The present invention relates generally to steam generators and, more particularly, to steam generators which are provided with a means for removing sludge deposits from its tubesheet.
A steam generator which is designed to be utilized within nuclear reactor systems generally comprises a cylindrical envelope which forms a pressure-confining shell. This cylindrical shell is usually disposed in such a way that its central axis is in a vertical position. Within the shell, a generally flat plate is disposed in such a way that it divides the internal portion of the steam generator into two major cavities. This plate, herein after referred to as a tubesheet, is further provided with a plurality of holes through which tubes are extended.
The lower portion of the steam generator, beneath the tubesheet, is formed into two essentially identical compartments. Each of these compartments is in the shape of a quarter-sphere and each of these compartments has the tubesheet as its upper boundary. Furthermore, the bottom portion of the steam generator shell is hemispherical and defines the lower portion of each of these two compartments. A vertical partition wall is provided to separate this hemispherical portion into the two quarter-spherical compartments.
Tubes, which extend through the tubesheet, provide fluid communication between the two quarter-spherical compartments by extending from one compartment, through the tubesheet, into the upper portion of the steam generator and back through the tubesheet in fluid communication with the other compartment. Since the two compartments are both located beneath the tubesheet, the tubes traverse a U-shaped path in order to provide fluid communication between the two compartments while extending into the upper regions of the steam generator.
In the upper region of the steam generator, secondary water is contained by the steam generator's shell and this volume of water is maintained in thermal communication with the outer surfaces of the tubes. In operation, a constant supply of water at a high temperature is provided in a first one of the two quarter-spherical compartments. The water passes into the tubes at the region where they extend, through the tubesheet, into that compartment. Due to differential pressures, the water passes upward through the tubes and along the U-shaped path defined by them. After passing through the tubes, the water flows into the second of the two quarter-spherical compartments and subsequently exits from the steam generator. As this hot water passes through the tubes, heat is transferred to the secondary water, by thermal conduction, through the walls of the tubes.
The primary water supply, which passes through both quarter-spherical chambers and through the tubes, is supplied from a nuclear reactor or some other heat producing apparatus. The secondary water, which is maintained within the shell of the steam generator in its upper portion and is in thermal communication with the primary water through the walls of the U-shaped tubes, is thereby heated and converted to steam which is eventually conducted to a steam turbine. It should be apparent from the above description that the primary and secondary water supplies are prevented from mixing together. This type of steam generator system enables a nuclear reactor to be used to generate heat for use by a steam turbine in such a way that prevents radioactive water from passing in fluid communication with the steam turbine of an electrical power generating station.
As the radioactive primary water passes through the U-shaped tubes, this water yields part of its heat by vaporizing the secondary water which is contained in the upper portion of the steam generator's shell and which surrounds the tubes. As this steam is produced, it is removed from the upper portion of the steam generator and conducted to a turbine which is associated with an electric generator. After being used to drive the turbine, the secondary water is condensed and eventually reintroduced into the upper portion of the steam generator. It has been found that, over long periods of operation, sediments can accumulate at the base of the tubes on the upper surface of the tubesheet. These sediments consist mainly of iron oxides although they may also contain precipitates of other compounds. The presence of a sludge, formed by these sediments on the upper surface of the tubesheet, can create tube corrosion phenomena which could potentially cause leaks in the tubes and permit the primary water to mix with the secondary water in the steam generator. Impurities, such as cobalt or entrained gaseous fission products, within the primary water supply are radioactive due to the flow of the primary water through the nuclear reactor.
If tube corrosion occurs, expensive repairs may be required and the steam generator may therefore be out of productive service for an unacceptable length of time. Since the primary water is typically under approximately 2200 psia and the secondary water is under approximately 1000 psia, a very small leak in a heat exchanger tube can lead to a flow of primary water into the secondary system. Larger leaks, which would occur if a tube ruptures, would cause a significant flow of primary water into the secondary system and, if not immediately identified and corrected, could lead to a major disruption of the power plant's operation. Therefore, periodic maintenance procedures are generally performed to remove sludge buildup at the base of the tubes where they are connected to the tubesheet at its upper surface.
U.S. Pat. No. 3,916,844 which issued to Cassell on Nov. 4, 1975, illustrates one possible approach for removing the sludge buildup in a steam generator. According to that invention, the sludge is removed by an arrangement of baffles within the steam generator's shell which define a settling chamber. The baffles retard the flow of secondary water and, essentially, effect an abrupt change in the secondary water's direction of flow so that suspended particles can settle out of the secondary water. By using a blowdown pipe, continuous or periodic flushing of this settling chamber can be effected. European Pat. No. 67,739 which issued to Jean-Claude Yazidjian on Dec. 22, 1982 illustrates a different concept in removing sludge from the upper portion of the tubesheet of a steam generator. In contrast to the Cassell patent, the Yazidjian invention comprises movable water lances which can be manipulated to direct a stream of fluid against the upper surface of the tubesheet in order to break up the sludge which had settled thereon. The sludge lances extend through the wall of the steam generator in a generally horizontal direction and can be manipulated by human effort in order to direct the stream of fluid towards different portions of the tubesheet. A portion of the steam generator's shell is shaped to receive the lance therethrough in sliding communication. Another fluid lancing method is disclosed in U.S. Pat. No. 4,079,701 issued to Hickman et al. on Mar. 21, 1978. This patent describes a technique which forces the sludge to the periphery of the tubesheet by maneuvering a movable fluid lance along the tubesheet diameter.
To date, efforts which have been directed to the removal of sludge from steam generator tubesheets have significant disadvantages. The methods which utilize stationary components generally require the steam generator to undergo elaborate design changes in order to form settling chambers in which the flow of the secondary water is retarded. This approach is illustrated in the Cassell patent described above. Other methods of removing sludge from the tubesheets of steam generators require interactive human participation in their operation and also require a means for extending a work tool through the wall of the steam generator in such a way that freedom of motion is provided but where human exposure to radioactive components is minimized. The Yazidjian patent is illustrative of this latter philosophy of operation.
In order to provide adequate removal of sludge buildup on the tubesheet of a steam generator without the necessity of elaborate design changes is a desirable goal. Since it is a goal in the nuclear industry to keep human exposure to radioactive components as low as reasonably achievable, it is advantageous to reduce or eliminate the requirement for human interaction with the sludge removing apparatus in order that radioactive exposure of human beings is significantly reduced or eliminated entirely.
The present invention provides a sludge removal apparatus which does not require a significant design change of typical steam generators and which, although it responds to the procedural dictates of a human operator, does not require the exposure of a human being to potentially radioactive components of the steam generator.
The present invention utilizes a manifold which is rigidly attached to the upper surface of a steam generator's tubesheet. The manifold is provided with a central conduit therethrough which has at least two termini. One of this conduit's termini is connected in fluid communication with a means for providing a flow of fluid from an external source into the manifold's conduit. Another termini of the manifold's conduit is connected in fluid communication with a means for directing a stream of fluid in a desired direction. Normally, this directing means is a nozzle through which the externally supplied fluid can pass. The function of the nozzle is to not only direct the fluid in a desired direction but also to increase the velocity of the fluid as it passes through the nozzle's aperture. In a sludge removing apparatus made in accordance with the present invention, the nozzle is directed in line with a lane which is formed by two rows of tubes extending from the tubesheet. The means for providing a flow of water from an external source to the manifold can be a length of tubing which extends from the manifold, through the steam generator's wall, to a water pump. At the region where thus tubing passes through the steam generator's wall, no relative movement between the wall and the tubing is required and, therefore, adequate sealing can be provided so that the manifold and its associated nozzle and tubing can remain in place during normal operation of the steam generator. Between the wall of the steam generator and the pump, valving means is provided so that the flow of water to the above-mentioned nozzle can be restricted at the desire of the operator. It should further be apparent that, by placing the pump at a significant distance from the steam generator, the valve can be placed at a distance from the steam generator which removes the human operator of the valve from any appreciable exposure from potentially radioactive components that are located within the shell of the steam generator.
By providing the manifold with a plurality of internal conduits, each being independent from one another, and by further providing each of these separate conduits with its own nozzles and tubing system, it should be apparent that a sludge removing apparatus made in accordance with the present invention can provide a plurality of independent fluid circuits that are independently operable. If each of the circuits is provided with its own valve, water can be selectively supplied to each of the nozzles independently from the operation of other nozzles. This characteristic permits a water pump of a given size to be used to serially remove sludge from different lanes between the tubes of the steam generator. This characteristic has the advantage of eliminating the need for an extremely large capacity pump that would otherwise be required if all of the nozzles of the present invention were operated simultaneously.
A preferred embodiment of the present invention comprises a plurality of the individual conduits within the manifold and each of these individual conduits is connected in fluid communication to a plurality of nozzles. Although a preferred embodiment of the present invention comprises approximately six nozzles connected to each of the individual conduits, this is not a requirement. The manifold can be extended along a center line of the tubesheet which would generally extend in the same direction as the vertical wall which separates the steam generators' two lower compartments described above. It should be understood that, depending on the particular geometry of the tube layout configuration, the manifold may be placed at other locations above the tubesheet in order to avoid obstruction of the tubes or other components within the secondary water portion of the steam generator.
Since the present invention provides a flow of water into the region above the tubesheet, some means for removing this water is required. In a preferred embodiment of the present invention, the water is removed from the upper surface of the tubesheet by providing at least one opening in the wall of the steam generator proximate the upper portion of the tubesheet. This opening permits the water, in which a quantity of sludge is entrained, to be removed from the upper portion of the steam generator and either filtered or discarded. In a preferred embodiment of the present invention, this water and sludge mixture can be filtered to remove the sludge and then recirculated to the pump. Depending on the quantity of sludge to be removed, it may not be practical to provide a closed water system in which the sludge is removed by filters. Instead, a constant supply of fresh water would be injected toward the sludge by the nozzles and, after being removed through the above-described opening, it would be stored for a later removal and disposal of the entrained sludge particles.
It should be apparent that the present invention is applicable to both new steam generator installations and, as a retrofit, to existing steam generators which are in operation. The manifold of the present invention is intended to be bolted or otherwise fastened rigidly to the upper surface of the tubesheet and to remain rigidly attached to the tubesheet during normal operation of the steam generator. It should be understood, however, that during sludge removal procedures, the secondary water would be drained from the steam generator prior to the operation of the present invention.