EP0178545B1

EP0178545B1 - Steam generator

Info

Publication number: EP0178545B1
Application number: EP19850112524
Authority: EP
Inventors: Jun Kashiwakura; Hiroshi Tsuda; Kouji Abe; Yasuo Tachi
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1984-10-05
Filing date: 1985-10-03
Publication date: 1988-07-06
Also published as: DE3563649D1; EP0178545A1; JPS6189401A

Description

The present invention relates to a steam generator of the kind referred to in the precharacterizing portion of claim 1. Such a steam generator is known from FR-A-2 379 881.
A conventional large scale spherical coiling type steam generator in a liquid metal fast breeder reactor is known for example, from JP-A-28467/1980.
High temperature liquid metal sodium serving as a heating medium flows into a longitudinal placement type steam generator through a sodium inlet nozzle provided at the upper end of a barrel of the steam generator. The liquid metal sodium descends through a tube bundle portion around which is vertically and helically wound a multiplicity of heat transfer tubes. This tube bundle portion is disposed in a region which is so formed as to be surrounded by a cylindrical inner shroud and an outer shround. The thus lowered sodium then flows out through a sodium outlet nozzle.
Feed-water is supplied through a feed-water inlet nozzle into a feed-water inlet chamber and is then led into a mulitiplicity of the heat transfer tubes, whereby the water is raised. At this time, the feed-water which has flowed in at low temperature is subjected to a thermal exchange with respect to the sodium which flows at high temperature on the outside of the heat transfer tubes.
With this step, the feed-water is preheated, boiled and superheated, thus vaporizing the feed- water at high temperature and at high pressure. The thus produced vapor is collected in a vapor outlet chamber and then fed out from a vapor outlet nozzle to a turbine.
Fig. 3B shows a general example of a secondary sodium system loop of a fast breeder reactor which includes the above-described steam generators 1a a and 1b. This system loop is constituted by an intermediate heat exchanger 23, the steam generators 1a and 1b, a circulating pump 24 and pipes 31a, 32a and 33a. The intermediate heat exchanger 23 performs the thermal exchange in regard to the primary sodium and the secondary sodium. The steam generators 1 a and 1b produce vapor by effecting a thermal exchange between the feed-water and the secondary sodium with high temperature which is transmitted from the intermediate heat exchanger 23. The circulating pump 24 circulates the secondary sodium.
A conventional steam generator, as shown in Fig. 3B is constituted by a superheater 1a a and an evaporator 1b which have the same structure with respect to each other. The evaporator 1 b evaporates the feed-water by virtue of the heat evolved by the sodium; and the superheater 1a has a function to convert the vapor produced by the evaporator 1b into the superheated vapor by further heating it with the sodium.
The secondary sodium which has been heated at high temperature by means of the intermediate heat exchanger 23 is transmitted via a hot leg pipe 31 a, a middle leg pipe 32a and a cold leg pipe 33a to the superheater 1 a or the evaporator 1b.
The middle leg pipe 32a extends from the lower end of the superheater 1a passes through a floor (from the point f₁ to the point f₂) on which the superheater 1a is installed and then descends. Furthermore, the middle leg pipe 32a reverses the piping direction thereof at the lower portion of the floor and again passes through the floor so as to reach the upper portion of the evaporator 1b. The cold leg pipe 33a extends from the lower end of the evaporator 1 passes through the floor (from the point f₃ to the point f4) on which the evaporator 1b is installed and then it descends.
In the large scale steam generator, as described above, the sodium pipe is large in diameter; hence, it is required to avoid an increase in the length of a supporting skirt. For this reason, through-holes are formed in the floor, and further, a bent portion of the sodium pipe is provided beneath the floor such as to be led round.
The large through-holes are formed respectively in the floor on which the superheater 1 a and the evaporator 1b are installed. In such a case, elavorations are required so as to obtain the strength of the floor when designing the structure of the reinforced concrete or disposing the reinforcing bar as compared with an arrangement wherein no through-hole is formed therein.
Moreover, the sodium pipe is led round on the upper, lower and lateral sides of the steam generator, this involving a large space of pipe arrangement. Namely, such arrangement is the very factor which enlarges the structure itself.
In the above-described pipe arrangement, if the cold leg pipe 33a in the vicinity of the sodium outlet nozzle of the evaporator 1 b is damaged, all of the sodium within the evaporator 1 b flows out into the atmosphere where it is virtually certain to ignite. Also, in so far as the through-holes are formed in the floor, some of the leaked sodium would flow into other equipment, spreading the damage and increasing the danger.
Moreover, as an another example, a large scale steam generator of having a coiling type in a liquid metal fast breeder reactor is known from the publication: ASME 80-C2/NE-31 "Steam Generator Design and Experience in the SNR-Project", page 10, Fig. 7.
The sodium within the longitudinal placement type steam generator of this publication flows in through the sodium inlet nozzle provided at the lower end of the barrel of the steam generator and is then raised within the inner shroud with which the sodium inlet nozzle is directly communicated, thereby reaching an upper sodium plenum chamber.
Thereafter, the flow direction of the sodium is converted, so that the sodium descends through the tube bundle portion around and then flows to the outside of the steam generator through the sodium outlet nozzle provided beneath the barrel separately the sodium inlet nozzle.
In this example, the sodium flows both inside and outside of the inner shroud, and there is a great difference in temperature between the sodium flowing inside and outside whereby a great thermal stress is presumably imparted to the inner shroud.
Since the tube bundle portion is supported by the inner shroud, the inner shroud must be shielded from thermal stress as far as possible.
Since the inner shroud is connected to the barrel, it is necessary to measure for the purpose of absorbing the difference in the thermal expansion between the inner shroud and the barrel.
Moreover, the sodium pipe is provided on the lower side of the apparatus; hence, it is reasonable to assume that all the sodium within the steam generator would flow out if the pipe would be seriously damaged, the flow mostly occurring in the damaged region.
As mentioned above, some technical problems still remain unsolved in both prior art steam generators. Further, there is no idea to utilize the inner shroud such that the sodium or heating medium flows out therethrough. If the heating medium flows out from the upper end portion of the inner shroud, it is difficult to reduce the difference in temperature about a region in the vicinity of the inner shroud.
From FR-A-2 379 881 a steam generator is known, comprising: a barrel having an inlet portion and an outlet portion for a heating medium; a multiplicity of heat transfer tubes being arranged vertically in a helical configuretion in said barrel; an inner shroud with a lower end opening being disposed such as to pass through the central portion of said helical heat transfer tubes and supporting said helical heat transfer tubes (2); the lower end opening being formed in said inner shroud such that the heating medium flows into said inner shroud through the lower end opening; an outer shroud being disposed on the outside of said helical heat transfer tubes and forming a passage for the heating medium between said inner shroud and said outer shroud; an inlet pipe being disposed on the inlet portion of said barrel; an outlet pipe being disposed on the outlet portion of said barrel; said outlet pipe being provided at the most upper end portion of said barrel; an opening being formed in the most upper end portion of said inner shroud; said outlet pipe passing through said barrel at the outlet portion for the heating medium and also passing through the most upper end opening of said inner shroud so as to the heating medium flows out through the inside of said outlet pipe; and a region between the inside of said inner shroud and the outside of said outlet pipe being filled with a stagnant heating medium or an inert gas.
An object of the present invention is to provide a steam generator wherein a difference in temperature about a region in the vicinity of an inner shroud and therefore thermal stress on an inner shroud can be reduced, a compact piping structure can be obtained such that space required for disposing the pipes can be reduced and wherein the amount of leaked heating medium can be minimized.
Furthermore object of the present invention is to provide a steam generator wherein no through-hole must be formed within the floor on which the steam generator is installed.
According to the present invention these objects are achieved with a steam generator as claimed in Claim 1.
The advantages offered by the present invention are a compact and secure steam generator which has effects wherein it is possible to reduce the difference in temperature about a region in the vicinity of the inner shroud, the thermal stress on the inner shroud, the space for installing the steam generator as well as the amount of pipe, and to decrease the amount of leaked heating medium if the pipes are damaged and further to minimize the damage from a heating medium fire, should one occur.

Fig. 1 is a longitudinal sectional view of a steam generator of a first embodiment according to the present invention;
Fig. 1A is a section view of along line a-a in Fig. 1 ;
Fig. 2 is a structural view which shows a steam generator constituted by a superheater and an evaporator in combination according to the present invention;
Fig. 3A is an isometric diagram of a secondary sodium system loop according to the present invention;
Fig. 3B is an isometric diagram of a secondary sodium system loop according to a prior art steam generator;
Fig. 4 is a longitudinal sectional view of a steam generator of a second embodiment according to the present invention.

A preferred embodiment in accordance with the present invention will hereinafter be described with reference to the accompanying drawings.
Fig. 1 shows a steam generator 1 according to a first embodiment of the present invention.
In the steam generator 1, a multiplicity of heat transfer tubes 2 pass through the lower portion of a cylindrical barrel 3, these heat transfer tubes 2 are communicated with feed-water inlet chambers 4. The feedwater inlet chambers 4 are provided at the lower outside portion of the barrel 3 of the steam generator 1.
The heat transfer tubes 2 are vertically and helically wound around a cylindrical inner shroud 5 having an upper end opening 5a and a lower end opening 5b in the barrel 3, thereby forming a helically coiled tube bundle portion 6. These helical heat transfer tubes 2 further pass through the upper portion of the barrel 3 so as to be communicated with vapor outlet chambers 7.
A cylindrical outer shroud 8 is provided on the outside of the helically coiled tube bundle portion 6, such outer shroud 8 being designed for the purpose of shielding the heat and forming an offtake for sodium or heating medium.
A dual-purpose sodium inlet-outlet pipe or nozzle is provided at a single upper portion above the barrel 3. A sodium outlet pipe or a sodium outlet nozzle 9 is coaxially provided within a sodium inlet pipe or a sodium inlet nozzle 10. This sodium outlet pipe 9 passes through a bent portion 11 of the sodium inlet pipe 10 which are connected to each other with the aid of metal bellows 12 as a displacement absorption mechanism.
The sodium inlet pipe 10 includes an opening 13 which is formed in an upper sodium plenum chamber 14 such as to be communicated therewith. The sodium outlet pipe 9 extends downwardly and passes through the upper end opening 5a of the inner shroud 5. Such sodium outlet pipe 9 includes another opening 15 which is formed in a lower sodium plenum chamber 16 such as to be communicated therewith.
The sodium outlet pipe 9 has a double pipe structure 18 with respect to the length ranging from the metal bellows 12 to the upper portion of the inner shround 5. An inert gas for medium as a thermal insulating material is encapsulated in a gap 17 formed between the inner pipe and the outer pipe of the double pipe structure 18.
A body supporting skirt 19 supporting skirt 19 supporting the steam generator 1 is provided at the lower portion of the barrel 3. No through-hole is provided in the concrete floor 20 on which the steam generator 1 is installed.
In this first embodiment, the steam generator 1 is of the non-liquid level type, and thus the inside portion of the barrel 3 is filled with the sodium. The initial method of filling the steam generator 1 with the sodium is usually one of two types: one is the pressurizing method wherein the pressurized sodium is injected into the barrel 3 of the steam generator 1; and the other is the "vacuum pull-in" method wherein a high vacuum inside of the barrel 3 of the steam generator 1 pulls the sodium into the barrel 3.
When the sodium is put in by employing the pressurizing method, the inner region of the inner shroud 5 and the similar region formed by the metal bellows 12 and the sodium outlet pipe 9 are each closed at end thereof, thereby enclosing the fluid. It is also possible for the sodium to be filled in by the vacuum pull-in method. The above described two regions are filled with stagnant or expended fluid by either method.
The high temperature sodium serving as a heating medium passes through a gap 21 formed between the sodium inlet pipe 10 and the sodium outlet pipe 9, both of which are provided on the upper side of the barrel 3. Such high temperature sodium flows into the barrel 3 of the steam generator 1 and then descends through the helically coiled tube bundle portion 6 around which it is helically wound with the helical heat transfer tubes 2.
Thereafter, the high temperature sodium reaches the lower sodium plenum chamber 16. The high temperature sodium is subjected to the thermal exchange with respect to the water and the vapor within the helical heat transfer tubes 2.
Feed-water is supplied through a feed-water inlet pipe or nozzle into a feed-water inlet chamber 4 and is then led into a mulitiplicity of the helical heat transfer tubes 2, whereby the water is raised. At this time, the feed-water which has flowed in at low temperature (about 240°C with about 150 ata) is subjected to a thermal exchange with respect to the sodium which flow at high temperature (about 500°C with about 4-5 ata) on the outside of the helical heat transfer tubes 2.
With this step, the feed-water is preheated, boiled and superheated, thus vaporizing the feed- water at high temperature (about 487°C with high pressure about 33 ata). Then the sodium becomes at low temperature (about 320°C with about 4-5 ata). The thus produced vapor is collected in a vapor outlet chamber 7 and then fed out from a vapor outlet pipe or nozzle to a turbine.
The low temperature sodium (about 320°C with about 4-5 ata) which has reached the lower sodium plenum chamber 16 changes its flow direction and flows through the opening 13 of the sodium outlet pipe 9 into the sodium outlet pipe 9. Then, this low temperature sodium is raised and flows out from the steam generator 1.
There is a great difference in temperature between the sodium descending outside the inner shroud 5 and the sodium ascending inside the sodium outlet pipe 9. However, the region which is surrounded by the inner shroud 5 and the sodium outlet pipe 9 is filled with the stagnant or expended fluid such as sodium or an inert gas, whereby a difference in temperature about the region between the inner shroud 5 and the outlet pipe 9 is reduced and also a great thermal stess is not imparted on the inner shroud 5.
The inner shroud 5 is joined to the barrel 3 and further to the sodium outlet pipe 9. Accordingly, the metal bellow 12 is provided on the sodium inlet pipe 10 in order to absorb the difference in the thermal expansion between the sodium outlet pipe 9 and the barrel 3. The sodium inlet-outlet pipe or nozzle has a double structure for the purpose of obtaining a preferable sodium flux in the upper sodium plenum chamber 14 and a uniform sodium flux at the upper end portion of the helically coiled tube bundle portion 6.
The sodium inlet pipe 10 and the sodium outlet pipe 9 are concentrated at the upper portion of the barrel 3 respectively. A bulkhead or partitioning plate 22 is provided in order to avoid the damage of the sodium inlet pipe 10 and the sodium outlet pipe 9 in case of a water-vapor system pipe rupture.
Fig. 2 shows a connecting arrangement of the pipes for sodium, this pipe arrangement being based on a superheater 1A and an evaporator 1B. The steam generator 1 is constituted by the superheater 1 A and the evaporator 1B which have the same structure.
Fig. 3A shows a general example of a secondary sodium system loop of a fast breeder reactor which includes the above-described steam generator 1 of the first embodiment of the present invention.
This system loop is constituted by an intermediate heat exchanger 23, the steam generator 1, a circulating pump 24, a hot leg pipe 31A, a middle leg pipe 32A and cold leg pipe 33A. The intermediate heat exhanger 23 performs the thermal exchange with regard to the primary sodium and the secondary sodium. The steam generator 1 produces vapor by effecting a thermal exchange between the feed-water and the secondary sodium with high temperature which is transmitted from the intermediate heat exchanger 23. The circulating pump 24 circulates the secondary sodium.
The high temperature sodium moved from an intermediate heat exchanger 23 flows in via the sodium inlet pipe 10 of the superheater 1A and then flows out via the sodium outlet pipe 9. The sodium outlet pipe 9 of the superheater 1A is communicated with the sodium inlet pipe 10 of the evaporator 1B through the cold leg pipe 33A. The sodium transmitted from the superheater 1A flows via the sodium inlet pipe 10 of the evaporator 1 B into the evaporator 1 B and then flows out through the sodium outlet pipe 9.
Where the steam generator 1 of the first embodiment according to the present invention is adopted, as shown in Fig. 2, there is no necessity for any through-hole to be formed in the floor 20 on which the steam generator 1 constituted by the superheater 1 A and the evaporator 1 B is installed; hence, the strength of the installation floor 20 can securely be maintained by adjusting the thickness thereof.
Furthermore, it is practicable to install the superheater 1 A and the evaporator 1 B in the same chamber by an arrangement wherein these two, that is the superheater 1A and the evaporator 1B, as a combined installation are brought closer each other.
In addition, no sodium pipe passes through the floor 20 on which the steam generator 1 is installed. With this arrangement, the space for installing the steam generator 1 is made smaller than that of the prior arts.
Moreover, all the sodium pipes, that is the hot leg pipe 31A, the middle pipe 32A and the cold pipe 33A, are provided on the upper side of the superheater 1A and the evaporator 1B. Therefore, the sodium within the steam generator 1 does not flow out at all even if the sodium pipes 31A, 32A and 33A are damaged. The amount of leaked sodium can be minimized as compared with the prior arts wherein the sodium pipe in the proximity of the sodium outlet nozzle is damaged.
No through-hole is formed in the floor 20 on which the steam generator 1 is installed, this preventing any leaked sodium from flowing into other parts other of the equipment, thereby greatly reducing the possibility of fire caused by the leaked sodium.
Fig. 3A is a schematic diagram of a secondary sodium system loop of one embodiment which employs the steam generator 1 constituted by the superheater 1A and the evaporator 1B according to the present invention.
As shown Fig. 3A, in comparison with Fig. 3B which shows the schematic diagram of the secondary sodium system loop of the prior art, the piping in this case can be eliminated when the above first embodiment of the present invention is adopted.
If the constitution of the steam generator takes only one unit, that is, in the case of the continuous percolation, it is possible to reduce the number of pipes as well as the space for disposing the pipes in regard to the secondary sodium system loop by adopting the present invention. Moreover, it obviously reduces the extent of damage when a sodium fire occurs.
Fig. 4 shows a steam generator according to a second embodiment of the present invention. It is shown an arrangement wherein an inner shroud 41 is provided with a metal bellow 42 and an metal bellow 42 as a displacement absorption mechanism is provided within the barrel 3. The inner shroud 41 has an upper end opening 41a and a lower end opening 41b. The metal bellows 42 absorb the difference in thermal expansion between the sodium outlet pipe 9 and the barrel 3. The sodium outlet pipe 9 is connected to an upper end opening 41a a of the inner shroud 41 by means of the metal bellow 42.

Claims

1. A steam generator (1, 1A, 1B) comprising: a barrel (3) having an inlet portion and an outlet portion for a heating medium; a multiplicity of heat transfer tubes (2) being arranged vertically in a helical configuration in said barrel (3); an inner shroud (5, 41) with a lower end opening (5b, 41b) being disposed such as to pass through the central portion of said helical heat transfer tubes (2) and supporting said helical heat transfer tubes (2); the lower end opening (5b, 41 b) being formed in said inner shroud (5, 41) such that the heating medium flows into said inner shround (5, 41) through the lower end opening (5b, 41 b); an outer shroud (8) being disposed on the outside of said helical heat transfer tubes (2) and forming a passage for the heating medium between said inner shroud (5,41) and said outer shroud (8); an inlet pipe (10) being disposed on the inlet portion of said barrel (3); and outlet pipe (9) being disposed on the outlet portion of said barrel (3); said outlet pipe (9) being provided at the most upper end portion of said barrel (3); an opening (5a, 41a) being formed in the most upper end portion of said inner shroud (5, 41); said outlet pipe (9) passing through said barrel (3) at the outlet portion for the heating medium and also passing through the most upper end opening (5a, 41a) of said inner shroud (5, 41) so as to the heating medium flows out through the inside of said outlet pipe (9); and a region between the inside of said inner shroud (5, 41 ) and the outside of said outlet pipe (9) being filled with a stagnant heating medium or an inert gas, characterized in that said inlet pipe (10) is provided at the most upper end surface portion of said barrel (3), said outlet pipe (9) is coaxially provided within said inlet pipe (10), said outlet pipe (9) extends downwardly through said opening (5a, 41a) of said inner shroud (5, 41), and said region is filled with the stagnant heating medium or the inert gas substantially overall in the space between the inside of said inner shroud (5, 41) and the outside of said outlet pipe (9).

2. A steam generator (1, 1A, 1B) as claimed in claim 1, characterized in that said outlet pipe (9) is formed in a double pipe structure (18) having an inner pipe and an outer pipe, and a gap (17) formed between the inner pipe and the outer pipe of the double pipe structure (18) is encapsulated with a thermal insulating material.

3. A steam generator (1, 1A, 1B) as claimed in claim 1 or 2, characterized in that a displacment absorption mechanism (12) is provided on said inlet pipe (9) so as to absorb the difference in the thermal expansion between said barrel (3) and said inlet pipe (10).

4. A steam generator (1, 1A, 1B) as claimed in claims 1 to 3, characterized in that a displacment absortion mechanism (42) is provided within said barrel (3) so as to absorb the difference in the thermal expansion between said barrel (3) and said inner shroud (41).