Steam generator for a pressurized-water power station

A generator for the production of both superheated steam and saturated steam has an outer casing formed by an inner cylindrical shell engaged within an outer cylindrical shell, the shells being closed by dished ends. An annular header for the admission of pressurized primary water is formed between an upper annular tube plate and a partition-wall while an outlet header for the discharge of primary water is formed by a central tube plate mounted within the inner shell. A bundle of parallel tubes for the circulation of primary water extends vertically upward from the central tube plate. The end portion of each tube has the shape of a crook in order to join the tubes to the annular tube plate in uniformly spaced relation. The secondary water is admitted into the casing by means of ducts formed in the inner shell near the central tube plate. The steam generated is discharged through ducts formed in the outer shell.

This invention relates to a steam generator in which a primary fluid 
consisting of pressurized heavy water or light water passes through a tube 
bundle and exchanges heat with a secondary fluid also consisting of water 
in order to convert this latter into saturated or superheated steam. A 
generator of this type is preferably adapted to operate in conjunction 
with an installation for the production of electric power by expansion of 
steam delivered by turbines in which the primary fluid is withdrawn from 
the cooling circuit of a nuclear reactor. As an advantageous feature, the 
water which constitutes the secondary fluid is supplied at least partly 
from a storage tank or station and partly from a circuit which serves to 
extract the water from the generator itself, the water being extracted at 
the vaporization temperature and employed especially for resuperheating 
the steam between the high-pressure and low-pressure sections of the 
turbines. 
Various designs of steam generators of this type are already known, 
especially those described and claimed in French patent Application No. 77 
14888 of May 16, 1977 in the name of Commissariat a l'Energie Atomique in 
respect of "Forced-circulation steam generator" in which the generator 
mainly comprises an outer casing and an inner skirt in coaxial relation 
which are associated with two separate horizontal tube plates consisting 
of a central plate and an annular plate, and with a U-tube bundle for the 
circulation of primary fluid. The tubes of the bundle are connected 
respectively to each of these two plates within the casing and the elbowed 
portions of the tubes pass beneath the end of the skirt. 
The present invention concerns an improvement made in the structure of a 
steam generator of the type mentioned in the foregoing so as to permit of 
utilization both as a superheated steam generator and as a saturated steam 
generator. 
To this end, the generator under consideration essentially comprises in 
combination an outer casing formed by two cylindrical shells having 
different diameters but a common vertical axis and consisting respectively 
of an inner shell and an outer shell which are closed at their opposite 
extremities by end-walls and engaged one inside the other, a horizontal 
annular tube plate disposed between the upper end of the inner shell and 
the outer shell, said annular plate being such as to delimit with the 
cylindrical shells and a parallel partition-wall an annular header for the 
admission of pressurized primary water, a central tube plate mounted in 
the inner shell near the bottom end-wall so as to delimit with said 
end-wall an outlet header for the discharge of primary water, a bundle of 
tubes which are parallel along the greater part of their length and 
through which the primary water is circulated. Said tubes extend 
vertically from the central tube plate and each have a crook-shaped end 
portion or bend having a downwardly-directed concavity for connecting them 
to the annular tube plate, said tubes being uniformly connected over the 
entire annular tube plate. The generator further comprises ducts which 
serve to admit the secondary water to be vaporized into the casing and are 
formed in the inner shell in the vicinity of the central tube plate, and 
outlet ducts which serve to discharge the steam produced and are formed in 
the outer shell or in the end-wall of this latter. 
By virtue especially of the arrangement adopted for the tubes of the bundle 
and for the tube plates on the one hand with respect to the inlet and 
outlet headers and on the other hand with respect to the secondary water 
inlet and outlet ducts, the primary water and the secondary water-steam 
circulate in countercurrent flow along practically the entire length of 
the tubes. 
The generator advantageously comprises at least one duct for extracting 
secondary water at the vaporization temperature and superheating the steam 
produced between the high-pressure and low-pressure sections of the 
turbines, the extracted water flow being then returned into the generator 
at a temperature which is even higher than that of the secondary 
feed-water. 
It is also preferably ensured that the inlet ducts for the admission of 
secondary water into the inner shell of the casing are associated with 
circumferential distribution elements which are mounted within said shell 
and serve to distribute the flow within this latter. Finally and in 
accordance with another particular feature, the tubes of the bundle are 
suitably spaced by means of grids provided with broached holes through 
which said tubes are passed.

In the example of construction illustrated in FIG. 1, the reference numeral 
1 generally designates the generator in accordance with the invention. The 
generator is mainly constituted by two cylindrical shells 2 and 3 
respectively which have a common vertical axis, the shell 2 being 
designated hereinafter as the "inner shell" and the shell 3 being 
designated hereinafter as the "outer shell". These two shells are joined 
to each other by means of a header 4 having the shape of a toric casing of 
rectangular section and are closed respectively at their opposite 
extremities by means of hemispherical end-walls 5 and 6. The top wall of 
the casing 4 is constituted by a horizontal tube plate 7 of annular shape. 
The inner and outer walls of said tube plate are joined respectively to 
the cylindrical shells 2 and 3 so as to form extensions of these latter 
whilst the bottom wall of the tube plate is a flat annular plate 8. A duct 
10 has its opening through the outer wall 3 of said header 4 and serves to 
admit into this latter the pressurized primary water which is discharged 
from the nuclear reactor. 
Provision is also made at the lower end of the inner shell 2 for a central 
tube plate 11 which is also horizontal and forms another header 12 or 
primary water discharge header with the bottom end-wall 6 which closes 
said cylindrical shell 2. The flow of primary water is thus collected in 
said header before being discharged from the steam generator through ducts 
13 and 14. 
In accordance with the invention, the primary water passes through the 
steam generator via a bundle of tubes 15, only some of which are 
illustrated in the figure. Over the greater part of its length, each tube 
has a vertical leg 16, the lower end of which is joined to the central 
tube plate 11. At the opposite end and above the open top portion of the 
cylindrical shell 2, each tube 15 has an extension in the form of an 
elbowed portion 17 having the shape of a shepherd's crook 
(downwardly-directed concavity), thus making it possible to join said 
tubes to the annular plate 7 which delimits the inlet header. By virtue of 
these arrangements, the pressurized primary water which is introduced into 
the header 4 through the duct 10 passes through all the tubes 15 and is 
then collected within the outlet header 12 prior to discharge from the 
steam generator through the ducts 13 and 14. Preferably, the tubes 15 of 
the bundle are braced with respect to each other by means of a series of 
spacer grids 18 shown diagrammatically in the drawing. At the point at 
which they are traversed by the tubes and especially by the vertical legs 
16 of these latter, said spacer grids are provided with broached holes 
(not shown) for maintaining said tubes in suitable positions without 
preventing circulation of the secondary water in counterflow to the 
primary water externally of the tubes. 
The production of saturated steam is carried out by admission of secondary 
water through a lateral duct 19; said secondary water penetrates into the 
inner shell 2 in the vicinity of its lower end above the central tube 
plate 11. Said duct 19 is associated within the interior of the shell with 
a deflector 20 of cylindrical shape for ensuring good distribution of the 
flow. Provision is also made in the opposite wall of the inner shell 2 for 
another admission duct 21 which is again associated with a deflector 22. 
As will be seen hereinafter, the secondary water which is introduced into 
the shell through said duct 21 corresponds to a return of water extracted 
from the generator itself in order to ensure resuperheating of the steam 
between the high-pressure and low-pressure sections of the turbines (not 
shown) which are associated with the installation. The temperature of the 
water which is returned into the shell 2 is at a substantially higher 
temperature than that of the feed water which is introduced through the 
duct 19 and is consequently intended to ensure efficacious sweeping of the 
tube plate 11 under the best thermal conditions. 
The secondary water within the inner shell 2 undergoes a progressive 
temperature rise in contact with the tube bundle 15, then vaporizes so as 
to form a mixture of droplets of water and of steam above the tube bundle 
15. The mixture then passes through separators 23 in which it is enriched 
with steam, then through driers 24 so that the steam collected in the 
header 25 located beneath the top end-wall 5 of the outer shell 3 is 
practically dry and finally withdrawn from the generator through the ducts 
26 and 27. The duct 28 and cylindrical deflector 29 are advantageously 
placed beneath the separators 23; as mentioned earlier, this arrangement 
makes it possible to draw-off part of the secondary water at the 
vaporization temperature for utilization in an external circuit before 
being returned to the generator through the duct 21. This arrangement has 
a further advantage in that it facilitates operation of the separators 23. 
The saturated steam generator in accordance with the design just described 
offers considerable advantages in regard to both performances and 
operation in comparison with conventional U-tube steam generators. From a 
performance standpoint, the fact of having a generator which provides 
systematic flow and good homogenization at a high secondary fluid velocity 
makes it possible to increase heat transfer to a very substantial extent. 
In consequence, the pressure within the generator can be increased, thus 
achieving enhanced efficiency of the installation as a whole, particularly 
when a superheating system is preferentially employed between the 
high-pressure and low-pressure sections of the turbines by extraction of 
secondary water from the generator itself. From an operational standpoint, 
and apart from excellent stability, the generator is perfectly adapted to 
follow all load variations by reason of the excellent coupling which 
exists intrinsically between the steam generator and the nuclear reactor 
which delivers pressurized primary water. Moreover, the separation of the 
tube plates and the resulting shape of the tubes permit a considerable 
reduction of thermal stresses within said plates; in addition, the 
crook-shaped design of the circulation tubes enables these latter to 
maintain a sufficient degree of flexibility to meet the problem of 
differential expansions. 
It should further be noted that the arrangements adopted make it possible 
to prevent the formation of zones of stagnation of secondary water or 
zones having unfavorable hydrodynamic characteristics. 
In a second alternative embodiment illustrated in FIG. 2, most of the 
arrangements adopted in the first alternative embodiment are again shown, 
although in this case the generator is intended to deliver superheated 
steam. In particular, there are again shown in this figure the annular 
tube plate 7 and toric inlet header 4 which provides a connection between 
the inner shell 2 and the outer shell 3, the bundle of tubes 15 with their 
vertical legs 16 and their crook-shaped end portions 17 as well as the 
dished end 5 which closes the top portion of the outer shell. 
In this alternative embodiment, however, the region located within the 
interior of the outer shell 3 above the end of the tube bundle 15 is 
provided with a transverse deflector 30 so as to ensure that the 
superheated steam of the region 31 carries out suitable sweeping of the 
upper portions of the tubes before collecting above said deflector 30 
within the header 32 and then being discharged from this latter through 
the ducts 33 and 34. In this alternative embodiment, withdrawal of water 
at the vaporization temperature is advantageously carried out through a 
lateral duct 35 associated with an internal cylindrical deflector 36. Said 
duct 35 is provided in a zone which corresponds to incipient vaporization 
of the secondary water in contact with the tubes for the circulation of 
primary water. As in the previous example, the secondary water which is 
withdrawn after resuperheating of the steam between the high-pressure and 
low-pressure sections of the turbines is returned to the bottomm of the 
generator in order to mix with the feedwater after sweeping of the tube 
plate 11. 
The superheated steam generator in accordance with this design has 
performances and operating characteristics which achieve a marked 
improvement over the performances of the straight-tube generators which 
are usually employed for the production of superheated steam. It should 
further be mentioned that the generator structure is such that the tubes 
are uniformly distributed within the inner shell and on the annular tube 
plate. In consequence, the creation of thermal gradients in a plane at 
right angles to the axis of the generator is prevented as far as possible. 
The following summary table gives examples of thermodynamic 
characteristics of steam generators in accordance with either of the two 
alternative embodiments described in the foregoing, in which the primary 
water is derived from a pressurized light water reactor. 
______________________________________ 
SATURATED STEAM SUPERHEATED STEAM 
(FIG. 1) (FIG. 2) 
______________________________________ 
Thermal power: 1387 MW 
Thermal power: 1387 MW 
Electrical power: 512 MW 
Electrical power: 511 MW 
Internal diameter: 
Internal diameter: 
2.80 m/4.50 m 2.80 m/4.50 m 
Height: 19 m Height: 20 m 
Number of tubes: 12,000 
Number of tubes: 12,000 
External diameter of 
External diameter of 
tubes: 16 mm tubes: 16 mm 
Heat-transfer area: 7,500 m.sup.2 
Heat-transfer area: 10,200 m.sup.2 
______________________________________ 
PRIMARY FLUID PRIMARY FLUID 
______________________________________ 
Inlet temperature: 330.degree. C. 
Inlet temperature: 329.degree. C. 
Outlet temperature: 292.degree. C. 
Outlet temperature: 293.degree. C. 
Flow rate: 5,900 kg/s 
Flow rate: 6,238 kg/s 
______________________________________ 
SECONDARY FLUID SECONDARY FLUID 
______________________________________ 
a) Feedwater a) Feedwater 
Inlet temperature: 243.3.degree. C. 
Inlet temperature: 232.5.degree. C. 
Flow rate: 746 kg/s 
Flow rate: 686.6 kg/s 
Steam Steam 
Generator outlet Generator outlet 
Pressure: 79 b Pressure: 74 b 
Temperature: 294.degree. C. 
Temperature: 310.degree. C. 
Flow rate: 746 kg/s 
Flow rate: 686.6 kg/s 
Turbine inlet Turbine inlet 
Pressure: 75 b Pressure: 75 b 
Temperature: 291.degree. C. 
Temperature: 308.degree. C. 
Flow rate: 746 kg/s 
Flow rate: 686.6 kg/s 
b) Superheating water 
b) Superheating water 
Inlet temperature: 260.degree. C. 
Inlet temperature: 250.6.degree. C. 
Outlet temperature: 295.degree. C. 
Outlet temperature: 290.degree. C. 
Flow rate: 622 kg/s 
Flow rate: 536.5 kg/s 
Total secondary-water flow 
Total secondary-water flow 
rate: 1368 kg/s rate: 1222 kg/s 
Mean velocity of water in 
Mean velocity of water in 
economizer section # 0.82m/s 
economizer section # 0.71 m/s 
______________________________________ 
It is readily apparent that the invention is not limited solely to the 
examples of construction wich are more especially described both in the 
foregoing and in the appended claims but extends to all alternative forms.