This invention generally relates to a device for delivering a plugging tool within a steam generator, and is specifically concerned with an improved robotic arm for delivering and positioning a plugging tool into the open end of a heat exchanger tube mounted within the tube sheet of a nuclear steam generator.
Robotic arms for delivering and positioning plugging tools are known in the prior art. One of the most practical and effective of these known devices is the robotic arm disclosed and claimed in U.S. Pat. No. 4,672,752 invented by Paolo R. Zafred et al and assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference. This particular robotic arm generally comprises a support member which is detachably connectable onto a man way of the channel head of a nuclear steam generator, and a base arm which is pivotally connected to the support member. The base arm includes a guidance mechanism and a positioning mechanism. The guidance mechanism aligns and inserts the tool into the open end of the tube to be plugged, and includes a lifting cylinder and a two-dimensional compliance coupling for this purpose. The positioning mechanism is connected to the guidance mechanism and serves to pivotally move the plugging tool between a plug installing and a plug loading position. A plug magazine is supported on the base arm for supplying the plugging tool with tube plugs. The plugging tool may incorporate the mechanism described and claimed in U.S. Pat. No. 4,369,662 to Reiben et al and assigned to the Westinghouse Electric Corporation. The tube plugs may be of the type disclosed in U.S. Pat. No. 4,390,042 to Kucherer et al and assigned to the Westinghouse Electric Corporation.
In operation, the support member of the robotic arm is bolted onto the channel head of a nuclear steam generator at the periphery of the man way. The base arm is pivoted upwardly and locked into a position that it is essentially parallel with the tube sheet of the steam generator. The base arm is rotatably manipulated by a gear train attached to an electric motor which is remotely controlled by an operator who monitors the position of the plugging tool by way of a TV camera. When the plugging tool is aligned with the open end of a tube to be plugged, a pneumatic cylinder included within the guidance mechanism swings the plugging tool 90 degrees from a horizontal, plug-loading position to a vertical, plug-installing position just under the open end of the tube to be plugged. The pneumatic cylinder included within the guidance mechanism lifts and inserts the plugging tool into engagement with the open end of the tube. The plug is then radially expanded in order to firmlY seal it around the open end of the tube, after which it is released by the plugging tool. The positioning mechanism then swings the plugging tool 90 degrees back into a horizontal position and into the plug magazine in order to load an additional plug into the plugging tool. After the loading operating has been completed, the process is repeated until all of the defective tubes are plugged.
While the robotic arm disclosed and claimed in U.S. Pat. No. 4,672,741 represents a major advance in the art, the applicants have observed that there are at least four limitations associated with this arm that have prevented it from realizing its full potential. First, while this tool does provide a compliance coupling between the plugging tool and the positioning mechanism which ultimately connects the tool to the base arm, the inability of this compliance coupling to afford three-dimensional movement to the plugging tool seriously impairs its ability to compensate for small axial misalignments between the center line of the plug held by the plugging tool, and the open end of a heat exchanger tube that receives this plug. Consequently, the two-dimensional compliance afforded by this particular coupling has caused an undesirable amount of friction and stress between the plug and the mouth of the tube during the plugging operation, which has in turn led to binding of the tool within the tube. Secondly, the fixed length of the base arm in this particular robotic arm makes it impossible to use the same robotic arm in different models of steam generators due to the different dimensions of the channel head of such generators. Thirdly, the applicants have observed that there is no convenient way to detach the plugging tool from the rest of the robotic arm in the event that the tool becomes bound or jammed with respect to a particular tube. This is a serious shortcoming, as the jammed tool can obstruct the man way of the channel head to such an extent that it becomes dangerous or impossible for a maintenance worker to enter the channel head and unjam the device, as the worker might have difficulties in leaving the radioactive environment of the channel head even if he were able to climb over the device and successfully enter the generator. Finally, the staggered clip-type tube plug magazine used in this prior art robotic arm does not always provide a completely smooth unloading of the tube plugs, which can cause scratches in the plugs. These scratches can in turn jeopardize the integrity of the resulting plug seal. Moreover, this clip-type magazine has a limited storage capacity for tube plugs, which necessitates a relatively frequent refilling of the magazine by a maintenance operator, thereby necessitating the exposure of an operator to significant amounts of radiation.
Clearly, there is a need for an improved robotic arm for effecting a plugging operation which is capable of smoothly and reliably plugging the open ends of heat exchanger tubes in the tube sheets of steam generators without the generation of binding or excessive stresses between the tube plugs and the heat exchanger tubes being plugged, and without the scratching of the plugs as they are withdrawn from the plug magazine. Ideally, such a robotic arm should be length adjustable so that it may be used in different models of steam generators. Such an improved arm should also have some sort of mechanism for disconnecting the plugging tool from the balance of the robotic arm in the event of a jam, so that the arm can be removed from the channel head. Finally, it would be desirable if the tube plug magazine was capable of storing a relatively large number of tube plugs in order to minimize the exposure of maintenance personnel to potentially harmful radiation.