Device For Closing Off Defective Heat Exchanger Tubes

Abstract

The device for closing off a defective heat exchanger tube includes a metallic plug having a blind hole in which a sleeve of plastic, cardboard or the like is mounted. The plug has a cylindrical cross-section at the forward end to fit in the tube and a conically tapered outer surface at the open end. The sleeve encloses an explosive charge within the plane of the conically tapered outer surface of the plug and is provided with an enlarged end portion which projects from the plug. This end portion is conically tapered and slotted to jam in the heat exchanger tube. Upon detonation, the explosive charge expands and welds the conically tapered surface portion of the plug to the heat exchanger tube to close off the end of the tube.

Description

FIELD OF THE INVENTION

The invention is concerned with plugs for closing off tubes and particularly heat exchanger tubes.

BACKGROUND OF THE INVENTION

In nuclear power plants, a primary coolant is usually circulated through a reactor system in order to cool down a reactor core by taking on heat. After passing from the reactor core, the primary coolant has been passed through a heat exchanger in heat exchange relation with a secondary coolant passing through the heat exchanger for various purposes, such as the generation of steam from the secondary coolant. In the course of time, the primary coolant can unavoidably become radioactively contaminated with a resultant depositing of radioactive particles in the area of the heat exchanger. Also, in some cases, there e is the danger that the heat exchanger tubes may break so that a direct connection then exists between the primary coolant and the secondary coolant. This has the immediate consequence that radioactivity can get into the secondary circuit and must, therefore, be avoided at all costs.

As it is practically impossible to repair these above tube faults due to the inaccessibility of their location, it is advisable to simply disable the heat exchanger tube in question, i.e., to close it off at both ends, for example, in the headers or plenum chambers of the heat exchanger. This could be done, for instance, by welding. For such a purpose, the plenum chambers must be entered via manholes which are provided in the heat exchanger. However, because of the intense radioactive radiation within these heat exchangers, the permissible dwelling time for the repair personnel is so short as to make it practically impossible to achieve a tight weld manually. Also, conventional welding can be attended by difficulties due to the presence of boron deposits which are caused by a boron-containing primary coolant as a boron incrustation can develop. This, in turn, can lead to leaks within the weld. Further, it is not practical to wait until the radioactivity has decayed to any substantial extent as the entire power plant would have to stand idle for too long a period of time.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a device for effecting a rapid closing off of a tube in a heat exchanger.

It is another object of the invention to provide a device for closing off a tube which can be easily manipulated by hand or by remotely controlled manipulators.

It is another object of the invention to provide a device of simple construction which can explosively weld a plug within a tube end to close off the tube end.

The initial problem arises of finding a welding method which requires the shortest possible time while assuring reproducible tightness. Such a method has been known in explosion welding, for example, as shown in U.S. Pat. No. 3,590,877 which consists of welding together two materials by means of a detonation shock wave. This method has also been used, for instance, in explosion plating. The present invention therefore concerns a device for closing off defective heat exchanger tubes within the area of the tube plates of heat exchangers by means of an explosion welding technique.

According to the invention, the device includes a metallic plug which is to be inserted into a tube of a heat exchanger to be sealed off. The plug which is of a cylindrical cross-section with a diameter to fit into the tube is provided with a blind hole as well as a conically tapered surface portion at the open end. In addition, the device includes a sleeve made of plastic, cardboard, or the like which is placed, in part, within the blind hole of the plug and has a conically enlarged end projecting from the plug. This enlarged end is slotted longitudinally to enable the device to be jammed into the heat exchanger tube. The sleeve contains a detonator cap in the part facing the end of the blind hole in the plug and an explosive charge in a central part which is located within the conically tapered surface portion of the plug.

In use, after entrance is gained to a bank of heat exchanger tubes, the device is inserted into the open end of the tube to be closed off. In this manner the forward end of the plug is slid into the tube until the enlarged end of the sleeve becomes jammed in the tube. Thereafter, the detonator cap is activated by a suitable means, such as through ignition wires. The detonator cap then activates the explosive charge which, in turn, causes a detonation shock wave to travel along the tapered surface portion of the plug. This shock wave circumferentially expands the tapered surface portion against the inside wall of the tube while also welding the plug to the tube. Thereafter, the materials remaining in the blind hole of the plug can be removed. The opposite open end of the tube is also closed off in a similar manner.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a heat exchanger in which a tube has been closed off in accordance with the invention;

FIG. 2 illustrates a cross-sectional view of a device according to the invention; and

FIG. 3 illustrates a cross-sectional view of a heat exhcnager tube and a plug welded within the tube to seal off the tube in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the heat exchanger in general consists of a pressure tank 1 which is divided into two parts by a tube plate 15. In addition, a plurality of U-shaped heat exchanger tubes 17 through which a primary coolant flows are mounted to one side of the tube plate 15 and have open ends which pass through the tube plate 15 to communicate with the opposite side. The space underneath the tube plate 15 is further subdivided by a partition 16 into two plenum chambers which serve to receive the entering and leaving primary coolant respectively. As shown, the primary coolant is fed into one plenum chamber via a connection 11 and, after passing through the tubes 17, is discharged via a connection 12. The tank 1 is also provided with suitable manholes 19 to provide access to the respective plenum chambers.

The pressure tank 1 also includes a connection 13 above the tube plate 15, as viewed, through which a secondary coolant is fed into the tank to flow around the heat exchanger tubes 17 in heat exchange relation with the primary coolant therein. Also, a connection 14 is provided for the discharge of the secondary coolant in the top of the pressure tank 1, as viewed. As the bundle of heat exchanger tubes 17 is packed extremely closely, a fault, for instance at point 18, is not directly accessible for repair. As such a fault occurs with greater probability in the curved parts of the heat exchanger tubes 17, it is practically impossible to make repairs even by means of special tools from the inside of the tube. Therefore, there remains only one possibility, namely, to close off the defective tube at both its ends, i.e., within the region of the tube plate 15, as is indicated by the plugs 2.

Referring to FIG. 2, each plug 2 is constructed in a very special manner to form a joint with the inner wall of a defective tube 17 according to the principle of explosion welding. The plug 2 consists of a generally cylindrical metal piece or body 21 which is provided with a central recess or blind hole 23. The plug 2 also has a conically tapered surface portion 22 at the open end which has an angular taper .alpha. of between 2.degree. and 6.degree..

In order to mount the plug 2 within the tube, each plug 2 is fitted with a sleeve 3 of plastic, cardboard or similar material within the blind hole 23. In addition, the sleeve 3 has a cone-shaped enlarged rear end 33 which extends away from the open end of the plug 2 in shoulder-like fashion. This enlarged end 33 is also provided with longitudinal slots 34 by which the sleeve 3 is welded or jammed into a tube to be sealed off. The plug 2 and sleeve 3 thus form a unitary device which can be inserted into the end of a heat exchanger tube 17.

In addition, a detonator cap 4 is mounted in the front end of the sleeve 3 and an explosive charge 5 is mounted in a central part of the sleeve 3. The explosive charge 5 is of a length corresponding approximately to the length of the tapered surface portion 22 of the plug 2 and is located concentrically within the plane of this portion of the plug 2. This device also has ignition wires 41 which are connected to the detonator cap 4 and are led out along the detonator cap 4 and the explosive charge 5, for instance, in a lateral slot of the sleeve 3.

In order to close off a defective heat exchanger tube, entry is first gained into the plenum chamber via a respective manhole 19 (FIG. 1). Thereafter, the end of the tube 17 to be closed is cleaned on the inside and the device, as shown in FIG. 2, is inserted in the tube. Next, the ignition wires 41 are led out through the manholes 19 and the explosive charge 5 is detonated by connecting the wires to an electric detonator. The shock wave of the detonation is then transmitted via the plastic sleeve 3 to the plug 2, whereby a progressive deformation of the plug 2 in the conically tapered portion 22 is achieved. Because of the very high pressures, which act only for a very short time, and the friction of the metal parts striking each other, the portions of the surfaces of the tube and plug to be joined are welded together. In this respect, is is noted that an intense heating within a narrow boundary zone is possible in the process. In addition, residual surface contamination as well as oxide layers are broken up in the process by the fine jet of molten metal which develops due to the shape of the plug, and are dislodged from the zone of the joint. The mechanics of the explosion weld carried out thus largely resemble the techniques of pressure or friction welding, respectively.

Referring to FIG. 3, after detonation and removal of the sleeve 3, the plug 2 is established in a close fit welded relation with the tube 17 along the expanded portion 22 in the area of the tube plate 15. Metallographic studies have shown that, in this region, a perfect weld, and not only parallel contact, has taken place.

The invention thus provides a device which can be used to close off defective tubes extremely fast. Further, the device of the invention can be easily handled manually and can, of course, also be used via remotely controlled manipulators should the radiation level in the plenum chambers be too high for operating personnel.

Further, by utilizing the device of the invention and the related welding technique, the aforementioned problems of covnentional welding are reliably avoided.

In order to gain a better picture of the dimensions of a plug 2 of this invention, it should be mentioned that, with an outside diameter of 19.4 millimeters (mm) the plug 2 may have, for instance, a length of 110 millimeters (mm). The blind hole 23 then has an inside diameter of 13 millimeters (mm) and a depth of about 100 millimeters (mm). The length of the explosive charge 5 and also of the conical part 22 of the plug 2 is about 30 millimeters (mm). The length of the sleeve 3 extending into the blind hole 23 depends on the size of the detonator cap 4 used. The quantity of explosive 5 must be determined by appropriate preliminary tests and also depends on the material of the heat exchanger tubes and of the plugs 2, respectively. Normally, both the tube and plug parts will be made of the same material. Generally, as a rule, the difference in the coefficients of thermal expansion between the material of the tube and that of the plug should be as small as possible. The magnitude of the angle .alpha. for the conical surface portion 22 at the plug 2 depends here on the detonation velocity of the explosive used, i.e., on its type. In general, the angle .alpha. is between 2.degree. and 6.degree. and will be the larger, the greater the detonation velocity of the explosive used. Aside from the type and amount of the explosive, the wall thickness of the plug as well as the conicity of the open end are essential for the success of this explosive-seal welding technique.

Claims

What is claimed is:

1. A device for closing off an end of a tube comprising

a metallic plug having a forward peripheral surface portion of cylindrical cross-section, a conically tapered rear surface portion of predetermined angle and a blind hole extending from a rear end thereof into the plane of said forward surface portion;

a sleeve having a forward end positioned in said blind hole and an enlarged rear end projecting away from said rear end of said plug, said enlarged end having a cone-shaped outer surface and a plurality of slots therein for jamming into the end of a tube to mount said plug in the tube;

a detonator cap within said forward end of said sleeve; and

an explosive charge of predetermined detonation velocity within said sleeve and within the plane of said conically tapered rear surface portion of said plug, said charge being of the same length as said conically tapered rear surface portion.

2. A device as set forth in claim 1 wherein said sleeve includes a slot therein and which further includes ignition wires connected to said detonator cap and extending through said slot of said sleeve out of said sleeve.

3. A device as set forth in claim 1 wherein said angle of said conically tapering surface portion of said plug is matched to said detonation velocity of said explosive charge and to the material of said sleeve whereby said angle increases with increasing detonation velocity.

4. A device as set forth in claim 1 wherein said angle is at least 2.degree. and not greater than 60.degree..

5. A device as set forth in claim 1 wherein said plug is of the same material as the tube to be closed off.

6. A device as set forth in claim 1 wherein said plug has a coefficient of thermal expansion equal to the coefficient of thermal expansion of the tube to be closed off.