Patent Application
20120152950
This invention is in the field of floating roofs for storage tanks that contain volatile fluid, and particularly for an emergency drain valve for water accumulated atop a double deck roof.
Storage tanks for oil and other liquid petrochemical products are typically provided with floating roofs that float atop the liquid product and seal the upper portions of the tanks to prevent the escape of harmful vapors off the surface of the contained liquid. Additional seals are provided between the outer periphery of each floating roof and the inner wall of the tank.
Such floating roofs are usually circular in shape, light weight and buoyant, typically may have a diameter of fifty feet or more, and are relatively thin and relatively weak.
A serious potential problem or danger with such floating roofs is that under rainstorm conditions water may accumulate on top of the roof and cause it to sink or break. Under these circumstances vapor from the liquid below the roof could escape into the atmosphere or into the space in the tank but above the roof, and produce undesirable and/or dangerous conditions.
In the prior art there are numerous drainage systems for such accumulated water atop a floating roof, and also emergency drainage systems which are intended to drain automatically whenever the accumulated water reaches a pre-determined dangerous level.
Disadvantages of prior art drainage systems include unreliable valves which stick in a closed or open position, and valves which are expensive or difficult to maintain, repair or install. With such prior art valves the sealing element may not reliably respond to a flow of accumulated water on the roof, or is too slow in reaction time to open the drain.
Prior art patent U.S. Pat. No. 5,758,792 discloses, for example, in FIG. 30 a floating ball check valve 220 on a floating roof where the plane of the closure is at the top surface of the roofs top deck sheeting 24. With this arrangement the ball 236 will not rise and open the valve until a substantial amount of water has accumulated upon the roof. During drainage, another feature of this device is that the ball will float down and close while a certain quantity of water still remains on the roof. Thus, this valve's main purpose is to block upward flow of vapors after the main drainage of water has occurred; it does not address the problem of massive accumulation of water before the valve opens or the problem of water remaining on the roof after the valve has closed.
The new invention is a floating plug emergency drain system for a floating roof, which typically has a double deck structure of top and bottom spaced apart surfaces with an aperture extending vertically through the roof to provide a drainage duct for water accumulated on the roof. This emergency drain system includes a valve element with a bottom part formed as a plug portion having a truncated conical shape that will drop into and seal with a mating corresponding conical shape valve seat, and an upper part which is a buoyant ball-like element. Extending further upward from the top of the ball-like element is guide pole generally coaxial with the central axis of the ball-like element and the truncated conical plug portion. This guide poll extends upward into a guide member which typically is a sleeve surrounding said guide poll, said guide element assuring that the guide poll will move accurately, vertically and keep the valve element's plug part properly aligned with the valve seat so that sealing is achieved quickly and reliably. The valve element has an overall weight which assures its descent to the valve seat at all times that water is not surrounding the buoyant ball-like part; the weight is small enough so that this part will float upward and unseat the plug part whenever a small amount of water flows into the sump area in the top of the roof.
In a preferred embodiment of this invention the ball element is positioned in the drain duct or drain shaft at an elevation lower than the top of the floating roof deck. Consequently, water beginning to accumulate on the top of the roof will flow into said drain duct and immediately cause the ball to float upward, opening the valve. Thus, draining will begin very quickly and before any significant quantity of water has accumulated on the roof. Obviously, the valve will remain open as long as water continues to flow downward from the roof.
In the preferred embodiment illustrated herein the drain duct extending through the roof has vertical length similar to the height of the ball element; however, the significance, as mentioned above, is that the elevation of the closure and valve seat is lower than the top surface of the floating roof where water begins to accumulate, and in this embodiment the valve seat is at the bottom of a sump in the roofs upper surface. Alternatively, said closure might be closer or farther from, but still below the top surface.
The drain duct extending from the top of the floating roof to the bottom may be a duct provided in the roof's structure or may be a cylindrical tube positioned to pass through the roof. In either case, such duct is a chamber in which said sealing element moves and through which accumulated water flows. The top and bottom ends of the duct are open obviously to allow accumulated water to flow into, through and out the bottom thereof. The top of the open duct is protected from debris by a screen element that generally overlies the entire open end. Such a screen may be formed generally as an upward extending hemisphere or other curved surface or other shape that may extend upward to accommodate the guide pole or may have an aperture through which the guide pole extends.
Finally, the guide element is situated above the top surface of the roof and obviously directly above the guide pole of the sealing element. This guide element is supported by and secured to the roof, preferably by support legs extending upward from the roof's top surface. Such support legs may conveniently be combined with the screen to support both the guide element and the screen, or to at least cooperate with the screen. The screen is preferably a non-rusting mesh which allows accumulated water to freely flow through it, but blocks debris which could interfere with the sealing surfaces of the plug and valve seat, or which would be undesirable if it flowed into the stored liquid below.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.
For convenience and clarity in describing these embodiments, similar elements or components appearing in different figures will have the same reference numbers.
In operation emergency drain assembly 15 is designed to allow flow of accumulated water when it occurs, but to otherwise close and seal as a valve, and thus prevent upward flow and escape of vapors from liquid 12 stored in tank 10. Thus, duct 14 has at its lower part a valve seat structure 23 having conical walls dimensioned to accommodate and seal with the outer surface conical walls of plug 17 when plug 17 has descended into and in contact with valve seat 23.
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In the above described preferred embodiment the valve materials are plastic for the ball, vulcanized rubber for the plug, plastic for the guide pole, plastic for the guide sleeve, stainless steel for the screen, and vulcanized rubber for the valve seat. In the embodiment of
While the invention has been described in conjunction with several embodiments, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the appended claims.
