This invention generally relates to methods for cleaning heat exchanger vessels, and is specifically concerned with an improved pressure pulse cleaning method for loosening and removing sludge and debris from the secondary side of a nuclear steam generator.
Pressure pulse cleaning methods for cleaning the interior of the secondary side of a nuclear steam generator are known in the prior art, and have been disclosed and claimed in U.S. Pat. Nos. 4,655,846 and 4,699.665. The purpose of these methods is to loosen and remove sludge and debris which accumulates on the tubesheet, heat exchanger tubes and support plates within the secondary side. In such methods, the secondary side of the generator is first filled with water. Next, the outlet of a gas-operated pressure pulse generator is placed into communication with the water. Such communication may be implemented by a nozzle which may be formed from either a straight section of pipe oriented horizontally over the tubesheet of the generator, or a pipe having a 90 degree bend which is oriented vertically over the tubesheet. Both of these prior art methods generally teach generating pressure pulses within the water by emitting gas through the nozzle that is pressurized to between 50 and 5000 pounds per square inch. The pulses are repeated at a frequency of one per second, and the succession of pulses may last anywhere from between 1 and 24 hours. The pressure pulses create shock waves in the water surrounding the tubesheet, the heat exchanger tubes and support plates within the secondary side of the generator. These shock waves effectively loosen and remove sludge deposits and other debris that accumulates within the secondary side over protracted periods of time.
While the cleaning methods disclosed in these patents represent a major advance in the state of the art, the applicants have found that there are limitations associated with these methods which limit their usefulness in cleaning nuclear steam generators. However, before these limitations may be fully appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
In the secondary side of such steam generators, the vertically-oriented legs of the U-shaped heat exchanger tubes extend through bores in a plurality of horizontally-oriented support plates vertically spaced from one another, while the bottom ends of these tubes are mounted within bores located in the tubesheet. The relatively small annular spaces between these heat exchanger tubes and the bores in the support plates and the bores in the tubesheet are known in the art as "crevice regions." Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator. The consequent reduced flow of water through these crevice regions results in a phenomenon known as "dry boiling" wherein the feed water is apt to boil so rapidly that these regions can actually dry out for brief periods of time before they are again immersed by the surrounding feed water. This chronic drying-out of the crevice regions due to dry boiling causes impurities dissolved in the water to precipitate out in these regions. The precipitates ultimately create sludge and other debris which can obstruct the flow of feed water in the secondary side of the generator to an extent to where the steam output of the generator is seriously compromised. Moreover, the presence of such sludges is known to promote stress corrosion cracking in the heat exchanger tubes which, if not arrested, will ultimately allow water from the primary side of the generator to radioactively contaminate the water in the secondary side of the generator.
To remove this sludge, many cleaning methods were used prior to the advent of pressure pulse cleaning techniques. Examples of such prior art cleaning methods include the application of ultrasonic waves to the water in the steam generator to loosen such debris, and the use of a high-powered jet of pressurized water to flush such debris out (known as "sludge lancing"). However, such techniques were only partially successful due to the hardness of the magnitite deposits which form a major component of such sludges, and the very limited accessibility of the crevice regions of the steam generator.
Since its inception, pressure pulse cleaning has been a very promising way in which to remove such stubborn deposits of sludges in such small spaces, since the shock waves generated by the gas operated pressure pulse operators are capable of applying a considerable loosening force to such sludges. However, the applicants have found that the methods disclosed in both U.S. Pat. Nos. 4,655,846 and 4,699,665 have fallen short of fulfilling their promise in several material respects. For example, research conducted by the applicants indicates that pressure pulses generated by gas pressurized at the lower end of the 50 to 5000 psi range are generally too weak to effectively dislodge significant amounts of such crevice-region sludges. While pressure pulses generated by gas pressurized at the upper end of to 50 to 5000 psi range would certainly be powerful enough to loosen and remove the sludges, this same research indicates that the shock waves resulting from such pulses are capable of generating momentary forces that would jeopardize the integrity of the heat exchanger tubes in the vicinity of the nozzle of the pressure pulse generator. Thus the prior art does not specifically indicate what range of pressure is the most effective. Still another shortcoming observed by the applicants was the lack of any means to remove dissolved ionic species from the water during such prior art cleaning processes. Such ionic species, if not removed, are capable of precipitating out in the form of new sludges after the termination of the pressure pulse cleaning process if no provision is made to remove them. Additionally, applicants observed that if the fine particulate matter that is dislodged from the crevice regions is not removed from the water during the pressure pulse cleaning method, these fine particles of sludge are capable of settling onto the tubesheet and densely depositing themselves into the crevice regions between the tubesheet and the legs of the heat exchanger tubes, thereby defeating one of the purposes of the cleaning method. The applicants have further observed that the usefulness of prior art pressure pulse cleaning processes is limited by the one pulse per second frequency that these methods teach. Specifically, the applicants have observed that the relatively rapid pulse frequency taught in the prior art does not give the nozzle and manifold of the pulse generator sufficient time to fill back with water, and thus leaves pockets of shock-absorbing gas in the nozzle of the pulse generator which limits the efficacy of later generated pulses in generating sludge-loosening shock waves. Finally, the applicants have observed that the maximum 24 hour time limit taught in U.S. Pat. Nos. 4,655,846 and 4,699,665 may not be sufficient to completely loosen and remove all of the sludges and debris from the interior of the secondary side of a typical steam generator.
Clearly, what is needed is an improved pressure pulse cleaning apparatus which overcomes the limitations associated with prior art pressure pulse cleaning methods and which is imminently practical for use in the secondary side of a nuclear steam generators.