WO2025073338A1

WO2025073338A1 - Method for replacing heat exchanging tubes of a tube bundle

Info

Publication number: WO2025073338A1
Application number: PCT/EP2023/025421
Authority: WO
Inventors: Giovanni MANENTI
Original assignee: Individual
Current assignee: Individual
Priority date: 2023-10-05
Filing date: 2023-10-05
Publication date: 2025-04-10
Anticipated expiration: 2026-04-05

Abstract

A method for completely replacing one or more existing straight heat exchanging tubes (1) with new tubes (23), connected at ends to two supporting means or tube-plates (2, 14) and part of a tube bundle apt for an indirect heat exchange between a tube-side and a tube-outside fluid. A damaged existing tube (1) is disconnected from tube-plates (2,14), said existing tube (1) and a new tube (23) are tied each other through at least one temporary butt-to-butt weld (25) and the new tube (23) is pushed inwards the tube bundle whereas the existing tube (1), acting as a guide for the new tube, is extracted and removed from the opposite bundle end.

Description

Method for replacing heat exchanging tubes of a tube bundle

DESCRIPTION

Field of the invention

The present invention describes a method for completely replacing one or more existing heat exchanging tubes with new tubes equivalent to the existing ones, installed in a tube bundle apt for an indirect heat exchange between two fluids.

More specifically, the present invention refers to tube bundles that are part of equipment like process gas boilers, quenchers or chemical reactors operating in chemical and petrochemical plants. Such equipment are mostly of shell-and-tube type.

The heat exchanging tubes covered by this invention are straight, welded at ends to supporting means or tube-plates.

Technical problem to be solved

Tube bundles of heat transfer equipment are often designed with straight tubes jointed or welded at ends to two supporting means or tube-plates. Process gas boilers installed downstream of steam methane reformers, quenchers installed downstream of hydrocarbons steam cracking furnaces and methanol reactors are examples. When one or more existing heat exchanging tubes are endangered or damaged, the damaged tubes are either taken out of service, for example by plugging them at both ends, or are removed and replaced with new tubes.

Replacing a damaged existing tube with a new tube is a desirable solution as the performance of the heat transfer equipment is not reduced and the remaining existing tubes are not overloaded. On the contrary, by plugging the damaged tubes, heat exchange surface and cross-area available for tube-side fluid are reduced. Consequently, at nominal operating conditions, an equipment with one or more plugged tubes has the tubes in service that are thermally/mechanically overloaded, and this is mostly critical for process gas boilers, quenchers and chemical reactors,

The replacement of an existing straight exchanging tube installed in a tube bundle that has already operated with high-pressure boiling water outside tubes, as for process gas boilers, quenchers and water-cooled chemical reactors, is a difficult and risky operation. Difficulties and risks arise because water-side or shell-side surfaces of tube-supports (baffles, grids), tube-plates and tubes are usually fouled and because tubes and tube-supports can be misaligned as a result of thermal expansions and mechanical stresses suffered during operations.

Consequently, the extraction of a damaged existing tube and, mostly, the insertion of a new tube from/into a process gas boiler, quencher or chemical reactor may cause local friction, stranding and finally damages to tube-supports, tube-plates and the new tube itself.

The present invention provides for an efficient, practical and relatively rapid method for replacing straight exchanging tubes, installed in process gas boilers, quenchers and chemical reactors, where difficulties and risks associated with the extraction of the damaged existing tubes and the insertion of the new tubes are reduced or eliminated.

The method disclosed here is particularly suitable for replacing operations at production plants, where tools/machines availability is limited and time-schedules are tight.

State-of-the-art

Patent document No. JP6291886B2, which is a prior-art document close to the present invention, describes a device and method for replacing straight tubes of a shell-and-tube equipment. According to this document, the existing tube is disconnected from the tubeplates and at one end is tied to the new tube through a double-head hydraulic spindle. The new tube is pushed inwards the bundle while the existing tube, acting as a guide for the new tube, is pushed outwards the bundle on the opposite side. The two connections executed by hydraulic expansion do not guarantee a complete continuity between the two tubes and, in the case of strong efforts and friction, may not be adequately robust.

Patent Document No. EP39643221 describes a method for repairing a tube to tube-plate weld and, more specifically, a tube to tube-plate weld of butt-to-butt type. According to this method, the damaged weld and related existing exchanging tube are removed, along with a portion of the tube-plate. The document does not describe how to extract the existing tube from the tube bundle and how to insert the new tube into the tube bundle.

Patent document No. US6247231B1 describes a method for repairing, or partially replacing, an existing U-shaped exchanging tube. According to this document, a damaged portion of the existing exchanging tube, near the tube-plate, is removed; a new tube portion is inserted into the bundle and indirectly juxtaposed to the existing tube portion by means of an intermediate welding element. A welding is performed from inside the new tubular portion, on the intermediate element, to join the two tubular portions; subsequently, the new tubular portion is connected to the tube-plate. Once the repair has been executed, the intermediate welding element remains in the bundle and the repaired tube is composed of at least two tubular portions joined by circumferential welds.

Patent document No. FR2282097A1 describes a method for repairing an existing damaged exchanging tube. The method essentially consists in removing the damaged portion of the existing tube and in inserting in the existing tube a repairing stub replacing the removed portion. The stub is therefore welded to the remaining portions of the existing tube.

Consequently, the methods described in documents No. US6247231B1 and No. FR2282097A1 substantially differ from the method disclosed here since these prior-art documents do not teach that the damaged existing tube is disconnected at both ends from tube-plate(s) and do not teach how to completely remove a damaged existing tube and insert a new tube from/into the tube bundle and, more specifically, how to insert a new tube in- one-piece-made, that is without intermediate elements and welds.

Brief description of the invention

The method here disclosed is aimed to replace one or more straight heat exchanging tubes with an outside diameter preferably equal to or greater than 31,75mm and, more preferably, equal to or greater than 38,1mm, or with a thickness equivalent to or greater than 3,05mm. Exchanging tubes with relatively large outside diameter and/or thickness are typical of process gas boilers, cracked gas quenchers and water-cooled chemical reactors like methanol or formaldehyde reactors. Process gas boilers and quenchers are often of shell- and-tube type and provided with thin tube-plates usually having a thickness not thicker than 40mm, often not thicker than 35mm.

A damaged existing heat exchanging tube, according to preferred embodiments of the present method, is completely removed from the bundle and the new tube inserted into the bundle has no intermediate circular welds; in other words, the new tube once inserted into the tube bundle is preferably made in one single piece. Therefore, the method here disclosed is specifically aimed to replace an existing tube with a new tube equivalent to the existing one.

The straight heat exchanging tubes object of the present method are jointed at ends to supporting means with permanent joints. Such permanent joints can be made by welding and/or by mechanical/hydraulic expansion. Said supporting means can correspond to two tube-plates having bores in fluid communication and common longitudinal axis with the exchanging tubes. In this description, tube joints are of permanent or temporary type; permanent joints are for normal operations, whereas temporary joints are for replacing operations. In this description, the term “permanent” may be omitted. In this description, the tube to tube-plate welds are considered, by definition, as permanent joints.

The present method can be implemented regardless the permanent joint type and, more specifically, regardless the tube to tube-plate weld type. In other words, the method here disclosed fits to any tube bundle having straight tubes and any combination of permanent joints or tube to tube-plate welds.

The present method can be implemented regardless the permanent joints of the tube ends are identical or different; more specifically, the present method can be implemented regardless the two tube to tube-plate welds are identical or different.

According to preferred embodiments of the present invention, the exchanging tubes are installed in a tube bundle that is part of a shell -and-tube equipment; accordingly, said supporting means are tube-plates and said permanent joints are tube to tube-plate welds. According to preferred embodiments of the present invention, said shell-and-tube equipment are process gas boilers, cracked gas quenchers and water-cooled chemical reactors.

Essentially, No. 3 types of tube to tube-plate welds are used in process gas boilers, cracked gas quenchers and water-cooled chemical reactors of shell-and-tube type with straight tubes: a) Outside groove and/or fillet welds, executed with the exchanging tube inserted into the tube-plate bore and the welding torch placed either in front of the tube-plate on tube-side or into the tube-plate bore from tube-side. Such welds are substantially executed on the tube outside surface; b) Inside fillet welds, executed with the exchanging tube substantially juxtaposed on shell-side to the tube-plate bore entry and with the welding torch inserted into tubeplate bore from the tube-side (inner bore welding or I.B.W.). Such welds are substantially executed on the tube cross-section; c) Butt-to-butt welds, executed with the tube not inserted into the tube-plate bore and abutted to a tubular neck made on shell-side of the tube-plate and with the welding torch inserted into the tube-plate bore from tube-side (I.B.W.). Such welds are substantially executed on the two tubular butt-ends.

It is to be noted that for the weld type a): - the groove depth may span from a portion of the tube-plate thickness to the full tube-plate thickness;

- the tube-plate bore can be fully or partially engaged by the tube, at least by 5mm approx.;

- the weld can be both of groove and fillet type, for instance when the tube protrudes from the tube-side surface of tube-plate.

It is to be noted that for the weld type b) the exchanging tube can be optionally engaged in the tube-plate bore by l-2mm approx.

It is to be noted that for the weld type c) the welding torch may also be positioned on shell-side or outside the tube.

According to the method here disclosed, the existing tube acts as a guide for the new tube during the insertion into the tube bundle. However, the present method substantially differs from the method taught by the abovementioned document No. JP6291886B2 because of, according to the present method, the existing and new tubes are tied each other through at least one temporary weld of butt-to-butt type. The use of butt-to-butt welded joints, instead of spindles and hydraulically expanded joints, allows to obtain more robust joints and to guarantee complete continuous joints, reducing risks of misalignment and local stranding during the insertion of the new tube.

The method covered by this invention essentially proceeds as follows:

- disconnection of the damaged existing tube from both supporting means or tubeplates;

- tying-up of the new tube to the existing tube, either directly or through a tube stub, by a butt-to-butt welding executed from outside the new tube,

- insertion of the new tube into the tube bundle and simultaneously extraction of the existing tube from the tube bundle on the opposite bundle end;

- execution of tube to tube-plate welds related to the inserted new tube.

As an expert in the field can understand, the method object of this invention is particularly appropriate when, at least at one end of tube bundle, there is sufficient space to place the new tube in length; on the contrary, the new tube must be divided into pieces that must be butt-to-butt welded one behind the other as the insertion of the new tube proceeds. However, this last solution, although feasible, is not recommended as the new tube inserted into the bundle would not be made in a single piece but formed by tubular portions joined with each other by circumferential welds. The detailed description of the method object of this invention is accompanied by following figures:

- Fig.lA-C, where the longitudinal sections of the three major types of tube to tubeplate welds are schematically shown;

- Figs.2A-C, where the longitudinal sections relating to figures Figs.lA-C are schematically shown after disconnection of the exchanging tube from tube-plate;

- Figs.3A-G, where the longitudinal sections of a portion of tube bundle subjected to the method of the present invention, according to a preferred embodiment, are schematically shown in sequence;

- Figs.4A-G, where the longitudinal sections of a portion of tube bundle subjected to the method of the present invention, according to a preferred embodiment, are schematically shown in sequence;

- Figs.5A-F, where the longitudinal sections of a portion of tube bundle subjected to the method of the present invention, according to a preferred embodiment, are schematically shown in sequence.

Detailed description of the invention

Figures from Fig.1 A to Fig.1C schematically show the longitudinal sections of the three major types of tube to tube-plate welds related to a tube bundle and object of preferred embodiments of the present method. Fig.lA, Fig. IB and Fig.lC respectively correspond to weld types a), b) and c) described above; therefore, detailed description of Fig.lA, Fig. IB and Fig. lC is partially omitted.

Figures from Fig.1 A to Fig.1C show a portion of an existing exchanging tube (1) having a first end (4) welded to a first tube-plate (2) partially shown. The first tube-plate (2) has a tube-side surface (5) and a tube-outside or shell-side surface (6), corresponding to surfaces respectively wetted and unwetted by tube-side fluid. The existing exchanging tube (1) is straight and has common longitudinal axis (7) with tube-plate bore.

Specifically:

- Fig.lA shows a tube to tube-plate weld (3) of outside groove and/or fillet type. The existing heat exchanging tube (1) has an outside diameter equivalent to or less than the diameter of the tube-plate bore;

- Fig. IB shows a tube to tube-plate weld (8) of inside fillet type. The existing tube (1) has an outside diameter equivalent to or less than the diameter of the tube-plate bore; - Fig.1C shows a tube to tube-plate weld (9) of butt-to-butt type between a first end (4) of the existing tube (1) and a tubular neck (10) present on shell-side surface (6) of first tube-plate (2). The existing tube (1) has an internal diameter equivalent or close to the diameter of tube-plate bore.

The second end of the existing exchanging tube (1) and the second tube-plate are intentionally not shown in Figs.lA-C; the tube to tube-plate weld related to the second tube end can present any of the welds described in Figs. lA-C, executed as described above.

Figures from Fig.2A to Fig.2C schematically show the longitudinal tube-plate sections related to Figs.lA-C after disconnection of the existing tube (1) from the first tube-plate (2) by mechanical processing.

Specifically:

- Fig.2A shows the existing exchanging tube (1) having the first end (4) free, that is without tube to tube-plate weld. The weld is removed, for example, with a cutter or grinding wheel placed in front of the first tube-plate (2) on tube-side;

- Fig.2B shows the existing exchanging tube (1) having the first end (4) free, that is without tube to tube-plate weld. The weld is removed, for example, with a cutter or grinding wheel placed in the tube-plate bore of the first tube-plate (2);

- Fig.2C shows a tubular portion (12) obtained from the first tube-plate (2) by means of a cut (13). The tubular portion (12) is obtained, for example, by a cup grinding wheel and cutting the first tube-plate (2) through its thickness. The tubular portion (12) has a first end (10) corresponding to the shell-side tubular neck welded to the first end (4) of the existing tube (4) by butt-to-butt weld (9), and a free second end (11). The tubular portion (12) has preferably an outside diameter equivalent or close to the outside diameter of the existing exchanging tube (1).

The second end of the existing exchanging tube (1) and the second tube-plate are intentionally not shown in Figs.2A-C; the tube to tube-plate weld related to the second tube end can be removed as described for the first tube-plate (2).

As an expert in field knows, the disconnections related to Figs.2A-C can be executed according to several, different operations or procedures. For instance, for Fig.2C the existing tube (1) can firstly be cut from inside the tube and then a portion of the tube-plate is cut and removed along with the tube to tube-plate weld, as described by aforementioned doc. No. EP39643221; nevertheless, this last solution is not recommended since, after the removal of the tube-plate portion, an end of the existing tube is not engaged therefore involving risks of misalignment and stranding.

Figures from Fig.3A to Fig.3G schematically show, in sequence, the longitudinal sections of a portion of the tube bundle subjected to the method of the present invention according to a preferred embodiment.

Figs.3A-G refer to an exchanging tube, two tube-plates and related tube to tube-plate welds as described in Figs.lA-C and Figs.2A-C. Figs.3A-G show the second tube-plate (14), having a tube-side surface (5) and a tube-outside or shell-side surface (6), and a shell-side baffle or grid (17) provided with bores which the exchanging tubes pass through. Fig.3A shows the first end (4) and the second end (15) of the existing exchanging tube (1) welded respectively to the first and second tube-plate (2,14) by means of respectively a first tube to tube-plate weld (8), corresponding to the weld shown in Fig. IB, and a second tube to tubeplate weld (16), corresponding to the weld shown in Fig.lA. The tube-side fluid flows in tubes (1) whereas the second fluid (e.g., boiling water) flows outside tubes (1), and the two fluids indirectly exchange heat with each other.

According to Figs.3A-G, the method for replacing an existing exchanging tube (1) according to this invention proceeds as per following operations:

- Fig.3 A: the existing heat exchanging tube (1), connected to tube-plates (2, 14) by first and second welds (8,16), has a damage or defect (18) and therefore it must be replaced with a new tube, equivalent to the existing one;

- Fig.3B: the first and second tube-plate welds (8,16) are removed by mechanical processing, as described for Figs.2A-C, and the existing tube (1) is disconnected from the tube-plates (2,14);

- Fig.3C: a first end (21) of a first tube stub (19) is abutted to the first end (4) of the existing tube (1). The second end (20) of the first tube stub (19) protrudes from the tube-side surface (5) of the first tube-plate (2). Preferably, the first tube stub (19) has an outside diameter equivalent or close to the outside diameter of the existing tube (i);

- Fig.3D : a second temporary butt-to-butt weld (22) is executed, with the welding torch (not shown in figure) placed inside the first tube stub (19), at abutted ends (4,21) to tie the first tube stub (19) to the existing tube (1); subsequently, a first end (24) of the new tube (23) is abutted to the second end (20) of the first tube stub (19); - Fig.3E: a first temporary butt-to-butt weld (25) is executed, with the welding torch (not shown in figure) placed outside the new tube (23), at abutted ends (20,24) to tie the new tube (23) to the first tube stub (19);

- Fig.3F: the new tube (23) is pushed inwards the tube bundle and concurrently the existing tube (1) is extracted from the tube bundle on the opposite bundle end and removed by means of one or more cuts (26);

- Fig.3G: the new exchanging tube (23) in completely inserted into the bundle; the existing exchanging tube (1) is completely removed, and the first tube stub (19), the first and second temporary welds (22,25) and an adjacent portion (27) of the new tube (23), corresponding to the heat affected zone of the first temporary weld (25), are removed as well by cutting (26). Subsequently, length and position of the new tube (23) are adjusted, and tube to tube-plate welds (not shown in figure) related to the new tube (23) are executed.

With reference to Figs.3 A-G, the length of the first tube stub (19) is preferably such that the second end (20) thereof protrudes from the first tube-plate (2) by a slice to make practicable the first temporary welding (25) executed from outside the new tube (23); the length of the first tube stub (19) is also preferably such that the second temporary welding (22) executed from inside the stub (19) is practicable. In other words, the first tube stub (19) preferably protrudes from tube-side surface (5) of the first tube-plate (2) by a minimum amount useful to place the welding torch and execute both the first and second temporary welding (22,25). Preferably, the first tube stub (19) protrudes from tube-side surface (5) of the first tube-plate (2) by 25^ 100mm.

Said protrusion of the first tube stub (19) is also useful for subsequent outside levelling of the welding bead related to said first temporary weld (25).

Figures from Fig.4A to Fig.4G show schematically, in sequence, the longitudinal sections of a portion of the tube bundle subjected to the method of the present invention according to another preferred embodiment.

Figs.4A-G refer to the same exchanging tube, tube-plates and related tube to tube-plate welds as described in Figs.3A-G. In particular, Figs.4A-B are equivalent to Figs.3A-B; therefore, for simplicity, description of Fig.4A-B is omitted.

According to Figs.4A-G, the method for replacing an existing heat exchanging tube (1) according to this invention proceeds as per following operations:

- Fig.4A: reference is made to the equivalent Fig.3 A; - Fig.4B: reference is made to the equivalent Fig.3B;

- Fig.4C: a first end (29) of a second tube stub (28) is abutted to the second end (15) of the existing tube (1). Preferably, the second end (30) of the second tube stub (28) protrudes from tube-side surface (5) of the second tube-plate (14). Preferably, the second tube stub (28) has an outside diameter equivalent or close to the outside diameter of the existing exchanging tube (1);

- Fig.4D: a third temporary butt-to-butt weld (31) is executed, with the welding torch (not shown in figure) placed either inside or outside the second tube stub (28), at abutted ends (15,29) to tie the second tube stub (28) to the existing tube (1); subsequently, the second tube stub (28) is pushed inward the tube bundle so that the first end (4) of the existing tube (1) protrudes from the tube-side surface of first tubeplate (2);

- Fig.4E: a first end (24) of the new tube (23) is abutted to the first end (4) of the existing tube (1) and then a first temporary butt-to-butt weld (25) is executed, with the welding torch (not shown in figure) placed outside the new tube (23), at abutted ends (4,24) to tie the new tube (23) to the existing tube (1);

- Fig.4F : the new tube (23) is pushed inwards tube bundle and concurrently the second tube stub (28) and the existing tube (1) are extracted from the tube bundle on the opposite bundle end and removed by means of one or more cuts (26);

- Fig.3G: the new exchanging tube (23) is completely inserted into the bundle; the existing exchanging tube (1) is completely removed, and the temporary welds (31,25) and an adjacent portion (27) of the new tube (23), corresponding to the heat affected zone of the first temporary weld (25), are removed as well by cutting (26). Subsequently, length and position of the new tube (23) are adjusted, and tube to tubeplate welds (not shown in figure) related to the new tube (23) are executed.

With reference to Figs.4A-G, said protrusion of the first end (4) of the existing tube (1) has an extension useful for placing the welding torch outside the new tube (23) and executing the first temporary weld (25), and for subsequent levelling of said first temporary weld (25) from outside the new tube (23).

Figures from Fig.5 A to Fig.5F schematically show, in sequence, the longitudinal sections of a portion of the tube bundle subjected to the method of the present invention according to another preferred embodiment. Figs.5A-F refer to an exchanging tube, two tube-plates and related tube to tube-plate welds as described in Figs.1 A-C and Figs.2A-C. Figs.5A-F show the second tube-plate (14), having a tube-side surface (5) and a tube-outside or shell-side surface (6), and a shell-side baffle or grid (17) provided with bores which the exchanging tubes pass through. Fig.5A shows the first end (4) and the second end (15) of the existing exchanging tube (1) respectively welded to the first and second tube-plate (2,14) by means of respectively a first tube to tube-plate weld (8), corresponding to the weld shown in Fig. lA, and a second tube to tube-plate weld (16), corresponding to the weld shown in Fig. IB. The tube-side fluid flows in tubes (1) whereas the second fluid (e.g., boiling water) flows outside tubes (1); the two fluids indirectly exchange heat with each other.

According to Figs.5A-F, the method for replacing an existing heat exchanging tube (1) according to this invention proceeds as per following operations:

- Fig.5A: reference is made to Fig.3A, with the caution to invert the tube to tube-plate weld types relative to the first and second tube-plates (2,14);

- Fig.5B: reference is made to Fig.3B, with the caution to invert the tube to tube-plate weld types relative to the first and second tube-plates (2,14);

- Fig.5C: a first end (24) of the new tube (23) is abutted to the first end (4) of the existing tube (1):

- Fig.5D: a first temporary butt-to-butt weld (25) is executed, with the welding torch (not shown in figure) placed outside the new tube (23), at abutted ends (4,24) to tie the two tubes (1,23). Positioning of the welding torch and execution of the first temporary weld (25) benefit from the tube to tube-plate weld groove.

- Fig.5E: the new tube (23) is pushed inwards the tube bundle and concurrently the existing tube (1) is extracted from the opposite bundle end and removed by means of one or more cuts (26);

- Fig.5F: the new tube (23) in completely inserted into the bundle; the existing tube (1) is completely removed, and the first temporary weld (25) and an adjacent portion (27) of the new tube (23), corresponding to the heat affected zone of the first temporary weld (25), are removed as well by cutting (26); subsequently, length and position of the new tube (23) are adjusted, and tube to tube-plate welds (not shown in figure) related to the new tube (23) are executed.

With reference to Figs.3A-G, Figs.4A-G and Figs.5A-F, it is emphasized that the existing tube (1), the new tube (23) and the tube stubs (19,28) are longitudinally abutted so to have same longitudinal axis (7). As an expert in the field can understand, the method described for Figs.3 A-G, Figs.4A- G and Figs.5A-F remains conceptually valid and therefore feasible in case the first end (4) and/or the second end (15) of the existing exchanging tube (1) are connected to the tubeplates (2,14) by different welds from the ones shown in Figs.3 A-G, Figs.4A-G and Figs.5A- F. Specifically, in case one or both the tube to tube-plate welds are of type c) as described above, the method of the present invention is essentially modified as follows:

- the existing tube (1) is disconnected from tube-plate(s) (2,14) according to Fig.2C;

- the first temporary weld (25) is executed either between the first end (24) of the new tube (23) and the free end (11) of the first tubular portion (12) or between the first end (24) of the new tube (23) and the free end (20) of the first tube stub (19) having the other end (21) jointed to the free end (11) of the first tubular portion (12) by said second temporary weld (22);

- the third temporary weld (31) is executed between the first end (29) of the second tube stub (28) and the free end of the second tubular portion obtained by cutting the second tube-plate according to Fig.2C.

With reference to Figs.3A-G, the welding butts (4,21) relating to the second temporary weld (22) are preferably bevelled to obtain the relevant welding chamfer. The welding chamfer is preferably "Y" shaped with the groove inwardly facing. Therefore, the second temporary weld (22) is preferably of partial penetration, relative to the thickness of the butts, so that the tube-plate (2) is not in contact with the welding fusion. However, the two butts (4,21) must be as much continuous as possible, relative to the outside surface, to minimize the risks of local stranding and friction during replacement.

With reference to Figs.3 A-G, Figs.4A-G and Figs.5A-F, the welding butts (4,24,20,11) related to the first temporary weld (25) are preferably bevelled to obtain the relevant welding chamfer. The welding chamfer is preferably "V" or "Y" shaped with the groove outwardly facing. The first temporary welding (25) is preferably executed so that the weld (25) is of full penetration, relative to the thickness of the butts.

Consequently, the method of the present invention may include the bevelling of welding butts for the temporary welds.

Said first temporary weld (25) is preferably levelled on the outside surface, for instance with a grinding wheel, to obtain a smoothed welding bead and thus to reduce or eliminate the risk of local stranding and friction during extraction/insertion of the exchanging tubes (1,23).

Consequently, the method of the present invention may include the outside levelling of the welding bead related said first temporary weld (25).

Said first temporary weld (25) is preferably continuous; the weld is executed so that the two welding butts (4,24,20,11) are completely fused with each other on the outside surface. A complete fusion on the outside greatly reduces risks of misalignment, stranding and friction during extraction/insertion of exchanging tubes (1,23).

According to a preferred embodiment of the present invention, after the disconnection of the existing tube (1) from the supporting means or tube-plates (2,14) and at any subsequent replacing operation, the existing tube (1) can be partially pushed inwardly the tube bundle from any bundle end so that the corresponding opposite tube end protrudes from the tubeside surface of tube-plate. This may realize to be advantageous for abutment, bevelling, welding or levelling operations. Consequently, the method of the present invention may also comprise partial pushes on the existing tube (1), from any bundle end, before or after any replacing operation. However, it is emphasized that, at any phase of the present method, the existing exchanging tube (1) should always be directly or indirectly engaged in the tubeplate bores to avoid a possible misalignment or local stranding. On this regard, an expert in the field can appreciate the fact that temporarily connecting a tube stub or a guiding element (19,28) to the existing tube (1) guarantees that during replacement operations the existing tube (1) remains always engaged in the tube-plate bores.

The method disclosed by this invention remains valid in case the first and/or the second tube-plate (2,14) are provided with a protective device or layer on the tube-side surface (5), such as a grid, a plate, or such as a welding, refractory or ceramic flame spray deposit, for protecting the tube-plate from high temperature or erosion.

According to an alternative embodiment of the method here disclosed, the first and/or the second tube stubs (19,28) are connected to the ends (4,15) of the existing tube (1) or to the free end (11) of a tubular portion (12) cut (13) from the tube-plate (2,14) by one or more of following operations: welding, interlocking, mechanical/hydraulic expansion, screwing. However, it is emphasized that a connection by welding (22,31) as described in Figs.3A-G and Figs.4A-G, and in particular for the second temporary weld (22), is recommended since it offers greater strength and guarantees much better continuity between the two butts. According to an alternative embodiment of the method here disclosed, the first and/or the second tube stubs (19,28) are substituted by guiding elements like tube pilots or spindles. However, it is emphasized that a tube stub (19,28) with an outside diameter equivalent or close to the outside diameter of the existing tube (1), as described in Figs.3 A-G and Figs.4A- G, offers a greater guarantee against possible misalignments and local stranding.

With reference to Figs.3A-G, Figs.4A-G and Figs.5A-F, as an expert in the field can understand, the disconnections of the existing tube (1) from the first tube-plate (1) and the second tube-plate (14) can be done either simultaneously or one after the other. According to a preferred embodiment of the present method, one tube disconnection is made once one or more temporary welds (22,31,25) have been executed.

According to the present method, the tube to tube-plate welds related to the new tube (23) inserted in the bundle may be of different type from those related to the existing tube (1). Preferably, the tube to tube-plate welds related to the new tube (23) are of the same type as those of the existing tube (1).

Finally, according to a preferred embodiment, the method here disclosed optionally comprises one or more operations of mechanical processing on the supporting means or tubeplates (2,14). Such mechanical processing on tube-plates (2,14) is mostly aimed to:

- create a new welding groove or modify an existing welding groove for executing the tube to tube-plate weld related to said new tube (23), and/or

- create a new groove or modify an existing groove for executing said first, second or third temporary welds (25,22,31). On this regard, an expert in field can appreciate that a new or larger groove on tube-plate (2,14) may be beneficial for placing the welding torch and executing the temporary welds.

As per above, the method for replacing one or more heat exchanging tubes of a tube bundle disclosed by this invention achieves the scope to provide for practical and efficient plant maintenance and repairing operations, being characterized by following advantages:

- The method requires neither special nor bulky maintenance/repairing tools;

- The existing tube (1) works as a robust and precise guide for the new tube (23) being their tying-up completely continuous and strong, eliminating risk of misalignment and stranding at tube-plates and tube-supports bores;

- The use of temporary tube stubs (19,28) makes easier and safer replacing operations;

- The method allows to completely replace the existing tube and to insert a new tube equivalent to the original one. The method object of this invention, as conceived and described, is subject in any case to numerous modifications and variations, all attributable to the same inventive concept. In addition, all details can be replaced with technically equivalent elements. In practice, construction materials, shapes and sizes, can be of any type according to the technical requirements.

The scope of protection of this invention is defined by the attached claims.

Claims

1. Method for completely replacing one or more existing heat exchanging tubes (1) with new tubes (23) of a tube-bundle apt for an indirect heat exchange between a tube-side and a tube-outside fluid, wherein said tubes (1,23) are straight and have a first and a second end (4,15) respectively connected to first and second supporting means (2,14) by respectively a first and a second permanent joint (3,8,9,16), wherein said supporting means (2,14) have a tube-side and a tube-outside surface (5,6) respectively wetted and unwetted by said tubeside fluid, comprising following operations not necessarily in the following sequence:

- The disconnection of said existing tube (1) from said first supporting means (2) by mechanical processing;

- The disconnection of said existing tube (1) from said second supporting means (14) by mechanical processing;

- The execution of a first butt-to-butt temporary weld (25) apt to tie said existing and new tubes (1,23) each other so that said tubes (1,23) have common longitudinal axis (7), wherein the first welding butt of said first temporary weld (25) corresponds to a first end (24) of said new tube (23) and wherein the welding torch is placed outside said new tube (23);

- The push on said new tube (23) inwards the tube-bundle for inserting said new tube (23) in the bundle and concurrently for extracting said existing tube (1) from the opposite bundle end, wherein said existing tube (1) acts as a guide for said new tube (23);

- The removal of said existing tube (1) and said first temporary weld (25), extracted from the bundle, by one or more cuts (26);

- The complete insertion of said new tube (23) in the bundle;

- The execution of a permanent joint between said new tube (23) and said first supporting means (2);

- The execution of a permanent joint between said new tube (23) and said second supporting means (14).

2. Method as per claim 1, wherein said first and/or second permanent joints correspond to a permanent weld (3,8,9,16) and/or a permanent mechanical/hydraulic expansion.

3. Method as per claim 1 or 2, wherein said first and/or second supporting means correspond to a tube-plate (2,14) provided with bores in fluid communication and common longitudinal axis (7) with said exchanging tubes (1,23), and wherein said first and/or second permanent joints correspond to a tube to tube-plate weld (3,8,9,16).

4. Method as per claim 3, wherein said tube bundle is part of a shell-and-tube equipment comprising a shell enveloping the tube bundle, wherein said tube-outside surface corresponds to a shell-side surface (6) of the equipment.

5. Method as per claim 3 or 4, wherein said permanent tube to tube-plate weld (3,8,9,16) is either of groove/fillet type (3,16) executed with the welding torch placed in front of the tube-side surface (5) of tube-plate (2,14) or in the tube-plate bore and with the exchanging tube (1,23) inserted into the tube-plate bore, or of fillet type (8) executed with the welding torch placed in the tube-plate bore and with the exchanging tube (1,23) substantially juxtaposed to the tube-plate bore entry on tube-outside and optionally engaged in the tubeplate bore by l-2mm approx., or of butt-to-butt type (9) executed with the welding torch placed in the tube-plate bore or on tube-outside and with the exchanging tube (1,23) butt-to- butt juxtaposed to a tubular neck (10) located on tube-outside surface (6) of the tube-plate (2,14).

6. Method as per claim 5, wherein said disconnection comprises a cut (13) through the thickness of the first and/or the second tube-plate (2,14) respectively forming a first and/or a second tubular portion (12) having a free end (11) and the other end (10) corresponding to said tubular neck.

7. Method as per anyone of previous claims, comprising the abutment of a first end (24) of said new tube (23) to the first end (4) of said existing tube (1), wherein the second welding butt of said first butt-to-butt temporary weld (25) corresponds to the first end (4) of said existing tube (1).

8. Method as per claim 6, comprising the abutment of a first end (24) of said new tube (23) to the free end (11) of said first tubular portion (12), wherein the second welding butt of said first butt-to-butt temporary weld (25) corresponds to the free end (11) of said first tubular portion (12).

9. Method as per anyone of claims from 1 to 6, comprising the abutment and subsequent execution of a second temporary connection (22) between a first end (21) of a first guiding element (19) and either the first end (4) of said existing tube (1) or the free end (11) of said first tubular portion (12), wherein said first guiding element (19) has a second end (20) corresponding to the second welding butt of said first butt-to-butt temporary weld (25).

10. Method as per claim 9, wherein said first guiding element is a first tube stub (19).

11. Method as per claim 9, wherein said second temporary connection (22) is made by one or more of following operations: welding, mechanical/hydraulic expansion, interlocking, screwing.

12. Method as per claim 10, wherein said second temporary connection corresponds to a second butt-to-butt temporary weld (22) executed with the welding torch installed either inside or outside said first tube stub (19).

13. Method as per anyone of previous claims, comprising the abutment and subsequent execution of a third temporary connection (31) between a first end (29) of a second guiding element (28) and either the second end (15) of said existing tube (1) or the free end of said second tubular portion, wherein said second guiding element (28) has a free second end (30).

14. Method as per claim 13, wherein said second guiding element is a second tube stub (28).

15. Method as per claim 13, wherein said third temporary connection (31) is made by one or more of following operations: welding, mechanical/hydraulic expansion, interlocking, screwing.

16. Method as per claim 14, wherein said third temporary connection corresponds to a third butt-to-butt temporary weld (31) executed with the welding torch installed either inside or outside said second tube stub (28).

17. Method according to anyone of previous claims, comprising one or more mechanical processing operations on said supporting means or tube-plates (2,14) for creating new grooves or enlarging existing grooves for subsequent execution of said permanent joints related to said new tube (31) or of said temporary welds (25,22,31).

18. Method according to anyone of previous claims, comprising one or more partial pushes on said existing tube (1) inwards the tube-bundle and from any bundle end so that the corresponding opposite tube end protrudes from said tube-side surface (5).

19. Method according to anyone of previous claims, comprising the beveling of one or more welding butts (4,21,20,24,11,15,29) related to said first, second or third temporary welds (25,22,31).

20. Method according to anyone of previous claims, wherein said first temporary weld (25) is executed without leaving a crevice at the outside surface of the welding butts thereof (4,24,20,11).

21. Method according to anyone of previous claims, comprising the leveling of the welding bead of said first temporary weld (25) at the outside surface.

22. Method according to anyone of previous claims, wherein said tube-side surface (5) is provided with a protection device or layer against erosion or high temperature.

23. Method according to anyone of previous claims, wherein said removal comprises also a portion (27) of said new tube (23) adjacent said first temporary weld (25).

24. Method according to anyone of previous claims, comprising the adjustment of length and position of the new tube (23) after said complete insertion.

25. Method according to anyone of previous claims, wherein one or both of said tubular portions (12) and/or one or both of said tube stubs (19,28) have an outside diameter equivalent or close to the outside diameter of the existing tube (1).

26. Method according to anyone of previous claims, wherein said exchanging tubes (1,23) have an outside diameter equivalent to or greater than 31,75mm and/or a thickness equivalent to or greater than 3,05mm.

27. Method according to anyone of previous claims, wherein said new exchanging tube (23), after said complete insertion, has no intermediate circumferential welds.

28. Method according to anyone of previous claims, wherein said tube-bundle is part of a heat exchanger or chemical reactor.

29. Method according to claim 28, wherein said heat exchanger is a process gas boiler installed downstream a chemical reactor or a quencher installed downstream a hydrocarbon steam cracking furnace receiving a process or cracked gas at a temperature higher than 500°C.

30. Method according to claim 29, wherein said process gas boiler or quencher is of shell- and-tube type comprising one or both tube-plates (2,14) with a thickness not larger than 40mm.