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(11) | EP 1 559 983 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method for forming a welded connection between a tubesheet and a number of tubes and a device produced by such method |
(57) The invention relates to a method for forming a connection between a tubesheet and
a number of tubes and a device manufactured according to such a method, the method
comprising:
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[0001] The present invention according to a first aspect relates to a method for performing a welded connection between a tubesheet and a number of tubes.
[0002] According to a second aspect the present invention relates to a device produced by the method according to the invention. Such a device finds application in e.g. heat exchangers. However, a useful application in other technical fields is not excluded.
[0003] In related heat exchangers there is an exchange in heat between the medium that runs through the tubes and the medium flowing at the outer side of the tubes. To ensure that these two media are separated from each other, adequate tube to tubesheet connections have to be executed. Such connections can for instance be obtained by expanding the tube ends in bores provided in the tubesheet. Usually such connections are liquid tight when proper expansion parameters are applied. However they are not guaranteed gas tight.
[0004] In those cases in which the usage of the heat exchanger requires extra certainty that the tube-tubesheet connections will be leak tight, tubes are welded to the tubesheet, whereby before or after the welding also expanding of the tubes may be executed.
[0005] An important characteristic of the welded connection is that the welds must provide sufficient leak path. The person skilled in the art will be familiar with the term leak path, being the shortest way of the end of the crevice between tube and bore to the medium inside the tube seen through the weld.
[0006] The weld-connections provided by the prior art methods are characterised by the fact that the weld-connection is made by melting the edge of the tubesheet-borezone together with the relatively thin tube-ends, which operation is very critical in terms of amongst others a) fulfilment of specified minimal leak path at all weldpositions, b) avoiding damage or penetration of the tube end by the welding-arc, c) avoiding the blocking of a centering-device, used for automated welding, at removal due to a narrowing of the tube diameter caused by the weld.
[0007] The invention is aimed at providing a method for forming a connection between a tubesheet and a tube, wherein the welding operation is far less critical. This object is reached by the method according to the invention which comprises:
- providing a tubesheet with a first surface and a second surface and a number of circular bores extending from the first surface to the second surface,
- providing a number of tubes of a pliable material with an exterior diameter, which enables the tubes to be placed in a bore,
- placing each tube in a bore, such that the tube protrudes on the side of the first surface,
- flaring the end of the protruded portion of each tube located at the side of the first surface of the sheet, thus forming a number of flares,
- making a welded connection between the tubesheet and each tube at the circumference of each flare.
[0008] In the method according to the invention the weld arc is aimed on the circumference of the flare and tubesheet itself, several millimeters away from the edge of the tubesheet-bore zone. This makes the welding operation far less critical.
[0009] In the method according to the invention the leak path is freely and better controllable opposed to the traditional welding details. This because the size of the welded connection is freely controllable as the weld arc is aimed at the circumference of the flare and the tubesheet itself.
[0010] Due to the fact that the weld-arc is now aimed at the circumference of the flare and the tubesheet, damage or penetration of the tube-end is less likely to occur. Also the possibility of a centering device getting blocked in the tube is avoided, as the welded connection does not influence the tube diameter.
[0011] Furthermore, the new method according to the invention provides benefits for execution of re-tubing. Re-tubing is some times executed to replace leaking tubes of a heat exchanger. Such leakage may occur after prolonged operation of the heat exchanger. Especially when operated under harsh conditions (e.g. very corrosive media).
[0012] In the re-tubing execution of the tubes connected to the tubesheet with the traditional weld details it is often necessary to clean up the bores due to local contamination that easily may influence the welding process in a negative way. Such clean up is normally executed by grinding the surface of the bores. Thus the diameter of the bores may run out of tolerance which influences the fit of the new tubing.
[0013] With the method according to the present invention retubing of a heat exchanger does not require cleaning of the bores in order to remove contaminations. It is sufficient to grind the surface of the tubesheet to remove any contamination which might be present.
[0014] In the method according to the invention a tubesheet with a first surface and a second surface is provided. The tubesheet can be formed by any material which can be subjected to a welding operation and which has suitable properties for the intended use. A selection of a suitable material is within the knowledge of the skilled person. Examples of suitable materials can be selected from metals and metal alloys, such as (stainless) steel, nickel (alloys), copper (alloys), titanium (alloys), zirconium. The material of the tubesheet may be cladded with a material which is more corrosion resistant then the base material. Suitable cladding materials may me selected from the group consisting of stainless steel, nickel (alloys), copper (alloys), titanium (alloys).
[0015] The tubesheet is provided with a number of circular bores extending from the first surface to the second surface. Within this specification and the claims it is to be understood that the wording "a number of" refers to at least one, viz. one or more, in all instances that it is used. The bores can be provided by any suitable technique known to the skilled person such as drilling. Preferably the bore has a diameter which enables standard tubing to be placed in the bore. The selection of suitable diameters of the bore in connection with the tubing used is within the knowledge of the skilled person. Suitable guides are given in tables RCB-7.41 and RCB-7.41 M of the eight edition of the standards of the Tubular Exchanger Manufacturers Association.
[0016] The number of tubes provided in the method according to the invention have an exterior diameter, which enables the tube to be placed in a bore. This can be either with a tight fit, a loose fit, or a temperature dependent fit. A temperature dependant fit can be reached by cooling the pipe prior to fitting it into the bore, or heating the tubesheet prior to fitting the tube into the bore. Preferably the tube is of a type which is commercially readily available. It is to be understood that the invention is not limited to the use of a specific type of tube.
[0017] The provided tubes are made from a pliable material, which enables them to be flared at their ends. Any pliable material can be used, as long as it can be subjected to a welding operation. The selection of suitable materials is within the knowledge of the skilled person. Examples of suitable materials can be selected from the group comprising metals, metal alloys, such as (stainless) steel, nickel (alloys), copper (alloys), titanium (alloys), zirconium.
[0018] Flaring of the end of the protruded portion of the tube can be done by any suitable methods known to the skilled person, e.g. by using a flaring tool. Preferably the flares are made by hydrolic pressing or forming.
[0019] It must be understood that if the other end of a tube is freely movable through the bore, flaring can be done before a tube is placed in the bore. If the other end of a tube does not permit passage through the bore, flaring will be done after the tube is placed in a bore.
[0020] The welded connection between the tubesheet and each tube, is made at the circumference of each flare. The welded connection can be made by using known welding techniques and equipment. It is possible to make the welded connection in a single passage of the weld arc around the circumference of the flare. During such a single welding passage the material of the tube and the tubesheet is melted together. During such a single welding passage preferably welding material is deposited to strengthen the welded connection. In addition to this in the method according to the present invention deposition of weld material will increase the leak path. However, such deposition of weld material is not strictly obligatory since for some applications the melting operation will provide a welded connection with sufficient strength and leak path.
[0021] It is also possible to use multiple welding passages, such as two or three or more welding passages. Such additional welding passages can be used to further strengthen the welded connection. When multiple welding passages are used, all welding passages can be similar or different welding modes can be used. For instance, when two welding passages are used, the first welding passage can be used to prepare the path for the second welding passage by e.g. evening it. In such a case the first welding passage only melts the material of the tubesheet and the tube together, and none or little filler material is deposited. During the second subsequent welding passage, welding material is deposited.
[0022] After the second welding passage it is possible to use a third welding passage in order to melt the deposited welding material in order to improve the weld shape. The use of such a third weld passage may provide a welded connection with a flattened surface. If electric welding is used preferably the third welding passage is then performed with a higher voltage than the previous welding passages.
[0023] According to a preferred embodiment of the method according to the invention, the flares are provided with a trumpet shape or a cone shape. It is also possible that the form of the flare is a combination of a cone shape and a trumpet shape. In such a case the trumpet shape is located closer to the circumference of the flare, then the cone shape. The trumpet shape or cone shape or the combined trumpet-cone shape of the flare provides a smooth flow of a fluid medium when accessing or leaving the pipe.
[0024] According to a different preferred embodiment of the method according to the invention the end of the bore at the first surface is rounded and/or has the shape of a cone. Such a shape of the bore end facilitated flaring the tube in a trumpet shape, a cone shape or a combined trumpet-cone shape.
[0025] When the provided tube has a loose fit in the bore, it is preferred to expand the tube to fit tightly in at least a part of the bore, prior to performing the flaring operation on the end of the tube. The expanded tight fit of the tube in at least a part of the bore fixates the tube in the bore which is beneficial for the flaring operation.
[0026] In a preferred embodiment of the method according to the invention a groove is located in the surface of a bore, which groove extends from the first surface in the axial direction of the bore for at least a portion of the axial length of the bore. In this case the tube has a tight fit in the bore, in the part where the groove is located, over a distance shorter then the axial length of the groove. The groove can be made in any form. A suitable form for making the groove is by making a screw thread. This because making a screw thread is a technique which is well known and for which many suitable devices are available. The function of the groove is to provide an open channel for a fluid between the second surface and the first surface despite the tight fit of the pipe in at least a part of the bore over a distance shorter then the axial length of the groove.
[0027] The presence of a groove in the surface of the bore prevents blowing up of the weld material during welding, especially when closing the circular weld, as there remains an open channel through which any enclosed gas can escape. The groove also makes it possible to conduct pressure using Non-Destructive Testing (NDT) to test if the welded connection of the tube to the sheet is sufficient leak tight. Examples of such pressure using NDT methods are the so called "soap test", a Helium leak test, and a pressure test. As these NDT methods are known to the skilled person they will be discussed below only briefly.
[0028] A suitable NDT method is the soap test in which a light air-over pressure is made around the tubes on the side of the second surface. The welds are than provided with a soap solution. On leaking, soap bubbles will develop which show the leak position.
[0029] Another suitable NDT is the He-leak test. In this test a light over-pressure of Helium gas is applied around the tubes on the side of the second surface. Subsequently the welded connections are sniffed with special equipment, which is able to detect break trough of the very penetrable helium.
[0030] A third suitable NDT is the pressure test. With this test water pressure is applied around the tubes on the side of the second surface. The level of the water pressure is related to the design pressure of the tube-tubesheet assembly. During the test the pressure is monitored and leakage is detected by any observed pressure drop event or any visual leakage.
[0031] After performing the pressure-using NDT method the remaining part of the tube not having a tight fit in the bore may be fitted tightly in the bore. This can be done by e.g. expanding the part of the tube not having a tight fit to fit tightly in the bore. During the expanding operation care is taken to avoid expanding of any part of the tube located outside the bore. In practice this can be achieved by not expanding the remaining 2-7 mm e.g. 5 mm of the tube located in the bore near the second surface. Hereby the channel formed by the groove is closed off. This provides additional sealing of the tube to tubesheet connection.
[0032] When more than one pipe and more than one bore are provided, it is beneficial if the bores are provided at such a distance of each other, that when the ends of the pipes are flared, the circumferences of the flares come in such proximity of each other that the welded connection at adjacent flares overlap. In this way a continuous surface is formed by the flares and the welded connections. Such a continuous surface forms a coverage over the tubesheet and can have the function of a cladding layer. Thus in applications in which the use of a cladded material for the tubesheet is suitable, the cladding layer may be formed by the continuous surface formed by the flares of the tubes and the welded connections. Any part of the tubesheet not covered by the continuous surface may be cladded in the usual way, or otherwise covered with a protective layer.
[0033] In order to form a smooth surface, the continuous surface formed by the flares and the welded connections is ground to a smooth surface and preferably subsequently polished. The presence of such a smooth continuous surface is beneficial in special applications wherein it is undesired if surface imperfections e.g. crevices etc. are present on the surface of the tubesheet. This e.g. provides a better septic quality of the tubesheet and in particular the tubesheet-bore-zone.
[0034] According to another aspect the present invention relates to a device comprising:
- a tubesheet with a first surface and a second surface and a number of circular bores extending from the first surface to the second surface,
- a number of tubes of a pliable material placed in each bore, wherein each tube has a flared portion on the side of the first surface, which flared portion is welded at its circumference to the first surface of the sheet.
[0035] The device according to the invention finds application in heat exchangers, although application in other technical fields is not excluded.
[0036] According to a preferred embodiment of the device according to the invention, the flares are provided with a trumpet shape or a cone shape. It is also possible that the form of the flare is a combination of a cone shape and a trumpet shape. In such a case the trumpet shape is located closer to the circumference of the flare, than the cone shape. The trumpet shape or cone shape or the combined trumpet-cone shape of the flare provides a smooth flow of a fluid medium when accessing or leaving the tube.
[0037] According to a different preferred embodiment of the device according to the invention the end of the bore at the first surface is rounded and/or has the shape of a cone. Such a shape of the bore end facilitates flaring the tube in a trumpet shape, a cone shape or a combined trumpet-cone shape.
[0038] It is preferred that the tube has a tight fit in at least a part of the bore. The tight fit of the tube in at least a part of the bore fixates the tube in the bore which is beneficial for the flaring operation. Furthermore, the tight fit provides additional sealing of the tube to tubesheet connection.
[0039] In a preferred embodiment of the device according to the invention a groove is located in the surface of the bore, which groove extends from the first surface in the axial direction of the bore for at least a portion of the axial length of the bore. In this case the tube can have a tight fit in the bore, in the part where the groove is located, over a distance shorter then the axial length of the groove. The function of the groove is to provide an open channel for a fluid between the second surface and the first surface despite the tight fit of the pipe in at least a part of the bore over a distance shorter then the axial length of the groove.
[0040] The presence of such a groove in the surface of the bore makes it possible to conduct pressure using Non-Destructive Testing (NDT) to test if the welded connection of the tube to the tubesheet is sufficient leak tight. Examples of such pressure using NDT methods have been discussed above.
[0041] It will be evident to the skilled person that the embodiment of the device according to the invention wherein there remains an open channel between the first surface and the second surface is not (optimally) suited for use in a heat exchanger. However, this embodiment is an important intermediate state in the production of a heat exchanger on which pressure using NDT methods can be performed. In order to make this embodiment of the device suitable for the application in a heat exchanger it is of importance to ensure that the tube has a tight fit over substantially the full axial length of the bore. If the tight fit of the tube in the bore is obtained by expanding the tube in the bore, in practice the remaining 2-7 mm e.g. 5 mm of the tube located in the bore near the second surface will remain unexpanded to prevent that any part of the tube located outside the bore is expanded.
[0042] The groove can have any form. A suitable form of the groove is a screw thread. This because making a screw thread is a technique which is well known and for which many suitable devices are available.
[0043] In a preferred embodiment of the device according to the invention more then one bore and more then one tube are provided and the welded connections between adjacent flares overlap. This embodiment of the device according to the invention comprises a continuous surface formed by the flares and the welded connections. Such a continuous surface forms a coverage over the tubesheet and can have the function of a cladding layer. Thus in application in which the use of a cladded material for the tubesheet is suitable, the cladding layer may be formed by the continuous surface formed by the flares of the tubes and the welded connections. Any part of the tubesheet not covered by the continuous surface may be cladded in the usual way, or otherwise covered with a protective layer.
[0044] Preferably the continuous surface is an even and smooth surface. Such an even and smooth surface is beneficial in special applications wherein it is undesired if surface imperfections such as crevices etc. are present on the surface of the tubesheet. This e.g. provides a better septic quality of the tubesheet, in particular the tubesheet-bore-zone.
[0045] The invention will be further explained with reference to the figures showing non-limiting embodiments of the invention.
Figures 1A-1D show cross sectional overviews of welded tubesheet to tube connections according to the prior art.
Figures 2A-2C show cross sectional overviews of different stages of the method according to the invention.
Figure 3 shows a cross sectional overview along the line III-III in fig. 4 of a preferred embodiment of the device according the invention.
Figure 4 shows a plan overview of a preferred embodiment of the device according to the invention.
[0046] Figure 1A shows a cross sectional overview of a so called "seal weld" known from the art. This figure shows a tube 1 located in a bore of a tubesheet 2. The end 3 of the tube 1 is melted together with the tubesheet 2 by the weld arc 4. In this weld detail the weld 5 is formed by a single passage of the weld arc, whereby mostly just a little amount of weld-filler material is added or none at all. Although this weld detail is popular because of the relatively low costs for machining, it has the disadvantage that the weld penetration is relatively small, which makes that the leak path is in principle also relatively small.
[0047] Figure 1B shows an alternative "seal weld". In this alternative the tubesheet 2 is provided with a circular incision 6 around the bore. The incision 6 ensures a larger weld penetration. Moreover it effects that the tubesheet gets less change on distortion by weld-shrinkage-forces, due to the fact that these forces more easily can be reduced by relaxation. The appliance of a stringent set of weld parameters such as velocity, current, voltage stays very important to ensure that a sufficient leak path is obtained.
[0048] Figure 1C shows a so called "strength weld". The tube 1 is located in the bore of a tubesheet 2. The end of the bore is enlarged relative to the rest of the bore. The tube 1 is fitted into the bore up to the enlarged section of the bore. The tube end 3 is melted together with the tubesheet by the weld arc 4 under suppliance of weld-filler-material by supply means 7. Usually the welding is performed by two weld passages, of which formation of the root layer 8 is the most critical part of the welding process. The benefit of this weld detail opposed to those shown in figures 1A and 1B, is that the leak path can be better controlled.
[0049] Figure 1D shows a so called "fillet weld" which is only applicable for relatively thick tubes, to avoid that the weld arc punctures the wall of the tube. The tube 1 is located in the bore of the tubesheet 2, such that it protrudes with its end 3 above the second surface 11 of the tubesheet 2. Again the end of the bore of the tubesheet 2 is enlarged and the weld connections 8,9 are made in two welding passages in the enlarged part of the bore. Here too formation of the root layer 8 is the most critical part of the welding process.
[0050] Now turning to a preferred embodiments of the present invention, figure 2A shows a tubesheet 2 with a first surface 10 and a second surface 11. The tubesheet 2 is provided with a bore 12, in which a screw thread 13, serving as a groove, is formed. The screw thread 13 starts from the first surface 10 and extends in the axial direction of the bore 12 towards the second surface 11. The axial length of the screw thread is shorter than the axial length of the bore. A tube 1 with an external diameter, that enables it to be placed in the bore 12 with a loose fit, is provided and placed in the bore by moving it in the direction of the arrow.
[0051] Figure 2B shows the position of the tube 1 in the bore 12 of the tubesheet 2, after it has been expanded to fit tightly in a portion of the bore, which portion is shorter then the axial length of the groove 13. As shown, the end 3 of the tube 1 protrudes above the first surface 10 of the tubesheet 2. The end of the bore in this situation is relatively unworked. However, it may be rounded of or have a trumpet, cone or a combined trumpet-cone shape. As the tube 1 remains to have a loose fitting in the bore on the side nearer to the second surface 11, there remains a connection 14 with the groove 13. Thus there is a free channel for passage of a fluid going from the second surface to the first surface 10.
[0052] Figure 2C shows the next step in the method according to the invention. As is shown, the protruding end 3 of the tube 1 is flared, whereby a flare 20 is formed. At the circumference 21 of the flare, the welded connection is made with the weld arc 4 under suppliance of filler material by supply means 7.
[0053] Figure 3 shows a cross sectional overview along the lines III-III as shown in figure 4 of a preferred embodiment of the device according to the invention. As is shown the remaining unexpanded part of the tube is expanded to provide a tight fit of the tube 1 in the bore 12. Thereby the connection 14 is closed off. As is also shown, the bores 12 in the tubesheet 2 are provided at a distance such that the flares 20 come in such proximity of each other that adjacent welds 22 overlap. Thus, a continuous surface is formed by the flares 20 and the overlapping welds 22. This surface may be ground to an even surface and further polished to a smooth surface (not shown in figure 3) .
[0054] Figure 4 shows a plan overview of a preferred embodiment of the device according to the invention. The device contains multiple tubes 1 connected to a tubesheet 2. At the first surface 10 of the tubesheet 2, a continuous surface 25, which is slightly elevated, is formed by the flares and the welded connections. The continuous surface 25 is ground and polished to an even and smooth surface. The continuous surface 25 may serve as a cladding layer, as it fully covers the first surface 10 of the tube plate 2 located directly underneath it. The remaining uncovered part of the first surface may be covered by a different material to protect it from corrosion. The device shown in figure 4 maybe applied in a heat exchanger.
1. Method for forming a connection between a tubesheet and a number of tubes comprising:
- providing a tubesheet with a first surface and a second surface and a number of circular bores extending from the first surface to the second surface,
- providing a number of tubes of a pliable material with an exterior diameter, which enables each tube to be placed in a bore,
- placing each tube in a bore, such that the tube protrudes on the side of the first surface,
- flaring the end of the protruded portion of each tube located at the side of the first surface of the sheet, thus forming a number of flares,
- making a welded connection between the tubesheet and each pipe at the circumference of each flare.
2. Method according to claim 1, wherein a number of flares have a trumpet shape.
3. Method according to claim 1, wherein a number of flares have a cone shape.
4. Method according to claims 1-3, wherein a number of flares have a form which is a
combination of a cone shape and a trumpet shape, the trumpet shape being located closer
to the circumference of the flare.
5. Method according to claims 1-4, wherein the end of a number of bores at the first
surface is rounded and/or has the shape of a cone.
6. Method according to claims 1-5, wherein each tube is expanded to fit tightly in at
least a part of each bore, prior to the flaring of the tube.
7. Method according to claim 6, wherein a groove is located in the surface of a number
of bores, which groove extends from the first surface in the axial direction of the
bore for at least a portion of the axial length of the bore, and wherein a tube is
expanded over a distance shorter then the axial length of the groove located in the
bore wherein it is fitted.
8. Method according to claim 7, wherein a number of grooves have the form of a screw
thread.
9. Method according to claims 7-8 further comprising testing a welded connection by means
of a pressure using non-destructive testing method, such as a non-destructive testing
method selected from the group comprising a soap test, a He-leak test, a water pressure
test.
10. Method according to claim 9, further comprising expanding any remaining unexpanded
part of a number of tubes over substantially the full axial length of the bore wherein
a tube is fitted, after the testing of each welded connection.
11. Method according to claims 1-10, wherein more then one pipe and more then one bore
are provided, which bores are provided at a distance of each other such that when
the pipes are flared, the circumferences of the flares come in such proximity of each
other that the welded connections at adjacent flares overlap.
12. Method according to claims 1-11, further comprising grinding the surface formed by
each flare and each weld to a smooth surface and preferably polishing the smooth surface.
13. Device comprising:
- a tubesheet with a first surface and a second surface and a number of circular bores extending from the first surface to the second surface,
- a number of tubes of a pliable material placed in each bore, wherein each tube has a flared portion on the side of the first surface, which flared portion is weld-connected at its circumference to the first surface of the sheet.
14. Device according to claim 13, wherein a number of flares have a trumpet shape.
15. Device according to claim 13, wherein a number of flares have a cone shape.
16. Device according to claims 13-15, wherein a number of flares have a form which is
a combination of a cone shape and a trumpet shape, the trumpet shape being located
closer to the circumference of the flare.
17. Device according to claims 13-16, wherein the end of a number of bores at the first
surface is rounded and/or has the shape of a cone.
18. Device according to claims 13-17, wherein a groove is located in the surface of a
number of bores, which groove extends from the first surface in the axial direction
of a bore for at least a portion of the axial length of the bore.
19. Device according to claim 18, wherein a number of grooves have the form of a screw
thread.
20. Device according to claims 13-19, wherein a number of tubes have a tight fit in at
least a part of the bore, wherein the tube is placed.
21. Device according to claim 18, wherein a number of tubes have a tight fit in the bore,
wherein the tube is placed, over a distance shorter then the axial length of the groove.
22. Device according to claims 13-21, wherein more then one bore and more then one tube
are provided and wherein the welded connections between adjacent flares overlap.
23. Device according to claims 13-22, wherein the surface formed by each flare and each
weld is ground to an even surface and wherein preferable the even surface is polished
to a smooth surface.