined depth of water on the roof. As will be understood by one of ordinary skill of the art, the height of the end of the drain above the roof deck will determine the maximum depth of water that will be allowed to accumulate on the floating roof. In order to provide a secure seal, the floatation members must be supported above the deck, i.e., they must not be resting on the deck. The buoyant force attributed to the floatation members must be sufficient to raise the cover from the end of the drain pipe when the accumulated water reaches a predetermined depth that is greater than the height of the top of the pipe. When the cover is lifted by the buoyant force, the excess water enters the drain and as the level of water drops, the cover eventually settles downwardly into position to reseal the drain. 
   In a particularly preferred embodiment, the buoyant members are configured so that the surfaces facing the drain pipe generally conform in shape to the exterior contour of the pipe. The buoyant member will thus serve to keep the cover generally centered over the pipe to assure a proper sealing relation when the water has been drained away. The exterior of the drain pipe can be coated with a low-friction industrial paint or polymer to further assure the free relative movement of the assembly. 
   The surface of the floatation material adjacent the drain pipe can be provided with an abrasion-resistant or low friction coating or finish to further assure free movement during the vertical movement of the cover structure. The floatation members can be constructed entirely of a molded foamed polymer, from molded or spun cast polyethylene, or from other materials that will be apparent to those of ordinary skill in the art. The buoyant members can be produced as a unitary element, joined by reinforcing structural members and/or bonded together by adhesive. 
   As noted above it is important to provide floatation members that will support the cover for resealing after excess water is drained from the roof. Gaps between the members must be provided to permit the water to flow freely into the open drain pipe. In preferred embodiments, two, three or four members are secured to the cover, evenly spaced about it periphery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in detail in conjunction with the accompanying drawings in which: 
       FIG. 1  is a cross-sectional view of a floating roof equipped with the drainage apparatus of the present invention and the closed or sealed position; 
       FIG. 2  is a top perspective side view of a circular drain cover with multiple pontoons; 
       FIG. 3  is an exploded view of the drain cover of  FIG. 2 ; 
       FIG. 4  is a perspective view of another embodiment showing four pontoons in spaced relation prior to attachment to the cover; 
       FIG. 5  is a side elevation view of the drain cover of  FIG. 2 ; and 
       FIG. 6  illustrates the drainage of accumulated water from the water on top of the roof of  FIG. 1 . 
   

   To facilitate an understanding of the invention, the same reference numerals have been used, when appropriate, to designate the same or similar elements that are common to the figures. Unless stated otherwise, the features shown and described in the figures are not drawn to scale, but are shown for illustrative purposes only. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a floating roof  10  is supported by, and floats on a liquid product  34  and sealingly engages the sidewall  31  of tank  30  with a resilient sealing structure  11 . The floating roof moves up or down vertically with the level of the liquid product inside the tank. The floating roof  10  has a substantially circular form which conforms to the cross-section of the tank and includes a top deck  12  and a bottom deck  13  joined by an intermediate structure. 
   A drain pipe  20  projects vertically from the top deck of the roof  10  and through the two decks  12 ,  13 , to provide a passage for drainage of accumulated water from the top deck  12 , into the liquid in the tank. 
   Referring to the embodiment shown in  FIGS. 1-4 , a circular drain cover assembly  50  is mounted on the buoyant pontoons  60  around the drain pipe  20 . A flat circular cover plate  51  has a diameter greater than that of the drain pipe  20 . A handle or other fitting  52  is secured to the top of the center of the plate  51  for lifting the cover assembly. 
   With reference to  FIGS. 2-4 , two arcuate pontoons  60  are secured, e.g. by adhesive bonding to the underside of the circular cover plate  51  at its rim. Each pontoon  60  has an inner wall  61 , an outer wall  62 , a top  63 , and a bottom  64  and sidewalls  65  adjoining the inner and the outer walls  61 ,  62 . The size of the pontoon  60  is determined in order to provide the necessary buoyancy, or force, to lift up the circular plate  51 . In a preferred embodiment, at least two pontoons  60  are attached to the plate  51  and a gap of predetermined distance is maintained between the pontoons to provide a water passage. The distance is determined based on the required flow rate of accumulated water on the top deck  12 . The greater the distance, the faster the flow rate. The number of pontoons  60  can vary as long as the spaces between them are adequate to permit the desired flow rate of accumulated water on the top deck  11  into the drain pipe  20 . 
   Referring again to  FIG. 1 , a gap is maintained between the bottom  64  of the pontoons and the top deck  12  in order to ensure that the drain cover will securely seal the drainpipe  20  and provide good vapor sealing. The gap is preferably 0.5 inch. A gap is also maintained between the drain pipe  20  and the inner cylindrical wall  61  of the pontoon to ensure free vertical movement of the drain cover assembly  50 . The gap is preferably 0.5 inch. 
   Referring to  FIG. 5 , the drain pipe  20  supports plate  51  and a seal  70  attached to the underside of the plate  51 . The seal  70  serves to substantially prevent fluid losses from the tank and drain pipe. 
   Referring now to  FIG. 6 , when the accumulated rain water on the top deck  12  of the roof reaches a predetermined level, the buoyant force of the pontoons  60  lifts drain cover assembly  50  and the accumulated rain water on the top deck  12  flows into the drain pipe  20  through the space between the pontoons  60  and the level of rain water on top deck  12  is thereby maintained below predetermined maximum level “X”. Thus, an automatic control of the water level on the roof is provided by the buoyancy of the pontoons and the roof is prevented from sinking into the stored liquid product. 
   Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail above, those of ordinary skill in the art can readily devise other and varied embodiments, and the scope of the invention is to be determined by the claims that follow.