EP0309361B1 - Fluid distributor in a pressurized container preventing thermal stratification - Google Patents

Description

The present invention relates to a fluid distributor in a pressure tank according to the preamble of claim 1, preventing thermal stratification in this distributor between two layers of immiscible fluids at different temperatures. It can in particular be applied to the water supply device for steam generators of nuclear power plants.

The water constituting the secondary fluid in the heat exchangers of nuclear power plants is vaporized in a steam generator, the wall of which is pierced by a water supply pipe. This pipe is terminated by a horizontal toroidal distributor internally running along the cylindrical wall of the steam generator. The interior of the steam generator and the interior of the torus communicate using distribution tubes crossing the wall of the latter and planted on its upper generator. Quite frequently these tubes are in the form of a butt, that is to say that their end external to the torus is terminated by an arc of a circle oriented radially with respect to the torus. As the case may be, the other end of the tubes penetrates inside the torus, possibly as far as the middle of its section, or is simply flush with the interior surface of its wall. This latter construction is in particular compulsory if the torus is completed by a tubular connecting section which is fitted into the supply pipe during assembly.

Such distributors meet several requirements: to promote a uniform flow of water in the steam generator, to subject the supply water to pressure drops limiting the violence of its flow, and above all, in the event of a lowering of the level of the liquid in the steam generator, avoid dewatering the pipes which would then lead to the creation of a water hammer.

Such an arrangement avoids this phenomenon, but on the other hand does not prevent the possible appearance of thermal stratification in certain exceptional circumstances of operation of the steam generator.

In transient or accidental regime, it may indeed be necessary to supply the steam generator with cold water at very low flow rate. It is then found that this water, of higher density than the hot water which previously flowed in the distributor, does not mix with it and circulates in the bottom of the supply pipe and the torus. The hot water remaining at the top of the supply water piping and of the torus is renewed following convection currents with hot water outside the torus; it therefore remains at high temperature. This hot water is evacuated only at the part of the torus diametrically opposite to the supply pipe, where the two streams of cold water meet, which increases their pressure. The level of cold water rises locally (opposite the inlet) in the torus until it reaches and fills the distribution tubes located there, and the cold water thus ends up passing through the steam generator. But this phenomenon remains local; everywhere else an interface remains between the two superposed layers of water. This interface corresponds to a thermal plateau which can exceed 100 ° C in this pressurized water. The walls of the torus and the supply pipe are therefore subjected to very high stress concentrations at these locations, which are all the more dangerous as they work in fatigue. Ruptures have been noted in particular at the place where the supply pipe crosses and is welded to the envelope of the steam generator: the metal is already subject to internal stresses there and has less latitude to deform .

The invention eliminates these drawbacks and consists in promoting the flow of water out of the distribution torus in the areas closest to the supply pipe where the hot water is liable to accumulate. In those regimes where the cold water flow which feeds the toroid is low, the invention makes it possible to limit the pressure losses produced by the passage of the distribution tubes mainly in areas where hot water tends to accumulate. This gives a much more balanced flow in the different tubes and the prospect of finally removing all the hot water. It is obvious that the invention does not only apply to a distribution device comprising a single supply pipe connected to a toroid, but that it can on the contrary apply to any equivalent device comprising a substantially horizontal pipe in the steam generator provided with distribution tubes passing through it by its upper generator.

More specifically, the invention relates to a fluid distributor in a pressure tank comprising a fluid supply pipe entering the tank and terminated by a substantially horizontal tubular distributor part communicating with the interior of the tank by tubes passing through its upper generatrix. and terminated by an open end remote from the dispensing part, characterized in that the tubes have a wall traversed by at least one opening outside the dispensing part between the dispensing part and the open end.

The invention can be implemented in two main aspects: the wall of the tubes can be notched by holes, or else, if these tubes are in the butt, the underside thereof can be notched.

In one important embodiment of the invention, the notches of the tubes are at increasing altitude away from the tubing of drinking water.

The invention can finally be implemented if the dispensing part is provided with holes or notches at its upper generatrix between the tubes and the pipe power supply.

• les figures 1 et 1a représentent une réalisation possible de l'invention,
• la figure 2 représente une autre réalisation possible de l'invention, et
• la figure 3 représente une vue plus générale montrant la mise en oeuvre de l'invention sur un tore d'alimentation complet.

We will now describe the invention more concretely with the aid of the appended figures, the following list of which is given by way of illustration and in no way limitative:

• Figures 1 and 1a show an embodiment possible of the invention,
• FIG. 2 represents another possible embodiment of the invention, and
• FIG. 3 represents a more general view showing the implementation of the invention on a complete feed toroid.

Figures 1 and 1a first show a pipe 1 seen in section, which can be toroidal and which is inside a steam generator, the envelope of which is traversed by a supply pipe which supplies line 1 with water. Line 1 is horizontal and provided with distributor tubes 2, only one of which is shown here, generally vertical and passing through the wall of line 1 at its upper generatrix. Part of the water flowing in the pipe 1 therefore passes through the distributor tube 2 to end up inside the steam generator. The distributor tube 2 shown here can be broken down into three parts: a rectilinear middle part 3, welded to the pipe 1 and passing through it, flush with a first end on the internal surface 1i of the wall of the pipe 1 and s' extending out of its outer surface 1e into the interior of the steam generator; a first end part 4 connected to the first end of the middle part 3 which it extends by extending into the pipe 1; and a second end portion 5 connected to the other end of the middle portion 3, therefore located outside of the pipe 1 and in the form of a semicircle or stick oriented radially with respect to the pipe 1.

The first end portion 4 is terminated at a bevel as can best be seen in FIG. 1a which is a side view of FIG. 1. The direction of flow of the water in the pipe 1 is indicated by the arrow Ec; it can be seen that the bevel arrangement makes it possible to introduce part of the water streams into the distribution tube 2 and thus to promote the flow by the latter. In the illustrated embodiment, the upper part of the bevel, oriented upstream of the flow Ec, is approximately three quarters of the height of the pipe 1 and the lower part of the bevel, downstream of the flow Ec, is approximately a quarter of this height.

It is clear that the distribution tubes can be constructed in many different ways. In particular, their proportions can be largely modified, especially with regard to the extreme parts 4 and 5 which are not compulsory and which can therefore be omitted entirely.

According to this embodiment of the invention, the distribution tube 2 is notched outside the pipe 1 by one or more bores. Here two holes were drilled, a lower hole 6 on the middle part 3 and an upper hole 7 on the second end part or butt 5. The number and arrangement of these holes can be varied, and their angular arrangement on the circumference of the distribution tube 2 is indifferent.

The operation of the distributor using such distribution tubes 2 will now be described. During normal operation of the steam generator, hot water circulates at high flow rate in the pipe 1. A portion passes through each of the distribution tubes 2 according to arrow E1; the feed water charge is sufficient to allow it to rise to the butt 5 to finally flow through the free end 8 of the latter in the steam generator. When the steam generator is supplied with a low flow of cold water, things happen quite differently: cold water is generally at low hydraulic load except at the end of the torus opposite the supply pipe where it accumulates and ends up acquiring a sufficient charge to penetrate entirely into the distribution tubes 2 at this location and flow into the steam generator. In the parts located further upstream of the flow, the load remains at a lower value which does not allow water to pass through the hooks 5. On the other hand, this load is sufficient to be able to gradually evacuate the hot water through the holes 6 and 7, so that the cold water ends up filling the pipe 1 to penetrate the distribution tube 2 and flow through these same holes 6 and 7 in turn according to arrows E2. The interface between hot water and cold water in line 1 is thus gradually eliminated, so that no abnormal stress concentration appears along its wall.

Another embodiment of the invention, which can be envisaged on supply tubes 2 provided with a stock 5, is illustrated in FIG. 2. Here no drilling is performed, but on the other hand the lower surface 5i (FIG. 1) of the stock 5 has been removed over its entire length, that is to say that the section of the butt 5 is now an open tube in its lower part.

The operation of this embodiment is very similar to that of the previous one: in normal operation, the hot water at high load is channeled by the upper surface 5e of the butt 5, gradually bending downward and in transverse direction relative to the pipe 1 to finally join the water present in the steam generator at the free end 8 of the butt 5. In the case of a low flow of cold water, the water rises gradually in the middle part 3 (and possibly the first end part 4 if it exists) of the distribution tube 2 to finally be flush with the top of the middle part 3 and flow out of the distribution tube 2 through the notched part of the lower surface of the butt 5. The arrows E′1 and E′2, of similar meaning to the arrows E1 and E2 in FIG. 1, symbolize these two different flows.

It is advantageous that all these notches made on the distribution tubes 2 are different depending on the location of the distribution tubes 2 along the pipe 1: FIG. 3 represents a complete supply device comprising a pipe 1 of toroidal shape connected to a feed pipe 10 passing through the envelope 11 of the steam generator to which it is welded. The pipe 1 can be welded to the supply pipe 10 or else provided with a fitting fitted in this supply pipe 10.

The feed water therefore enters the pipe 1 and flows in two symmetrical flows between the two left and right branches of the torus. The flows pass successively near several distribution tubes 2a, 2b, 2c, etc. from upstream to downstream. As, for reasons which have been explained earlier, the load of cold water decreases appreciably upstream in the case of a low flow, it is essential to favor the flow out of line 1 especially in the upstream parts by reducing there more strongly the pressure losses produced by the crossing, over their entire length, of the distribution tubes 2. This is why the holes 6a, 6b, 6c, etc ... operated on the tubes of corresponding distribution 2a, 2b, 2c, etc ... are found at decreasing heights on the tubes located upstream. It is also possible to make holes 9 on the upper generator of the pipe 1 between the most upstream tube 2a and the flow pipe 10. This latter arrangement is even necessary in the case of distribution tubes 2 comprising a first end portion 4 extending internally to the pipe 1, because one cannot prevent part of the hot water from remaining trapped in the upper part of the pipe.

In this figure 3 there is shown distribution tubes 2 devoid of stock 5; the result would be the same if they had one, and the invention could be implemented in exactly the same way, with the difference that it would be possible to have notched tubes 2 as shown in FIG. 2 near the downstream part of the flow (part of the torus diametrically opposite to the supply pipe 10).

For a practical implementation of the invention, it is advisable to stagger the heights of the holes 6a, 6b, 6c, etc. according to a linear law as a function of the curvilinear abscissae of the upper generatrix of the pipe 1 in which the tubes 2a, 2b, 2c, etc. are implanted.

The invention therefore makes it possible to establish in the pipe 1 a flow and evacuation of water much better distributed than with the systems used until now. The low-flow cold water first slides under the hot water which originally fills the pipe 1 and the supply pipe 10; the interface formed by these two immiscible water layers is at substantially constant altitude along the length of the pipe 1 and gradually rises until the cold water completely fills the pipes. This therefore avoids any thermal stratification between two very different temperature layers.

Claims (5)

1. Fluid distributor in a pressurized reservoir (11) incorporating a supply pipe (10) for the fluid entering the reservoir and terminated by a substantially horizontal distributing part (1) communicating with the interior of the reservoir (11) by tubes (2) traversing its upper generatrix and terminated by an open end (8) remote from the distributing part (1), characterized in that the tubes have a wall traversed by at least one tying outside the distributing part between the latter and the open end.

2. Fluid distributor in a pressurized reservoir according to claim 1, characterized in that the openings in the wall of the tubes are pierced openings (6, 7).

3. Fluid distributor in a pressurized reservoir according to any one of the claims 1 or 2 in which the tubes (2) are terminated outside the distributing part by a crook (5), characterized in that the openings of the wall of the tubes (2) extend along the lower surface of the crooks.

4. Fluid distributor in a pressurized reservoir according to any one of the claims 1 to 3, characterized in that the openings of the wall of the tubes (3) are at increasing height moving away from the supply pipe (10).

5. Fluid distributor in a pressurized reservoir according to any one of the claim 1 to 4, characterized in that the distributing part (1) is provided with openings (9) on its upper generatrix between the tubes (2) and the supply pipe (10).

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