EP0045034B1 - Device to avoid the formation of fissures on the inner side of feedwater inlet nozzles of a pressure vessel - Google Patents

Description

The invention relates to a device for avoiding the formation of cracks on the inner surfaces of feed water line nozzles opening into pressure vessels, in particular nuclear reactor pressure vessels or steam generators, according to the preamble of claim 1.

Such a device is known for steam generators of nuclear reactors from US-A-3661 123.

If cooler medium is replenished in a system (container, pipeline) that is filled with an elevated temperature medium, i.e. a feed water pipe socket, the horizontal connecting piece will be stratified by colder and warmer medium if the Feeding with low throughput in relation to the size of the connection cross-section, d. H. small flow velocities. The same happens when the medium in the system to be fed is already in an evaporated state. The stratification occurs because the lighter (warmer) medium can flow back into the upper part of the flow cross-section of the nozzle due to its buoyancy against the colder water to be fed. The respective temperature differences of the two media lead to thermal stresses in the connecting piece or in the connecting line, which are usually already highly stressed by the internal pressure of the system, so that material fatigue and thus crack formation can occur with a sufficiently high number of cycles of the feeding processes. The phenomenon of temperature stratification could be demonstrated by temperature measurements on the feed water pipe socket of a steam generator for pressurized water reactors.

In the known device, the thermal voltage problems mentioned at the outset can still occur in partial areas of the feed water line connecting piece. Decisive for this is - as was found - a certain dimensioning of the feed water pipe socket, namely its horizontal distance An from the pressure vessel inner wall to the overflow edges, and its inside diameter D _i , whereby the ratio A _{i /} D _i should be as small as possible.

The invention has for its object to provide the device of the type mentioned in such a way that with weak feeding of the feed water pipe connection, as occurs in part-load or no-load operation of a system, material fatigue and thus crack formation on the load cycles even under higher load cycles Trim is excluded with certainty. According to the invention, the object is achieved by the features listed in the characterizing part of claim 1. An advantageous further development of the subject matter of claim 1 is described in dependent claim 2. The advantages achievable with the invention can be seen above all in the fact that a backflow of the specifically lighter medium in the fed system against the heavier (still cold) medium to be fed in is prevented. The invention solves not only the problems of thermal stresses and cracking that occur in particular under no-load and low-load operation, but also the problems that arise during start-up and shutdown operation.

• Fig. 1 in einem Längsschnitt einem Dampferzeuger für Druckwasserreaktoren mit einem nach Anspruch 1 ausgebildeten Speisewasserleitungsstutzen;
• Fig. 2 die Einzelheit X aus Fig. 1 vereinfacht;
• Fig. den Schnitt nach der Linie III-III aus Fig. 2;
• Fig. 4 in entsprechender Darstellung zu Fig. 2 eine andere Ausführung der Einrichtung, die in Stutzenachsrichtung besonders flach baut;
• Fig. 5 den Schnitt nach der Linie V-V der Fig. 4;
• Fig. 6 eine weitere Ausführung, wobei ein Teilstück der Druckbehälterwand mitgezeichnet ist, besonders hohen Strömungsquerschnitts;
• Fig. den Schnitt nach der Linie VII-VII der Fig. 6 und
• Fig. 8 eine vierte Version mit einem abwärts führenden Zuleitungsstück und Auffangtasse sowie
• Fig. 9 eine Tabelle der dargestellten Größen An und D_i.

In the following, the invention is explained in more detail with reference to the drawing showing several exemplary embodiments and the mode of operation is described. Therein shows in a schematic, simplified representation omitting the parts not necessary for understanding the invention:

• Figure 1 is a longitudinal section of a steam generator for pressurized water reactors with a feed water pipe socket designed according to claim 1.
• Fig. 2 simplifies the detail X from Fig. 1;
• Fig. The section along the line III-III of Fig. 2;
• FIG. 4, in a representation corresponding to FIG. 2, another embodiment of the device which is particularly flat in the direction of the nozzle axis;
• 5 shows the section along the line VV of Fig. 4.
• 6 shows a further embodiment, a section of the pressure vessel wall being drawn, with a particularly high flow cross section;
• Fig. The section along the line VII-VII of Fig. 6 and
• Fig. 8 shows a fourth version with a downward lead piece and collecting cup and
• Fig. 9 is a table of the sizes An and D _{i shown} .

The steam generator DE for pressurized water reactors according to FIG. 1 (hereinafter abbreviated as DE) has a pressure vessel housing 1 with a primary chamber region 1.1, an evaporator region 1.2 with the U-shaped heat-exchanging tubes 2 and a conically widening housing - Transition area 1.3 adjoining separator area 1.4. The tube sheet 3 welded into the housing 1 and the heat exchanger tubes 2 welded into it and held by it separate the primary chamber I from the secondary chamber 11 in a gas-tight manner. The primary chamber is formed by a base cap 4 welded to the tube sheet 3 and having an inlet connection E and an outlet connection A, whereby the inflow space e1 of the primary chamber is separated from the outflow space a1 by an arched partition 5. Of the tubes 2 of the tube bundle 2 ', only the outer and inner are indicated by lines; the pipe bends are marked 2.1, the inner pipe lane is marked 2.2.

The primary medium (water) heated in the core of the pressurized water reactor, not shown is supplied to the primary chamber 1 via the inlet connection E at a temperature of approx. 316 ° C. and under a pressure of 155 bar, flows through the heat-exchanging pipes 2 and is discharged via the outlet chamber a1 and the outlet connection A at a temperature of approx. 290 ° C. fed back to the reactor pressure vessel.

The tube bundle from the heat-exchanging tubes 2 is held in a vibration-proof manner by means of tube holding grids 6 spaced axially from one another; it is surrounded by a hollow cylindrical jacket 7 which, together with the wall 1, forms an annular fall space 8. Since the jacket 7 is arranged at a distance a2 from the tube sheet 3, the drop chamber 8 has a flow connection at its lower end via the flow passages 8.1 with the evaporation chamber inside the jacket 7. The jacket 7 is closed at its upper end by an attachment 9, which carries on its top a battery of water separators 10, into which the water-steam mixture from the evaporation chamber II enters through appropriate flow channels. The ejected water, the water level of the circulating water is indicated at 11, is fed directly to the drop room 8. The ring line 12, which is arranged at the upper end of the drop space, is used via openings (not shown) for introducing the feed water from a feed water line connection piece 13 via a substantially perpendicular connecting pipeline 14. The largely dewatered steam emerging from the water separators 10 on the upper side then passes still in fine separator 15 and from these via the live steam line connector 16 of the steam dome 17 to the steam turbines, not shown.

The DE works according to the natural circulation principle. The feed water and the separated water flow mixed in the fall chamber 8 down into the evaporation chamber II and rise in this under evaporation (wet steam). The water-steam mixture then passes into the coarse separator 10 and then into the fine separator 15, as already explained. For the introduction of the feed water via the nozzle 13 and the connecting pipe 14, a very specific flow guide is provided, which is explained in more detail with reference to FIGS. 2 and 3.

The feed water is introduced into the water-steam space of the DE via an essentially horizontally running line part 140 and a subsequent rising line part 141, which is designed as a pipe bend, up to overflow edges Ü at the end of the flow path of the rising line part 141. From there the feed water, see flow arrows f1, via a downward line piece 142 and the feed ring line 12 connected thereto (FIG. 1) to the water-steam space, i. H. in this case, the drop room 8 of the DE, mixed. The line part 142 is dome-shaped and comprises the line part 141 as a kind of bell. The line part 140 can be held in the connection piece 13 (FIG. 1) in the manner of a thermosleeve pipe. Due to the described line routing, a return flow of already warmed feed water back into the line part 140 can no longer take place, whatever the load condition, because the colder inflowing feed water, because of its higher specific weight, first has to completely fill the nozzle cross-section before it overflows the higher overflow edges Ü reached.

The flat, compact design according to FIGS. 4 and 5 is recommended for steam generators or reactor pressure vessels in which there is little space available in the direction of the nozzle axis. The same parts have the same reference numerals. Here, the rising line part is formed by a trough 141 'with a rectangular cross section, which is flat and box-shaped, that is to say forms a water box, on the two narrow side upper edges of which the overflow edges U are arranged. This water box 141 'is surrounded by a likewise box-shaped structure 142, rounded at its upper edge, for the downward line piece, which can also be curved in accordance with the inner circumferential curvature of the pressure vessel, and opens into the ring line 12 at its lower end via a narrowed neck piece 143 .

6 and 7 is intended for an even higher feed water throughput at full load. The overflow edges Ü are not only the upper side edges 141.1, but also the edge 141.2 on the long side of the line part 141. Accordingly, the flow cross section of the downward line section 142 is larger than that of FIGS. 4 and 5. In addition, it can be seen that the Inner circumference of the feed water pipe socket 13, which is welded into the housing wall 1 of the DE by means of a ring weld 18, is lined with the essentially horizontally running pipe part 140, which is designed as a thermosleeve pipe. This thermosleeve pipe can be of a conventional design or configured in accordance with FIG. 2 of DE-A-2 346 411.

In Fig. 8 it is also shown that an essentially horizontally extending line part 140 can be followed by a substantially downwardly extending line part 145, which opens into a collecting cup 141 ", which opens the ascending flow paths up to the overflow edges Ü has.

As tests have shown, it is essential for the prevention of temperature stratification due to a backflow of warmer water that a certain ratio A _n / D _i is maintained. An means the horizontal distance of the pressure vessel inner wall 1i from the center line M _ü running through the center of gravity of the cross-sectional area F _ü spanned by the overflow edges Ü. D; means the inside diameter of the feed water line 140 opening into the pressure vessel DE. The stated ratio A _{n /} D; should be as small as possible and is within the limits between 0.5 and 2.

In Figures 2 and 3, the pressure vessel inner wall 1 is dashed and the center line M _{u is shown in} dash-dot lines, and the dimension lines for the distance A _n = A ₁ and the inner diameter D; registered. For the exemplary embodiment according to FIGS. 2 and 3, the table according to FIG. 9 shows A ₁ = 4.70, D _i = 3.70 and accordingly A _{n /} D _i = 1.27.

Even more favorable values for the ratio A _n / D _i result for the exemplary embodiments according to FIGS. 4, 5 and 6, 7. When comparing FIG. 4 and the table according to FIG. 9, a value A ₂ results = 2.15, a value D _i = 3.55 and accordingly a ratio value A _n / D _i = 0.61. This favorable value results from the compact, flat design of the pipeline parts 141 ', 142. Correspondingly favorable values result for the exemplary embodiment according to FIGS. 6 and 7 with A ₃ = 1.50, D _i = 2.50 and A _n / D _i = 0.60.

The embodiment according to FIG. 8 shows in connection with FIG. 9 that the ratio A _n / D _i is in the range of the upper limit value of 2. An advantage of this example is the relatively high flow cross-section and the cylindrical symmetrical shape, the latter of which is also given in the exemplary embodiment according to FIGS. 2 and 3. In general it can be said that cylindrical symmetrical forms allow a higher pressure load; box-shaped cross sections, on the other hand, are lower in the compressive strength for a given wall thickness, but the extension in the direction An is lower. 1 to 3, where a relatively low ratio A _n / D _i of 1.27 is achieved and a relatively high compressive strength with a sufficient flow cross-section are given. In comparison, the other exemplary embodiments can be regarded as special designs in which either the ratio A _n / D _i is kept particularly low (FIGS. 4 to 7) or the flow cross section in the overflow area is particularly large (FIG. 8).

Overall, short flow paths up to the overflow edges Ü can be achieved by the invention in relation to the line cross section, so that the feed water has no opportunity to warm up appreciably on the way to the overflow edges. The warming is counteracted by the volume flow of the feed water, which is characterized by the value D _i , which is the denominator of the ratio. The invention makes it possible in a relatively simple manner to make the path and thus the dwell time of the feed water, with a small throughput in relation to the flow cross-section, on the way of the feed water to the overflow edges so small that harmful heating and thus the consequences of temperature stratification and a circulation flow up to the feed water nozzle are avoided.

The table according to FIG. 9 is based on the cm values for A _n and D _i , as can be seen from the approximately true-to-scale drawing. In order to get an idea of practical dimensions in natural size, one must know that the pressure vessel of the steam generator DE according to FIG. 1 has an outer diameter of approximately 4800 mm in its separator area 1.4 (steam dome), so that it can be used for enlargement 2 and 3 entered sizes A, and D _{i give} values of about 500 mm and 400 mm, respectively. The same applies to the natural sizes of the A _µ and D _j values of the other figures. The steam generator shown in Fig. 1 serves z. B. together with three other steam generators in a 4-loop arrangement to generate the working steam in a 1200 MW _el pressurized water nuclear power plant.

Claims (2)

1. An arrangement for avoiding the formation of cracks at the inner surfaces of feed water piping end pieces opening into pressure containers, in particular nuclear reactor pressure containers, or steam generators, in which the introduction of the feed water into the water- steam-space of the pressure container is effected through a piping section (140), which runs essentially horizontally, and a succeeding rising piping section (141), up to overflow edges (U) at the end of the flow path of the rising piping section, from where the feed water is admixed with the liquid medium to be evaporated in the pressure container (DE), by way of a downwardly directed piping section (142) and if necessary by means of an annular feed pipe (12) connected thereto, characterised in that the ratio A_n/D_i lies in the range of from 0.5 to 2, so that a temperature stratification as a result of a return flow of warm water in the feed water piping end piece is avoided, where

A_n represents the horizontal distance of the inner wall of the pressure container from the central line (M_ü) running through the central point od the cross-sectional area (Fü) defined by the overflow edges (Ü), and

D; represents the internal diameter of the feed water piping section (140) opening into the pressure container.

2. An arrangement according to Claim 1, characterised in that the ratio A_n/D_i ist from 0.6 to 1.5.

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