GB2148374A

GB2148374A - Liquid-storage tank with floating roof

Info

Publication number: GB2148374A
Application number: GB08327996A
Authority: GB
Inventors: Andre Marquet
Original assignee: BSL Bignier Schmidt Laurent SA
Current assignee: BSL Bignier Schmidt Laurent SA
Priority date: 1983-10-19
Filing date: 1983-10-19
Publication date: 1985-05-30
Also published as: GB8327996D0

Abstract

A known tank shown on the right has a cylindrical wall 1 and a floating roof 3 (shown in upper and lower positions) whose edge is coupled to the wall 1 by a diaphragm 6 which retains a counter-pressure liquid 12. In the new tank shown on the left the roof is a spherical dome 3A with an upstanding cylindrical wall 3B and sufficient counter-pressure liquid is used to come above the top of the dome (up to overflow 18 in the upper position). The surface of the counter-pressure liquid is thus of constant area and can be covered with a protective layer. At the junction of the dome 3A with the diaphragm provision is made for release of air bubbles formed inside the tank and these rise to the top of the tank to escape through a valve. An emergency pressure relief valve 15 is located above the counter-pressure liquid 12. <IMAGE>

Description

SPECIFICATION Liquid storage tank with floating roof The present invention concerns tanks of floating roof type.

Such tanks are used in particular in nuclear power stations for storing certain substances and in particular demineralised waters, in such a way as to exclude contact with air. The liquid contained in the tank is permanently in direct contact with the roof and the roof moves with the level of liquid in the tank.

An air-tight condition is provided by the roof on the one hand and by a flexible diaphragm which air-tightly connects the periphery of the roof and the side wall of the tank. In order to prevent the flexible diaphragm forming folds or being pressed against the wall of the tank by the pressure of liquid therein, it is known for the space between the diaphragm and the facing wall of the tank to be filled with a counter-pressure liquid.

In the known constructions, the structure of the floating roof is relatively complicated, for various reasons, the roof generally being of a double-wall construction and also including stiffening members.

The manufacture of such a roof requires many assembly operations which result in the production cost being high.

In addition, the free surface of the counter-pressure liquid varies substantially, by virtue of the geometry of the roof, when the roof is displaced in the heightwise direction of the tank, and that may be a disadvantage when the free surface of the counter pressure liquid is to be covered in order to reduce evaporation of that liquid.

It is also known for the air bubbles which finally arrive under the junction between the diaphragm and the tank to be removed by one or more blowoff valves which are fixed to the wall of the tank.

However, the known roofs suffer from the disadvantage of requiring arrangements, which are difficult of access, for removing any air bubbles which finally arrive just below the junction between the flexible diaphragm and the roof.

A tank of floating roof type, in accordance with the present invention, is essentially characterised in that the part of the roof which is in contact with the liquid in the tank is a wall in the general form of a spherical dome, the convexity of the dome being directed upwardly.

By virtue of its shape, that structure is strong and there is no need for it to be supplemented by stiffening members.

In accordance with another feature of the invention, it is proposed that the roof be provided with a raised portion preferably formed by a wall of revolution about a vertical axis, which is fixed on the dome, the axis of the raised portion coinciding with that of the tank.

Those features and other features and advantages of the invention will be described hereinafter with reference to the Figures of the accompanying drawing, which relate to a typical construction of a tank according to the invention, the drawing being limited to the parts of the tank which are relevant for comprehension of the invention: Figure 1 is a vertical half-section of a tank according to the invention, in the region of the floating roof thereof, Figure 2 is a detail view of the manner of fixing the diaphragm to the floating roof, and Figure 3 shows in vertical half-section a tank in accordance with the invention on the left and a known tank on the right, in each case with the roof shown both in its uppermost and in its lowermost position.

The tank is for example a tank for storing water which is used in a nuclear power station, and essentially comprises a cylindrical wall 1 made of steel, a bottom 2 and a floating roof 3 (see Figures 1 and 3).

The floating roof 3 essentially comprises a wall 3A in the form of a spherical cap or dome which is in contact with the liquid in the tank, the convexity of the dome being directed upwardly and the dome being surmounted by a cylindrical wall 3B, the upper edge of which is possibly stiffened by a ring member 4.

The dome 3A is bordered by a ring 5 which serves for fixing one edge of a peripheral diaphragm 6, the other edge of which is fixed to the cylindrical wall 1 of the tank at a selected location, and, preferably, at a region 7 which is disposed substantially halfway up the useable part of the tank.

Figure 2 is a view on an enlarged scale showing the manner of fixing the diaphragm 6 to the ring 5 which is an integral part of the wall 3A. It will be seen from Figure 2 that the diaphragm 6 is fixed to the ring 5 in such a way that an air passage 8 in the ring 5 permits an air bubble which might occur in the space 9 just below the top of the diaphragm 6 to escape towards the spherical dome 3A, as shown by the arrow 10.

The upper edge of the diaphragm 6 is therefore fixed to the ring 5 substantially at the same level as the wall 3A is fixed or below that level, and one or more air passages 8 is or are disposed just below the location at which the diaphragm 6 is fixed to the ring 5. The diameter of the raised cylindrical portion 3B and its height are such that it is able to contain a sufficient quantity of counter-pressure liquid when the roof is in its lowermost position to ensure that the surface of the counter-pressure liquid is above the level of the top of the domed roof when the roof is in its uppermost position. In this way the level of the counter-pressure liquid is always above the level of the liquid in the tank. A valve 11 is disposed in the central region of the dome forth release of air bubbles which may rise under the dome.Discharge valves of that kind are already used in the known floating roofs, and the invention does not relate to a particular valve structure.

In per se known fashion, counter-pressure liquid 12 is provided around the diaphragm 6 and the raised portion 3B. When the roof moves (see the left-hand part of Figure 3), the counter pressure liq uid 12 is always in contact with the cylindrical portion 3B so that its surface area is constant. Therefore, it is possible for that surface to be covered with floating panels or balls 13 in order to reduce evaporation thereof.

By way of indication, the right-hand part of Figure 3 shows a tank with a conventional floating roof, and it will be seen that the free surface of the counter-pressure liquid varies substantially when the roof is displaced, so that it is dlfficult to conceive covering the liquid with elements intended to prevent evaporation thereof.

Tanks of floating roof type are generally provided with a safety means to permit liquid contained within the tank to be discharged by way of the roof, in the event of an increase in pressure. In the present structure such discharge means terminate above the counter-pressure liquid, which can easily be achieved by using suitable conduits. By way of example, the left-hand part of Figure 3 shows a conduit 14 which connects the roof to a discharge valve 15 for the internal liquid 17, that being disposed above the counter-pressure liquid 12.

The side wall of the tank is provided with an overflow 18 disposed at a level lower than the level attained by the raised portion on the floating roof when the roof is in its position of maximum upward movement.

Claims

1. A tank for liquid, of floating roof type, the roof being sealingly connected to the wall of the tank which surrounds it by way of a flexible diaphragm which, with the wall of the tank and the roof, defines a space for receiving a counter-pressure liquid, characterised in that the roof is in contact with the liquid in the tank by way of a wall in the general form of a spherical dome, with its convexity direction upwardly.

2. A tank of floating roof type according to claim 1 characterised in that said spherical dome is surmounted by a raised portion which limits the space available for the counter pressure liquid and which ensures that the level of the counter pressure liquid is above the top of the spherical dome.

3. A tank of floating roof type according to claims 1 and 2 characterised in that said roof essentially comprises a cover wall in the form of a spherical dome and a cylindrical raised portion.

4. A tank of floating roof type according to claim 2 or claim 3 characterised in that said raised portion is a cylindrical wall having a vertical axis.

5. A tank of floating roof type according to claim 4 characterised in that the diameter and height of the raised portion are such that the level of the counter-pressure liquid is above the top of the spherical dome.

6. A tank of floating roof type according to one of the claims 1 to 5 characterised in that said spherical dome is bordered by an annular wall to which said diaphragm is fixed.

7. A tank of floating roof type according to claim 6 characterised in that, below the junction between said annular wall and the spherical dome and below the level at which the diaphragm is fixed to said annular wall, said annular wall comprises at least one passage in order for any air which may be trapped under the top of the diaphragm to be able to pass below the spherical dome.

8. A tank of floating roof type according to one of claims 1 to 7 and which comprises a valve for discharge of the internal liquid, in the event of an increased pressure therein, character ised in that means are provided for discharge of said valve to be above the counter-pressure liquid.

9. A tank of floating roof type according to one of claims 1 to 8 characterised in that the side wall of the tank is provided with an overflow disposed at a level lower than the upper level of the raised portion of the floating roof when the roof is at its maximum height.

10. Use of a tank of floating roof type according to one of claims 1 to 9 as a storage tank in a nuclear power station.

11. A tank for containing liquid having a floating roof which is connected in a sealed manner to the wall of the tank by way of a flexible diaphragm which, together with the wall of the tank and the roof, defines an external space for receiving a counter-pressure liquid characterized in that the roof is domed and surmounted by a raised central portion which inwardly limits the space available for the counter-pressure liquid and is of such a height that the counter-pressure liquid can be above the top of the dome in the uppermost position of the roof without overflowing the central portion in the lowermost position of the roof.