EP0296064A1 - Steam generator comprising a water purification tank - Google Patents

Abstract

The tank consists of a bottom (9), of a lateral wall (31) and of a roof (32). It is provided with a chimney (33) which includes the lower end (21) of a drain (20) collecting the water to be purified. Lateral orifices (35) allow a calm flow inside the purification tank (30) and the deposit by gravity of the pollutant particles. The device as a whole is simple and effective. <??>Application in thermal nuclear power stations. <IMAGE>

Description

The subject of the present invention is a steam generator water purification tank.

In the steam generators, the water in the secondary circuit contains particles which are mainly oxidation products such as magnetite. These particles tend to settle in areas with low flow velocity, and in particular near the tube plate, thus promoting corrosion of the exchange tubes.

This is why we have already undertaken to build devices comprising purification tanks in which these particles accumulate while waiting to be purged. The prior art in the matter includes three patents registered in the name of the Westinghouse Company and extended respectively in France under the numbers 2 462 655 and 2 564 949, and in Europe under the number 0 185 174.

The inventions of these three patents relate to purification tanks placed on the horizontal roof of an envelope containing the tubes in which the primary fluid circulates. The water from the secondary circuit of the steam generator, which is the subject of purification, circulates between the shell of the steam generator and the envelope of the tube bundle downwards, then inside the envelope towards the top ; it then vaporizes on contact with the tubes and rises in vertical pipes piercing the roof of the envelope. Various devices placed at the top of the steam generator, in the two-phase zone, make it possible to dry the steam produced by separating it from the water it still contains. Such devices are cyclones that act by centrifugation and herringbone dryers.

This water, separated from the vapor phase, is more charged with particles than the water in the water returns or at the bottom of the beam because the boiling concentrates the solid particles in non-vaporized water. In general, it flows to a purification tank located on the roof of the bundle envelope where it settles before being recycled. The particles are deposited at the bottom of the tank, and they are thus left to remain until the tank is drained. These purges can take place between two operating cycles or during a cycle using a special device comprising a water spray hose and a purge or draw pipe.

In the first of these patents, the purification tank is open at the top; its bottom is formed by the roof of the envelope and its side wall is vertical cylindrical. The water collects until it overflows and cascades over the top edge of the side wall. It is then recycled. The purge is ensured by a withdrawal pipe opening into the center of the bottom of the tank.

This device has the following drawback: as the cyclones and the dryers are distributed over the entire surface of the steam generator, the side wall of the tank must be of large diameter, in practice almost as large as that of the shell of the generator. steam itself, to be able to encompass all the runoff paths of the water separated from the steam. As a result, a very large plate must be moved and installed in the upper parts of the steam generator which are difficult to access during construction.

The other drawback is due to the withdrawal pipe which requires drilling the envelope in two places: through the roof and on the side wall, and also piercing the shell of the steam generator itself, which creates weakened areas. .

The second patent proposes a lighter solution since the single container is replaced by small closed containers whose top is perforated. These tanks are placed entirely in the liquid phase. The particle-laden water passes over them and part of it enters. As the flow speed is very low inside, the particles settle at the bottom of the tanks. The racking device is completed by a device sprinkler consisting of a pipe entering the tank from its top and terminated by radial nozzles spraying its interior like a shower head.

This device makes it possible to escape the disadvantage of the large bin. On the other hand, as the withdrawal device is the same as that of the previous solution, it has the same drawbacks, and it must be recognized that another disadvantage appears with this solution: the water laden with particles circulates erratically over the above the perforated bottoms of the various bins and we do not know the quantity that actually passes through it. The performance of the device is therefore uncertain.

In the third patent, the envelope which contains the bundle of tubes has a triple apex in the form of three parallel horizontal plates. The upper plate is perforated, the middle plate includes a central orifice and does not extend radially to the periphery of the envelope, and the lower fluid-tight plate corresponds to the roof of the envelope.

The water to be purified circulates, as in the previous solution, above the upper perforated plate and penetrates through it. It goes towards the central orifice of the middle plate which it therefore also crosses downwards. It then enters a baffle device which guarantees a laminar flow conducive to the deposition of particles; the water then circulates towards the periphery of the envelope and is recycled after having made a vertical movement which causes it to pass externally to the middle plate and through the peripheral holes of the perforated plate. In this device, no particular means of purging or drawing off is provided.

The disadvantages of this solution can be summarized as follows: as in the previous solution, nothing can confirm that the water loaded with particles actually penetrates through the holes in the perforated plate and is not recycled directly; and most importantly, baffle devices should be placed in relatively narrow spaces between two horizontal plates and the large pipes carrying the water-steam mixture which pass through these plates. We can therefore conclude that the assembly of these baffles will be very complicated.

Finally, the absence of a purge device risks requiring relatively short operating cycles.

The object of the invention is to avoid these various drawbacks and more particularly the drawback associated with perforated covers which do not guarantee a good performance of the treatment tank.

A particular object of the invention is therefore to propose a purification tank which is practically a closed envelope, with the exception of orifices, possibly small, used for the evacuation, and of an upper chimney allowing a pipe in which flows the water loaded with particles to lead into the tank.

More specifically, the invention relates to a water purification tank circulating in a steam generator, separated from the steam by means of dryers and having flowed into a water supply pipe into the tank, characterized in that 'It includes an envelope consisting of a bottom, a side wall pierced with water discharge orifices and a roof pierced with a central chimney into which penetrates the lower end of the pipe.

The invention can also be applied to certain steam generators, the construction of which requires that the water contained in the tank may be rapidly available for cooling purposes. Under these conditions, the orifices are extended by vertical slots extending towards the bottom and thus possibly allowing rapid and almost complete emptying of the tank.

In the preferred embodiment, the purging device comprises a water spray pipe tangent to the bottom, passing through a median line of this bottom and pierced with holes allowing sprinkling tangent to the bottom, as well as a collecting pipe communicating with the tank by fittings passing through said tank near the junction of its bottom and its side wall.

• - la figure 1 est une coupe diamétrale d'un générateur de vapeur équipé du dispositif de l'invention ;
• - la figure 2 est une coupe suivant la ligne II-II de la figure 1 ;
• - la figure 3 et la figure 4 représentent deux variantes possibles de construction de l'invention ;
• - et la figure 5 représente une construction un peu différente du générateur de vapeur et du bac.

The description will now be made with the aid of the following figures, figures given by way of illustration and in no way limitative:

• - Figure 1 is a diametral section of a steam generator equipped with the device of the invention;
• - Figure 2 is a section along the line II-II of Figure 1;
• - Figure 3 and Figure 4 show two possible construction variants of the invention;
• - And Figure 5 shows a slightly different construction of the steam generator and the tank.

First of all, briefly recalling with the aid of FIG. 1 the main outlines of the operation of a steam generator and of the water recovery mode.

The primary water circulates according to the arrows A: it first penetrates inside the bottom of the casing 1 by a primary inlet pipe 2 and circulates inside inverted U-shaped tubes 3, the end of the branches is planted in a tubular plate 4. The primary water leaves the tubes 3 to leave the steam generator by a primary outlet pipe 5. A separator plate 6 is formed between the tubular plate 4 and the enclosure 1 of the steam generator so as to force the primary fluid to pass through the tubes 3.

The secondary water circulates according to the arrows B, C and D. It firstly penetrates inside the enclosure 1 of the steam generator by a secondary inlet pipe 19 supplemented by a supply device not shown here. The liquid phase water flows down the steam generator, between the enclosure 1 and an envelope 7 inside thereof, which is concentric with it and which includes the tubes 3. The secondary water is stopped by the tube plate 4 and then flows radially towards the interior of the steam generator passing through openings 8 formed between the bottom of the envelope 7 and the tube plate 4. The secondary water then rises, is reheated on contact with the tubes 3 and vaporizes partially. The circulation of wet steam is more particularly represented by the arrows C: the water-steam mixture rises towards the roof 9 of the casing 7, which is also the bottom of the purification tank which will be described below. , and circulates in vertical pipes 10 communicating with the interior of the casing 7 through holes in which the roof 9 is pierced. The water-steam mixture rises towards the top of the steam generator 1, but it is still very loaded with moisture. This is why there are devices for separating steam and water. First of all, cyclones 11 are provided at the top of the tubes 10 which operate a first separation of the water droplets suspended in the vapor by centrifugation. The water thus collected passes through collecting devices 12 and then flows by gravity (downwards), along the pipes 10 outside of them until reaching the liquid / vapor interface H.

In general, the tops of the cyclones are braced by a solid plate 44, equipped with drains 13 which serve to evacuate the droplets not separated by the cyclones and falling by gravity. These drains 13 can open onto the vertical drain 20 which itself opens into a purification tank 30 described below. In this case, the water separated by gravity is purified by the device proposed by the inventor.

The wet steam continues to rise according to the arrows C until it leaves the enclosure 1 of the steam generator by a secondary outlet pipe 14, but in the meantime it passes through the dryers 15, which are structures suspended at the top of the envelope 1 and in which the droplets contained in the vapor and not centrifuged by the cyclones 11 are captured by corrugated sheets. This residual water is loaded with solid particles which must undergo purification. This water can even entrain particles which detach from these sheets following an accidental oxidation during a stop. In this case, the water from the dryers is more charged than the water recirculated by the cyclones, and the proposed device has the advantage of purifying the water from the dryers before it mixes with the rest of the recirculated water. The droplets flow by gravity, first along the dryers 15, then inside a collection tank 16 suspended from the dryers 15 where they collect, and finally inside a vertical drain 20 which collects the water from the collection tank 16 and possibly that which falls from the drains 13. The path of this water is represented by the arrows D.

The drain 20 opens at its lower end 21 into a purification tank 30 located below the liquid-vapor interface H, which constitutes the object of the invention and will now be described in detail.

The purification tank 30 is of generally cylindrical shape and comprises a bottom 9 here merged with the roof of the envelope 7, a side wall 31 and a roof 32. The bottom 9 and the roof 32 can be crossed, in the case of a large diameter tank 30, by the vertical pipes 10 which convey the water-steam mixture. The roof 32 is pierced with a chimney 33 consisting of a cylindrical sleeve placed in the center of the roof 32. The drain 20 penetrates into the interior of the chimney 33 and makes with the sleeve a slight clearance in order to tolerate thermal expansions and location uncertainties due to manufacturing.

The chimney 33 is obstructed by a grid 34 which rests on the roof 32. The lower end 21 of the drain 20 is above the grid 34. The utility of the grid 34 is to create pressure losses of the water flowing in the drain 20 and in particular to rapidly reduce the vertical component of its speed.

According to another conception of the invention, shown in FIG. 3, the same result can be achieved by means of a drain 20 ′ penetrating more deeply into the purification tank 30 and whose lower end 21 ′ is clearly at below the roof 32. The section of the drain 20 ′ is not open but obstructed by a plate 29, and the flow of water out of the drain 20 ′ is made radially by orifices 22 drilled in the circumference of the drain 20 ′, under the roof 32.

The water entering the drain 20 -or 20′- in the purification tank 30 loses its polluting particles in suspension by depositing them on the bottom 9. This is achieved by creating a flow that is as laminar and calm as possible, especially at the bottom of the tank 30, which allows not to entrain again the particles already deposited. Evacuation orifices 35 regularly distributed along the side wall 31 and at the same level in the upper part thereof ensure a radial flow of the water meeting these characteristics. The radial flow allows the water entering the purification tank 30 to be slowed down very sharply, and the long transit time promotes the deposition of particles on the bottom 9. In addition, the lower part of the tank 30 is little affected by the flow and therefore constitutes a calm zone.

The purified water leaving through the orifices 35 then mixes with the general flow represented by the arrows B.

In some steam generators, it may however be necessary to have very quickly the major part of the water contained in the purification tank 30 to ensure cooling in the event of overheating of the assembly. The discharge orifices 35 shown in FIG. 1, which are small, then prove to be insufficient. This is why, as shown in FIG. 4, these orifices 35 can be replaced by orifices 36 of larger dimensions and made up of two parts. The upper part, which can extend up to the roof 32, is a circular or rectangular hole 37; the lower part is a slot 38 extending downward and extending the hole 37. It can extend to the bottom 9. This arrangement guarantees that a very rapid flow of water out of the purification tank 30 can be obtained and the bin will empty almost completely. It is clear that the efficiency of the purification is slightly reduced by the more turbulent flow in the purification tank 30, but this loss can be accepted because the width of the slots 38 remains sufficiently small. so as not to allow excessive flow at the bottom of the purification tank 30 under steady conditions.

The purification tank 30 according to the invention therefore has various advantages over the solutions known and disclosed in the patents mentioned.

As it is almost completely closed, the particles deposited are protected from external flows in the balloon, and the risks of re-emulsion during accidental dewatering of the tank are very low.

The closed nature of the device makes it possible to maintain the same efficiency whatever its environment, and in particular if the tank 30 is entirely in the liquid phase.

Thanks to the direct supply via a drain 20 passing through a chimney 33 and opening at the center of the tank, a radial internal flow is created, the speed of which decreases rapidly from the center to the periphery.

• a) un débit trop important conduisant à un temps de parcours trop court pour que les particules aient le temps de se déposer.
• b) un débit liquide, et donc de particules, négligeable par rapport au débit total d'eau recirculée par le système de séparation.

Both in speed and in direction, the flow is thus much better controlled than in the two other patents cited above. This control of flow avoids two pitfalls:

• a) too high a flow rate leading to too short a travel time for the particles to have time to settle.
• b) a liquid flow rate, and therefore of particles, negligible compared to the total flow rate of water recirculated by the separation system.

We are therefore entitled to expect from the installation described above a yield much higher than that of the designs described in the cited prior art.

The purging or drawing off of the particles collected in the purification tank 30 by deposition can be carried out in various ways. We will now present the preferred solution, but others are obviously possible. We comment on Figures 1 and 2 simultaneously.

A spray hose 40 passes inside the purification tank 30 and on the bottom 9. The spray hose 40 is straight in the purification tank 30 and arranged in a central or diametral course. It is pierced with horizontal spraying orifices 41, thus allowing a flow of spraying water which is horizontal and tangent to the bottom 9 when purging.

The particles flow in the form of mud, repelled by the spraying water towards the periphery of the purification tank 30. It then passes through fittings 42 arranged along the side wall 31 and near the bottom 9, and which communicate the interior of the purification tank 30 with a recuperator pipe 43 of annular shape around the side wall 31 outside of the latter. The mud can then be vacuumed and collected.

• a) on supprime le risque de laisser de l'eau stagner sur le toit quand on vidange le générateur de vapeur lors de l'arrêt annuel. On facilite ainsi le conditionnement à sec du générateur de vapeur par rapport au toit plat, qui peut présenter des défauts de planéité, même légère, susceptibles de retenir de l'eau.
• b) Le tourbillon de recirculation qui se forme à la périphérie favorise le dépôt des particules, ce qui favorise l'évacuation par aspiration.

In general, the bottom 9 of the tank is flat and horizontal. However, when space permits, it can be given a concave shape and more specifically a cone, the point at the top of the vertical axis and large opening (angle at the top between 170 and 160 °). This is what FIG. 5 represents, where this new bottom is referenced 49. For a given depth at the center, the slight slope thus obtained has several advantages:

• a) the risk of water stagnating on the roof is eliminated when the steam generator is emptied during the annual shutdown. This facilitates dry packaging of the steam generator relative to the flat roof, which may have flatness defects, even slight, likely to retain water.
• b) The recirculation vortex which forms at the periphery promotes the deposition of particles, which promotes evacuation by suction.

The suction pipes 40 and recuperator 43 do not pass through either the casing 7 or the enclosure 1, but are bent upwards and open above the interface H near a hole secondary man 50. When the operation of the steam generator is interrupted, it is thus easy to open the cover of the manhole 50 and to connect the sprinkler and recovery pipes 40 and 43 to pumps.

Claims (9)

1. Water purification tank (30) circulating in a steam generator, separated from the steam by means of dryers (15) and having flowed into a pipe (20) for supplying water to the tank (30 ), characterized in that it comprises an envelope consisting of a bottom (9, 49), a side wall (31) pierced with orifices (35, 36) for discharging water and a roof (32) pierced with a central chimney (33) into which the lower end (21) of the pipe (20) penetrates. 2. Water purification tank according to claim 1, characterized in that the tank is located below a water-steam interface (H) in the generator. 3. Water purification tank according to any one of claims 1 or 2, characterized in that the chimney (33) comprises a cylindrical sleeve surrounding the lower end (21) of the pipe (20). 4. Water purification tank according to claim 3, characterized in that the chimney comprises a grid (34) disposed under the cylindrical sleeve (33) and below the pipe (20). 5. Water purification tank according to any one of claims 1 to 3, characterized in that the lower end (21 ′) of the pipe (20 ′) is obstructed by a plate (29) and provided with bores (22) below the roof (32) allowing a horizontal flow of the water leaving the pipe (20). 6. Water purification tank according to any one of claims 1 to 5, characterized in that the orifices (35) are small and regularly distributed at the same level close to the roof (32). 7. Water purification tank according to any one of claims 1 to 5, characterized in that the orifices (36) are extended by vertical slots (38) extending towards the bottom (9, 49). 8. Water purification tank according to any one of claims 1 to 7, characterized in that it comprises at least a water spray pipe (40) tangent to the bottom (9, 49), passing through a center line of this bottom (9, 49) and pierced with holes (41) allowing tangent sprinkles to the bottom (9, 49 ), as well as a recovery pipe (43) communicating with the tank (30) by fittings (42) passing through said tank (30) near the junction of its bottom (9, 49) and its side wall (31) . 9. A water purification tank according to any one of claims 1 to 8, characterized in that its bottom (49) is concave.

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