EP0216667B1 - Apparatus to retain liquid in a substantially horizontal conduit, open at one end, when the liquid flow rate drops below a given level - Google Patents

Description

The present invention relates to a device for retaining a liquid in a pipe of substantially horizontal axis and having an open outlet end, when the flow of this liquid is stopped or its flow drops below a threshold given.

In a substantially horizontal pipe whose outlet end opens into a gaseous medium such as water vapor, a significant drop in the flow has the effect of a gas entry into the piping. When the flow of liquid in the piping rises to its normal operating level, this gas present in the piping has the effect of producing a water hammer.

Furthermore, it may happen that the temperature of the liquid circulating in the pipe suddenly drops and this phenomenon is accompanied by a drop in the flow rate. Given the density effects between the cold liquid and the hot liquid, a stratification of this liquid then takes place and progresses along the piping. Severe thermal stresses can thus be generated.

Such situations can arise in particular in pressurized water nuclear reactors fitted with steam generators of a particular type. In fact, in certain steam generators such as that which is the subject of patent FR-A 2 428 787, the edible water from the water-steam circuit of the reactor is admitted into the generator by a horizontal pipe opening into a toroidal manifold pierced with holes along its interior generatrix. This collector is placed in the atmosphere of water vapor prevailing in the upper part of the generator, so that water hammer may occur due to the entry of water vapor into the horizontal pipe during '' a significant drop in flow in the water-steam circuit.

In addition, under certain operating conditions, the temperature of the feed water injected into the steam generator can be reduced and the flow rate can be reduced. The previously mentioned stratification phenomenon is therefore likely to occur, causing severe thermal stresses on the connection pipe between the envelope of the generator and the feed water inlet pipe.

In document BE-A 828 942, a solution to this problem has been proposed, consisting in successively placing three plates forming a diaphragm in the pipe which opens into the steam generator. However, such a device considerably disturbs the flow of water.

The subject of the invention is precisely a device making it possible to solve these problems by ensuring at low flow rate the retention of the liquid in an open pipe of substantially horizontal axis, without however disturbing too much the flow of the liquid in this pipe in normal operating conditions.

To this end and in accordance with the invention, there is provided a device for retaining liquid in a substantially horizontal axis pipe having an outlet end opening into a gaseous medium when the flow rate of said liquid drops below a threshold. given, this device being characterized in that it comprises four tubes placed in the pipeline and wound helically around a central hub disposed along the axis of the pipeline, each of these tubes having at least one low point for which the tube is located entirely below the axis of the pipe and a high point for which the tube is located entirely above said axis and has an opening communicating with the outlet end of the pipe, the high point being closer to the outlet end than the low point, the tubes as well as the central hub being supported inside the pipe by a sealed support plate interposed between the pipe and the said tubes, at one end of the latter opposite to said outlet end of the pipe.

In such a device, the tubes remain permanently filled with water on the side of their inlet end. In addition, each of the tubes having an opening at its highest point, no siphon effect occurs. Therefore, the circulation of the liquid in each tube is determined only by the flow in the pipeline. When this flow is canceled out or becomes very low, stagnant liquid remains in each tube and somehow forms a plug opposing the flow of the liquid it contains. The desired effect is thus obtained. Conversely, the device is designed to disturb as little as possible the flow of the liquid under normal operating conditions, that is to say when the flow rate is high. In addition, the use of four helically wound tubes makes it possible to reduce the passage section as little as possible.

To further reduce the flow resistance, the ends of the central hub are preferably tapered.

In a first embodiment of the invention, said opening is a hole formed in the highest part of the wall of the tube.

In a second variant embodiment of the invention, the other end of the tube is located at said high point and forms said opening.

• - la figure 1 représente de façon schématique un générateur de vapeur dont la canalisation d'arrivée d'eau alimentaire est équipée d'un dispositif conforme à l'invention,
• - la figure 2 est une vue en coupe longitudinale partielle représentant à plus grande échelle le dispositif de retenue du liquide équipant la canalisation représentée sur la figure 1,
• - la figure 3 est une vue en coupe selon la ligne III-III de la figure 2, et
• - la figure 4 est une vue comparable à la figure 2 représentant une variante de réalisation du dispositif selon l'invention.

A preferred embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings in which:

• FIG. 1 schematically represents a steam generator whose feed water supply pipe is equipped with a device according to the invention,
• FIG. 2 is a partial longitudinal section view showing on a larger scale the liquid retaining device fitted to the pipe shown in FIG. 1,
• FIG. 3 is a sectional view along line III-III of FIG. 2, and
• - Figure 4 is a view comparable to Figure 2 showing an alternative embodiment of the device according to the invention.

FIG. 1 very schematically represents a steam generator ensuring the heat exchange between the primary water circuit and the secondary water-steam circuit in a pressurized water nuclear reactor.

The steam generator 10 represented in FIG. 1 is of the same type as that which is the subject of patent FR-A 2 428 787. It will therefore not be described in detail.

The generator 10 comprises a cylindrical casing 12, of vertical axis, in the lower part of which is disposed a bundle of inverted U-shaped tubes 14. The ends of the tubes 14 are fixed on a horizontal tube plate 16 delimiting with the end hemispherical lower part of the casing 12 a primary water inlet chamber 18 and a primary water outlet chamber 20. Tubing 22 and 24 opening respectively into the chambers 18 and 20 make it possible to connect these to the rest of the primary circuit.

The water 26 of the secondary circuit, in which the tubes 14 of the bundle bathe, is introduced into the upper part of the casing 12 by a substantially horizontal pipe 28. This pipe 28 opens into a feed toroid 30 having perforated holes along its lower generatrix. This torus 30 is placed in the water vapor formed by the heating of the water 26 of the secondary circuit resulting from the circulation of the primary water inside the U-shaped tubes 14. The water vapor thus formed is evacuated through the dome formed at the upper end of the casing 12, by an outlet pipe 32, after having passed through the drying devices 13. The secondary circuit also comprises, in a known manner, a turbine 34 and a circulation pump 36.

In such a configuration, the horizontal pipe 28 which opens through the torus 30 into the atmosphere of water vapor prevailing in the upper part of the casing 12 has the drawbacks mentioned above. Indeed, a significant stop or drop in the flow of water admitted by the horizontal pipe 28 inside the steam generator leads to penetration of the water vapor inside the torus 30 and the pipe horizontal 28. In addition, in certain operating configurations, a lower flow of cold water than previously can be sent to the steam generator. Given the density effects between cold and hot water, stratification then tends to take place and progress along the piping. This stratification then creates severe thermal stresses, in particular on the connection pipe between the casing 12 of the steam generator and the piping 28.

To overcome these drawbacks, it is proposed in accordance with the invention to place a liquid retaining device 38 in the pipe 28, near the open end of this pipe.

This device shown in more detail in FIGS. 2 and 3, comprises a central hub 40, of relatively small cylindrical section, arranged along the axis of the pipe 28. In order to reduce the resistance to the flow of the liquid (arrow F on Figure 2) presented by this central hub 40, its ends are tapered.

In the annular space formed between the central hub 40 and the pipe 28 are placed four tubes of circular section 42a, 42b, 42c and 42d wound in a helix. The hub 40 and the ends of the tubes 42a to 42d opposite the open outlet end of the pipe 28, that is to say the right ends in FIG. 2, are welded to a sealed support plate 44. This plate 44 supports the tubes and the hub inside the pipe 28, while forcing the liquid flowing in the pipe to pass through the inside of the tubes 42a to 42d.

The ends of the tubes 42a to 42d closest to the open outlet end of the pipe 28, as well as the corresponding end of the central hub 40 are supported by a second plate 46 preferably arranged with some play inside of the pipe 28. Unlike the plate 44, this plate 46 is not waterproof. The passage of the fluid on either side of this plate can be done by the annular space formed between the periphery thereof and the pipe 28 and / or by perforations passing through it.

As illustrated in particular in FIG. 3, the tubes 42 have an outside diameter of very little less than the difference between the inside diameter of the piping 28 and the outside diameter of the hub 40. The passage section of the device 38 is therefore also as large as possible, which makes it possible to present the minimum flow resistance inside the piping 28.

In the alternative embodiment shown in FIG. 2, the tubes 42a to 42d are all of the same length, this length being such that each of them extends over at least one pitch of the propeller that it forms around the hub 40. Thus, each of the tubes 42 passes through at least one high point for which it is entirely above the axis of the pipe 28 and at least one low point for which it is entirely below of this axis, this low point being located upstream of the high point, if we consider the direction of flow of the liquid indicated by the arrow F.

As illustrated in FIG. 2, the high point of the tube 42a placed downstream from its low point is coincident with the end of this tube fixed to the plate 46. It therefore communicates directly with the open outlet end of the line 28, so that any siphon effect is impossible.

On the other hand, the high points of the tubes 42b, 42c and 42d are followed downstream of a portion of downward tube, so that a siphon effect is likely to occur. To eliminate this risk, a hole 48b, 48c and 48d is formed in the highest part of the wall of each of these tubes, at the location of this high point.

In Figure 2, there is also shown the particular case where the pipe 28 is made in two parts designated by the references 28a and 28b in the figure. This configuration facilitates the establishment and fixing of the device 38 inside the piping. Indeed, the opposite ends of the parts 28a and 28b of the piping are provided with flanges between which can be fixed, in particular by welding, the plate 44.

The operation of the device which has just been described with reference to Figures 2 and 3 is as follows.

In normal operation, that is to say when a liquid flows in the direction of arrow F with a relatively large flow, the pressure drop introduced into the pipeline by this device is sufficiently low so that the flow liquid is not significantly disturbed.

When the flow stops or drops below a determined minimum threshold, the gaseous medium (in this case, water vapor) into which the left end of the pipe in Figure 2 tends to penetrate by this end inside the pipeline. The device 38 being located near this open end, the steam quickly reaches this device, at least in the upper part of the pipe. Due to the non-sealed nature of the plate 46, the steam enters the high point of each of the tubes through the holes 48b to 48d for the tubes 42b to 42d and through the open left end of the tube 48a. This introduction of vapor into the high point of each of the tubes has the effect of avoiding any phenomenon of siphoning of the liquid present upstream of the device 38 towards the open end of the pipe 28.

Each of the high points of the tubes 42a to 42d thus filled with vapor being preceded upstream by a low point of this same tube, a plug of liquid is thus formed in each of the tubes. The desired effect is thus obtained in a simple manner, without significantly disturbing the flow rate of the liquid under normal operating conditions.

The variant embodiment shown in FIG. 4 essentially differs from the variant embodiment which has just been described with reference to FIGS. 2 and 3 in that the admission of gaseous fluid through the high points of each of the tubes 42 ' b at 42'd is no longer made by means of a hole but by using tubes of different lengths, the end closest to the open outlet end of the pipe 28 (i.e. the left end in Figure 4) is located at the location of this high point. The left ends of each of the tubes 42'a to 42'd are thus regularly spaced between the plate 46 and the plate 44. Consequently, only the tube 42'a is fixed to the plate 46.

The operation of the device shown in FIG. 4 is also completely identical to that of the device described above with reference to FIGS. 2 and 3.

In FIG. 4, a variant has also been shown of the case where the device 38 is installed inside a pipe 28 in one piece, the periphery of the plate 44 then being welded directly inside this pipe. .

Of course, the invention is not limited to the embodiments which have just been described by way of example, but covers all variants thereof.

Thus, the tubes of circular section described can be replaced by tubes of different section.

Finally, even if such a device has particularly advantageous applications in the case of the secondary water supply pipe of a steam generator such as that of FIG. 1, this application is in no way limiting. In particular, such a device can be used in all hydraulic circuits comprising a substantially horizontal pipe, one end of which opens into a gaseous fluid, to avoid water hammer or thermal problems in such a pipe.

Claims (5)

1. Device for maintaining a liquid in a substantially horizontally axed duct (28), having an outlet end issuing into a gaseous medium, when the flow of said liquids below a given threshold, characterized in that the device (38) comprises four tubes (42a to 42d) placed in the duct (28) and wound helically about a central core (40) arranged along the axis of duct (28), each of these tubes having at least one low point for which the tube is totally positioned below the duct axis and a high point for which the tube is completely above said axis and has an opening (48b to 48d) communicating with the outlet end of the duct, the high point being closer to the outlet end than the low point, the tubes (42a to 42d) and central core (40) being supported within duct (28) by a tight support plate positioned between the duct (28) and said tubes at one end of the latter opposite to said duct outlet end.

2. Device according to claim 1, characterized in that the ends of the central core (40) are streamlined.

3. Device according to either of the claims 1 and 2, characterized in that the said opening is a hole (48b to 48d) formed in the highest point of the wall of tube (42b to 42d).

4. Device according to either of the claims 1 and 2, characterized in that the other end of tube (42a, 42'a to 42'd) is located at said high point and forms said opening.

Images