Abstract:
A polyethylene floating roof design equipped with height adjustable bases. Allowing full discharge of the tank and having people enter the tank and the space below the roof in case of adjusting the bases in maintenance mode for dredging and repairs.

Description:
BACKGROUND 
       [0001]    Due to high heat value and ease of transportation, oil products play a major role in the global energy cycle. On the other hand, due to the high flammability and blast ability materials, maintenance and using them is seriously dangerous and requires meeting critical principles and standards. Accordingly, the equipment used in the oil industry are highly expensive, which are designed with much precision. Since the discovery of oil and over the last hundred years with the increasing growth of science and technology, the oil facilities and equipment have been constantly modernized; the old equipment are replaced with new equipment benefiting from higher efficiency and more accurate performance. With introduction of polyethylene in the industry, having unique characteristics including high strength and low weight, these materials could replace metals in manufacturing equipment in many cases. The use of these materials in the petroleum industry is increasingly expanding, and in some cases, they have been used to build the underground pipelines or small tank. 
       SUMMARY 
       [0002]    Oil storage tanks are one of the most important and most expensive equipment in the oil depots, refineries and other oil industries. These tanks are divided into two categories of fixed and floating roofs in terms of evaporation rate of their content. In floating roof tanks, to prevent evaporation, the tank roof is always floating on the surface of the oil product and moves up and down with the level of the product within the tank. 
         [0003]    The body of these tanks are made of metal plates up to 4 cm in thickness due to tolerating too much pressure. The floating roof of the tank is presently made from the material of the tank body. Since it does not need high strength and its lower weight would increase the volume of the storage tank, the roof can be built of polyethylene. 
         [0004]    Building the roof by this method, meanwhile significantly reducing the costs of construction and maintenance will reduce the risks for tank in cases such as earthquake, since this roof does not damage the tank body. Also, considering the adjustable bases for reparations and operational purposes, all operational process required for the tank by this construction method could be done. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0005]      FIG. 1 , displays a side view of tank with a polyethylene floating roof 
           [0006]      FIG. 2 , a more accurate view of the roof and the placement of polyethylene pipes and sheets and their adjustable bases are shown. 
           [0007]      FIG. 3  shows how to connect pipes to the polyethylene sheets. 
           [0008]      FIG. 4 , the roof top view, in the case were the polyethylene sheets are removed and with their pipes and junctions can be seen. 
           [0009]      FIG. 5 , the details of installation and functioning of the operational- reparations bases are displayed. 
           [0010]      FIG. 6  shows how to connect the roof to its sealed tube around it. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The roof ( FIG. 2 ) comprises of large number of polyethylene pipes ( 3 ) that are connected together in parallel and at a distance of one meter from each other with pipe clamps ( 4 ). These pipes ( 3 ) are placed floating on an oil product ( 5 ). Two sides of the pipes ( 3 ) are closed and filled in order to avoid the entry of the oil product. The pipes ( 3 ) are covered up with polyethylene sheets ( 1 ) with a thickness of at least  1  cm and  120  cm in width. As can be seen in  FIG. 1 , an escalator ( 21 ) is embedded from the top of the tank ( 20 ) to the roof in order to visit the floating roof and change the position of reparations and operational bases ( 2 ). 
         [0012]    To drain rainwater, a flexible tube ( 22 ) made of Polyethylene, 6 inches in diameter, is directed from the middle of the roof toward out of the tank body ( 20 ) at the lowest point. As seen in  FIG. 3 , these sheets ( 1 ) are tightened to the pipes ( 3 ) by plastic clamps ( 6 ). The polyethylene straps ( 7 ), 20 cm in width, are used to connect the sheets ( 1 ) to each other with bolts and rivets ( 8 ). 
         [0013]    As seen in  FIG. 4 , the pipes diameter ( 3 ) decreases from 12 inches ( 11 ) to 10 inches ( 10 ) and 8 inches ( 9 ) to create appropriate 1% (at least) slope on the floating roof while approaching more to the center of the roof (the diameters may change as needed however in the preferred embodiment the start point is 12 inches). These pipes ( 3 ) are connected in parallel to each other by pipe clamps ( 4 ) at intervals of one meter. In order to prevent the infiltration of rain water into the tank, the surroundings of the roof are fully closed as existing metal roofs with tube sealing material with a width of 20 cm ( FIG. 6 ). 
         [0014]    In order to attach the tube seal, another layer of polyethylene sheet ( 1 ) is connected to the existing sheet ( 1 ) by plastic rivets ( 8 ) at the junction site to reinforce the strength of the roof. Finally, both polyethylene sheets are screwed to the upper and lower points of the rubber ( 19 ) on the tube seal with rivets. This connection is done at 50 cm intervals around the roof, and thus, the tube seal ( 18 ) is connected rigidly to the roof sheets. Inside the tube seal ( 18 ) is filled with kerosene ( 17 ). To avoid the full coming down of the roof (because the input and output valves of the tank are placed at the lowest point on the tank, also in case of full lowering of the roof, rainwater drain pipe will be damaged), some bases ( 2 ) are provided in different parts of the roof ( FIG. 4 ). The bases ( 2 ) are adjustable at two heights of 1 m and 2 m, respectively for operating mode and reparations mode. 
         [0015]    These bases ( 2 ) are connected to the roof as follows: Two layers of polyethylene sheets ( 1 ) with dimensions of 50×50 cm are connected above and below the main sheet with plastic rivets ( 3 ) at the junction point to reinforce the roof strength. A hole 2.5 inches in diameter is opened in the middle of them, through which a 2.5-inch polyethylene tube ( 12 ) with a length of 50 cm is passed. To tighten this tube ( 12 ) to the roof, both sides of the tube ( 12 ) at the top and bottom of the roof are connected with the tube clamps ( 15 ). Above the roof, the sidewall of the tube is pierced to pass the holder pin ( 13 ) of the bases. 
         [0016]    Through these tubes ( 12 ), the 2 inches rods ( 5 ) (reparations and operational bases) are passed. These rods are also pierced at two distances of 1 m (operating) ( 14 ) and 2 meters (reparations) from the bottom. During normal operation of the roof, where the bases should be in operational mode, the holder pin ( 13 ) is passed into the hole with a height of 1 m. In this case, when the oil product tank becomes less than 1 m in height, the roof is placed on these bases ( 2 ) and does not come down to the bottom. 
         [0017]    When there is a need to do dredging and reparations inside the tank, the pin ( 13 ) is passed into the hole with a height of 2 meters. In this case, the roof will be placed at a height of 2 m on its bases ( 2 ), and people and equipment can easily enter the tank. In addition, to prevent entering water rain into the tank, the top of the holder sheath ( 12 ) of the bases ( 2 ) is covered by a plastic shroud layer ( 16 ). All polyethylene parts of the roof that are exposed to the product and its vapors are covered with polyamide coating. 
         [0018]    The advantages of this design are as follows:
       The capability of lodgment the roof on adjustable bases in both operational and reparations modes, which makes it easy to perform repair and dredging inside the tank and under the roof. Also, in case of complete discharge of the oil product, the roof does not come down to the bottom of the tank floor, and no damage would occur to the rainwater drainage hose and internal connections of inlet and outlet valves of the tank.   Very low weight of the roof, which increases the storage volume of the tank.   Significant reduction in manufacturing costs due to price differences of polyethylene materials compared to metal materials of available roofs   enhancement of tank safety, especially in case of earthquakes (At the time of the earthquake, metal roof collide the tank shell and cause ruptures)   Avoiding the possibility of metal corrosion and rusting of floating roof   Reduced cost and time required to repair the roof(due to the ease of manufacturing and low costs of the parts)   Physical strength necessary for roof functioning range