EP0178210B1 - Liquid tank with a floating roof, such as a storage tank used in the electronuclear field - Google Patents

Description

The invention relates to floating roof tanks for liquids.

Such tanks are used in particular as storage tanks in the electro-nuclear field for liquids such as degassed demineralized water.

In this field, high reliability is required for the design of the tanks, in order to eliminate any risk of stoppage of operation, or worse still of deterioration of the essential constituent elements; it is also essential to have a safe operation, without risk of false operation. In addition, when such floating roof tanks are used to add water to the primary circuit of a nuclear reactor, it is necessary to have storage in the absence of air which ensures very strict quality. for the stored liquid, the degassed water must, in this particular application, have an oxygen content not exceeding 100 ¡.Lg / I to avoid any risk of oxidation.

It is known to produce a floating roof tank comprising a cylindrical wall, a floating roof tightly connected to said wall which surrounds it by a flexible membrane, this membrane defining with the wall of the tank a space suitable for receiving a liquid of back pressure, in accordance with the preamble of claim 1. A typical example of such a tank is illustrated in FR-A-2 526 405. To preserve this flexible membrane connecting the periphery of the roof and the side wall of the tank, avoiding in particular the formation of folds and / or an uncontrollable application against the wall of the tank under the effect of the pressure of the liquid contained in the tank, it is usually provided that the space defined by the membrane and the wall of the tank , space communicating with the outside, receives a counterpressure liquid essentially acting as a lubricant for the membrane during the movements of the floating roof, and incidentally allowing, at least in certain positions, a centering of said roof relative to the cylindrical wall of the tank.

One of the problems posed by the design of such floating roof tanks lies in controlling the actual volume of backpressure liquid, and in particular the emptying of this volume.

Indeed, if this volume is excessive, the back-pressure liquid overflows excessively on the periphery of the roof, thus being able to invade it, when the latter is in the low position, which can cause a certain instability and / or pouring. said roof with the risk of damaging the membrane, or even the side wall of the tank; and if this volume is insufficient, the membrane risks sticking to the side wall when the roof is in the high position, this friction which is difficult to control can cause deterioration of the membrane. In addition, the volume of the back-pressure liquid is subject to an evaporation phenomenon which is variable depending on the site and the type of operation, essentially as a function of the temperature and the humidity, and which is difficult to determine. precisely.

In the document FR-A-2 526 405 already cited, it has been proposed to cover the surface of the backpressure liquid with balls or floating panels to reduce evaporation, in order to combat the phenomenon of evaporation. Furthermore, the side wall of the tank is provided with an overflow located at a level below the level reached by the raising of the floating roof when the latter is in the maximum height position: this overflow constitutes only a safety of the water guard, and can in no case act as a drain, as clearly shown in the upper position illustrated in Figure 3 of this document.

In fact, with floating roof tanks of the aforementioned type of tank, there is in practice a periodic addition of backpressure liquid very empirically, without serious control of the action of evaporation. If we want to have a real known volume of the back pressure liquid, we then proceed to a total emptying of the tank, we eliminate the back pressure liquid, and after these empties, we carry out a filling of a predetermined volume of back pressure fluid, before refilling the tank. This technique has the disadvantage of requiring total emptying of the tank, with the risks involved in the movements of the floating roof if these are insufficiently controlled, and with a significant loss of operating time, incompatible in particular for an application in the electro-nuclear field.

In an attempt to overcome these drawbacks, it has been proposed to drain the backpressure liquid when the roof is in the high position, using a drain device provided just above the level of the membrane attachment points, that is to say halfway up the side wall of the tank. These attempts were not satisfactory because the emptying was imperfect due to the sticking against the side wall of a lower zone of the membrane above the emptying device. Furthermore, the accessibility of the device at a medium level enabled potential users to draw backpressure liquid, the volume of which could then become insufficient.

The invention aims to avoid the drawbacks of the previously mentioned prior techniques.

An object of the invention is therefore to provide a floating roof tank, the structure of which makes it possible to carry out in a completely reliable manner a total emptying of the volume of back pressure, when the roof is in the high position.

Another object of the invention is to have a simple structure and a reasonable manufacturing cost for the tank.

Another object of the invention is to permanently dispose of the liquid stored in the reservoir, this in particular for application in the electro-nuclear field.

A final object of the invention is to avoid the risk of inadvertent withdrawal of the backpressure liquid.

According to the invention, the technical problem described above is solved by providing a floating roof tank for liquids, characterized in that it includes means making it possible to drain the backpressure volume when the roof is in position. high, these means comprising a drain outlet orifice passing through the wall of the tank, said orifice being disposed in the upper part of the tank, and being connected to a conventional drain pipe external to said wall, as well as a means preventing obstruction of said drainage intake orifice by the flexible membrane, so that the flexible membrane allows the total expulsion of the backpressure liquid under the effect of the pressure exerted by the liquid contained in the tank, in accordance with the characterizing part of claim 1.

In particular, the means preventing obstruction essentially consists of a flat element fixed on the internal lateral wall of the reservoir, and having a projecting portion so as to avoid any direct contact at the level of said wall between the intake orifice. and flexible membrane.

Preferably, the drain outlet orifice is arranged in the vicinity and below the level occupied by the liquid in the tank when the roof is brought, when filling the tank, pressing against high stops defining the maximum height position. of said roof, and at a distance from the level of the water line represented by the ratio of said maximum volume to the area of the crown formed by the plane of the backpressure liquid.

• - la figure 1 est une coupe verticale d'un réservoir à toit flottant conforme à l'invention;
• - la figure 2 est une coupe partielle du réservoir dont le toit est en position haute;
• - la figure 3 est une coupe partielle illustrant le détail des organes de vidange du liquide de contre-pression; et
• - la figure 4 est une coupe selon IV-IV de la figure 3.

Other characteristics and advantages will appear more clearly in the light of the description and the figures of the appended drawing, illustrating particular embodiments of the invention, with reference to the figures where:

• - Figure 1 is a vertical section of a floating roof tank according to the invention;
• - Figure 2 is a partial section of the tank whose roof is in the high position;
• - Figure 3 is a partial section illustrating the detail of the bodies for draining the back pressure liquid; and
• - Figure 4 is a section on IV-IV of Figure 3.

In FIG. 1, a floating roof tank for liquids 1 comprises a generally cylindrical wall 2, a floating roof 3 produced in an open box on the top, and a flexible membrane 4 sealingly connecting the periphery of the floating roof to the side wall of the tank. It should be noted that the membrane 4 is here hooked in 5 at mid-height of the tank, but the invention would also apply to the case of a membrane hung on the upper part of the tank. According to the usual technique, the space 6, open to the outside, defined by the membrane 4 and the wall 2 of the tank is suitable for receiving a back-pressure liquid essentially serving as a lubricant for the flexible membrane 4, avoiding in particular the formation of folds during the various movements of the floating roof 3, and incidentally allowing, at least in certain positions, a centering of said roof relative to the wall of the tank.

FIG. 1 schematically illustrates a certain number of devices which are usually provided on floating roof tanks for liquids these devices will be recalled succinctly, given that the person skilled in the art is well accustomed to the presence of these devices.

There is thus provided, for the floating roof 3, six guide wheels 7 with spring 8, mounted in a regular distribution at the upper periphery of the roof and intended to be applied against the wall 2 of the tank in all positions of the roof; an advantageous measure consists in providing a dimensioning of the floating roof, with its guide wheels, which always remains less than the diameter of the tank so as to make possible, although this is not desirable, a complete reversal of the roof without jamming by the wall of the tank. The floating roof 3 is also equipped with an analog inclinometer 9 to which a cable 10 is connected, equipped with a slack and provided with a counterweight, for returning the reading and the alarms to the control room, which allows you to follow the inclination of the roof in case of hanging in the intermediate position, operating in the lower part or emptying the tank. The floating roof 3 is also equipped, in the vicinity of the membrane-roof junction, with peripheral vents 11 connected by a venting manifold 12 provided with a valve 13, and possibly with an air purge 14 in the center of the roof in case it does not have sufficient taper to collect air at the periphery. The floating roof is finally fitted with a vacuum breaker tube 34 forming a natural valve which protects the reservoir against excessive depression of the liquid during emptying when the said roof is in the low position, or when the latter gets stuck in an intermediate position (moreover, in this case, this tubing advantageously serves as an outlet in the event of overpressure).

As for the tank itself, it is equipped with a manual venting below the level of the attachment points 5 of the membrane, with several vents distributed equally on the periphery and connected by a manifold 15 to an outlet pipe to a valve. 16; provision is also made at a low level for the inlet 17, outlet 18, and drain 19 pipes for the reservoir liquid. A circular gangway 20 at the top of the tank makes it easier to visit the roof and the membrane.

The movement of the floating roof is limited by low stops 21 and high 22, and indication and alarm equipment (not shown) are provided for checking the extreme high and low levels in the control room.

As explained at the beginning of the description, it is important to check the actual volume of the back-pressure liquid contained in the space 6, and in particular the emptying of this volume.

According to the invention, the tank is equipped with means making it possible to drain the volume of backpressure liquid when the roof is in the high position. As illustrated in FIGS. 1 and 2, these means comprise a drain outlet 23 passing through the wall 2 at the top of the tank and connected to a conventional drain pipe 24 provided with a valve 25, and external to said wall.

To drain the counterpressure liquid, the tank is filled, thereby raising the floating roof until it rests against the upper stops 22 (Figure 2). To protect the tank against overfilling, an external overflow pipe 26 in the form of a butt, can be provided, provided in the upper part with a siphon-breaking conduit 27; in this case, the filling of the reservoir will be continued until the level of the waterline 28 of the liquid, which moreover corresponds substantially to the surface level of the back-pressure liquid, reaches the level 29 of the excess full, slightly above the level occupied at the start of the top stop, which is additional security to be sure that the floating roof is at its maximum height.

The tank is then isolated by closing the valve of the inlet pipe 17, then the valve 25 of the drain outlet orifice 23 is opened; this, provided for this purpose at a level lower than that 28 of the waterline when the roof is in high stop, allows a complete draining of the volume of backpressure liquid by the natural effect of the pressure of the liquid of the reservoir which tends to radially push the flexible membrane 4, and therefore to expel this liquid until the whole part of the membrane which is lower than the drain orifice is applied against the interior wall of the reservoir.

Thanks to the device of the invention, it is therefore possible to carry out this total draining of the backpressure liquid with the roof in the high position. It then suffices to actuate means for filling with back-pressure liquid, means preferably arranged in the upper part of the reservoir at a level close to that of the emptying means, and constituted for example (FIG. 1) by a pipe 30, a valve 31 and a volumetric counter 32 allowing the admission of a predetermined volume.

It is important to insist on the position of the drain opening, which is chosen according to the maximum volume of back-pressure liquid, and preferably at a distance from the waterline level represented by the ratio V _max / S where V _max is this maximum volume and S the area of the crown formed by the plane of the back-pressure liquid. The importance of this level comes in particular from the fact that, when the drain valve 25 is opened, the roof drops slightly as the back pressure liquid is expelled; it is therefore necessary to be certain that the level 28 of the waterline remains higher than that of the drain outlet orifice 23, so that the emptying is complete. In addition, the high position of this orifice also prevents inadvertent withdrawal of the backpressure liquid due to a location generally located outside the normal range of travel of the roof.

Figures 3 and 4 illustrate a means for avoiding the obstruction of the drain outlet orifice by the flexible membrane during the emptying of the back pressure liquid; this means is for example made in the form of a flat element 33 fixed on the side wall 2 and the projecting portion of which avoids the obturation of the drain outlet orifice 23 while allowing the liquid to pass laterally.

Note that the volume of the backpressure liquid is predetermined when filling the empty tank for the first time, as in use after draining the backpressure liquid, with the roof in the high position.

For an application in the electro-nuclear field, demineralized water will advantageously be used as back pressure liquid; the design of the floating roof tank of the invention also makes it possible to avoid bringing the liquid from the tank into contact with the air, which avoids having to redegas this liquid (communications in the open air by the siphon breaker of the overflow piping and the vacuum breaker manifold introduce only a negligible disturbance, because the siphon breaker, which is very far from its communication with the tank in the lower part, and the vacuum breaker tubing encloses a lower cap of liquid which forms an effective screen).

Claims (5)

1. A floating roof tank for liquids, the tank comprising a cylindrical wall (2), a floating roof (3) surrounded by said cylindrical wall, a flexible membrane (4) connecting said floating roof to said wall in watertight manner, said membrane defining together with the wall of the tank a space (6) which is suitable for receiving a counter-pressure liquid, characterized by the fact that it is fitted with means (23, 24, 25, 33) for emptying the volume of counter-pressure liquid when the roof (3) is in its high position, said means comprising an emptying orifice (23) passing through the wall of the tank (2), said orifice being disposed in the upper portion of the tank, and being connected to a conventional emptying duct (24) outside said wall, together with means (33) preventing the emptying orifice (23) from being closed by the flexible membrane (4), in such a manner that the flexible membrane (4) serves to expel all the counterpressure liquid under the effect of the pressure exerted on the membrane by the liquid contained in the tank.

2. A floating roof tank according to claim 1, wherein the means for preventing the orifice being closed are essentially constituted by a flat member (33) fixed on the inside wall of the tank, and having a portion projecting into the tank in such a manner as to prevent any direct contact between the emptying orifice (23) and the flexible membrane (4) in the vicinity of said flat member.

3. A floating roof tank according to anyone of claims 1 and 2, wherein the emptying orifice (23) is disposed close to and below the level (28) occupied by the liquid in the tank (3) when the roof is brought, by said tank being filled, into abutment against top stops (22) defining the highest position of said roof.

4. A floating roof tank according to claim 3, wherein the level of the emptying orifice (23) is chosen as a function of the maximum volume of counter-pressure liquid, and is preferably at a below the level of the liquid-line (28) by a distance obtained by dividing said maximum volume by the area of the ring formed by the plane of the counter-pressure liquid.

5. A floating roof tank according to anyone of claims 1 to 4, further including counter-pressure liquid filling means (30, 31, 32), said means being disposed at the top of the tank at a level close to that of the emptying means.

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