Electricity: measuring and testing – Magnetic – Magnetic sensor within material
Patent
1993-06-30
1995-08-15
O'Shea, Sandra L.
Electricity: measuring and testing
Magnetic
Magnetic sensor within material
33302, 335441, 376249, G01N 2790, G01B 520, G01B 726, G21C 1700
Patent
active
054422841
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for inspecting heat exchanger tubes of a heat exchanger according to the preamble of claim 1.
Especially of a steam generator within a primary circuit of a power plant, with which heat exchanger a heat transfer from a first medium to a second medium that is gas- and liquid-tightly separated from the first medium is performed, with the tubes fixed in their position by spacers, wherein a measuring probe is inserted into the tube to be inspected, with which probe the tube section to be inspected is subjected to an electromagnetic alternating field of a predetermined frequency, and wherein the local eddy currents generated within the tube section induce a voltage signal within an eddy current sensor that is slidable in the longitudinal direction of the tube and rotatable in the circumferential direction of the tube, the voltage signal varying locally according to the condition of the material and the thickness of the wall section. A nuclear power plant has approximately 3,000 to 6,000 heat exchanger tubes which are fixed in their position over their length with five to ten spacers. The spacers have predetermined fixed distances and support one tube over a portion of its circumference. When the spacers are loosened or displaced during operation, the heat exchanger tubes can be induced to perform undesirable vibrations which result in a premature material fatigue, respectively, in a wall thickness reduction of the tube. The vibrations can be so strong that the heat exchanger tubes mechanically impact one another possibly causing a leak. However, especially in the case of nuclear power plants it is a primary requirement that the radioactively loaded primary circuit is gas- and liquid-tightly separated from the secondary circuit. Accordingly, such a heat exchanger is subjected regularly to repeated inspections. Each individual heat exchanger tube is inspected with an eddycurrent measuring probe which is inserted into the tube to be inspected. An electromagnetic alternating field of a predeterminable frequency generates within the tube wall local eddy currents which induce voltage signals in a sensor that is entrained with the probe. By rotating the sensor in the circumferential direction of the tube and by guiding it along the longitudinal direction of the heat exchanger tube, each location of the tube wall is inspected with the eddy current sensor. For a uniform wall thickness and homogenous material the eddy current sensor emits a uniform, approximately constant signal. However, wall thickness reductions, material fractures etc. lead to changed sensor exit signals and can thus be recognized and locally fixed. Changes of the sensor exit signal caused by other effects, as, for example, the spacers, are suppressed in the known method because they prevent an evaluation of the condition of the wall. Since the position of the spacers in the longitudinal direction as well as within the circumferential direction of the tube is known, the suppression of such a disturbing signal is easily accomplished.
It is however disadvantageous that with the known method only damage that has already occurred due to a loose or wrongly positioned spacer can be detected. It is furthermore possible that initially only one spacer along one of the heat exchanger tubes is loose and that the resulting vibrations of the tube do not lead immediately to any significant changes of the wall thickness, respectively, of the condition of the tube material. This critical location within the heat exchanger thus cannot be detected with the known method. When in addition another spacer neighboring the first loose spacer comes loose, a relatively long section of the heat exchanger tube is insufficiently supported and thus prone to considerable vibrations, which within a short period of time, can result in a fracture and thus in a radioactive contamination of the secondary circuit. In practice, such a case has occurred in a nuclear power plant in Japan.
It is therefore an object of the present
REFERENCES:
patent: 3302105 (1967-01-01), Libby et al.
patent: 3694740 (1972-09-01), Bergstrand
patent: 4182985 (1980-01-01), DeWolfe et al.
patent: 4194149 (1980-03-01), Holt et al.
patent: 4235020 (1980-11-01), Davis et al.
patent: 4341113 (1982-07-01), Gutzwiller, Jr.
patent: 4625165 (1986-11-01), Rothstein
patent: 4687992 (1987-08-01), Bernus et al.
patent: 4814702 (1989-03-01), Driggers et al.
patent: 4851773 (1989-07-01), Rothstein
patent: 4876506 (1989-10-01), Brown et al.
patent: 5237270 (1993-08-01), Ceceo et al.
Gemeinschaftskernkraftwerk Neckar GmbH
O'Shea Sandra L.
Patidar Jay M.
LandOfFree
Process for inspecting spacers of heat exchanger tubes in a heat does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for inspecting spacers of heat exchanger tubes in a heat, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for inspecting spacers of heat exchanger tubes in a heat will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2184777