Patent Application
20140261246
1. Field
This invention relates to steam generators and more particularly to methods for removing sludge deposits from the top of tube sheets (secondary side) of steam generators.
2. Description of Related Art
A nuclear steam generator is a pressurized vessel divided into a primary and a secondary side. The primary and the secondary sides are separated by the “tube sheet”. As in any heat exchanger, both primary and secondary sides have an inlet and an outlet. In order to increase the heat exchange surface, the tube sheet is drilled with a plurality of holes organized in two groups. The primary side is divided in two sections by the “divider plate” in a way that one group of hole communicates with the primary side inlet (to form the “hot leg”) and the second group of hole communicate with the primary side outlet (to form the “cold leg”). U-shaped tubes attached to the tube sheet extend in the secondary side and connect the holes from the hot leg to the holes from the cold leg. These U-shaped tubes form the tube bundle. The primary hot water can now enter the hot leg, travel through the tubes where the heat transfer takes place and leave the steam generator through the cold leg. On the secondary side, cold water (“feedwater”) enters through the secondary side inlet (“feed water nozzle”), turns into steam from the heat transfer through the tubes and it exits through the secondary side outlet (“steam nozzle”). This configuration is described for example by U.S. Pat. Nos. 8,238,510; 5,036,871; 4,273,076; and 4,079,701 (Haberman; Ruggieri et al.; Lahoda et al. and Hickman et al., respectively), many of which also relate to top of tube sheet sludge removal.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tube, the U-tube walls are the boundary for isolating these radioactive particles from the secondary side. It is, therefore, important that the U-tubes be maintained defect-free so that no leaks/breaks will occur in the U-tubes.
A variety of degradation mechanisms have been experienced on the shell side of steam generators. These degradation mechanisms may be loosely divided into two categories; mechanical degradation, such as wear or denting and chemical induced degradation such as stress corrosion cracking (SCC) or Inter/transgranular attack. High caustic levels found in the vicinity of the cracks in tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions, have identified high caustic levels as the possible cause of the intergranular corrosion, and thus the possible cause of the tube cracking. Acid conditions have also empirically demonstrated the ability to cause tubing degradation. Elevated concentrations of deleterious species such as lead or copper and situations with elevated electrochemical potential are also catalysts for tubing Accelerated degradation as a result of localized mechanical stresses from deformation of tubing via in situ formation of magnetite, known as denting, is also a cause of accelerated tubing degradation. These degradation mechanisms typically occur in the vicinity of a sludge pile present on the top of tubesheet on the shell side of the steam generator. The sludge is mainly iron oxide particulates and copper compounds along with traces of other minerals that have settled out of the feedwater onto the tube sheet, and into the annulus between the tube sheet and the tubes. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and the tubing degradation location strongly suggests that the sludge deposits provide a site for concentration of impurities at the tube wall that results in the onset of tubing degradation.
To remove these deposits, sludge lancing is performed every one to two refueling outages. Currently, standard practice involves spraying high pressure water through the tube bundle and directing the flow to suction hoses where the loose deposits can be removed and filtered. These suction hoses may be located at a substantial distance from the completely separate high pressure lance. This prior art process typically requires large pumping and filtration systems which use several hoses to deliver the cleaning media, which can be located over 500 feet away. The high pressure water is typically delivered from the “no” (central) tube lane (lane without tubes separating the hot leg side from the cold leg side) of the steam generator and “pushes” the deposits into the suction hose system. The lancing process requires the tube sheet to be lanced several times to ensure satisfactory cleanliness results, which is time consuming and not cost effective.
In most nuclear steam generators in service today, there are usually 6 inch (15.2 cm.) diameter hand holes in the shell of the steam generator near and above the tube sheet that has an associated hole in the wrapper providing access to the tube sheet for removal of the sludge deposits.
In regard of the description of the related art made above, there is a need for a method that can clean the top of the tube sheet with low cost and efficiency without second or third application, and a main object of this invention is to provide such a method.
The above mentioned problems are solved and object accomplished by providing a method of sludge removal from the top of tube sheet surface in a tubular steam generator having a plurality of entry handholes allowing access to the no tube lane and to the circumferential annulus, consisting of 1) opening at least one handhole allowing access to the no tube lane and/or to the circumferential annulus. 2) introducing at least one moveable sludge suction apparatus within the no tube lane and/or the circumferential annulus, said apparatus includes a suction head with at least one vacuum inlet fitting within a row of tubes of the tube bundle; and 3) sludge vacuuming the hot leg side and cold leg side of the tube bundle and top tube sheet surface with the moveable sludge suction apparatus, allowing continuous and complete vacuuming of sludge in the hot and cold sides of the tube bundle without the introduction of pressurized cleaning water during sludge removal activity.
The invention also resides in use of a moveable sludge suction apparatus. The said apparatus is able to deliver a wand and a suction head assembly in bundle and has an optional light and visual means. The wand connects the suction head to the suction apparatus and the optical device acts to inspect sludge removal from the top of the tube sheet.
The method proposed in this invention approaches sludge removal through a local, in-bundle suction method. This method gives the ability to clean specific areas of interest allowing for less time to be spent on areas already cleaned or more time for heavier loaded regions. The method preferably includes visual inspection capabilities to provide “live” cleanliness results, eliminating the need to perform a separate inspection that follows typical prior art sludge lancing. The inspection capabilities also provide 100% accessible in-bundle tube sheet inspections. Currently, in-bundle inspections are performed separately, following the acceptance of sludge lancing results, and on a limited scope basis.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the invention, it is believed the invention will be better understood from the following description, taken in conjunction with the accompany drawings, wherein:
This invention describes a method for deposit removal from the top of tube sheet of steam generators. The method will be implemented through a suction wand that is able to be delivered in the majority of current SG tube gaps. The suction wand will be delivered from the handhole, the no tube lane or from the circumferential annulus of the SG. It will consist, preferably, of at least one suction head capable of removing soft sludge deposits. Abilities also include lighting and video inspection for viewing cleaning results and potential tool position.
This method would only require the use of a single air operated diaphragm pump for a vacuum flow source. This replaces the current method of delivering high pressure water with flows from 25 to 48 GPM (gallons/minute) and up to 3,000 psi (204 atmospheres). The method (and its delivery system) can perform live cleanliness inspection, eliminating the need for several platform set ups that are currently in use.
In order to better understand the invention, we must refer to the prior art at this time. In a U-tube type steam generator, a tube sheet supports a bundle of heat transfer U-tubes. During operation, sludge forms on the tube sheet around the U-tubes and in the annulus, causing potential failure of the tubes. Failure of the tubes may result in a release of radioactive particles from the primary reactor coolant into the secondary side system. The invention, herein described, is a method for removing this sludge accumulation by a vacuum process rather than a high pressure process.
Referring now to
Again, referring to
During operation, inlet feedwater W enters steam generator 10 through feedwater inlet nozzle 46, flows through a feedwater header, and out of feedwater header through discharge ports. The greater portion of the feedwater exiting discharge ports, flow down annular chamber 44 until the feedwater contacts tube sheet 34. Once reaching the bottom of annular chamber 44 near tube sheet 34, the feedwater is directed inward around tubes 38 of tube bundle 40, which itself is enclosed by wrapper 42, where the feedwater passes in a heat transfer relationship with tubes 38. The hot reactor coolant fluid H being in tubes 38 transfers heat through tubes 38 to the feedwater thereby heating the feedwater. The heated feedwater then rises by natural circulation up through the tube bundle 40. In its travel around tube bundle 40, the feedwater continues to be heated until steam S is produced and passes through steam nozzle 20.
Referring now to prior art
Then, according to one aspect of the prior art, again shown in
Referring now to
The vacuum head(s) provide(s) a vacuum sufficient to remove aqueous sludge from the top surface of the tube sheet 34′. The vacuuming takes place after the SG has been drained, but with a sufficient volume of water still being present on top of the tubesheet. The vacuum is delivered through a nozzle on one side, or both sides or the bottom of the suction head, providing cleaning capability throughout the tube column as the suction head is advanced through the tube bundle. The apparatus must fit through the no tube lane 60 or annulus 85.
In operation, both the hot leg side 31 and the cold leg side 33 are vacuumed individually with the moveable sludge vacuum apparatus 120 with its wand 144 and suction head 146. Header 64′ is optional. As water is removed with the sludge, clean water will have to be pumped back in order to maintain a constant water level. The moveable sludge suction apparatus 120 can be moveable robotic delivery cart or other device connected to vacuum pump 128 by outlet sludge/control umbilical 150. Arrows 130 show sludge removal. An optional mounting mechanism 78 for apparatus 120 is shown. Vacuum pump 128 can extract sludge 130 to provide exit line 132.
Referring now to
In
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
