A DEVICE FOR STEAM GENERATORS

Abstract: The invention relates to a device for steam generators of the type which in a roof-forming upper part (1) comprises a nozzle (2) in order to lead out water steam from the interior of the generator into a subsequent outlet pipe (3). On the bottom side of the roof part (1), a cup-shaped part (10) placed centrally in relation to the nozzle (2) is arranged with a plurality of mutually separated openings (11) having the purpose of distributing the flow of water steam passing through the cup part into a plurality of equivalent part flows with the object of bringing about a stable main flow through the nozzle; all with the purpose of counteracting unstable flows in the nozzle together with the pipe.

Description: A DEVICE FOR STEAM GENERATORS Technical Field of the Invention This invention relates to a device for steam genera- tors of the type which in a roof-forming upper part comprises a nozzle having the purpose of leading water steam out from the interior of the generator in a subsequent outlet pipe and in conjunction with this bringing about pressure recovery, and which has an inlet neck with a smallest hole area from which an upwardly diverging cavity extends, forming a transition to the outlet pipe, as well as a guide surface widening downwards and having an arched, trumpet-like shape which forms a transition to a lower surface on said upper part. Background of the Invention A problem, which is difficult to master in the steam generators of nuclear power plants, is that unstable flows during certain circumstances may arise in the nozzle included in the roof of the generator building and in the subsequent, outgoing pipe. This risk for unstable flows is especially marked when said pipe is angular; something which usually is the case, because of the fact that the building-in space above the generator is limited. In the technology area in question, there are different theories about the reasons for the emer- gence of the unstable flows as well as how the problems should be fought. An undisputed factor, which contributes to the emergence of unstable flows, is constituted by the so called separation in association with the diverging, usually cone- shaped wall surface in the nozzle. Such separation manifests itself in that individual particles included in the steam suddenly and abruptly comes off from the nozzle wall and are thrown into the flow, where they give rise to vortex formations and/or turbulence phenomena. Another factor is that irregularly shaped and placed construction details are built- in in the upper part of the generator house. Thus, it is usual that box-like components for separating water from the steam are present in the upper space in the generator house. Such construction details gives rise to the fact that the flow of steam, which is brought in the direction to the centrally placed nozzle, becomes uneven or inhomogeneous . In case the steam main flow passing the nozzle is markedly inhomogeneous, the tendency of separation of the above-mentioned type is increased and thereby also the emergence of unstable flows in the pipe. The consequence of unstable flows is that vibrations arise in large parts of the flow-carrying structure in which the pipe and the nozzle are included. These vibrations in turn expose the structure for mechanical stresses that may be risky. In particular, there is a risk that existing welds are fatigued. Objects and Features of the Invention The present invention aims at managing or at least reducing the risk for emergence of problems of the above-men- tioned type. Thus, a primary object of the invention is to provide a device which, with simple and inexpensive means, can counteract irregularities in the inlet flow of the steam to the nozzle with the ultimate purpose of eliminating the risk for unstable flows in the outlet pipe and other parts of the flow-carrying structure. Another object is to provide a device which not only may be integrated in newly built steam generators, but also be mounted afterwards in existing steam generators . According to the invention, at least the primary object is attained by the features defined in the characterizing clause of claim 1. Preferred embodiments of the invention are furthermore defined in the dependent claims. Brief Description of the Appended Drawings In the drawings: Fig 1 is a vertical section through the upper part of a steam generator together with a nozzle included therein and a device according to the invention mounted in association therewith, Fig 2 is a planar view from below in fig 1 illustrating a first embodiment of the device according to the invention, Fig 3 is an analogous planar view showing a second, alternative embodiment, Fig 4 and 5 are partial views from below showing two further alternative embodiments of the device according to the invention Fig 6 is a section corresponding to fig 1 showing a further alternative embodiment of the device, and Fig 7 is a perspective exploded view of the device shown in fig 6. Detailed Description of Preferred Embodiments of the Invention In fig 1 numeral 1 generally designates a roof-forming upper part of a steam generator which is not closer shown in other respects. In this roof part a nozzle designated 2 is included. To the nozzle 2, an outgoing angularly shaped pipe 3 is connected. Fluid in the form of water steam is fed with high speed (usually more than 100 m/sec) from the interior of the generator through the nozzle 2 and further on through the outgoing pipe, as is outlined by the arrows. The nozzle includes a diverging cavity 4, which in the example is assumed to have a cut down cone-shaped basic shape, i.e. the cavity is delimited by a cone-shaped wall surface 5. In the area of the bottom part of the nozzle there is an inlet 6, most often denominated neck, having a smallest diameter. The cavity 4 widens on the downstream side of said neck. There is a rotary symmetric, arched guide surface 7 which widens in a trumpet- like way from the neck in the direction to the interior of the steam generator. It should also be mentioned that the nozzle 2 is centrally placed in relation to the generator house. The neck 6 may have a diameter of the magnitude of 400 mm at the same time as the wider outlet mouth 8 may have a diameter of 550 mm. The cone angle of the cavity 6 may then amount to 15°. At such dimensions, the pipe 3 may have an inner diameter of 750 mm, the thickness of the tube wall being within the range of 50-80 mm. It should also be pointed out that the inlet end of the pipe 3 is united to the outlet end of the nozzle by a circumfering weld 9. At a certain distance under the roof part 1 of the generator house, there are irregularly placed construction details built-in in the house which are not closer shown, since they are trivial. However, the consequence of said con- struction details is that the flow of steam approaching the nozzle becomes irregular or inhomogeneous to its nature. According to the present invention, a cup-shaped part or structure 10 is arranged on the bottom side of the roof part 1, centrically placed in relation to the nozzle 2. Said cup-shaped part has a plurality of mutually separated openings 11 having the purpose of distributing the flow of water steam passing through the cup part in a plurality of part flows with the purpose of bringing about an even or homogeneous main flow through the nozzle. These openings may be uniformly distributed. In the embodiment shown in fig 1 and 2 the cup part 10 is in the shape of a lattice work in which the openings 11 are wedge-shaped and defined between long narrow elements 12 extending between a central area and a peripherical area of the cup part. In the example, the individual elements 12 are at their interior ends jointly connected by means of a central hub-like member 13 having at least some radial extension. At their outer ends, the elements 12 are directly connected to the lower surface 14 on the roof part 1. In other words, the free end of each element is pointwisely united to the roof part, e.g. by means of welding. This means that the flow of steam is not disturbed in the area between two adjacent element ends, in that the surface 14 there is smooth. It should also be noted that the elements 12 in the embodiment according to fig 1 and 2 are radially directed and equidistantly placed. In this case, ten long narrow elements are included in the device, the angle between adjacent elements throughout being 36°. As may be seen in fig 1, the cup part 10 composed of the elements 12 has an arched shape. However, this arched shape is not spherical but rather determined of a bending radius that is largest in the central area so as to thereafter successively decrease in the direction towards the periphery. In that manner, the water steam will flow in in a similar way irrespective of the direction of flow. Therefore, a peripheral flow which sweeps under the bottom side of the roof part will meet the geometric arch of the cup part at an angle of approximately 90° similar to a central flow in the direction vertically from below. The embodiment according to fig 3 differs from the embodiment according to fig 2 only in that the elements 12 at the interior ends thereof extend tangentially out from the hub member 13 made as a trundle. By the fact that the elements are positioned in this way, the device will also operate as a rotary producer which applies at least some rotary movement to the passing flow of steam before it enters the nozzle. In the drawing, the long narrow elements 12 which together forms a cup-shapedly arched lattice are shown only schematically in the shape of evenly thick bars. In practice, however, the cross-section shape of the individual element may be realized in many different ways. The elements may, for instance, be made as wings having a flattened cross-section shape . Reference is now made to fig 4 and 5 which schematically illustrate how the cup part 10 may also be in the shape of an arched sheet-metal plate or board in which a plurality of holes 11' are recessed which form the steam distributing openings. In the two the embodiments, these holes 11' are arranged in a plurality of ring-shaped sets which are radially separated. In the individual set, the holes should be confor- mal and of the same size as well as be equidistantly placed. In the embodiment according to fig 4, the holes 11' in the outer ring set are somewhat larger than the holes in the outermost but one set, and then the holes in the sets being inside thereof successively decreases in size. In the embodiment according to fig 5, all holes are substantially of the same size irrespective of the radial location thereof. It should be pointed out that the holes 11' according to fig 4 and 5 may also be distributed in such geometric patterns that the water steam during the passage thereof through the cup part is subjected to a certain rotary movement. When the irregularly flowing water steam arrives towards the device according to the invention, the flow will be distributed in a plurality of equivalent part flows which after passage is reintegrated in a continuous main flow being substantially homogeneous or conformal irrespective of the flow taking place linearly or with some rotary movement. This counteracts the risk for separation along the nozzle wall and thereby the risk for emergence of unstable flows in the nozzle together with the subsequent pipe. In fig 6 and 7 an alternative embodiment is shown of the device at which the arched, substantially cap-shaped cup part 10 is composed of a plurality of wedge-shapedly tapering, sector-shaped part boards 15 each one which having an arched shape. In practice, these part boards may consist of sheet- metal plates which along converging side edges has bent flanges 16 which may be interconnected by means of suitable joints, e.g. welding or screw oints. Thus, a number of holes 17 are outlined in the drawing for screw joints which are schematically illustrated by a screw 18 and a nut 19. In said sheet-metal plates 15, radially separated sets of holes 11" are provided. As may be clearly seen in fig 7, tne hole^ which are located on one hand closest to the peripheral edge of * αe part board and on the other hand near tne taperj ng tip of the board are smaller than the holes which are placed in the area approximately halfway between the peripheral edge and the tip, respectively. As is schematically illustrated by the curve graph A in fig 6, these varying hole sizes result m the fact that the share of holes per surface unit becoming largest in an area between the centre and periphery of the cup part. In practice, this hole area distribution entails that the liquid flow becomes larger in a ring area, the diameter of which occurs within the range of 40-70 % of the outer diameter of the cup part. This is something which has shown to give an optimally stable main flow through the nozzle. Feasible Modifications of the Invention The invention is not solely restricted to the embodiments described above and shown in the drawings. Thus, the cup part made in the shape of a lattice may be made in another way than those which are exemplified in fig 2 and 3. For instance, individual, substantially radial elements may be mutually united by means of rings having different diameters instead of by a central hub member. The lattice may also have a net-like character or be provided similar to a floor board lattice. Also the board-shaped basic embodiment according to fig 4 and 5 may be modified in many ways, above all in respect of the choice of shapes and dimensions of the individual, small holes. Here it may be specially mentioned that the holes in the external, peripheral set of holes may also be outwardly open instead of being concluded at some distance inside the peripheral edge of the arched board.

Claims: Claims 1. Device for steam generators of the type which in a roof- forming upper part (1) comprises a nozzle (2) having the pur- pose of leading water steam out from the interior of the generator in a subsequent outlet pipe (3) and in conjunction with this bringing about pressure recovery, and which has an inlet neck (6) with a smallest hole area from which an upwardly diverging cavity (4) extends, forming a transition to the out- let pipe (3), as well as a guide surface (7) widening downwards and having an arched, trumpet-like shape which forms a transition to a lower surface (14) on said upper part (1), c h a r a c t e r i z e d in that on the bottom side (14) of the roof part (1) a cup-shaped part (10) is arranged cen- trally in relation to the nozzle (2) said part having a plurality of mutually separated openings (11, 11') having the purpose of distributing the flow of water steam passing through the cup part into a plurality of part flows with the object of bringing about a stable main flow through the noz- zle, the arched shape of the cup part being determined by a bending radius which is largest in a central area so as to thereafter successively decrease in the direction towards a peripherical area of the cup part. 2. Device according to claim 1, c h a r a c t e r i z e d in that the cup part (10) is in the form of a lattice in which openings (11) are delimited between long narrow elements (12) which extend between the central area of the cup part and the peripheral area thereof. 3. Device according to claim 2, c h a r a c t e r i z e d in that inner ends of the individual elements (12) are interconnected by means of a central, mutual hub member (13) and, at outer ends thereof, directly connected with the bottom side (14) of the roof part (1) . 4. Device according to claim 2 or 3, c h a r a c t e r i z e d in that the long narrow elements (12) are radially directed. 5. Device according to claim 3, c h a r a c t e r i z e d in that the long narrow elements (12) at the inner ends thereof extend tangentially out from the hub member (13) in order to apply rotary movement to the main flow. 6. Device according to claim 1, c h a r a c t e r i z e d in that the cup part consists of an arched sheet in which holes (11') are recessed which form the steam distributing openings. 7. Device according to claim 6, c h a r a c t e r i z e d in that the holes (11') are arranged in a plurality of ring- shaped sets which are radially separated. 8. Device according to claim 6 or 7, c h a r a c t e r i z e d in that the share of holes (11) per surface unit is largest in an area between the centre and periphery of the cup part. 9. Device according to any one of claims 6-8, c h a r a c t e r i z e d in that the arched sheet forming said cup part is composed of a plurality of wedge-shapedly tapering part boards (15) .

Inventors: HENRIKSSON MATS, HENRIKSSON, MATS
Applicants: HENRIKSSON MATS, VATTENFALL AB PUBL, HENRIKSSON, MATS, VATTENFALL AB (PUBL)
Application year: 1998
Main classification: F22B37
IPC codes: F22B37