Plug Assembly for Use in Method and Apparatus for Repairing Heat Exchangers

The present invention relates to a method and apparatus for repairing heat exchangers. More particularly, the present invention relates to a method and apparatus for sealing off a defective tube in a shell and tube type heat exchanger.

Shell and tube type heat exchangers customarily comprise a bundle of tubes mounted in spaced parallel relation at opposite ends in tube sheets. The tube bundle is surrounded by a shell through which a medium is flowed between an inlet and an outlet of the shell. Heads are provided on opposite ends of the shell to afford the flow of another medium through the interior of the tube bundles. In multiple pass heat exchangers, one head end of the heat exchanger may be provided with flow reversing means to cause the fluid flowed within selected tubes to make several passes before exiting the heat exchanger.

Heat exchangers of the above type have been known to fail in use and to require repair. Generally this is accomplished by removing the heads at opposite ends of the tube, and either removing and replacing the defective tube or plugging the defective tube. Since such heat exchangers are often integral components of continuous plant processes, it is important that they be repaired quickly with a minimum of down-time.

Various types of plugs are known for sealing off defective tubes. For instance, plugs marketed by Expando-Seal Tools, Inc. the Assignee of the present application, include a pair of rubber plugs which are expanded into tight engagement with the interior of the tube in the zone of the tube sheet. Such equipment is sold under the trade designation TBT Tools.

While the aforementioned TBT Tools function satisfactorily in many applications, they have certain limitations. For instance, the rubber stoppers limit the applicability of such tools to a certain temperature and pressure range and service life. In addition, the tools require that the actuating mechanism for the stoppers be left in place inside the tube bundle and head ends. Not only is this an uneconomic use of the actuator, but it also may effect adversely the flow of fluid in the head ends.

In U.S. Pat. No. 2,856,963, there is disclosed a plugging device for use in shell and tube type heat exchangers as described above. In the patented assembly, a two-piece plug is inserted in a defective tube in substantial alignment with the tube sheet and is connected by a breakaway connection to a pull rod which extends longitudinally inside the tube. The outer portion of the plug is restrained against axial movement of the tube by means of a series of fingers which engage spaced locations around the periphery of the tube adjacent the tube sheet. While the patented structure may function satisfactorily in certain applications, the proclivity for the fingers to straighten when substantial tension is applied to the pull rod limits the amount of tension which can be applied and hence the amount of sealing pressure. Moreover, since there is often welding present around the periphery of the tube where it engages the tube sheet, it is not always possible for the fingers to grip the tube properly with an even amount of stress applied to each finger.

While the plugging of heat exchanger tubes is difficult when the tubes are relatively free from scale and corrosion, it is even more difficult to provide a completely fluid-tight seal when the tubes have inner layers of scale and/or corrosion. In such cases, there may be a tendency for fluid to flow between the plug and the scale/corrosion covered tube. Moreover, it is important for plugs to be able to accommodate variations in manufacturing tolerances of heat exchanger tubes. Hence, a plug assembly which can handle all these conditions is particulary desirable.

A device for plugging holes in other applications is disclosed in U.S. Pat. No. 4,091,841.

With the foregoing in mind, it is a primary object of the present invention to provide improved apparatus for plugging the ends of tubes in heat exchangers.

It is another object of the present invention to provide a novel method for plugging heat exchanger tubes efficiently and with a minimum of labor and exchanger down-time.

As another object, the present invention provides a unique method and apparatus for plugging tubes against relatively high pressures for relatively long periods of time.

As a still further object, the present invention provides an economical method and apparatus for securely plugging tubes in a heat exchanger in a manner which minimizes interference with internal flow patterns within the heat exchanger.

Yet another object of the present invention is to provide an improved plug assembly which is particularly suited for sealing off tubes having scale and/or corrosion as well as accommodating variations in the inside diameters of the tubes due to manufacturing tolerances.

More specifically, the present invention provides an improved method and apparatus for efficiently and effectively plugging defective tubes in heat exchangers. The apparatus comprises a two-piece tapered plug assembly which includes a ring adapted to be installed inside a defective tube in the zone of the far tube sheet and expanded into place when an inner wedge member is pulled axially of the ring by an hydraulic ram assembly located at the opposite end of the defective tube. For this purpose, a pull rod is connected to the wedge portion of the plug assembly via a breakaway member which is received in a positioning socket. The positioning socket slidably receives the pull rod and abuts a compression tube which surrounds the pull rod inside the defective tube. The pull rod and compression tube are operatively connected to the hydraulic ram assembly by adjustable releasing means. With this structure, the defective tube may be plugged by displacing the two-piece plug assembly axially inside the defective tube from one end thereof and positioning the same in the zone of the tube sheet. Thereafter, upon actuation of the hydraulic ram assembly, the pull rod pulls the wedge portion of the plug assembly into the ring portion thereof to expand the ring radially outward, applying pressure until the breakaway piece separates and is pulled against its stop inside the positioning socket, whereupon the socket, pull rod and compression tube may be withdrawn axially from inside the defective tube. The near end of the defective tube is plugged in a similar manner utilizing different length compression and tension applying means.

A plug assembly which is particularly suited for closing off badly scaled and/or corroded tubes has a sleeve with a series of circumferential cutting edges alternating with grooves. The cutting edges penetrate the scale and displace the same into the grooves when the sleeve is expanded radially by the wedge.

The foregoing and other objects, features and advantages of the present invention should become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a conventional sheet and tube heat exchanger with a portion of the shell having been broken away to illustrate the tube bundle therewithin;

FIG. 2 is an exploded side elevational view of apparatus embodying the present invention which is particularly suited for plugging defective tubes in accordance with the method of the present invention;

FIG. 3 is a horizontally-foreshortened, partially sectioned view illustrating the apparatus of FIG. 2 installed in a defective tube prior to plugging the far end thereof;

FIG. 4 is an enlarged, fragmentary sectional view of a portion of the apparatus illustrating the relative forces applied during plugging of the tube;

FIG. 5 is a view similar to FIG. 4 but illustrating the relative positions of the various elements of the invention after the plug has been installed;

FIG. 6 is a fragmentary side elevational view of the hydraulic ram portion of the apparatus with a portion of the ram broken away to expose interior details and illustrating the manner in which tension is applied during the plugging operation;

FIG. 7 is a greatly enlarged exploded perspective view illustrating certain of the components of the apparatus of the present invention, excepting the hydraulic ram assembly;

FIG. 8 is a sectional view illustrating a modified form of adjusting mechanism which may be used satisfactorily in the present invention;

FIG. 9 is a fragmentary sectional view of a plug which is particularly suited for sealing tubes having interior scale and/or corrosion;

FIG. 10 is a sectional view taken on line 10--10 of FIG. 9;

FIG. 11 is a greatly enlarged fragmentary view illustrating the disposition of the plug sleeve against the inside wall of a heat exchanger tube as the sleeve is being expanded; and

FIG. 12 is a view similar to FIG. 11 but showing the disposition of the scale cutting edges on the sleeve after full expansion of the sleeve against the inside of the tube.

Referring now to the drawings, FIG. 1 illustrates a heat exchanger 10 of the shell and tube type. As illustrated therein, the heat exchanger 10 comprises a bundle of tubes such as tubes 11 and 12 mounted horizontally in spaced parallel relation at opposite ends in tube sheets 24 and 25 (FIG. 3) located in substantial alignment with flanges 14 and 13 extending outwardly from opposite ends of the shell 15. The shell 15 is provided with an inlet 16 and an outlet 17 for the purpose of flowing fluid transversely with respect to the bundle of tubes 11, 12. Opposite ends of the shell 15 are provided with heads 18 and 19 for the purpose of flowing another fluid through the insides of the tubes 11, 12, the head 18 having an inlet 20, and the head 19 having an outlet 21. The head 18 has a flange 22 which is bolted to the shell flange 13, and the head 19 has a similar flange 23 bolted to the shell flange 14 to afford removal of the heads.

In the event that one of the tubes in the tube bundle should become defective, as by having developed a leak or burst, it has generally been necessary for both of the heads 18 and 19 of the heat exchanger 10 to be removed to enable the defective tube to be removed and replaced. For emergency repair purposes, however, depending upon pressure, temperature and service life, it has been possible temporarily to plug both ends of a defective tube from one end simply by removing one of the heads. As discussed, heretofore, however, known prior art devices utilized for this purpose have had certain limitations.

In accordance with the present invention, it is now possible quickly and easily to plug a defective one of the tubes, such as the tube 12, simply by removing one of the heads, such as the right or near head 19, and the plugging can be effected with a minimum of downtime for the heat exchanger 10. Moreover, the plugs are capable of resisting substantial pressures and sustaining elevated temperatures for long periods of time without failure or leaking.

To this end, the present invention comprises apparatus 30 (FIG. 2) which is particularly suited for plugging tubes in accordance with the method of the present invention. As illustrated in FIG. 2, the apparatus 30 comprises a two-piece plug assembly 31 which is connected via a breakaway member 32 to a pull rod 33 which is received by compression tube means 35 comprising a socket 36 for receiving the breakaway means 32 and a compression tube 37. The pull rod 33 and compression tube 37 are operatively connected to a ram assembly 34. As will be described hereinafter, the apparatus 30 is designed to securely install the plug assembly 31 inside the far end of defective tube in the zone of its tube sheet, i.e. from a location remote from the open end of the heat exchanger, such as the righthand end in FIG. 1 when the head 19 is removed. The apparatus 30 is also designed to plug the near end, i.e., the end of the tube 11 adjacent the removed head 19, with minor changes in certain of the components of the apparatus 30.

Referring now to FIG. 7, the plug assembly 31 comprises a ring 40 and a tapered wedge 41 received within the ring 40. In the illustrated embodiment, the ring 40 is cylindrical and is shaped to fit snugly within the cylindrical tube which is to be repaired. The wedge 41 has a frusto-conical shape with a narrow end 41a and an enlarged end 41b and a constant taper between the ends. The ring 40 has an internal bore with a taper which mates with the taper of the plug 41. The ring 40 and wedge 41 may be fabricated of brass, type 304 or 316 stainless steel or cold rolled steel, or any other metals depending upon the environment in which the plug assembly 31 is to be installed, including pressures, temperatures, service life, etc. It is important, however, for the ring 40 to be deformable relative to the wedge 41 so as to be capable of being expanded radially outward when the wedge 41 is displaced axially within the ring 40 as will be described. Also, when certain hard metals are used, the inner bore of the ring 40 is preferably stepped, i.e., relieved inwardly for about one-half its length from the end having the larger internal diameter.

The plug assembly 31 is connected to the other components of the apparatus 30 in the manner which provides an automatic release after a predetermined sealing pressure has been applied by the wedge 41 to the ring 40. For this purpose, the breakaway means 32 is provided. As best seen in FIG. 7, the breakaway means 32 comprises a section of bar stock having a necked-down portion 32a which is located intermediate threaded portions 32b and 32c and which has a diameter smaller than the root diameters of the threaded portions 32b and 32c. The threaded portion 32b threadedly engages within a threaded bore provided in the narrow end 41a of the wedge 41. The threaded portion 32c threadedly engages within a threaded bore provided in the far end 33a of the pull rod 33. An enlarged section 32d having wrench flats is provided intermediate the threaded portions 32b and 32c on the side of the necked-down portion 32a remote from the wedge 41. The wrench flats 32d enable the breakaway means 32 to be screwed into the wedge 41 and the pull rod 33 to permit the pull rod 33 to apply tension to the wedge 41 via the breakaway means 32. The wrench flats also permit the ruptured portion of the breakaway member 32 to be removed from the pull rod 33 for replacement with another breakaway member 32.

In order to prevent the ring 40 from moving axially of the tube 12 as tension is applied to the wedge 41, the compression tube means is provided. As best seen in FIG. 7, the compression tube means comprises a compression tube 37 and a socket 36 which has an end wall 36a having a bore 36' which slidably receives the far end 33a of the pull rod 33. The socket 36 has a tubular wall 36b which terminates in an annular surface 36c adapted to abut against the inner end 40a (FIG. 2) of the ring 40 in the manner illustrated in FIG. 4. As best seen in FIG. 4, the socket 36 has a cylindrical interior chamber which is sufficiently large as to receive the enlarged portion 32d of the breakaway member 32 while the socket end wall 36a projects radially inward toward the bore 36' to provide an annular shoulder which engages the enlarged portion 32d to limit the motion of the breakaway means 32 in the rightward direction relative to the socket 36 after the breakaway means 32 has ruptured, as in the manner illustrated in FIG. 5. This structure also permits the breakaway means to be withdrawn from within the tube 12 along with the compression tube 37 and pull rod 33, and the plug positioner socket 36. When used with small-diameter sockets to plug small-diameter tubes, it is preferable for the wrench flats 32d to be eliminated and an enlarged continuous peripheral flange substituted at the same location in order to provide sufficient bearing surface between the enlarged section 32d and the annular shoulder defined by the inside of the socket end wall 36a.

Pressure is applied in the leftward direction to the socket 36 to counter the tension of the pull rod 33. To this end, as best seen in FIG. 4, the left or far end of the compression tube 37 abuts the end wall 36a of the socket 36, and the right or near end of the compression tube 37 extends into proximity with the right-hand tube sheet 24 a distance sufficient as to be engaged by the ram assembly 34. See FIG. 6. Thus, with this structure, compression can be applied in the leftward direction in FIG. 4 to the ring 40 and tension applied in the rightward direction to the wedge 41 for the purpose of pulling the wedge 41 into the ring 40 until radial pressure on the ring 40 causes the necked-down portion 32a of the breakaway means 32 to yield and separate in the manner illustrated in FIG. 5. Thereupon, the compression tube 37, socket 36, and pull rod 33 can be withdrawn rightward in the direction indicated by the arrows in FIG. 5. This leaves the plug assembly 31 installed in the defective tube in the zone of the tube sheet 25.

Substantial tension must be applied to the wedge 41 in order to drive the ring 40 outwardly into leak-resistant fluid-tight engagement with the interior of the tube 12. For this purpose, the hydraulic ram means 34 is operatively connected to the pull rod 33 and the compression tube 37. As best seen in FIG. 6, the hydraulic ram means 34 comprises a frame 50 having a pistol grip 51 and an interior chamber 52 adapted to be filled with an hydraulic fluid via an inlet 53 connected to a source of hydraulic fluid under pressure. The ram 34 has a driving yoke 54 which is connected by tension springs 55a, 55b to the frame 50. A conventional hydraulic ram which has these components and which has been used satisfactorily is presently marketed by the Owatonna Tool Co. of Owatonna, Minn.

A retaining collar 45 slidably receives the pull rod 33 and has a neck 45a which is received within a counter bore in the ram frame 50. The retaining collar 45 has an inner bore 45b which terminates in an interior shoulder adapted to engage the outer or near end 37a of the compression tube 37 and which is sized to receive the compression tube 37 axially. Thus, the retaining collar 45 is interposed between the outer end 37a of the compression tube 37 and the frame 50 of the ram assembly 34 and centers the compression tube 37 with respect to the tube 11 and the ram assembly 34.

In order to apply tension to the pull rod 33, the outer end 33b of the pull rod 33 is connected to the ram assembly 34 in a manner which permits looseness between the various parts to be taken-up prior to the application of compression and tension. For this purpose, the outer end 33b of the pull rod 33 is internally threaded for threadedly engaging an externally threaded stem 60a of an adjusting wheel 60 which has an enlarged portion 60b with an annular shoulder 60c. A circular collar or spacer 62 having a friction lock 62a is adapted to be interposed between the movable yoke 54 of the ram assembly 34 and the shoulder 60c. The spacer 62 has a bore 62a which is sufficiently large as to accommodate the threaded portion 60a of the adjusting wheel 60 but which is sufficiently small as to engage the annular shoulder 60c.

With this structure, the apparatus 30 may be assembled and installed in the manner illustrated in FIG. 3, and the adjusting wheel 60 rotated until its shoulder 60c abuts the spacer 62 to eliminate any slack or play in the system prior to the application of the fluid pressure in the ram chamber 52. Thereafter, hydraulic fluid under pressure is pumped into the inlet 53 via a flexible hose (not shown) from a source, such as a manual pump, to displace the ram yoke 54 rightward for applying tension to the pull rod 33 and compression to the compression tube 37. After the breakaway member 32 has ruptured and the plug assembly 31 has been installed in the manner described, the ram assembly 34 may be disconnected by rotating the adjusting wheel 60 in the opposite direction to dismantle the apparatus. It is noted that if several tubes are to be plugged at the same time, it is desirable to retain the same length of compression tube and pull rod and to plug the far ends of all tubes at the same time, simply replacing the breakaway means 32 after each tube has been plugged. Thereafter, in order to plug the near ends of the tubes, a shorter length of pull rod is substituted for the pull rod 33, and either the socket 36 or a short length of compression tube may be utilized for the purpose of providing the requisite compression forces to the ring of the plug assembly used to plug the near end of the tube. Thus, it should be apparent that the steps employed in plugging the near end of a defective tube is essentially the same steps as used in plugging the far end of the same tube.

The apparatus of the present invention can be used to plug tubes of various diameters. To this end, compression tubes and pull rods, as well as plug assemblies, are provided in different diameters, depending upon the inside diameter of the heat exchanger tube to be plugged. In order better to accommodate variations in the diameter of the pull rod while providing accurate adjustment, a modified adjusting mechanism is provided and is illustrated in FIG. 8.

For purposes of clarity, only the rear portion of the ram yoke 54 is illustrated in section, it being understood that the other aspects of the ram are the same as illustrated and described heretofore. In this embodiment of the adjusting means, the rear end of the ram yoke 54 is provided with an internal threaded bore 54' which threadedly receives an adjusting wheel 65 having a threaded stem 65' which engages the internal threads 54' of the ram yoke 54. The knurled wheel 65 has an internal bore 66 which slidably receives a portion of the rear end of the pull rod 33. The outermost end of the pull rod 33 is provided with an externally threaded protrusion 33' which threadedly engages the outermost threaded section 67' of a compression rod extension 67. The compression rod extension 67 has internally threaded bore sections 67" and 67'" of a smaller diameter than the threaded bore 67' to accommodate smaller diameter protrusions 33' of smaller diameter pull rods 33.

The outermost end of the pull rod extension 67 has a reduced diameter neck portion 69 providing an annular shoulder 69'. The neck 69 is adapted to removably mount a horseshoe-shaped spacer 70 which extends outwardly from the pull rod extension 67 to engage the outer surface 65a of the adjusting wheel 65 when the same has been rotated outward of the ram yoke 54. Thus, when the ram 34 is actuated so that the ram yoke 54 moves rightward, the rear surface 65a of the adjusting wheel 65 engages the horseshoe-shaped spacer 70 which in turn engages the shoulder 69' on the pull rod extension 67 to apply tension to the pull rod 33 for purposes described heretofore. After the desired tube has been plugged, the spacer 70 is disengaged from the pull rod extension 67 to enable the ram assembly to be pulled outward relative to the pull rod before it is withdrawn from inside the plugged tube.

In view of the foregoing, it should be apparent that the present invention now provides an improved method and apparatus for plugging defective tubes of heat exchangers with a minimum of labor and down time. By virtue of the apparatus of the present invention, such plugging can be achieved from only one end of the heat exchanger, thereby making it unnecessary for access to be provided to both ends, as by removing both heads. Moreover, the apparatus of the present invention provides a fluid-tight seal which is capable of remaining leak resistant even under relative high pressures (up to 5000 psi.) and temperatures and which does not affect adversely internal flow patterns within the heat exchanger.

By way of example, and not by way of limitation, it has been discovered that a breakaway member 32 having a cylindrical necked-down zone 32a with a diameter of 0.165 inches when used in conjunction with a plug assembly 31 fabricated of brass, cold-rolled steel or stainless steel ruptures at a hydraulic pressure of about 800 psig. applied to the ram assembly to provide leak resistance at a pressure in excess of 5000 psi. when installed inside a heat exchanger tube having an inside diameter of 0.500 inches. The actual amount of tension applied at 800 psig. is 2800 pounds due to the 3.5 to 1 force to pressure ratio provided by the hydraulic ram assembly.

While the plug assembly described thus far functions effectively in many applications to provide a fluid-tight seal, it has been determined that when the plug is used in tubes which have a substantial amount of internal scale and/or corrosion small leaks can develop around the periphery of the plug assembly due to the porosity and irregularity of the scale and/or corrosion. Moreover, it is possible for small leaks to occur even in relatively new tubes because the inside diameters of the tubes can vary from 0.001 to 0.005 inches and still be within commercially acceptable tolerances. Accordingly, in order to accommodate scale and tube tolerances, an improved plug assembly has been developed.

Referring now to FIG. 9, the plug assembly 131 comprises a ring or sleeve 140 and a wedge 141 mounted for sliding movement axially relative to the ring 140 as described heretofore with respect to the plug assembly 31. A breakaway member 132 is threaded into the narrow end of the frusto-conical wedge 141 for purposes which have been discussed.

The plug assembly 131 is shown installed inside a tube 112 which has a layer of scale or corrosion 112a on its inner periphery. As has been discussed, it has been difficult to completely seal off such tubes because the porosity and irregularities provided by the scale enables fluid to seep axially along the outer periphery of the ring 40 illustrated in FIG. 7. In order to overcome this problem, the sleeve or ring 140 is provided with a plurality of cutting edges 140a, 140b and a plurality of grooves, such as the groove 140c, which alternate between the cutting edges. The cutting edges 140a, 140b are disposed in axially spaced planes normal to the central longitudinal axis of A--A of the ring 140. Each cutting edge, such as the cutting edge 140a (see FIG. 11) is defined by a pair of outwardly converging surfaces 140', 140" which intersect at the edge 140a and define a substantially V-shaped cross-section. The surfaces 140" and 140'" cooperate to define therebetween the groove 140c.

The cutting edges are disposed in the outer surface of the ring 140 at a frequency in a range of about 14 to about 18 cutting edges per axial inch of length of the ring 140. The depth of the grooves 140c is less than 0.10% of the outside diameter of the ring 140. The number of cutting edges per inch of ring length depends on the wall thickness of the ring 140. As the wall thickness of the ring 140 increases, the number of cutting edges and grooves decreases so that the depth of the grooves increase.

The plug assembly 131 is installed in the tube 112 in essentially the same manner as discussed heretofore. The wedge 141 is pulled leftward relative to the ring or sleeve 140 using the apparatus illustrated in FIGS. 3-6. Thus, when the plug 131 is pulled leftward into the ring or sleeve 140, the cutting edges 140a and 140b are expanded radially outward relative to the inside of the tube 112. The cutting edge surfaces 140" and 140'" cooperate to displace the scale 112a into the groove 140c between the cutting edges as they move outwardly. This may be seen by reference to FIG. 11.

Continued outward motion of the cutting edges 140a, 140b causes their tips to deform under pressure in the manner illustrated in FIG. 12. Such deformation is beneficial in that it insures conformance of the periphery of the ring 140 with the inside diameter of the tube 112 and provides a series of closed annular pockets about the outer periphery of the ring 140. Since the cutting edges are disposed in axially spaced planes (as contrasted with being disposed helically) each of the grooves is separate from the other so that a leak-resistant labyrinth type of seal is provided along the outer periphery of the ring 140. Thus, the plug assembly 141 has been found to be particularly suited for use in providing a fluid-tight joint in scale-laden tubes, such as the tube 112. Moreover, because the cutting edges are capable of being deformed upon maximum outward expansion of the ring 140, they are capable of accommodating variations in the inside diameter of the tube as may be caused by variations in manufacturing tolerances of the tube 112. Of course, the plug assembly 141 can be used effectively in providing a fluid-tight joint in tubes which do not have interior scale or corrosion.

While a preferred method and apparatus have been described in detail, various modifications, alterations and changes may be made without departing from the spirit and scope of the present invention as defined in the appended claims.