로터 중심공 비파괴검사 및 정량평가

Abstract

Large equipment-structures such as nuclear and thermoelectirc power plants, aging aircrafts, rapid transit railway, petrochemical plants, and vessels are national infrastructure that are produced with enormous amount of funding, manpower and time. However, cracks and corrosions are bound to occur since it is exposed to poor surroundings like high temperature, high pressure, high energy, fatigue and corrosion. Also, if such defects are not detected at the in-service-inspecion period will progress rapidly in the elaborated poor surrounding and consequently result in a big problem as a country not only in the form of enormous amount of human and economic loss but also in the form of environmental pollution and such. Meanwhile, if the size of the defect, that is tolerance size, does not affect the safety of the structure and the continuous operation is economic, the service life can be extended even after the design life has been reached. In such aspect of Damage Tolerance, the life of a total of 57 nuclear power plants in the United States have been extended from 40 years to 60 years and currently 18 plants are being examined for an extended use. Even in the case of Japan, the design life of 18 nuclear power plants has been allowed to be extended. The two important factors in Damage Tolerance are the permissible size of damage (tolerance size) and the quantitative analysis of damage using nondestructive testing (quantitative nondestructive testing). Generally nondestructive testing of large equipment-structure is done by national code and damaged equipment parts that were detected during the periodic inspection period is repaired or replaced after evaluation. However, an unforeseen accident may occur if the crack within the structure grows and exceeds the permissible limits during usage. Thus, it is crucial that the smaller defects are detected in nondestructive testing and are assessed quantitatively. According to the type of energy source used for defect detection, nondestructive testing can be classified into Radiographic(RT), Ultrasonic Testing(UT), Eddy Current Testing(ECT), Magnetic Flux Leakage Testing(MFLT), Penetrant Testing(PT), Leak Testing(LT), Visual Testing(VT), Magnetic Particle Testing(MPT), Acoustic Emission(AE), Neutron Radiographic Testing(NRT), and Infrared Thermography Testing, IRT). Meanwhile, a large part of the nuclear, thermoelectric and petrochemical power plants are pipe structures. Pipe structures are vulnerable to stress and corrosion because is transmit fluids of high temperature and high pressure. Also, because the form is narrow and long, there is the drawback that it is not easy to detect the defects with the above-mentioned nondestructive testing methods. For example, testing method is extremely limited when it complexly occurs both axially and circumferentially in a narrow and long pipe inner wall like a roter bore. As an example, until now the inside of the rotor bore has been magnetized by a high voltage current of about 1000A which goes through the rotor bore and the MT has been carried out. Meanwhile, while observing the magnetic particles in the inner walls of the rotor bore using long endoscope for industrial use in order to observe the shape of the magnetic particles, there was a need to solve (1) the non-uniformity of the magnetic particle distribution due to gravity, (2) the necessity of pre-treatment and post-treatment process, (3) the exposure to danger of a worker due to high voltage current power usage, and (4) the extension of labor hour. Meanwhile, although the method of scanning using UT installed to a robot that makes a spiral movement automatically inside the rotor bore which is about 50 to 300mm narrow and 10 to 30m long was proposed and actually used, the shortcomings that (1) due to the nature of the UT, the sensor needs to be drawn right up against the inner wall of the rotor bore, (2) it is not fit for exploring cracks that are very little because the special resolution that can be acquired by 1 sensor of UT is about 5mm big, (3) the defect exploring ability can be lowered according to the direction of the crack (axial or circumferential), and that (4) the exploring time is too long are problems that have to be solved urgently. In the research, we recommend the NDT method which gave ways to (1) the solution to the non-uniformity of the exploring ability due to gravity, (2) the minimization of the pre-treatment and post-treatment process, (3) the rejection of high voltage current power usage, (4) the reduction of labor hours, (5) the practical use of non-contact sensor, (6) the enhancement of special resolution, and (7) the minimization of the dependence on the crack’s length and direction of the exploring ability which was the problem that was to be solved in the elaborated MT and UT in exploring the defects that exist within the piping with the narrow and long shape. Also, we have verified its usefulness by exploring and evaluating defects of various sizes and directions that were introduced in the piping proof piece.