Cross flow baffle for a steam generator

[57] ABSTRACT

A steam generator having a U-shaped tube bundle and a baffle and tube support which has circulation holes drilled in a selective pattern to promote cross flow across the hot leg of the tube bundle to eliminate chemical hideout and poor flow distribution.

Claims, 7 Drawing Figures

<img file="US3867908A_D0001.tif" />

PATENTED FEB 2 51875

3,867,908

<img file="US3867908A_D0002.tif" />

3,867,908

CROSS FLOW BAFFLE FOR A STEAM GENERATOR

BACKGROUND OF THE INVENTION

This invention relates to steam generators for nuclear power generating plants and more particularly to a cross flow baffle disposed adjacent to tube sheet to eliminate chemical hideout and improve flow distribution.

Nuclear steam generators, like fossil fuel steam generators, have been subjected to tube failures. Recent compilation of data for numerous installations has shown a tendency for tube failures to occur within a region adjacent the center of the hotleg of the steam generators tube bundle.

It is hypothesized that a treatment void is created at the center of the hotleg adjacent the tube sheet due to buoyancy effects which subject the tube to cyclic temperature excursions which produce thermal stresses that result in a premature tube failure. Thus, by eliminating these voids the tube life can be appreciably extended.

SUMMARY OF THE INVENTION

Among the objects of this invention may be noted the provision of a baffle which will channel the flow of the secondary fluid toward a central portion of the hotleg of the tube bundle to eliminate chemical hideout and improve flow distribution over the hotleg thus minimizing premature tube failure.

In general, a vapor generator, when made in accordance with this invention, comprises a shell portion, a tube handle having a plurality of tubes disposed within the shell portion, a wrapper disposed within the shell so as to generally enwrap the tube bundle, a tube sheet disposed on one end of the tube handle, a head portion cooperatively associated with the shell and tube sheet, a primary fluid inlet nozzle for supplying primary fluid to the inside of the tubes, a secondary, fluid inlet nozzle for supplying secondary fluid to the inside of the shell and the outside of the tubes, and a baffle disposed parallel to the tube sheet and cooperatively associated with the wrapper so that the influent secondary fluid sweeps cross the tube sheet as it flows between an area between the baffle and the tube sheet. The baffle has a plurality of tube holes through which the tube pass and a plurality of circulation holes through which the influent secondary fluid flows. The circulation holes are disposed in a plurality of zones, a first zone adjacent the center of the baffle which generally has a maximum number and size of circulation holes to generally form a maximum amount of open area for the flow of secondary fluid through the first zone and a second zone disposed adjacent the first zone, the second zone having circulation holes arranged to form substantially less than the maximum amount of open area for the flow of secondary fluid therethrough.

BRIEF DESCRIPTION OF THE DRAWING

The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings, in which corresponding reference characters indicate corresponding portion throughout the drawings, and in which:

FIG. 1 is a sectional view of a steam generator having a pre-heater and a baffle made in accordance with this invention;

FIG. 2 is a section taken on line II—II of FIG. 1;

FIG. 3 is a partial sectional view of a steam generator without a pre-heater and having a baffle made in accordance with this invention;

FIG. 4 is a sectional view taken on line IV—IV of FIG. 3;

FIG. 5 is an enlarged partial sectional view showing an arrangement of flow holes;

FIG. 6 is an enlarged partial sectional view showing an alternate arrangement of flow holes; and

FIG. 7 is an enlarged partial sectional view showing still another alternate arrangement of flow holes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in detail, FIG. 1 shows a steam or vapor generator 1 having a vertical shell portion 3 and a plurality of U-shaped tubes 5 forming a tube bundle 7 which is disposed within the shell 3. The shell has a lower portion 9 having one diameter, an upper portion 11 having a larger diameter, and a frustoconical portion 13 serving as a transition member between the upper and lower portion 11 and 9, respectively. The lower portion 9 of the shell 3 is enclosed by a spherical head portion 15. The head portion 15 has an integral tube sheet 17 which receives the ends of the U-shaped tubes 5. A dividing plate 19 separates the head into two generally equal compartments 21 and 23. The inlet compartment 21, the one on the left as shown on the drawings, has an inlet nozzle 25 for primary fluid and the outlet compartment 23, the one on the right as shown on the drawings, has an outlet nozzle disposed therein, however, the outlet nozzle is not shown. A sleeve or wrapper portion 27 enwraps the tube bundle and forms an annular chamber 29 between the wrapper 27 and the shell 3. A pre-heater portion 31 is disposed adjacent the tube sheet 17 on the coldleg portion of the tube bundle 7, the portion shown on the right in the drawings. The pre-heater 31 is an enclosed chamber formed by a portion of the wrapper 27, a bottom circular closure plate 33 disposed adjacent and parallel to the tube sheet, an upper closure plate 34 disposed parallel to the bottom closure plate, and a vertical closure plate 35 extending between the bottom and upper closure plates 33 and 34, respectively. A secondary fluid or feed water inlet nozzle 37 is disposed in the shell adjacent the pre-heater and has a sleeve 41 which directs the inlet water to the pre-heater and impact plate 43 which directs the inlet flow downwardly and prevents the high velocity inlet secondary fluid from impinging on the tubes. A plurality of horizontal baffles 45 are so disposed within the pre-heater to cause the influent secondary fluid to follow a sinuous path as it flows upwardly to an outlet portion 47, which discharges the pre-heated secondary fluid into the annular chamber 29.

A horizontal baffle and tube support 51 which is disposed adjacent and parallel to the tube sheet, generally extends over the hotleg portion of the tube bundle 7 and has a plurality of tube holes 53 and circulation or flow holes 55 disposed in prearranged pattern.

The end of the wrapper 27 adjacent the tube sheet 7 has a plurality of notches 57 so disposed to direct the secondary fluid so that it sweeps across the tube sheet

3,867,908 prior to flowing upwardly through the circulation holes 55 and then along the outer surface of the tubes not contained within the pre-heater. Some of the notches 57 are provided with adjustable flow gates 59 which are adapted to change the open area of the notches to adjust the flow of the pre-heated secondary fluid across the tube sheet 17.

The wrapper 27 generally parallels the shell and flairs outwardly along with the transition portion 13 and has a domed end enclosure 60 disposed in the lower portion of the upper portion 11 of the shell 3. The domed enclosure 60 and the wrapper 27 thus forms a chamber 61 above the tube for collecting steam or vapor produced by the steam generator. The upper portion 11 of the shell is closed by an elliptical head portion 63. A steam or vapor outlet nozzle 65 is centrally disposed in the elliptical head portion 63 and provides a discharge port for vapor produced in the steam generator. Two diametrically opposed man-ways 67 provide access to the interior of the upper portion of the shell which houses two separate moisture or liquid separators 69 and 71. The first separator 69 is a centrifugal type separator and comprises a plurality of riser tubes 73 in communication with the chamber 61 and extends upwardly from the domed end portion 60 of the wrapper 27. The riser tubes 73 have swirl vanes 75 disposed therein. The swirl vanes 75 extend radially from a centrally disposed hub portion 77 which is disposed adjacent the upper end of the riser tube 73. A closure plate 79 is disposed slightly above the upper end of the riser tubes 73 and has orifices 81 which register with the riser tubes 73. Collars 83 extend through the orifices 81 and extend downwardly therefrom. Sleeves 85 encircle the riser tube and extend from the closure plate 79. The lower end of the sleeve 85 is disposed above the domed enclosure 60 providing a lower discharge port. The upper ends of the sleeves have tangential discharge ports 87 disposed adjacent the closure plate 79.

The second moisture separator 71 comprises two groups of hook and pocket vane-type or chevron separators disposed in parallel, one group being disposed above the other. The hook and pocket vane-type separators 71 have drip-leg 89 depending therefrom, the lower end of the drip-leg 89 has a seal cup 91 connected thereto and the discharge from the seal cups 91 joins with the moisture from the centrifugal separators and flows into the annular chamber 29.

As shown in FIG. 2, the baffle 51 is generally semicircular shaped and has circulation holes 55 disposed in zones Zl, Z2, and Z4. Each zone incorporates circulation holes 51 which are a particular size and are arranged to provide an open area which is a percentage of the maximum allowable open area that can be obtained and still provides a matrix which will support the tubes 3.

Zone Zi, the first zone, is disposed adjacent the center or in the central portion of the baffle 51 and generally contains the maximum number and size of circulation holes to generally provide a maximum amount of open area for the flow of secondary fluids through the circulation holes 55 disposed in zone Zi.

Zone Z₂, the second zone, is disposed adjacent and generally encircles zone Zj. Zone Z₂ generally has circulation holes 55 arranged to form substantially less than the maximum amount of open area for the flow of secondary fluids through the circulation holes contained in zone Z₂. The zone Z₂ preferably provides an open area of about 20 to 50 percent of the maximum open area that could be provided. Such a zone may be formed by deleting a certain number of circulation holes 55 as shown in FIG. 6 or by reducing the size of the circulation hole 55 as shown in FIG. 7 or by a combination of deleting circulation holes and reducing their size.

A peripheral zone Z₄ comprises a band of circulation holes 55 and is disposed adjacent the peripheral edge of the baffle 51. Zone Z₄ contains circulation holes 55 arranged to generally provide the maximum open area for the flow of secondary fluid, as shown in FIG. 5.

FIGS. 3 and 4 show a portion of a steam generator which does not have a pre-heater. Therefore the baffle 51' adjacent the tube sheet is generally circular.

The second fluid inlet nozzle 37 directs the influent secondary fluid to the annular chamber 29 between the wrapper 27 and the shell 3. The influent secondary fluid flows downwardly through the port 57 and sweeps the tube sheet 17 and then flows upwardly through the circulation holes 55 in the baffle 51'.

The circulation holes 55 are arranged in a plurality of zones Z_n Z₂, Z₃, and Z₄, each with a predetermined number of circulation holes 55 of various size to provide zones having different percentages of open areas through which the secondary fluid can flow.

Zone Zj generally has circulation holes arranged to generally provide the maximum amount of open area through which the secondary fluid can flow and is generally disposed in the central portion of the baffle 51. Zone Z₂ is disposed adjacent zone Zi and generally encircles zone Ζ_υ Zone Z₂ generally has circulation holes 55 arranged to generally provide about 20 to 50 percent of the maximum amount of open area that could be provided for the flow of secondary fluid therethrough.

Zone Z₃ generally has circulation holes 55 arranged to generally provide about 0 percent to 30 percent of the maximum amount of open area that could be provided for the flow of secondary fluid therethrough.

Providing a baffle plate 51 or 51', as hereinbefore described, with a plurality of zones having various amounts of open area advantageously channels influent secondary fluid to areas which have been particularly susceptible to premature failure to reduce chemical hideout and improve the flow distribution of the secondary fluid to substantially reduce premature failure of the tube adjacent the tube sheet.