US3555656A - Method of explosively plugging a leaky metal tube in a heat exchanger tube bundle - Google Patents

Abstract

THIS INVENTION PROVIDES AN EXPLOSIVE PLUGGING DEVICE FOR INSERTION INTO A LEAKY METAL TUBE OF A HEAT EXCHANGER BUNDLE TO PLUG THE TUBE BY EXPLOSIVE WELDING AND A METHOD OF EXPLOSIVELY WELDING A METALLIC PLUGGING DEVICE HAVING AN EXPLOSIVE CHARGE TO A METAL TUBE BY DETONATING THE CHARGE WITH A DETONATION INITIATOR TO PERIPHERALLY EXPAND THE PLUG INTO WELDING ASSOCIATION WITH THE TUBE.

Description

Jan. 19, 1971 BROWN ET AL 3,555,656

METHOD OF EXPLQSIVLLY PLUGGING A LEAKY METAL TUBE IN A HEAT EXCHAN TUBE BUNDLE v Filed May 1967 FIG.|.

WQIITNESSES INVENTOR 57 5: BY QQSL H W WZQU R V 7 I w \L\\\\\\E i- 9 United States Patent 3,555,656 METHOD OF EXPLOSIVELY PLUGGING A LEAKY METAL TUBE IN A HEAT EXCHANGER TUBE BUNDLE Francis X. Brown, Broomal, and Samuel W. Wismer, Jr., Springfield, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 25, 1967, Ser. No. 641,381 .Int. Cl.B22d 19/10; B23p 7/00 U.S. Cl. 29-401 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The invention relates to a method and device for plugging leaky tubes in heat exchangers of the tube and shell type and has for an object to provide a method and device of the type that provides a positive, metallurigcally bonded seal that is leak-proof when exposed to pressurized fluids, and that is relatively safe and expeditions to employ in all types of tube and shell type heat exchangers such as condensers, feed Water heaters and steam generators.

Heat exchangers of the above type are employed to transfer heat from one fluid to another and are usually provided with a bundle of tubes, the open ends of which extend through a suitable tube sheet disposed within a suitable channel head to which one of the fluids is directed for circulation through the tubes. The tube bundle and tube sheet are encompassed by a shell structure through which the other fluid is circulated in a manner to pass over the outer surfaces of the tubes. One of the fluids is hotter than the other, hence during operation the cooler fluid is heated by transfer of heat from the hotter fluid, and vice versa.

Usually one or both of the fluids are pressurized and, to prevent intermingling of the two fluids, the ends of the tubes are welded or otherwise sealingly secured to the tube sheet.

4 However, in operation, a tube may spring a leak through its wall or a leak may develop at the tube-to-tube sheet seal, thereby causing undesirable flow of the more highly pressurized fluid into the lower pressure fluid and contamination or adulteration of the lower pressure fluid.

Many solutions have heretofore been proposed to block or plug such a leaky tube at both ends, thereby to inactivate the tube and terminate such leakage. Such solutions have usually involved mechanically expansible plugs for insertion in the leaky tube. Such solutions necessarily required a workman to be in close proximity with the tube sheet in order to make such repair and entailed working entirely within the channel head, in the larger industrial heat exchangers of the types mentioned above. Also, such prior plugging arrangements were subject to incipient leakage leading to corrosion and eventually even catastrophic failure after prolonged service.

SUMMARY Briefly, one aspect of the invention resides in the provision of an explosive plugging device for insertion into a metal tube to plug said tube by explosively welding of the plugging device to the tube. In accordance with the in- Patented Jan. 19, 1971 vention the plugging device comprises a cylindrical body formed to be slidably inserted into the tube and having a solid cylindrical portion to block the flow of fluid and a hollow cylindrical portion containing an explosive chemical charge which is detonated by a suitable explosive initiator to peripherally expand the hollow cylindrical portion into violent abutment with the inner wall of the tube with attendant welding or metallurgical bonding, thereby effecting a positive leak-proof seal.

Explosive welding techniques are generally known in the welding art and many schemes have been employed or proposed to solve specific problems in the manufacture of heat exchange apparatus, as indicated in C. C. Simons and R. I. Carlson patent application No. 467,244, filed June 28, 1965, now Pat. No. 3,409,969, and R. J. Carlson, C.C. Simons and R. L. Bradford patent application No. 488,670, filed Sept. 20, 1965 and now Pat. No. 3,402,870. Both of these applications are assigned to the same assignee as this invention and more fully describe the phenomena attendant in explosive welding.

Another aspect of the invention resides in a method of explosively welding a plugging device of the above type in a leaky tube of a tube and shell heat exchanger. Briefly the method comprises insertion of a. plugging device of the above type into the leaky tube within the length of the tube confined in the tube sheet but preferably in close proximity with the open end of the tube and detonating the explosive charge to eflect the peripheral welded seal. The detonation is effected preferably from a position external of the heat exchanger channel head, thereby rendering the method inherently safe for personnel, especially in repairing nuclear power steam generators.

The above method is subject to modification involving chamfering of the end of the tube and adjacent tube sheet and emplacement of the plugging device in flush relation with the surface of the tube sheet, so that, on detonation of the explosive charge, a portion of the plugging device is expanded into the flared opening formed by the chamfermg.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial sectional view of a typical heat exchanger of the tube and shell type prepared for the plugging of a leaky tube, in accordance: with the invention;

FIG. 2 is an enlarged axial sectional view of an explosive tube plugging device in accordance with the invention;

FIG. 3 is an enlarged fragmentary axial sectional view illustrating the plugging device in inserted position within a leaky tube of the heat exchanger shown in FIG. 1, in preparation for sealing by welding;

FIG. 4 is a view similar to FIG. 3 but illustrating the tube and plug after welding;

FIG. 5 is a fragmentary axial sectional view illustrating another leaky tube condition;

FIG. 6 is a view similar to FIG. 5 but illustrating preparation of the tube and tube sheet for plugging;

FIG. 7 is a view similar to FIG. 2 but illustrating a further plugging device modification;

FIG. 8 is a view illustrating the modified plugging device of FIG. 7 inserted in the prepared tube of FIG. 6, in preparation for welding; and

FIG. 9 is a view similar to FIG. 8 but illustrating the tube and plug after welding.

DESCRIPTION OF THE INVENTION Referring to the drawings in detail, in FIG. 1 there is shown a typical heat exchanger 10 of the tube and shell type comprising the usual components, as follows a bundle oftubes 12 of the hairpin type having theiropposite end portions 14 and 15 extending through asuitable tube sheet 16.

As illustrated, thetube sheet 16 may be of the clad type, including a relativelythick plate portion 17 of one type of metal and a relatively thin clad layer 1 8 of a different type of metal. As more fully explained in F. X. Brown and L. K. Poole Pat. 3,257,710, issued June 28, 1966, and assigned to the same assignee as this invention, the metal for theclad layer 18 may be selected for weld compatibility with the metal of thetubes 12, to permit welding of thetube end portions 14 and 15 to thetube sheet 16, as more clearly shown at 19 in FIG. 3.

Thetube bundle 12 is enclosed by a tubular shell structure 20 disposed in fixed sealing abutment along its peripheral end portion with the periphery of thetube sheet 16 and jointly therewith forming achamber 21. The shell 20 has a fluid inlet opening 22 and a fluid outlet opening 23 communicating with thechamber 21.

A channel head 24 having apartition 25 is disposed in fired peripheral sealing abutment with the opposite face of thetube sheet 16 and jointly therewith forms aninlet chamber 26 in communication with thetube end portions 14 and anoutlet chamber 27 in communication with thetube end portions 15. The channel head 24 has a fluid inlet opening 28 communicating with thechamber 26 and a fluid outlet opening 29 communicating with thechamber 27.

In operation, as well known in the art, pressurized fluid at one temperature is admitted to thechannel head chamber 26 through theinlet 28 and directed through the bundle oftubes 12 to thechamber 27 and thence directed through the outlet 29. A second fluid at a different temperature and pressure is directed into thechamber 21 through theinlet 22 and thence through theoutlet 23. During its flow across the outer surfaces of thetubes 12, heat is exchanged between the two fluids with resulting heating of one fluid and cooling of the other.

Since the heat exchanger described above employs different fluids at different pressures, it is desirable, and in many cases imperative, to prevent intermingling of the two fluids. This is especially so in heat exchangers such as steam generators employed in nuclear turbine power plants, since the fluid flowing through thetubes 12 is usually the hotter fluid and has been heated in the nuclear reactor, and therefore may contain some radioactivity.

Accordingly, if leaks should occur in the tubes, it is present practice to interrupt operation of the heat exchanger 10 and plug bothends 14 and 15 of atube 12 that is leaking fluid.

In accordance with the invention, there is provided anexplosive plugging device 30, as best shown in FIG. 2, for insertion into atube 12 that is leaking through arupture 31 in the tube wall as indicated in FIG. 3, and a method of plugging the leaky tube in a positive leakproof manner.

The plug device is of symmetrical circular crosssection about its longitudinal axis A-A and includes ametallic body member 33 preferably formed of metal having weld compatibility with the tube metal, and of generally thimble form having oneend portion 34 of solid cylindrical shape and anopposite end portion 35 of hollow cylindrical shape jointly forming anaxial cavity 36 of cylindrical shape. As shown in FIGS. 2 and 3 thesolid portion 34 is of such a diameter as to permit a slidable fit with thetube end 14 to be plugged, while thehollow portion 35 is of smaller diameter than the solid portion and of about the same wall thickness as the tube.

More specifically thesolid portion 34 is on the order of about .002" smaller diameter than the internal diameter of thetube end 14, while thehollow portion 35 is on the order of about .0 30.050 smaller diameter than the diameter of the solid portion to provide an annular stand-off distance or space S of about .015"- .025" from the internal wall of thetube end 14.

Within thecavity 36 there is received an explosivechemical charge 37 of cylindrical shape and provided with anaxial opening 38. Thecharge 37 is preferably of integral or cast form and extends axially about 50% of the length of the cavity. The charge may be of any suitable high detonation velocity, explosive material for example, TNT (trinitrotoluene) or PETN (pentaerythritol tetranitrate) and of such axial length and cross-sectional area as to provide about 6 grams per square inch (of cross-sectional area) of such explosive.

Anend cap member 40 of any suitable plastic material having anaxial opening 41 extending therethrough is provided with aninner end portion 42 of a diameter to be snugly received in thecavity 36 and maintain thecharge 37 in position and anouter end portion 43 of about the same diameter as the outer diameter of thetube end portion 14.

Anelectrical detonation initiator 44 of cylindrical form and having a pair of externallead wires 45 is slidably received in the registeringaxial openings 41 and 38 and extending through thecharge 37 into endwise abutment With the end Wall of thecavity 37.

Theexplosive plug device 30 is inserted into thetube end portion 14 to the fullest extent permitted by abut-.

ment of theend portion 43 of thecap member 40 with the end of thetube portion 14. Accordingly, thebody member 33 and theexplosive charge 37 are properly positioned within the tube portion in the optimum position indicated in FIG. 3.

Thelead wires 45 are then connected to a supply of electrical current and energized to fire theinitiator 44, which, in turn detonates theexplosive charge 37. As thecharge 37 is detonated, its explosive forces explosively expand thehollow end portion 35 of the body in peripherally outward direction across the annular stand-off space S into impinging abutment with the inner wall of thetube portion 14, as shown in FIG. 4. The velocity of radially outward movement of theend portion 35 is of suflicient magnitude to metallurgically bond or explosively weld the periphery of thehollow end portion 35 to the inner periphery of the tube and provide a seal that is leak-proof and reliable, even when subjected to fluid pressure, as incurred in operation of the heat exchanger.

Any refuse remaining in thecavity 36 from the detonated explosive, theplastic cap member 40 or theinitiator 44 may be easily removed.

Theopposite end portion 15 of the leaky tube may be plugged in the same manner described above to completely isolate the tube from the system.

In FIG. 5, there is shown a tube end portion 14a, similar to thetube end portion 14 shown in FIGS. 3 and 4, and extending through thetube sheet 16. In this case the tube has not been ruptured, but the annular tube-totube sheet weld joint 19 has incurred a small break causing a leakage flow path 31a to to be established between the outer surface of the tube and the tube sheet, so that fluid in thechamber 21 and the fluid inchamber 26 may intermingle.

The above type of leaky tube may be plugged in the same manner described in conjunction with FIGS. 2-4, incl., since the explosive forces of thecharge 37 are so great that a peripheral portion of the tube and 14a will be expanded into tight abutment with the tube sheet to interrupt the flow path 31a. However, a positive and reliable seal may not be obtained in every instance, since most of the explosive force may be absorbed in Welding theplug body 33 to the tube.

Accordingly, in cases of leakage as shown in FIG. 5, there is provided a modified explosive plugging device 50', shown in FIGS. 7 and 8. This plugging device may be identical in all aspects to the pluggingdevice 30, except that theend cap 51 has anouter end portion 52 of greater diameter than theend portion 43 for a purpose subsequently to be described.

As shown in FIG. 6, the preparation for plugging the tube end portion 141: comprises the step of machining away (by chamfering or countersinking, for example) a portion of the tube, the weld joint 19 and a peripheral portion of the cladlayer 18 surrounding the tube to provide anenlarged opening 53 that is flared or beveled at any suitable angle on. The machining is performed to a depth sufficient to cause theend 54 of the tube to terminate within the cladlayer 18 and present a flush continuation of the flaredopening 53.

To plug the tube end portion 14a, theplug 50 is inserted thereinto to the fullest extent permitted by theend cap 51. Theend portion 52 is formed with a larger diameter than the maximum diameter of the flared opening, so that insertion of the plug is limited by abutment of theend portion 52 with theouter surface 56 of the tube sheet. Accordingly, as in the first embodiment, theplug body 57 and theexplosive charge 58 are properly located in the optimum position.

After, the explosive charge is detonated, theplug body 57 is explosively deformed as shown in FIG. 9 to form an annular explosive weld or metallurgical bond between the periphery of the hollowtubular body portion 59 and the peripheral surface of the flared opening in the tube sheet, thereby effectively interrupting the leakage path 31a.

It will now be seen that the invention provides a simple, yet highly eifective method and apparatus for plugging a leaky heat exchanger tube by explosively welding the plug in position.

It must be pointed out that in both disclosed schemes, the plugging device is positioned in the leaky tube within the confines of thetube sheet 16. Accordingly, the large mass of the tube sheet is eflective to greatly restrain the explosive forces of the exploding charge and enhance the explosive welding effect.

Also, since the tube portion subjected to the explosive forces is not greatly enlarged in cross-section, due to the restraining effect of the tube sheet, the plugged tube may be readily removed at a later date and replaced in any conventional manner, if so desired.

It will further be seen that since thelead wires 45 may be passed through the inlet oroutlet openings 28 or 29, or a suitable man-way opening (not shown), the tube plugging may be conducted in a manner safe to personnel.

We claim:

1. The method of plugging a leaky metal tube in a heat exchanger tube bundle and tube sheet structure, wherein the open ends of the tubes in the tube bundle extend through said tube sheet, said method comprising inserting a metallic plug device having an explosive charge into the opening end of said leaky tube, inserting a detonation initiator into said plug in axial juxtaposition with said eXposive charge, and

firing said initiator to detonate said charge in a manner to peripherally expand said plug into peripheral welding association with said tube.

2. The method recited in claim 1 and further includchamfering the open end of said leaky tube to provide a flared opening in the tube sheet surface, and

positioning the metallic plug device adjacent said flared opening before detonation of the initiator, thereby to peripherally expand an annular portion of the plug device into welding association with said flared opening.

3. The method recited in claim 1 wherein the tube sheet is clad with a layer of metal having weld compatibility with the metal tubes, and

the metallic plug device is formed of metal having weld compatibility with the metal tubes and/or said layer of metal.

4. The method recited in claim 1, wherein the tube sheet is clad with a layer of metal having weld compatibility with the metal tubes, and the metallic plug device is formed of metal having weld compatibility with the metal and! or said layer of metal, and including chamfering the open end of said leaky tube to provide a flared opening in said layer of metal, and

positioning the metallic plug device in substantially flush relation with said layer of metal, thereby to peripherally expand an annular portion of the plug device into welding relation with said flared opening.

References Cited UNITED STATES PATENTS FOREIGN PATENTS l/l957 England 29-421 PAUL M. COHEN, Primary Examiner US. Cl. X.R.

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