The present invention relates generally to the field of pressure vessels of the type used in nuclear reactors and in particular to a new and useful device and method for reducing the difficulty and cost of repairing stud holes in pressure vessels.
Openings in pressure vessels are needed for a variety of reasons. These openings must be tightly sealed to maintain pressure in the vessel during operation of a nuclear reactor.
FIGS. 1 and 2 display a circular opening 12 through the wall of a pressure vessel 10 having a cover 50. Covered circular openings 12 in pressure vessels 10 typically have a joint between the opening 12 and the cover 50 sealed by a gasket 40. A diaphragm 45 across the opening 12 is provided to help seal the vessel opening 12. Pin holes 24 are provided to help align the cover 50 over the opening 12. The cover 50 is held to the pressure vessel 10 using several studs 20 positioned around the circumference of the cover 50.
The studs 20 are threaded into stud hole 26, an internally-threaded, blind bore in the vessel 10 shell. Threaded nuts 22 are threaded over the exposed ends of studs 20 and tightened against the top of the cover 50 (or closure head flange) for securing the cover 50 to the pressure vessel 10.
It is not unusual for an internally threaded blind hole such as stud hole 26 on the pressure boundary opening 12 to be damaged. In these cases, it can be repaired using a helical coil threaded insert.
To repair the damaged threaded stud hole 26, the current technology requires first drilling out the existing damaged threads using the drill size specified for the diameter of the helical coil insert being installed. Care must be taken not to drill the hole deeper than the original hole depth specified on the manufacturing drawing. Dimensional tolerance on the drilling diameter and depth must be guaranteed.
Once the damaged hole 26 is drilled out, then new internal threads must be machined in the hole 26 in order to install the helical coil inserts. This job is often done in a poor working environment where space is limited and the physical environment can be quite hazardous to workers. As a result, stud repair is a time-consuming, hazardous and costly process.
A further problem arises when a stud 20 becomes stuck in the bore 26. Removing a stuck stud is also very time-consuming and costly for current pressure vessels. In fact, it is sometimes impossible to unthread a stuck stud merely by turning the stud with a normal amount of torque, because of the hardened debris and galled threads.
Sometimes the only practical way to remove the stud from the threaded bore is to drill-out the stud from the bore, a more time-consuming and costly process. U.S. Pat. No. 5,703,916 indicates that at the time the patent was filed, the expense of drilling out a stuck stud can be as high as $300,000 per stud. At the same time, the damage prevents returning the pressure vessel to power until it can be repaired.
U.S. Pat. No. 5,703,916 describes a solution for repairing stuck studs of pressure vessel opening covers using hydraulic technology. The prior art solution does not change the connection of the studs with the blind bore from the known structure described above. Rather, hydraulic feed lines communicate with the bore through the stud. A pulsating fluid can be introduced through the feed lines to eliminate hardened debris and wash it from the bore. A suction pump is provided to remove the fluid and debris as it is forced from the stud/bore threaded joint. The cleaning effected by the hydraulic fluid then hopefully permits normal turning of the stud so that it can be removed.
Obviously, this solution can not eliminate the possibility of damaging the internally threaded blind hole, especially as the hydraulic fluid causes the stud to vibrate up and down against the threads to remove debris. As a result, the above repair process using helical coil inserts are still needed.
None of the prior art provides a good solution for easily repairing damaged bores or stuck studs in a cost-efficient manner. Thus, an improved connection for the cover of a pressure vessel opening which makes bore repair and/or stud removal easier and less costly is still needed.
It is an object of the present invention to provide a device which permits easier repair of stud bores in pressure vessels for connecting opening covers.
It is a further objection of the invention to provide a method and device for efficiently removing stuck studs and speeding bore repair for the closure of a pressure vessel opening.
Accordingly, a segmented nut is provided for inserting into a blind bore in a pressure vessel wall to receive one end of a stud used to secure a cover on an adjacent opening of the pressure vessel. The segmented nut is internally threaded on each part of the nut. The parts making up the nut join together to form a continuous thread and outer surface. The upper end of the nut has a conical shape; the end is tapered outwardly from the opening for the internal threads toward the outer side surface of the nut.
A method for using the segmented nut in a pressure vessel of a nuclear reactor includes forming a bore with a tapered upper end in the pressure vessel wall, and inserting the segmented nut into the bore oriented so that the tapered end of the nut can contact the tapered end of the bore. A stud is threaded into the internal threads of the segmented nut the stud and nut are tightened, forcing the tapered surfaces of the nut and bore together. The load on the stud is thereby translated to the pressure vessel.
The stud and segmented nut are easily replaced when needed. Relieving the load on the stud causes the segmented nut to loosen as they can expand outwardly because of the tapered surfaces. The stud can be easily removed, and replacing the segmented nut is all that is needed to repair damaged threads.