Horizontal Steam Generator for Nuclear Power Plants and Its Assembly Method

Abstract

This invention relates to steam generators, and more particularly to horizontal steam generators for nuclear power plants with a water-water energetic reactor (VVER). We claim a horizontal nuclear power plant steam generator comprising a cylindrical vessel, two elliptical bottoms, at least one feed water supply and steam removal connection pipe, an inlet header and an outlet header, a heat-exchange tube bundle connected to the same, wherein number Ntb of heat-exchange tubes in the bundle is selected depending on outer diameter dtb of the heat exchange tubes according to formulae. The technical result of the invention is an increased heat transfer efficiency in the steam generator with a limited number and maximum length of heat exchange tubes, which allows to use tubes employed in the industry.

Claims

1. A horizontal nuclear power plant steam generator comprising a cylindrical vessel, two elliptical bottoms, at least one feed water supply and steam removal connection pipe, an inlet header and an outlet header of the primary circuit coolant, a heat-exchange tube bundle connected to the same, wherein number Ntb of heat-exchange tubes in the bundle is selected depending on outer diameter dtb of the heat exchange tubes as follows: if dtb≦14 mm: $\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}\le \mathrm{Ntb}\le \frac{1.211.Math.{10}^6}{\pi .Math.\mathrm{dtb}}$ if dtb>14 mm: $\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}\le \mathrm{Ntb}\le \frac{1.111.Math.{10}^7}{\pi .Math.{\left(\frac{4.Math.\mathrm{dtb}}{5}+0.2\right)}^2}$ and the gap between the adjacent heat-exchange tubes in the vertical direction does not exceed the vertical spacing between the heat-exchange tubes in the bundle.

2. A steam generator according to claim 1, wherein seamless solid-drawn austenitic stainless steel pipes are used as heat-exchange tubes.

3. A steam generator according to claim 1, wherein in the ratio between the clear area of a heat-exchange tube and the heat-exchange tube bundle installation area per tube in the heat-exchange bundle is selected based on the following criterion: $0.1\le \frac{S_{\mathrm{tb}}}{\mathrm{ftb}}\le 0.8$ where: S.sub.tb is the clear area of a heat-exchange tube, mm.sup.2, f.sub.tb is the heat-exchange tube bundle installation area per tube, mm.sup.2.

4. A steam generator according to claim 1, wherein the heat-exchange bundle tubes are grouped in banks separated by vertical inter-tubular tunnels.

5. A steam generator according to claim 4, wherein the heat-exchange tube banks are separated along their sides by baffles forming riser and downtake sections of boiler water circulation.

6. A steam generator according to claim 5, wherein the heat-exchange tube banks located on the inlet primary circuit coolant header side are separated along their sides by baffles forming riser and downtake sections of boiler water circulation.

7. An assembly method for a nuclear power plant horizontal steam generator including manufacture of a cylindrical vessel, two elliptical bottoms, at least one feed water supply and steam removal connection pipe, an inlet header and an outlet header of the primary circuit coolant, heat-exchange tubes with outer diameter dtb and in number Ntb, operations for installation and welding of the headers, heat exchange tube supports to the vessel, forming of a heat-exchange tube bundle and their connection to the inlet and outlet headers of the primary circuit coolant, as well as installation and welding of the bottoms to the vessel, wherein the heat-exchange tube bundle is formed so as to provide vertical gaps between the adjacent heat-exchange tubes does not exceed the vertical spacing between the heat-exchange tubes and number Ntb of heat-exchange tubes in the bundle is selected depending on outer diameter dtb of a heat exchange tube as follows: if dtb≦14 mm: $\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}\le \mathrm{Ntb}\le \frac{1.211.Math.{10}^6}{\pi .Math.\mathrm{dtb}}$ if dtb>14 mm: $\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}\le \mathrm{Ntb}\le \frac{1.111.Math.{10}^7}{\pi .Math.{\left(\frac{4.Math.\mathrm{dtb}}{5}+0.2\right)}^2}$

8. A method according to claim 7, wherein before installation into the steam generator vessel, through holes are drilled in the side surface of the inlet and outlet headers in accordance with the number of heat-exchange tubes in the bundle (Ntb).

9. A method according to in claim 8, wherein the heat-exchange tubes are secured in the holes in the side surface of the primary circuit coolant header by round-welding of the tube ends on the inner surface of the headers, followed by hydraulic expansion of the heat-exchange tubes over wall thickness of the headers and mechanical curling near the external surface of the headers until the of gap between the headers and the heat-exchange tubes is closed.

10. A method according to claim 7, wherein the heat-exchange tubes are bundled directly in the vessel from the bottom upwards.

11. A method according to claim 7, wherein seamless solid-drawn austenitic stainless steel tubes not longer than 30 m are used as heat-exchange tubes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] A possible embodiment of the claimed horizontal steam generator and its assembly method is detailed below with references to figures.

[0045] FIG. 1 shows the general view of the steam generator.

[0046] FIG. 2 shows the sectional view of the steam generator from the elliptical bottom.

[0047] FIG. 3 shows the heat-exchange tubes with spacing elements.

[0048] FIG. 4 shows the cross-section of staggered arrangement of heat-exchange bundle tubes.

[0049] FIG. 5 shows the cross-section of in-line arrangement of heat-exchange bundle tubes.

[0050] FIG. 6 shows the arrangement of baffles separating heat-exchange tube banks.

DETAILED DESCRIPTION

[0051] A steam generator is a horizontal vessel heat-exchange unit designed for arrangement of a submerged heat transfer surface in the same consisting of the following structural elements shown in the attached figures: a vessel 1, a heat-exchange tube bundle 2 (also referred to as tube bundle), inlet and outlet headers 3 of the primary circuit coolant, a feed water supply and distribution device 4, an emergency feed water supply and distribution device 5, an overhead perforated sheet 6, a submerged perforated sheet 7, a chemical reagent supply device 8.

[0052] The vessel 1 is a component part of the steam generator, it accommodates inlet and outlet headers 3 of the primary circuit, a heat-exchange surface in the form of heat-exchange tube bundle 2 and vessel internals. The vessel 1 accommodates secondary circuit manholes 9 for servicing of primary circuit inlet and outlet headers 3.

[0053] The vessel 1 is a horizontally elongated welded cylinder container with elliptical bottoms 10 with manholes 11 for access to the secondary circuit volume positioned on them welded to its both ends.

[0054] The vessel 1 also contains primary circuit coolant supply and removal connection pipes 12, steam removal connection pipes 13 feed water supply connection pipes 14 and other connection pipes and access manholes.

[0055] Headers 3 of the primary circuit coolant are thick-walled cylinders of varying diameters and thickness. They are made of high-strength pearlite grade steel, and their internal surfaces have a protective anti-corrosion build-up. The central cylinder part of headers 3 has perforations for fastening the ends of heat-exchange tubes 15. The upper part of headers 3 has a split for access inside through manholes 9 of the secondary circuit.

[0056] The heat-exchange surface of the steam generator is formed by seamless solid-drawn heat-exchange tubes (15) made of austenitic stainless steel. Heat-exchange tubes are formed into U-shaped coils arranged in bundle 2 and installed with a slope towards headers 3 in order to provide possibility of complete draining of heat-exchange tubes 15. Heat-exchange tubes 15 are fixed in headers 3 by counter welding of the ends with the internal surface of the headers 3. Hydraulic expansion of heat-exchange tubes 15 is performed over wall thickness of the headers 3 with mechanical curling near the external surface of the headers 3 until the gap (split) between the headers 3 and the heat-exchange tubes 15 is closed. Heat-exchange tubes 15 are installed at certain intervals from one another (spaced in bundle 2) using spacing elements 16, such as wave-shaped bands and flat plates (FIG. 3). This fixing structure allows the heat-exchange tubes 15 to move during thermal expansion.

[0057] Internal devices located in the vessel 1 include the following:

[0058] feed water supply and distribution device 4 located above heat-exchange tube bundle 2. The said device 4 consists of pipelines and distributing pipes with orifices for feed water removal along their full length. The main material used for manufacture of the above device is stainless steel, emergency feed water supply and distribution device 5 located in the steam space and consisting of a header and distributing pipes with orifices for feed water removal along their full length, The material used for its manufacture is stainless steel, device 8 for chemical reagent supply during steam generator flushing located in the steam space and consisting of a header with orifices for chemical reagent removal along its full length. The material used for its manufacture is stainless steel, overhead perforated sheet 6 located in the upper part of the steam generator and designed to decrease the header effect during steam removal from the steam generator. The material used for its manufacture is stainless steel, submerged perforated sheet 7 with alternating perforation located above the heat-exchange tube bundle 2 and designed to level the evaporation surface steam load. The material used for its manufacture is stainless steel.

[0059] To improve boiler water (secondary circuit coolant) circulation, the heat-ex-change bundle tubes of the steam generator can be grouped into banks separated from each other by vertical inter-tubular tunnels shown in FIG. 2 and FIG. 6. Besides, as shown in FIG. 6, the banks of heat-exchange bundle tubes of the steam generator can be separated along their sides by baffles 17 forming riser and downtake sections of boiler water circulation. In this case, the steam generated between the heat-exchange tubes does not reach the inter-tubular tunnels and does not prevent colder boiler water from moving downwards. Boiler water circulation becomes more intensive.

[0060] In another embodiment, the baffles forming the boiling water riser and downtake sections can close only the heat-exchange tube banks located on the primary circuit coolant inlet header side. The above baffles are made of metal sheets without perforation.

[0061] The operation principle of the steam generator structure is as follows. Coolant heated in reactor is supplied to the inlet or distributing header of the primary circuit coolant (one of headers 3). From the distributing header, the coolant is fed to heat-exchange tubes 15 grouped into a bundle 2, moves through them transferring the heat through the heat-exchange surface wall to the boiler water, and is collected in the outlet or collecting header of the primary circuit coolant (the other header 3). The coolant is returned to the reactor from the collector header by a circulating pump. The steam generator vessel 1 is filled with boiler water to a certain level which is to be maintained during operation. Feed water is supplied to the steam generator by the feed water supply and distribution device 4. The feed water flowing out of it is mixed with the boiler water and heated to the saturation temperature. The heat transferred from the coolant is spent on boiler water evaporation and steam generation in the inter-tubular space of the steam generator. The generated steam is ascending to the separation part of the steam generator comprising a free volume, separation devices or a combination thereof. After passing the separation part of the steam generator, the steam has the design rated humidity. Then it is removed from the steam generator through steam removal devices comprising steam removal connection pipes 13 and overhead perforated sheets 6 installed in front of them. The steam generated by the steam generator is used in steam-power process cycle of electric power generation.

[0062] In the general case, an emergency feed water supply and distribution device 5, a chemical reagent supply device 8, an overhead perforated sheet 6, a submerged perforated sheet 7 are optional (not obligatory) components of a steam generator. They are required to improve the steam generator operation reliability, durability, etc., and may either included or not in different horizontal steam generator structures. An emergency feed water supply and distribution device 5 is used to supply water to the steam generator if the main feed water line is damaged and during cooldown of the reactor plant through the secondary circuit in case of a design basis accident. A chemical reagent supply device 8 is used during regular flushing of the steam generation for removal of the accumulated depositions and corrosion products. This device is used to supply chemical reagents to the steam generator. A submerged perforated sheet 7 is used to level the steam load in the steam generator steam space. This is required to provide separation parameters of the steam generation and is only relevant for high-power steam generators. An overhead perforated sheet 6 is used to form an even profile of steam velocities in the steam generator steam space by creating resistance on its way, which is required to provide reliable steam separation in the steam generator.

[0063] A horizontal steam generator for a nuclear power plant is assembled as follows: A cylindrical vessel 1 of the steam generator is manufactured from a set of steel shells, followed by its heat treatment and machining. Elliptical bottoms (11), a feed water supply and distribution device (4), steam removal connection pipes (13), inlet and outlet headers (3), U-shaped heat-exchange tubes (15) with outer diameter dtb in quantity Ntb are manufactured. Then the welded vessel is installed onto the supports. Holes are drilled and tooled in the side surface of the inlet and outlet headers, then headers are installed inside the steam generator vessel and secured by welding. Supports for the coolant tube bundle are installed and a bundle 2 of heat-exchange tubes is formed directly in the vessel in rows from the bottom upwards. Each tube is secured in the coolant header, curled and welded from the inner side of the header. Other internals are installed. The elliptical bottoms 11 are installed and welded onto the vessel 1. The following devices may also be installed in the vessel: an emergency feed water supply and distribution device 5, a chemical reagent supply device 8, an overhead perforated sheet 6, and a submerged perforated sheet 7. These elements are optional for the steam generator, however, as it was mentioned above, they are designed to improve steam generator operation, in particular, to increase the operation reliability.

[0064] The heat-exchange tube bundle 2 is formed so that it is filled with heat-exchange tubes 15 continuously from the top downwards. Spacing elements 16 ensure gaps in the heat-exchange tube bundle 2 that do not exceed the vertical spacing of heat-exchange tubes 15 in the bundle 2.

[0065] Number Ntb of the heat-exchange tubes is selected depending on outer diameter dtb of a heat-exchange tube based on the relations above. On the one hand, reduction of the outer diameter with a simultaneous increase in the number of tubes and with gaps between the heat pipes not exceeding the vertical spacing in the bundle increases the heat transfer surface, and, as a result, the steam generator output. On the other hand, it is necessary to ensure reliable circulation of the boiler water (secondary circuit coolant) between the heat-exchange bundle tubes

EXAMPLE 1

[0066] A steam generator with the following parameters is manufactured:

[0067] thermal power Q=750 MW,

[0068] coolant flow rate G=22,000 m.sup.3/h,

[0069] heat-exchange surface area H=6000 m.sup.2,

[0070] heat-exchange tube outer diameter dtb=21 mm and wall thickness δ=1.5 mm,

[0071] coolant pressure P=16 MPa,

[0072] tube arrangement is staggered (k=1),

[0073] vertical and horizontal spacing of tubes in a bundle Sv=Sh=36 mm.

[0074] According to the claimed technical solution, the lower acceptable limit of the number of heat-exchange tubes that can be fitted into the vessel of this steam generator is:

[00010]$\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}=\frac{1.944.Math.{10}^6}{\pi .Math.{\left(\frac{4.Math.21}{5}+0.8\right)}^2}=1988.$

[0075] The upper acceptable limit of the number of heat-exchange tubes that can be fitted into the vessel of this steam generator is:

[00011]$\frac{1.111.Math.{10}^7}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.2\right)}^2}=\frac{1.111.Math.{10}^7}{\pi .Math.{\left(\frac{4.Math.21}{5}+0.2\right)}^2}=11,417.$

[0076] Let us compare the coolant flow velocity W (m/s) for a steam generator design with a number of heat-exchange tubes within the derived range 1988≦Ntb≦11,417, and that for a steam generator with a number of heat-exchange tubes outside the derived range.

[00012]$W=\frac{G}{\frac{\pi }{4}.Math.{\left(\mathrm{dtb}-2.Math.\delta \right)}^2.Math.\mathrm{Ntb}}=\frac{22,000}{3600.Math.\frac{\pi }{4}.Math.{\left(21-2.Math.1.5\right)}^2.Math.\mathrm{Ntb}}$

[0077] The coolant flow velocity for a steam generator with a number of heat-exchange tubes within the derived range will be as follows:

2.1 m/s≦W≦12 m/s.

[0078] Let us calculate the coolant flow velocity for a steam generator with a number outside the set range 1988≦Ntb≦11,417.

[0079] For a number of heat-exchange tubes greater than the specified one, for example, for 13,000 heat-exchange tubes, the coolant flow velocity will be as follows:

W.sub.13,000=1.84 m/s.

[0080] The example shows that the coolant flow velocity is very low and does not allow to ensure efficient heat transfer.sup.1, therefore, the technical and economic performance of the steam generator with a number of heat-exchange tubes outside the set range will be worse than that of the claimed one. .sup.1 Data on the nominal coolant flow velocity can be found in the following book: “Hydrodynamic and Thermo-chemical Processes in Steam Generators of NPP with VVER” by: N. B. Trunov, S. A. Logvinov, U. G. Dragunov, Moscow, Energoatomizdat, 2001, p. 50, stating: “Practices in design of horizontal steam generators allow us to assume the optimal coolant flow velocity in pipes to be 4-6 m/s”.

[0081] If the number of heat-exchange tubes used in the steam generator design is less than the specified one, for example, 500 heat-exchange tubes, then W.sub.500=48 m/s. The said coolant flow velocity during steam generator operation will lead to severe erosive wear of heat-exchange bundle tubes and their frequent damage, which reduces the technical and economic performance of the steam generator.

[0082] Let us compare the required length of heat-exchange tubes Ltb for a steam generator design with the parameters specified in the example and a number of heat-exchange tubes within the set range 1988≦Ntb≦11,417, and that of a steam generator with a number of heat-exchange tubes outside the range.

[0083] Assuming that

[00013]$\mathrm{Ltb}=\frac{H}{\pi .Math.\mathrm{dtb}.Math.\mathrm{Ntb}},$

the range of tube length sufficient to manufacture a steam generator with a number of heat-exchange tubes within the preset range 1988≦Ntb≦11,417 will be:

7.97 m≦Ltb≦45 m

[0084] It shall be further noted that to manufacture a steam generator according to the claimed invention, seamless solid-drawn austenitic stainless steel pipes with maximum length generally not more than 30 m are used. According to the metal industry development trends, it can be expected that manufacture of seamless solid-drawn or hot-rolled tubes with a length up to 45 m will be possible in the foreseeable future.

[0085] For a given steam generator design with a number of heat-exchange tubes less than specified, for example, 500 heat exchange tubes, the required length of heat-exchange tubes will be: Ltb=182 m.

[0086] Seamless 182 m long heat-exchange tubes are not available in the industry and their manufacture is not expected in the near future.

EXAMPLE 2

[0087] A steam generator with the following parameters is manufactured:

[0088] thermal power Q=1000 MW,

[0089] coolant flow rate G=36,000 m.sup.3/h,

[0090] heat-exchange surface area H=9000 m.sup.2,

[0091] heat-exchange tube outer diameter dtb=12 mm and wall thickness δ=1.1 mm,

[0092] coolant pressure P=17 MPa,

[0093] According to the claimed technical solution, the lower acceptable limit of the number of heat-exchange tubes that can be fitted into the vessel of this steam generator is:

[00014]$\frac{1.944.Math.{10}^6}{{\pi \left(\frac{4.Math.\mathrm{dtb}}{5}+0.8\right)}^2}=\frac{1.944.Math.{10}^6}{\pi .Math.{\left(\frac{4.Math.12}{5}+0.8\right)}^2}=6000.$

[0094] The upper acceptable limit of the number of heat-exchange tubes that can be fitted into the vessel of this steam generator is:

[00015]$\frac{1.211.Math.{10}^6}{\pi .Math.\mathrm{dtb}}=\frac{1.211.Math.{10}^6}{\pi .Math.12}=32,000.$

[0095] Let us compare the coolant flow velocity W (m/s) for a steam generator design with a number of heat-exchange tubes within the derived range 6000≦Ntb≦32,000, and that for a steam generator with a number of heat-exchange tubes outside the derived range.

[00016]$W=\frac{G}{\frac{\pi }{4}.Math.{\left(\mathrm{dtb}-2.Math.\delta \right)}^2.Math.\mathrm{Ntb}}=\frac{22,000}{3600.Math.\frac{\pi }{4}.Math.{\left(12-2.Math.1.1\right)}^2.Math.\mathrm{Ntb}}$

[0096] The coolant flow velocity for a steam generator with a number of heat-exchange tubes within the derived range will be as follows: 4 m/s≦W≦22 m/s.

[0097] Let us calculate the coolant flow velocity for a steam generator with a number outside the set range 6000≦Ntb≦32,000.

[0098] For a number of heat-exchange tubes greater than the specified one, for example, for 40,000 heat-exchange tubes, the coolant flow velocity will be as follows:

W.sub.40,000=3.3 m/s.

[0099] The example shows that the coolant flow velocity is very low and does not allow to ensure efficient heat transfer, therefore, the technical and economic performance of the steam generator with a number of heat-exchange tubes outside the set range will be worse than that of the claimed one.

[0100] If the number of heat-exchange tubes used in the steam generator design is less than the specified one, for example, 4000 heat-exchange tubes, then W.sub.4000=33 m/s. The said coolant flow velocity during steam generator operation will lead to severe erosive wear of heat-exchange bundle tubes followed by plugging of the damaged tubes, which reduces the technical and economic performance of the steam generator.

[0101] Let us compare the required length of heat-exchange tubes Ltb for a steam generator design with the parameters specified in the example and a number of heat-exchange tubes within the set range 6000≦Ntb≦32,000, and that of a steam generator with a number of heat-exchange tubes outside the range.

[0102] Assuming that

[00017]$\mathrm{Ltb}=\frac{H}{\pi .Math.\mathrm{dtb}.Math.\mathrm{Ntb}},$

the range of tube length sufficient to manufacture a steam generator with a number of heat-exchange tubes within the preset range 1988≦Ntb≦11,417 will be:

7.46 m≦Ltb≦39 m

[0103] For a given steam generator design with a number of heat-exchange tubes less than specified, for example, 4000 heat exchange tubes, the required length of heat-exchange tubes will be: Ltb=59 m.

[0104] Seamless 59 m long heat-exchange tubes are not available in the industry, therefore, manufacture of NPP steam generators with heat-exchange tubes of the said length is not possible.

EXAMPLE 3

[0105] A steam generator is manufactured with the input parameters same as those in example 1, wherein the outer diameter of heat-exchange tubes is dtb=21 mm and the wall thickness is δ=1.5 mm. According to the claimed invention, number Ntb of heat-exchange tubes is selected from the range of between 1998 and 11,417 pcs. Clear area S.sub.tb of a heat-exchange tube will be:

[00018]$\mathrm{Stb}=\frac{{\pi \left(\mathrm{dtb}-2.Math.\delta \right)}^2}{4}=254.5.Math..Math.\mathrm{mm}$

[0106] Heat-exchange tube bundle installation area ftb per tube in case of in-line arrangement (k=1) and at equal vertical spacing S.sub.v and horizontal spacing S.sub.h (FIG. 2) of 36 mm will be:

[00019]$\mathrm{ftb}=\frac{S_V.Math.S_H}{k}=1296.Math..Math.{\mathrm{mm}}^2$

[0107] Then the relation between clear areas Stb and bundle installation area ftb will be:

[00020]$\frac{S_{\mathrm{tb}}}{\mathrm{ftb}}=0.2$

[0108] Consequently, a heat exchange bundle cell formed by four adjacent tubes with ftb=1296 mm.sup.2 has 80% of its area available for circulation of the boiler water (secondary circuit coolant), which enables its unhindered flow.

[0109] Compliance with this criterion increases the technical and economic advantages of the claimed steam generator, as in combination with the criteria of limited heat exchange tube number and length it contributes to improvement of the steam generator operation reliability.

[0110] The latest relation also confirms that the claimed steam generator design has heat-exchange tube bundle filling the inner volume of the steam generator vessel at uniform spacing without filling gaps, and the gap between the adjacent heat-exchange tubes in the vertical direction does not exceed the vertical spacing between the heat-exchange tubes in the bundle.