Title of Invention

AN IMPROVED SATURATED STEAM GENERATOR (SG) ADAPTABLE TO PRESSURIZED HEAVY WATER COOLED NUCLEAR REACTORS

Abstract

This invention generally relates to a natural circulation, vertical and integral steam drum type steam generators generating dry saturated steam. Particularly the invention relates to steam generators adaptable to pressurized heavy water cooled nuclear reactors. Particularly, the invention concerns the modifications and changes in design configurations employable on typical SG' s so as to augment their performance and up-rate their capacity and performance.

Full Text

FIELD OF THE INVENTION This invention generally relates to natural circulation, vertical and integral steam drum type steam generators (hereinafter referred to as SG) generating dry saturated steam. Particularly the invention relates to steam generators adaptable to pressurized heavy water cooled nuclear reactors. Particularly, the invention concerns the modifications and changes in design configurations employable on typical SG's so as to augment their performance and up-rate their capacity and performance. BACKGROUND OF THE INVENTION Natural circulation steam generators are increasingly being used to generate steam from the heat available with process fluids, which is further utilized to drive a turbine and generate electricity. The transport of energy from this type of steam generators is usually through saturated steam. One such typical general arrangement of a steam generator is shown in Fig. 1. The configuration of the Boiler and Drum sections in such a typical steam generator is influenced by the various device characteristics for example, the primary side fluid that usually flows inside the steam generator tubes, its mass flow rate, the fluid quality, temperature and its pressure; the secondary side water flow, which usually flows outside the tube bundle of the SG, its flow rate, its feed temperature; capacity of the steam separator, re-circulation ratio, steam pressure and the water level in the drum. During the "high-reactor-power" phase, owing to hotter primary fluid conditions, more steam is likely to be produced around the tube bundle. Since the volume of the steam generator is fixed, owing to swelling vapour volumes, the liquid level rises in the drum; i.e. increased power output is reflected by an increase in water level. Alternatively, in the "low-reactor-power" phase, the water level in steam drum reduces. Thus, for a given reactor and with a selected SG capacity, and a predetermined operating condition, the normal water level is somewhat fixed. The power rating of the continuous steam producing capacity of such a SG is subject to technical limitations as explained hereinunder. The sizing of the drum section is normally larger than that of the boiler section, the sizing being primarily decided by the number of steam separators required for a given evaporation rate corresponding to the maximum rating of steam generator. The height of the drum is primarily decided by the recirculation ratio of the feed water and the water level needed to be maintained for safer operation of the steam generator at various power loads. Size of the drum is further influenced by the number of separators including the disposition of the separators allowing optimum access to the drum internals. A still further factor influences the sizing of the drum which constitutes provisions for a feed water line including space provisions for fabrication, maintenance and inspection of the above components. For the steam generator shown in Fig. 1, atleast 23 steam separators are disposed on the deck plate with a provision for accommodating feed water pipe and the deck plate manhole. The separators are eccentrically arranged with reference to the drum axis as the feed water line crosses the separator deck plate. Because of the above interference, the configuration of the cone is also made asymmetric and eccentric. For any chosen configuration and the chosen operating parameters, the SG can generate rated steam flow corresponding to a specified power output value, say X Mw. With all other parameters remaining unchanged, the power output can be said to be tightly linked to the number of steam separators and the drum size. If only any one of them or both can be increased, the steam generator rate and hence the power output will also increase. However, other limitations for example, structural strength and design complication usually does not allow drum size to increase. Alternatively, there is a possibility of accepting the hot process fluid at a still higher heat condition or at higher 'enthalpy', which can be fed into the SG for production of higher flow rate of steam in the boiler section. Parametric studies show that any boiler section of the SG has an inherent capability of accepting higher input and can generate higher steam water mixture and send it to the drum section. A typical boiler section as shown in fig. 1 has the capability to generate higher steam water mixture to the tune of 30-40% more in terms of mass flow rate. To accommodate the higher steam water mixture so produced in the drum section, the drum size as well as number of separators have to be increased. Such a solution, however, gives rise to a plurality of basic constraints, for example, to accommodate higher number of separators and higher capacity internals, the size of the separator deck plate and drum inner diameter have to be in turn increased. The changes in the drum size can have several repercussions on the plant design and the manufacturing process of higher diameter drum shell and dished ends by forging process. Since the primary source is capable of supplying higher quantity of energy it is often desired to harness this quality heat energy and thereby generate higher power from the same steam generator. It is also a standard industry practice to keep high redundancy on all critical components, so as to ensure uninterrupted availability. Normally nuclear utility operators keep a couple of spare pressure boundary components of steam generators always. Costlier pressure boundary components like drum shell, drum dished ends, tube-sheet and boiler shells pertaining to a particular Steam Generator of standard capacity are often procured and kept as a ready stock. To be capable of using the stocked and costlier pressure boundary components indirectly makes it mandatory to consider up-rating without any change In dimensions of the SG components. This invention precisely explains how the task of up-rating is accomplished with the existing components itself and minimal changes. USPAT# 4,200,061: Titled, "Steam generator for nuclear power plant", discloses an arrangement, especially for pressurized water reactors. This patent deals with introduction of flange connection at the tube sheet and the middle section boiler drum for easy assembling and dismantling in case of maintenance of steam generator. USPAT# 6,408,800: Titled "Separator for water/steam separating apparatus", deals with design details of separator components. USPAT#6,044,804: Titled "Method and device for monitoring feed water supply to a steam generator", deals with the method and device for monitoring a feed water supply and control for a fossil-fuel fired natural circulation type steam generator. Thus, none of the existing art suggests or teaches an improved steam generator device adaptable to pressurized heavy-water cooled nuclear reactors which has been capacitivity upgraded without replacing or modifying the critical components of the steam generator. Up-rating with the existing configuration of standard sized steam generator poses perceptible limitations of steam separation capacity. It is generally not possible to accommodate a higher number of steam separators that are fixed on to the separator deck plate (SDP), which in turn is fitted on to the upper portion of the conical shroud. Accommodating larger number of steam separators warrant increasing the shroud diameter. But the presence of feed water inlet nozzle pipe within the drum internal space severely restricts such an increasingly size of shroud. Larger openings cut on the SDP, to accommodate larger feed water line would also further reduce the space availability for placing of separator evenly on the SDP, which eventually could result in non-uniform steam separation performance. As a general practice, the separator deck plate (SDP) and the cone used at the upper part of the shroud are arranged eccentrically with respect to steam generator axis. Manufacturing of such an asymmetrical cone will be difficult and costlier. The gap width between the SDP and steam generator wall also varies with height. This would cause different re-circulation water flow in the annular down comer. To accommodate a larger number steam separators to generate higher power output the drum diameter has to increased. However, for an operating plant that needs to up-rated, accommodation of larger drum diameter will be very difficult within the given plant layout. OBJECTS OF INVENTION Accordingly, it is an object of the invention to provide an improved steam generator adaptable to pressurized-heavy water-cooled nuclear reactors which utilizes the heat energy of the primary side fluid to generate higher power from the steam generator. A further object of the invention is to provide an improved steam generator adaptable to pressurized-heavy water-cooled nuclear reactors which increases the capacity of the steam generator without causing redundancy of the critical components of the generator, thereby exercising economy in capacity up- gradation. A still further object of the invention is to provide an improved steam generator adaptable to pressurized-heavy water-cooled nuclear reactors which is capable of generating comprehensively higher steam-production rate without necessitating replacement, or major reconfiguration of the critical components of the steam generator. An yet another object of the invention is to provide an improved steam generator adaptable to pressurized-heavy water-cooled nuclear reactors which is configured basically through optimization of components layout. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1- Shows a typical conventional steam generator. Figure 2- Shows an improved steam generator incorporating the modifications according to the invention. SUMMARY OF THE INVENTION Accordingly, there is provided an improved saturated steam generator (SG) adaptable to pressurized heavy-water-cooled nuclear reactors, the steam generator (SG) comprising a drum section; a boiler section; a primary fluid passage formed by a bundle of inverted 'U'-tubes passing through a tube sheet to which the bundle of inverted 'U'-tubes are rigidly fixed, the primary fluid being a hot medium containing substantial heat energy; a secondary fluid passage configured by the outer surface of the U-tubes, the outer shell of the boiler section, and a thin down-comer shroud, the secondary fluid exchanging heat from the primary fluid to form water-steam mixture, the shroud being fixed with a separator deck plate and accommodating a plurality of steam separators; and atleast two numbers feed flow nozzles provided on the drum section being disposed in diametrically-opposed locations, the improvement is characterized in that: the height of the drum section is selected to be H + 2/5 H wherein 'H' is the standard height of the drum section and the incremental length is determined by the thermal design, the thin down-comer shroud is configured as a regular conical section being disposed concentric to the drum-section axis, and having a length atleast of L + 2/5 L, wherein 'L' is the length of the shroud corresponding to the standard height of the drum section , the atleast two feed- water inlet nozzle provided each being configured to have reduced diameter thereby occupying lesser overall space around the shroud; said separator deck plate accommodating atleast n + 3/10 n standard sized steam separators, thereby increasing the steam production rate of the improved steam generator (SG). The drum diameter remaining the same, its length is increased above the knuckle transition. The conical shroud length is also made to increase. The feed nozzle is split into two pipes, mounted diametrically opposed to each other on either end. Doubling the feed line quantity causes individual feed line diameter to reduce. As the conical shroud top goes above the feed nozzle location, its radial size is enlarged to the maximum that can be accommodated in the annular space. This in turn enables the separator deck plate to accommodate additional numbers of steam separators which enhance the dry steam production rate, thereby increasing the overall steam rate. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Constructional features of a typical conventional steam generator can be described with fig. 1. The steam generator consists of two major functional sections. The upper portion forms the drum section (1) where the saturated steam water mixture from the boiler section collects and gets separated. The lower portion of the steam generator is termed 'Boiler section' (2) where the hot process fluid passes through the inverted vertical U-tube bundle (4) fixed to the tube sheet (3). The primary fluid entering a right-side bottom inlet chamber (10) and flows through the bundle of U-tubes (4) connected to the tube sheet (3) at the bottom of the steam generator and flows out through an outlet (left) chamber. On the secondary side feed water enters at the middle section of the steam generator, i.e. above the end of the U-end of the tube. Feed water mixes with the water returned by the steam separator and flows down to the bottom end of steam generator through an annular path formed between a shroud (5) and a linear surface of the steam generator (2). The length of the shroud (5) is predetermined in relation to the length of the boiler section (2), for example, 'L'. The heat exchanged from the primary fluid heats up and boils the secondary side fluid creating a two-phase flow around the tube bundle (4). The vapour phase of flow gradually increases in quality (Quality of vapour is definable as the mass ratio of vapour quantity to the overall fluid quantity) towards the steam drum section (1). When the water and steam mixture moves past the tube bend at the top, the quality of the mixture reach approximately 25% (i.e., 25% of the mixture is dry steam by weight). The mixture then passes through a separator deck plate, SDP (8), and a plurality of centrifugal steam separators (7) for example, n- numbers fixed on to the SDP (8), where the primary stage of separation takes place. And the steam reaches around 95% purity at this juncture. The steam drum section (1) primarily consists of plurality of steam separators (7), for example, n- number steam drier (9), a feed water supply line (6), a distribution header (13), several drain lines and other internals housed in the upper section. The steam water mixture from the boiler (2) enters the steam separator (7), where major quantity of water is separated from the mixture. The distribution header (13) having a pre-selected diameter (D). The relatively higher quality vapour mixture then enters a corrugated plate type steam drier (9) and then through a perforated sheet (not shown in figure), which is kept at the top section of the steam drum, the liquid phase of the fluid is separated here resulting in 99.75%, dry steam passing out of the steam generator through a nozzle (12) and to be ducted to a steam turbine in turn. The 'make up' feed water flow, to compensate the steam that has been evaporated from the water, along with the saturated water separated at the steam separators (7) and the return water from the turbine are admitted at the top section of the boiler (2) through a nozzle (6) and further conveyed down to the bottom of tube bundle (4) via a ring header (13) and the down corner (5) completing the circulation loop. The water separated from the primary separator and the drier mixes with the feed water injection and flows down through an annular gap all the way down up to the tube sheet (3). The annular gap is made up of the space between the thin shroud (5) around and the tube bundle (4) and the inside surface of the boiler section (2). The difference in the static head between the saturated fluid from the down-comer and less dense two-phase flow in the riser section creates the driving force for natural circulation within the steam generator. The feed water flows to the bottom and takes an upward direction and rises over the tube bundle (4) again and continues to pick up the heat from the primary coolant. A liquid level (15) inside the drum section (1) is critically maintained by matching the steam evaporation rate and the feed water flow rate. Fig.2 shows only the salient features of an up-rated steam generator, which incorporates the inventive modifications to aid-up rating process. Compared to the configuration in fig. 1, the arrangement as depicted in fig.2 significantly varies in the following aspects. The drum section (1) is made taller, the liquid level (15) is significantly higher measured from the knuckle transition of the boiler and drum sections. The conical shroud (5) is also made larger in diameter at the top and longer from the boiler top end, the funnel like top of the shroud and the separator deck plate (8) being coaxial rather than eccentric to the drum axis, the feed water inlet nozzle (6) is made smaller in diameter, but instead of one, a pair of nozzles cater to the feed water make up process. This also promotes a smaller sized ring header (13) to be employed and a more uniform circumferential distribution of feed water. The separator deck plate (8) accommodates more than the 30% additional number of steam separator assemblies (7), the top end of the conical shroud (5) goes above the feed water nozzle entry location, thereby avoiding complicated 'cut-outs' made in the SDP and space left out to facilitate this pipe's location, the steam separators (7) themselves being located at a higher elevation and in an unencumbered manner. The higher number of steam separators, dual feed nozzles and larger diameter shroud together make the steam generator accept inflows of hotter primary fluid and also sustains higher rates of boiling on the secondary fluid side, thus feeding a higher steam (vapour) flow throughput to the steam turbine feed line. As indicated, the collective effect of the modifications achieve a higher performance for any chosen primary/secondary fluid combinations. WE CLAIM 1. An improved saturated steam generator (SG) adaptable to pressurized heavy- water-cooled nuclear reactors, the steam generator (SG) comprising a drum section (1); a boiler section (2); a primary fluid passage formed by a bundle of inverted 'U'-tubes (4) passing through a tube sheet (3) to which the bundle of inverted 'U'-tubes (4) are rigidly fixed, the primary fluid being a hot medium containing substantial heat energy; a secondary fluid passage configured by the outer surface of the U-tubes (4), the outer shell of the boiler section (2), and a thin down-comer shroud (5), the secondary fluid exchanging heat from the primary fluid to form water-steam mixture, the shroud (5) being fixed with a separator deck plate (8) and accommodating a plurality of steam separators (7); and atleast two numbers feed flow nozzles (6) provided on the drum section (1) being disposed in diametrically-opposed locations, the improvement is characterized in that: - the height of the drum section (1) is selected to be H + 2/5 H wherein 'H' is the standard height of the drum section (1); - the thin down-comer shroud (5) is configured as a regular conical section being disposed concentric to the drum-section axis, and having a length atleast of L + 2/5 L, wherein 'L' is the length of the shroud (5) corresponding to the standard height of the drum section (1); - multiple feed-water inlet nozzle (6) each being configured to have reduced diameter thereby occupying lesser overall space around the shroud (5); - said separator deck plate (8) accommodating n + 3/10 n standard sized steam separators (7), thereby increasing the steam production rate of the improved steam generator (SG). 2. The steam generator as claimed in claim 1, comprising multiple corrugated plate steam drier (9) fixed at a top end of the drum section (1). 3. The steam generator as claimed in claim 1, wherein the steam separators (7) are symmetrically disposed to achieve higher steam separation and steam flow rate. 4. The steam generator as claimed in claim 1, wherein an upper part of the shroud (5) configured with a uniform cone angle. 5. The steam generator as claimed in any of the preceding claims, wherein a uniform gap is established between the deck plate (8), and the walls of the drum section (1), and the boiler section (2) to achieve an uniform re-circulation of feed water flow. 6. The steam generator as claimed in claim 1, comprising a nozzle (12) for exiting the dry steam from the generator (SG) for ducting to a steam turbine. 7. The steam generator as claimed in claims 1 to 4, comprising a ring header (13) for admitting and conveying the return water from the steam turbine. 8. An improved steam generator adaptable to pressurized heavy water cooled nuclear reactors as substantially described herein with reference to the accompanying drawings. This invention generally relates to a natural circulation, vertical and integral steam drum type steam generators generating dry saturated steam. Particularly the invention relates to steam generators adaptable to pressurized heavy water cooled nuclear reactors. Particularly, the invention concerns the modifications and changes in design configurations employable on typical SG' s so as to augment their performance and up-rate their capacity and performance.

Documents:

412-KOL-2005-FORM-27-1.pdf

412-KOL-2005-FORM-27.pdf

412-kol-2005-granted-abstract.pdf

412-kol-2005-granted-claims.pdf

412-kol-2005-granted-correspondence.pdf

412-kol-2005-granted-description (complete).pdf

412-kol-2005-granted-drawings.pdf

412-kol-2005-granted-examination report.pdf

412-kol-2005-granted-form 1.pdf

412-kol-2005-granted-form 18.pdf

412-kol-2005-granted-form 2.pdf

412-kol-2005-granted-form 3.pdf

412-kol-2005-granted-gpa.pdf

412-kol-2005-granted-reply to examination report.pdf

412-kol-2005-granted-specification.pdf