US2981437A - Floating roof tank - Google Patents

Description

April 25, 1961 l. 1.. WISSMILLER 2,981,437

FLOATING ROOF TANK Filed Feb. 24, 1960 2 Sheets-Sheet 1 JZYT/(BWT: jpmzJMaawm/k'r,

April 1961 I. L. WISSMILLER 2,981,437

FLOATING ROOF TANK Filed Feb. 24, 1960 2 Sheets-Sheet 2 United States Patent FLOATING ROOF TANK Ivan L. Wissmiller, Chicago, IlL, assignor to Chicago Bridge & Iron Company, Chicago, Ill., a corporation of Illinois Filed Feb. 24, 1960, Ser. No. 10,605

4 Claims. (Cl. 220-26) This invention relates to an improved type of seal for a floating roof tank for the storage of volatile products.

For many years, one of the most eflicient storage containers for volatile liquid products has been the flat bottomed cylindrical walled floating roof type of tank that is well known in the art. In such a tank there is generally provided a circular roof having a diameter somewhat less than the diameter of the cylindrical tank which is adapted to float upon the surface of the liquid stored in the tank. A suitable sealing device of annular shape is arranged about the periphery of the floating roof within the shell of the tank for the purpose of sealing the annular space defined by the rim of the floating roof and the inner surface of the tank shell against the escape of volatile fractions of the stored product. vIt is not economically feasible to construct the floating roof of exactly the same diameter as the inner surface of the cylindrical shell so as to fit together as a piston fits in an automobile cylinder because in a tank of the size under consideration the cylindrical shell is of relatively thin plate material which is subject to local distortion caused by such factors as thermal shrinkage of welded joints, wind forces, uneven foundation settlement and fabrication and erection tolerances. For this reason the diameterof the floating roof may be from twelve to twentyfour inches less than the diameter of the tank shell, and in such cases the annular space between the rim of the roof and the shell presents a significant source of evaporation loss if not properly sealed.

According to the practices well known in the art, sealing devices for the annular space between the floating roof rim and the tank shell have been of the mechanical or liquid tube type. In the mechanical type, relatively thin flexible metal shoes are placed in sliding contact with thetank shell, supported by various mechanical linkages from the tank roof, with a curtain of seal material extending from the rim of the tank roof to the metallic shoes. This type of sealing device has certain inherent disadvantages, among them being corrosion and abrasion of metallic parts, distortion of the flexible metal shoes causing imperfect contact with the tank shell with resulting high evaporation loss of the stored volatile product, and sparking resulting from electrical charges arcing across various metallic components thus increasing the hazard from fires if not properly protected.

On the other hand, the liquid filled tube type of sealing device known to the art, which in its basic form comprises a rubberized fabric seal material extending across between the roof and the tank shell and distended by means of a liquid filled annular tube, also presents certain inherently undesirable characteristics. For example, the pressure exerted by an adequate depth of liquid may 'cause excessive abrasion of the fabric material in sliding contact with the tank shell. Furthermore, it frequently happens that the weight of the liquid causes the seal to sag rather than distend outward against the tank shell.

In my copending application Serial No. 855,788, filed November 27, 1959, there is disclosed a gas inflated rubberized fabric type sealing system for a floating roof tank that is not subject to the difficulties outlined above with respect to conventional types of sealing devices. There is provided a constant gas pressure within the seal irrespective of variations in temperature of the tank or its surrounding atmosphere and also irrespective of a moderate loss of gas by diffusion through the fabric and by leakage. This sealing system requires a constant-pressure gas reservoir for maintaining a uniform gas pressure within the seal and a supply of gas for the constant-pressure reservoir to replace that loss from the system.

In accordance with the instant invention there is provided automatic means for supplying the gas required in the above-described sealing system. The invention utilizes the daily atmospheric temperature variations to supply air under suflicient pressure to satisfy the normal requirements of the constant-pressure gas reservoir. Briefly, the invention comprises a constant volume chamber or container provided with inlet and outlet means each of which is equipped with a suitable check valve. The inlet valve permits gas flow into the container and the outlet valve permits gas flow out of the container while neither valve permits gas flow in the direction opposite to the normal direction of flow. The output of the thermal pump is supplied to the constant-pressure reservoir used in the aforementioned gas-inflated seal.

The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawings, in which:

Figure 1 is a vertical cross-sectional view of a flat bottomed cylindrical liquid storage tank with a pontoon type floating roof and a sealing device of the type with which the invention is advantageously used;

Figure 2 is a similar view, on an enlarged scale, of a portion of the tank shell, the pontoon portion of the roof and the related sealing device with the pressure control means positioned in its normal operating position;

Figure 3 is a plan view of a portion of a floating roof showing the pontoon and the thermal pump system of the invention; and

Figure 4 is a sectional view taken along the line 44 of Figure 3. 4 1

In Figures 1 and 2 there is shown a cylindrical tank having a flat bottom 10 and vertical cylindrical sidewalls 11, within which is positioned a floating roof 12 supported by the stored liquid. In these figures the floating roof is of the pontoon type, having a central single deck 13a and a plurality of pontoons 13 arranged annularly about the single deck so as to provide buoyancy for the roof even when great amounts of rain water may have accumulated thereon.

As shown more clearly in Figure 2, an annular tube 14 completely encircles the rim 15 of the floating roof and, when inflated, bears against the interior surface of the tank shell 11. A connection 16 to the tube 14 is provided as a source for supplying gas under pressure to the tube.

As shown in Figure 2, a gasholder 17 is located inside one of the pontoons 13 and is connected by a suitable conduit 18 to the connection 16 supplying gas to the tube 14. This gasholder constitutes a reservoir for the storage of a supply of gas under suitable pressure to accommodate the change in volume of the gas resulting from thermal contraction and expansion. The gasholder 17 consists essentially of a sump 17a and a cover 19, said sump and cover being sealed to each other by a flexible diaphragm 20. The diaphragm 20 permits the cover to rise and fall within rather wide limits to accommodate varying'volumes of gas stored in the gasholder 17. The weight of the cover 19 is selected so as to place the gas stored within the gasholder at the optimum pressure desired for maintaining the tube 14 in proper sliding relationship with the tank shell 11.

A suitable inlet valve 21 permits replacement of gas within the gasholder as needed. Valve 21 is preferably equipped with actuating means for opening when the roof 19 is near its lowest operating position. Also provided is a venting valve 22 connected by means of a suitable conduit 23 to the gasholder 17 and to connection 16. Venting valve 22 is preferably supplied with automatic mechanical means 22a for opening when the gasholder cover nears its fully inflated position.

The automatic mechanical means for actuating the gas supply valve 21 and the venting valve 22 may be quite simple, consisting merely of vertical rods 21a and 22a extending from the operating mechanism of the valve downwardly to a selected point of contact with cover 19 or the bottom of pontoon 13.

The cover 19 of the gasholder 17 shown in Figure 2 is in a position within the range of normal operation, at

which time there is a suflicient supply of gas within the gasholder to maintain the desirable pressure for most efiective operation. In such position, both the supply valve 21 and the venting valve 22 are closed.

In the condition shown in Figure 2 the gasholder maintains the proper operating pressure within the tube 14 despite tendencies for the pressure to increase or decrease because of breathing caused by such factors as daily temperature variations, changes in climatic conditions, stretching or other dimensional changes in the tube, moderate loss of gas from diflusion or leakage, and the like. The capacity of the gasholder should be at least equal to the difference between the maximum and minimum volumes occupied by the gas contained within the system.

The synthetic rubber impregnated fabric tube 14 is selected of such quality and dimensions that the desirable inflating pressure range for the tube which will permit maintaining the tube at all times in sliding contact with the shell 11 without locking the roof into place and inhibiting sliding, is 1 to 3 inches of water. A gasholder cover is selected having a total weight (including appurtenances) that creates this level of pressure in the gasholder. A sufficient supply of gas is then let into the gasholder to inflate the tube to the desired pressure, which may be of the order of approximately 2 inches of Water, and to provide a reserve supply within the gasholder at a time during the day when the average temperature of the system is, for example, 70 F. It may be expected that, after the sun has set and nocturnal cooling to about 50 F. has taken place, the volume of gas within the system at the established pressure is reduced by a substantial amount, thus permitting the cover to descend towards its low operating position. No additional gas, however, need be supplied to the system to maintain its pressure unless the volume is decreased to such an extent that the cover 19 descends to its lowest position and actuates supply valve 21. As the next day progresses, the temperature may increase until in the early afternoon the average temperature within the system may be as high as 100 F., at which time the gas at the selected pressure will have expanded so as to raise the cover 19 to a much higher level without increasing the pressure. If this expansion raises the cover to its highest operating position, then the venting valve 22 will automatically be actuated to vent a sufficient quantity of gas to prevent the pressure from exceeding the selected level.

It is obvious from the foregoing that the dimensions of the gasholder should be selected so as to provide a reserve capacity of gas sufiicient to accommodate the maximum volume change of the gas at the selected pressure which may be expected to result from normal temperature variations. In a typical example of a floating roof tank having a diameter of 120 feet designed for use in a temperate zone Where daily temperature variations may be as much as 50 F., the amount of gas reserve to be provided in the gasholder is approximately 31 cubic feet. Floating roof tanks having a smaller diameter or designed for use in other areas where the temperature variation is not so extreme will require a commensurately smaller gas reserve, whereas larger tanks or larger temperature variations will necessitate a larger gas reserve.

The thermal gas pump system shown in detail in Figures 3 and 4 is used to provide the supply of gas needed for replacing the losses from the sealing system on a routine basis. When this gas pump system is used, gas from valve 21 need be supplied only on an emergency basis so that under suitable conditions it may be found possible to eliminate valve 21.

As seen in Figures 3 and 4, the thermal gas pump system comprises a suitable closed container 31, formed in this case by bulkheads 32 and 33 and the top 34 and bottom 36 surfaces of pontoon 13. Container 31 is equipped with an inlet check valve 37 and an outlet check valve 38 for permitting the flow of gas into and out of container 31 respectively. The check valves shown consist of a housing 39 and 41 containing a supply of a liquid 42 and 43, suitably water. Suitable control systems, not shown, may be used to maintain a constant liquid level. Inlet conduits 44 and 46 extend a short distance below the surface of the liquid in the valve housings, permitting the flow of air through each valve in one direction only as will be apparent to those skilled in the art. Inlet conduit 44 of valve 39 is open to the atmosphere; outlet conduit 47 leads from the valve to container 31. Inlet conduit 46 of valve 41 communicates with container 31; outlet conduit 48 of this valve leads to gasholder 17.

The operation of the thermal gas pump system is as follows. Assume as a starting condition a time during the day when the ambient temperature is at its daily maximum. As the day progresses and the temperature begins to fall, the air trapped in container 31 contracts, causing the pressure in the container to fall below atmospheric. As a result of the pressure differential, air from the atmosphere passes through inlet valve 39 into container 31 to re-establish a pressure equilibrium. This process con- .tinues until a minimum temperature is reached at some time in the evening, after which the temperature begins to rise. The rising temperature causes the air trapped in container 31 to expand, thus increasing the pressure.

Provided that the pressure within container 31 exceeds the pressure in gasholder 17 plus the hydrostatic pressure exerted by the liquid in valve 41, air will pass from container 31 through valve 41 to fill gasholder 17 with a reserve supply of air. In this manner the daily atmospheric temperature variation causes air to be pumped into the tank sealing system to replace losses.

The gas which may be introduced through inlet valve 21, if and when needed, may come from any one of a number of convenient sources, including a replaceable pressure vessel, more commonly called a gas bottle; a pressure vessel commonly called an air receiver used in combination with a compressor to maintain the gas pressure; a vessel containing a liquefied gas such as propane; an electrically powered compressor operated by means of limit switches arranged to operate when the gasholder roof nears its extreme bottom position; or a compressor powered by the vertical roof movement caused by filling and emptying the tank.

The gasholder shown in the figures is located within a pontoon of a pontoon type floating roof. Although this 1s a preferred embodiment, it is obvious that the gasholder can be positioned elsewhere on the floating roof, or used in conjunction with a floating roof other than the pontoon type, all without departing from the scope of the present invention. Likewise, although a section of pontoon has been used in the thermal gas pumping system, it will be obvious that any other closed container of sufiicient volume can also be used.

fhe utilization of a gasholder and the thermal gas pump system of this invention permits trouble-free and maintenance-free operation of a floating roof having a gas inflated seal. Such an arrangement, being almost entirely self-operating and automatic, does not require the constant attention of maintenance personnel that would otherwise be required. When a gasholder is used, the only maintenance required is the periodic checking of the working parts and replenishment of the emergency gas supply. This maintenance-free feature of the roof is of particular importance at bulk terminals and marketing terminals, where maintenance and repair personnel are not ordinarily employed.

The foregoing, detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In a cylindrical tank for the storage of a volatile liquid, a floating roof having a diameter less than the internal diameter of said tank and adapted to fioat on said liquid, an annular impervious flexible tube disposed between said floating roof and the interior surface of said tank, inflating means for inflating said tube by gas pressure, pressure-regulating means for maintaining in said tube a gas pressure within a predetermined range comprising a constant-pressure reservoir of variable volume in gaseous communication with said tube, said reservoir having inlet means for supplying gas thereto and outlet means for releasing gas therefrom, and a thermal pump system for supplying gas to said reservoir comp-rising a constant volume container having an inlet and an outlet, a first check valve permitting ingress from the atmosphere into said container through said inlet, a second check valve permitting egress from said container through said outlet, and a conduit communicating from said second check valve to said reservoir.

2. The device of claim 1 in which said reservoir com prises a bottom, sidewalls sealed to said bottom, and a cover flexibly sealed to said sidewalls, said cover being adapted to rise and fall in relation to said bottom to permit changes in volume of the gas contained in said reservoir without substantial change in the pressure thereof.

3. The device of claim 1 in which said floating roof comprises a pontoon which provides buoyancy for said roof, and said constant volume container comprises a portion of said pontoon.

4. The device of claim 1 in which said check valves are liquid seal valves comprising a closed container, a quantity of liquid enclosed within said container, a vertically disposed dip-leg inlet conduit extending below the surface of said liquid, and an outlet, conduit leading from said container.

No references cited.

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