Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Utility Patent
1999-06-24
2001-01-02
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Utility Patent
active
06167764
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a stud tensioning method and more particularly to a method for determining the elongation of a stud in order to verify the load on the stud.
Removable closure heads of large vessels of the type employed to contain fuel rod assemblies in commercial nuclear power plants for generating electric power have large circumferential flanges which are secured to mating circumferential flanges on the vessels by nuts rotatably engaged with threaded studs extending upwardly from the vessel flanges and through aligned stud holes in the head flanges. As is disclosed by U.S. Pat. No. 3,851,906 to Frisch et al., there may be up to 54 such studs or more and the studs may have diameters of about 7 inches. The nuts may be as long as about 6 inches or more.
These heads are removed every one to two years during scheduled refueling outages to replace spent fuel rod assemblies with fresh fuel rod assemblies and to shuffle the still useful fuel rod assemblies in the reactor vessel. About a third of the fuel rod assemblies are replaced during these outages. Once the fuel rod assemblies have been reloaded and shuffled, the closure heads are replaced on the flanges and secured to the reactor vessels by tightening the nuts with commercially available tensioners to load the studs extending through the flanges of the heads to the extent that the studs are elastically stretched. All of these refueling and associated activities are performed on critical path schedules and involve daily costs of hundreds of thousands of dollars or more for every day the plant is in the outage and not generating electric power in addition to the cost of performing the refueling. Accordingly, it is highly desirable to rapidly perform these activities.
The Frisch et al. Patent discloses a stud tensioning method for securing the studs on the heads of nuclear reactor vessels which is generally employed today. According to the Frisch et al. Patent, one or more stud tensioners are employed to tighten the nuts on the studs. Measurement of the elongation of each stud is employed to verify that the stud has been properly tightened. The elongation measurements are intended to minimize uncertainties due to variations in residual loads on the studs caused by variations in the actual pressure applied to the studs by the tensioner, high friction in the stud threads and possibly operator error. The elongation of each stud is determined by measuring the length of the stud with a measuring rod inserted in an axially extending bore in the stud before and then after the tensioning step. The elongation of the stud is determined by the difference in the measured lengths. These measurements normally are performed in series with the tensioning step because the measuring rods are normally removed to protect them from being bent by the large, heavy, suspended tensioners as they are manipulated around the studs by the operators.
Because the measurements are performed in series with the tensioning step, the measurement times for up to 54 studs or more can require up to about two and one half hours if no serious discrepancies develop. However, if a discrepancy is found, the tensioning and elongation measurements must be repeated until the measurements satisfy the specifications, which can add hours to the critical path schedules.
Other tightening techniques may be employed, but they may be subject to small but unacceptable inaccuracies. Thus, the nuts may be torqued to a predetermined setpoint, but friction in the threads may result in a lower tensioning force being transmitted to the studs. Nut rotations may be determined in other practices, but the initial points may not always be determined with sufficient accuracy.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a stud tensioning method which measures the elongation of the stud contemporaneously with the stud tensioning step rather than serially. It is a further object to provide a method which accurately measures elongation.
With these objects in view, the present invention resides in a method of determining the elongation of a tensioned stud. A hydraulically actuated tensioner is attached to a stud extending from a body. The pressure of the hydraulic fluid of the tensioner is increased from a first low pressure to a higher pressure for elongating the stud while correlating the increase in pressure of the hydraulic fluid either with the volume of hydraulic fluid in at least a portion of the tensioner or with the displacement of a tensioner member for at least a portion of the pressure increase. The nut is tightened on the stud. The pressure of the hydraulic fluid is reduced from the higher pressure to a second low pressure while correlating the decrease in pressure of the hydraulic fluid either with the volume of hydraulic fluid in the tensioner or with the displacement of the tensioner member. The elongation of the stud is then determined as a function of either the difference in the correlated volumes of hydraulic fluid or the correlated differences in displacement of the tensioner member at the first and second low pressures. Finally, the tensioner is detached from the stud.
REFERENCES:
patent: 3851906 (1974-12-01), Frisch et al.
patent: 3994158 (1976-11-01), Weinhold
patent: 4047456 (1977-09-01), Scholz
patent: 4333351 (1982-06-01), Bickford
patent: 4454790 (1984-06-01), Rieben
patent: 4513497 (1985-04-01), Finch
patent: 4548103 (1985-10-01), Orban
patent: 4552038 (1985-11-01), Heiermann
patent: 4676109 (1987-06-01), Wallace
patent: 4686859 (1987-08-01), Wallace
patent: 5005424 (1991-04-01), Markowski
patent: 5408509 (1995-04-01), Ruzga et al.
patent: 5515294 (1996-05-01), Mohr et al.
patent: 5589640 (1996-12-01), Butler
Noori Max
Westinghouse Electric Company LLC
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