1. Field of the Invention
The present invention relates generally to a method of inspecting a heat exchanger and, more particularly, to a method of using unique, tube-specific tubesheet eddy current signal characteristics to verify the identity of individual steam generator tubes having ends disposed in a tubesheet. The invention also relates to a computer program product for performing a method of inspecting steam generator tubes.
2. Background Information
Heat exchangers, such as, for example, steam generators used in pressurized water nuclear powered electric generating systems, generally include thousands of U-shaped heat exchanger tubes disposed within a generally cylindrical pressure vessel. The ends of the heat exchanger tubes are secured within a transverse plate called a tubesheet, which separates the steam generator into a primary side and a secondary side. Heated primary fluid from the nuclear reactor is passed through the tubes to effectuate a heat transfer with the secondary working fluid which, in turn, drives the turbo-machinery used to generate electricity. The primary fluid can be radioactive. Accordingly, to prevent leakage of the reactor coolant into the secondary side of the generator, which would contaminate the steam, the heat transfer tubes must be periodically inspected for flaws and degradation such as cracks, pits, dents and tube wall thinning. If a degraded tube is discovered, it is typically plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few tubes does not appreciably affect the efficiency of the heat transfer.
Eddy current testing is a well known, commonly used method of nondestructive testing of steam generator tubes. Generally, in performing an eddy current test on steam generator tubes, a sensor or probe is advanced through the tube as signals are generated and recorded for later analysis. See, e.g., U.S. Pat. No. 3,302,105 (illustrating and describing the eddy current signatures of various types of tube defects); see also U.S. Pat. Nos. 3,693,075; 4,194,149; 4,207,520; and 4,631,688. U.S. Pat. No. 4,763,274, which was filed on Jun. 24, 1986 and issued to the assignee hereof, discloses eddy current inspection processes for nuclear steam generator tubes and computer analysis of the eddy current data for automatically detecting flaws in the heat transfer tubes of a steam generator.
The Electrical Power Research Institute (EPRI) is developing new criteria for data quality acceptance in eddy current steam generator inspection. One such requirement is to verify and ensure that the eddy current sensor or inspection probe is delivered to the correct tube for inspection thereof. Conducting efficient tube inspection and/or repair while minimizing the impact of such inspection on the steam generator operations (i.e., minimizing the downtime required for such inspection), requires quick and accurate tube identification and the ability to record information with respect to each tube for future reference and comparison.
Historically, heat exchanger tube inspection involved the use of technicians who would conduct tests using a variety of inspection devices (i.e., an eddy current probe) and then record the results for future reference. In view of the thousands of tubes in a typical generator, this was an extremely time consuming, arduous and tedious procedure, the results of which were highly susceptible to human error. Ensuring the accurate identity of an individual tube or relocating to a tube was also very difficult. Accuracy was dependent on the analyst correctly counting tubes to locate the desired tube. Moreover, it was a dangerous process, potentially exposing the technicians to an unacceptable dose of radiation.
A relatively recent improvement upon such manual inspections involves the use of computer controlled robotic arms with specialized end-effectors capable of conducting the inspection or repair. Generally, the robotic arm is secured beneath the tubesheet with the end-effector mounted at the end of the arm. The end-effector typically comprises an assembly mounted on the end of the robotic arm and including, for example, a television camera, one or more illumination sources, inspection tools, such as an eddy current sensor or probe, and tooling to plug effected tubes. A computer, under the control of an operator, is used to control the robotic arm. The operator can move the end-effector across the tubesheet using, for example, a joystick or by specifying a target tube destination using x, y cartesian coordinates. U.S. Pat. Nos. 5,751,610, 5,838,882 and 5,878,151, disclose several representative prior art robotic arm end-effectors for use in inspecting steam generator tubes. However, while such robotic systems have improved the inspection process, they are relatively deficient at pin-pointing or relocating to a specific tube, in order to, for example, check on the status of the tubes degradation or the integrity of a prior tube repair.
Accordingly, there have been several attempts to improve the accuracy and efficiency with which industrial steam generator tubes are identified. Known prior art methods of tube identification involve, for example, physical modification of the tube ends to create a computer readable marking system or use of a computerized tube position visualization and verification system.
U.S. Pat. No. 5,321,887, for example, discloses a process of marking each tube with a binary bar code formed from circular impressions and the absence of such impressions. The bar code is embossed on the exterior of the tubes during manufacture or alternatively on the interior of the tubes after they have been fixed in the tubesheet. A cartography is then produced in the factory or after installing the steam generator in the nuclear power station, using the bar codes to associate position information, in Cartesian coordinates, with the location of each tube in the tubesheet. When it is subsequently desired to locate a given tube, a computer is used to access the cartography and to provide automated control of the robotic arm to the desired tube. However, such a method requires physical modification of the tubes and is timely and costly to implement, particularly when modifying an existing steam generator to implement the technology.
U.S. Pat. No. 6,282,461, which was filed on Jul. 9, 1999 and issued to the assignee hereof, discloses an independent tube position verification system including a television camera mounted on the robotic arm end-effector, in order to visually track changes in position as the robotic arm changes position with respect to the tubesheet. The camera outputs successive image frames in order to track the physical displacement of a recognizable reference artifact, such as a plugged tube, as the end-effector moves across the tubesheet. The physical displacement information is then converted into velocity and direction information and compared against position information maintained by the robotic arm encoder. Mismatched readings indicate a loss of tracking integrity requiring re-calibration of the system.
Such image capturing and analysis systems are susceptible to several disadvantages. Factors such as illumination and associated shadows, robot arm bending, absence of an adequate reference artifact from which to calculate displacement and camera image distortion caused by, for example, the speed at which the end-effector is traveling and the end-effector's location relative to the tubesheet, can negatively impact the accuracy of the system. Calibration of robotic inspection systems can be problematic. The accuracy of the robotic arm and end-effector movement depends on the robot system design, calibration, the robot's onboard encoder or analyzer and communication of the robot with the computer control system. For example, it is not uncommon for the robotic arm to be subject to bending forces, particularly when it is fully extended with respect to its mounting location. Such bending can result in the robotic arm being located at a distance and location different from that calculated by the robot's onboard tube locating encoder or analyzer. It is, therefore, possible for the wrong tube to be inspected and/or repaired, thereby resulting in the potential of plugging or repairing a perfectly good tube or leaving a degraded tube un-inspected or un-repaired and returning it to service in its degraded condition. Additionally, known prior art methods of tube identification, whether from the robot or from the tube position verification system, do not update the data collection system to include the current status of the tube for updated future reference.
There is a need, therefore, for a method of quickly and accurately identifying and verifying the identity of the tubes of a steam generator for inspection and/or service thereof, that overcomes the aforementioned disadvantages.
Accordingly, there is room for improvement in heat exchanger inspection and in heat exchanger tube identification and verification.