WO2008065478A1

WO2008065478A1 - Process for welding special-steel tubes to a tube plate coated with a dpw28 duplex steel comprising machining of resting and welding seats in the spigots of the tube plate; heat exchanger comprising a tube bundle obtained by said process

Info

Publication number: WO2008065478A1
Application number: PCT/IB2006/054514
Authority: WO
Inventors: Fausto Foroni; Roberto Bernardi
Original assignee: Belleli Energy Cpe S.P.A.
Priority date: 2006-11-29
Filing date: 2006-11-29
Publication date: 2008-06-05

Abstract

The invention concerns a process for welding, with the Internal Bore Welding (IBW) method, duplex-steel tubes (3) to a tube plate made of carbon steel or high-yield- point steel, on whose surface provided for fitting the tubes (3) there is formed a welding coating layer of DPW28 duplex steel. On this coating layer there are formed, by mechanical machining, projecting spigots (4) forming tube resting and welding seats. According to the invention, this process comprises the steps of : a) executing said mechanical machining in the duplex-steel coating layer, to a shorter depth than the thickness of the layer and forming a retaining edge, said edge projecting upwards from the outer contour of the top of each spigot (4) and having an inner diameter larger than the outer diameter of the tube by about 0.15 mm; b) bringing one of the ends of each tube (3) into contact with said resting and welding seat and wedging it into said retaining edge; c) finally, executing the welding of said tube end to said resting seat of the spigot (4), with the IBW method. Preferably, said welding is performed in the presence of a protective gas, consisting of a mixture containing at least 90% argon and at least 1% nitrogen. The invention further concerns a heat exchanger consisting of a tube bundle manufactured with the welding method as described above.

Description

IMPROVED PROCESS FOR WELDING SPECIAL- STEEL TUBES TO A TUBE PLATE , AND HEAT EXCHANGER COMPRI S ING A TUBE BUNDLE OBTAINED BY SAI D PROCESS

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The present invention concerns a process for the fixing and welding of special-steel tubes to a tube plate of a tube bundle assembly. It is for example an "AUTOMATIC GTAW" process with no material input, with IBW (Internal Bore Welding) technology, already used in particular for heat exchangers intended for plants processing highly-corrosive and highly-pressurised liquids, such as urea manufacturing plants . The invention further concerns a heat exchanger structure which incorporates a tube bundle made of high-tensile, anti- corrosion steel, manufactured with the process according to the invention .

Tube-bundle heat exchangers of this type are widely known in the art, in particular in large steam generation plants or, as provided in the specific case of the present invention, in urea manufacturing plants, wherein a urea solution is circulated.

It is known that in these types of plants the tube bundle undergoes not only strong mechanical strains due to the high pressures at play, but also high corrosion levels owing to the aggressive liquids used here.

The Japanese companies Toyo Engineering Corporation and Sumitomo Metal Industries, Ltd. have devised a duplex stainless steel described in document EP-I .340.829-B1 and known commercially as DPW28 - which has remarkably improved features of resistance to mechanical stress and to corrosion with respect to those of previously-known materials .

As known, so-called duplex steels are hybrid chrome steels wherein the chrome content ranges between 18 and 26% and the nickel one is comprised between 4.5 and 6.5%, such quantities being insufficient for determining a fully austenitic microcrystalline structure. Almost all contain between 2.5 and 3% molybdenum. The main properties are: specific microcrystalline structure, known precisely as duplex, austenitic and ferritic, which imparts greater resistance to breakages caused by stress corrosion; higher degree of passivation, due to the larger chrome content (and to the presence of molybdenum) ; good weldability and forgeability; high resistance to traction and to yield stress .

The steel marketed as DPW28 is a duplex steel with improved features, especially as concerns resistance to corrosion, in particularly aggressive environments . The present invention addresses the problem of how to obtain a tube bundle using tubes made of DPW28 duplex steel. As a matter of fact, it has been evidenced that by using a steel of the above- mentioned DPW28 type, the anti-corrosion features of the material improve dramatically, but at the same time welding becomes increasingly difficult.

In a previous Italian patent no. 1.200.406, filed on 8.3.1985 by the same Applicant, reference whereto can be made for better understanding of the invention, it is suggested to manufacture a tube plate consisting of a thick stainless steel plate, equipped with a plurality of through-holes, in correspondence of each whereof the tubes of the tube bundle are anchored by means of IBW (Internal Bore Welding) welding technique.

More precisely, according to what is set forth in this patent, for manufacturing a tube bundle of a heavy-duty heat exchanger, it is suggested to use a carbon-steel tube plate - which, as known, has strong features of mechanical resistance, in addition to other technical and economic advantages over stainless steel - and ferritic stainless-steel tubes, which have better anti-corrosion features. In order to overcome the fact that the welding of the ferritic stainless- steel tubes to the carbon-steel tube plate - due to the different nature of these materials - displays serious joint safety problems, up to the impossibility of carrying out said welding, the above-mentioned patent no. 1.200.406 suggests a welding process consisting in: applying a welding coating of ferritic stainless steel on the surface of the tube plate; executing the simultaneous machine drilling of the tube plate and of said coating layer; forming by machining the spigots provided by the IBW method for anchoring the tubes; and in welding with the IBW method the tube ends to the housings of said spigots . With this process the welding is performed on same-type materials - coating layer and tubes, both of ferritic stainless steel - and hence with no problems of incompatibility between the materials .

The main object of the present invention now consists in the suggestion of a process capable of allowing, in particular, the perfect welding of DPW28 steel tubes to a carbon-steel or high-yield- point-steel tube plate, provided with a coating layer equally made of DPW28. This object is achieved through a development of the technology described in the above-mentioned patent no. 1.200.406, whose features are mentioned in the characterising portion of claim 1. According to a first aspect of the invention, firstly a high precision in the fitting of the tubes to the plate must be achieved, which is accomplished by the special shape of the housings on the connection spigots; moreover, a perfectly even welding line must be achieved, as free from defects as possible, which may allow the infiltration of corrosive liquids from one tube assembly chamber to the other, which perfection is achieved through an accurate definition of the welding parameters .

Further features and advantages of the invention are in any case more evident from the following detailed description of some preferred embodiments, given purely by way of a non-limiting example and illustrated with reference to the accompanying drawings, wherein: fig. 1 is a plan view of a tube plate portion, with holes for the tubes arranged quincunx-wise; fig. 2 is a section view according to line II-II of fig. 1 showing the shape of a spigot for fitting a tube; fig. 3 is a partial perspective view of an area of the tube plate, with spigots in sight and tubes welded to a part of the spigots; and fig. 4 is an enlarged diagrammatic section of the welding area between the tube and its respective spigot of the tube plate.

Figs. 1 to 3 show a tube plate 1 of a heat exchanger, in particular an exchanger (not shown, since fully known per se) intended for urea processing plants, wherein the plate undergoes strong stresses both in terms of mechanical stresses - since the liquid flowing within the exchanger may reach pressures in the order of 150 bar - and of corrosion, since the same liquids are chemically very aggressive.

As can be detected in fig. 1, the plate is equipped with a plurality of holes 2 arranged quincunx-wise, to each whereof a tube of the heat exchanger is welded.

Plate 1 is made of thick - for example up to 300 mm - carbon-steel or high-yield-point steel in order to withstand the strong mechanical stresses it experiences. On the surface of this plate, whereto the tubes of the tube bundle are to be welded, a welding coating layer is applied (with the above-mentioned technique described in Italian patent no. 1.200.406, and whereto reference is made for a deeper understanding of the present invention) . This layer and the tubes of the tube bundle both consist, as said, of DPW28 duplex steel.

Fig. 2 shows, in a section II-II according to the tube axis, that this tube 3 is welded with its lower end (with respect to the drawing) on the top of a spigot 4. This spigot is entirely made of DPW28 duplex steel, being formed by machining the welding coating layer, without this machining affecting the underlying thickness of the carbon-steel tube plate. As a matter of fact, the machining is carried out by removing part of the welding coating layer found in the gap between one spigot and the other, by a shorter height than the thickness of such a layer. Fig. 3 shows the same tube plate of figs. 1 and 2, in a perspective view; from all these drawings a first fundamental feature of the present invention is clear, i.e. that all the surfaces of the spigot and of the opposite tube end are finely machined to form, in particular, on the spigot side, a retaining edge of the tube to be welded and, on the tube side, a contour of the end thereof which fits snugly into said edge. More precisely, this seat consists of a planar resting surface 4a, surrounded by a retaining edge 4b. This retaining edge 4b has an inner diameter substantially equal to the outer diameter of the end of tubes 3, so that, when such tubes are brought to rest on surface 4a, in order to be welded, they are practically wedged in said edge 4b. This edge has a radial thickness and an axial height, sufficient to ensure a stable retaining action of the respective tube: for example, the edge has a radial thickness in the order of 1.0 mm, and a height in the order of 1.25 mm According to a possible embodiment, given here purely by way of example, tube 3, hole 2 in the tube plate, and spigot 4 have the following dimensions :

Tube material DP 28W

Tube outer diameter mm 25.4 Radial thickness of tube wall mm 2.7

Hole diameter in the plate and in the spigot mm 20.0 Spigot radial thickness mm 3.3

Retaining edge radial thickness mm 1.0

Retaining edge axial height mm 1.25 The welding is performed, as already said, with the IBW (Internal Bore Welding) technique or method, by introducing a welding torch 5 upwards into hole 2, and by fusion-welding the material making up both tube 3 and spigot 4, with no further material input.

According to a further important aspect of the present invention, the welding is performed, after washing with a degreasing solvent, in the presence of a protective gas and respecting the following parameters :

EXAMPLE I

Type of process: AUTOMATIC GTAW (IBW technology) Protective gas to the torch: mixture of 98% argon and 2% nitrogen Flow (litres/min) : from 6 to 10

Gas outside the tube 99, 99% nitrogen

Flow (litres/min) : from 18 to 23

Moreover, the importance of maintaining the features of power supply to the welding torch within the following parameters has been ascertained:

Polarity of current: DC

Current min: A from 50 to 65 max: A from 120 to 140 Tension V from 10 to 13

Starting current A from 35 to 45

Final current A from 8 to 12 A

Initial rising time sec from 3 to 5

Final dropping time sec from 4 to 6 Pre-flow time sec from 10 to 15

Finally, the importance of guaranteeing the following physical parameters of the welding has been ascertained: Type of tungsten to the torch EW Ce-2 Tungsten size mm 2.4 Torch tip distance from the tube wall mm 1.5

Tungsten distance from the tube support plane mm 1.0

Cord type RECTILINEAR Ceramic diameter mm from 10 to 12

Welding time sec. from 55 to 65

No. of pulses per revolution no. from 110 to 130 CHECK OF THE PHYSICAL CHARACTERISTICS OF THE WELDING

After having effected the welding in the above-mentioned operating conditions, a welding check through axial sectioning of the tube and of its respective spigot has been carried out, detecting a substantial welding continuity and uniformity, with the following parameters, referred to the drawing of fig. 4:

Thickness reduction, neck area "g" : mm 2.9 to 3.0 Thickness increase, widening area "h" : mm 0.3 to 0.4 which fall within perfectly acceptable limits from a quality point of view .

The above-described embodiment has allowed to obtain welding joints qualifying as "perfect" in 100% of the tubes welded according to the invention. The different welding checks have produced the results listed below: CHECK I

A welding quality check has been carried out by means of penetrant liquids as follows :

- brush application of penetrant liquid NEOSOL - penetration time 20 min.

- removal of penetrant liquid through water

- spray application of the crack detector

- air drying

- detection time 7 min. - reading time 30 min.

RESULT: no cracks detected.

CHECK II

A welding quality check has been performed by means of X-rays on 11 samples, as follows: - welding thickness: 2.4 mm

- welding type : IBW method

- photographic film: individual KODAK M, size 3.5 x 12

- radiation source: Ir. 192

- Power: KW 0.2 - 0.6 - Intensity: Cl

- focal spot : 0.5 x 0.5 mm

- exposure time: 15 sec. - minimum radiographic sensitivity: W14/0.16, W15/0.125, W16/0.10 - centre depth: 0.26, 0.35, 0.52

RESULT: all 11 samples display a welding classified as "perfect" CHECK III

A welding seal test by means of the tensile test to breaking point has been executed, with the following result: Sample no. 1 (tube) Sample no. 2 (tube)

Outer radius 12.7 mm Outer radius 12.7 mm

Inner radius 10.0 mm Inner radius 10.0 mm area 192.5 mm area 192.5 mm temperature +2O⁰C temperature +2O⁰C Breakage Pos . BW Breakage Pos . BW

Breakage T. S. 951 MPA Breakage T. S. 935 MPA

Total breakage 183,000 N Total breakage 180,000 N CHECK IV A macrographic and visual test of the welding has been performed, which has highlighted a welding profile as appears from the diagrammatic section of fig. 4, wherein the following measurements can be seen:

Sample 1 g = 2.9/3.0 mm h = 0.3/0.4 mm

Sample 2 g = 2.9 mm h = 0.3 mm

Sample 4 g = 2.9/2.9 mm h = 0.3/0.3 mm Sample 5 g = 2.9/3.0 mm h = 0.3/0.3 mm

Sample 7 g = 2.9/2.9 mm h = 0.3/0.3 mm

Sample 8 g = 2.9/3.0 mm h = 0.3/0.4 mm

Sample 11 g = 3.1 mm h = 0.3 mm

RESULT: fully satisfactory welding structure CHECK V

A HUEY TEST has been performed on the welding; with a demanded value <= 0.15 gr/m .h the following values have been read for subsequent corrosion cycles:

1st = 0.0000 2nd = 0.0544

3rd = 0.0325

4th = 0.0143

5th = 0.0210 average = 0.0244 The results are hence more than satisfactory. Nevertheless, it can be assumed that, during production, when welding thousands of tubes, some defective tube may occur and hence the need for repairs may arise. The invention hence provides the opportunity of performing a repair welding by refusion; in this case, the same welding parameters seen earlier are used, however, with the following variant:

EXAMPLE II

All the other parameters unchanged, the distance of the torch tip from the tube wall is brought to mm 1.2 The welding check, by axial sectioning of the tube and of the respective spigot thereof, has allowed to detect still a substantial welding continuity and uniformity, with the parameters being substantially unchanged over those of the original welding (always referred to the drawing of fig. 4) : Thickness reduction, neck area "g" : mm 2.9

Thickness increase, widening area "h" : mm 0.5

Despite the care with which this second refusion welding is performed, the welding of one of the tubes may still not be perfect.

According to the present invention, in this case it is still possible to execute a third refusion welding; the same welding parameters are then used as seen in connection with example I, however, with the following variant:

EXAMPLE III

All the other parameters remaining unchanged, the distance of the tungsten tip from the tube support plane is brought to mm 1.5

The different checks of these welding lines performed by refusion, carried out in the conditions seen for the check of the original welding, have given substantially equivalent results . However, it is intended that the invention must not be considered limited to the specific conditions illustrated above, which represent only an example embodiment thereof, but that various changes are possible, all within the reach of a person skilled in the field, without departing from the scope of protection of the invention, as defined in the following claims .

Claims

1) Improved welding process, with IBW technology, of duplex steel tubes to a tube plate made of carbon-steel or of high-yield-point steel, comprising the steps of: a) depositing, on the surface of the tube plate provided for the fitting of the tubes, a welding coating layer made of DPW28 duplex steel, b) executing a first mechanical machining to drill the tube plate, with holes running through both said plate, and said coating layer of DPW28 duplex steel, mutually coupled, wherein

- the holes are arranged quincunx-wise, and

- the inner diameter of the holes is equal to the inner diameter of the tubes which will form the tube bundle; c) executing a second mechanical machining of the tube plate surface surrounding each hole, in order to form spigots projecting from said surface and representing the seat on which said tube rests and is welded to, wherein said resting seats comprise, for each tube, a planar, annular, resting surface of the tube lower end, characterised in that d) said second machining

- is performed in the DPW28 duplex steel coating layer, at a smaller depth than the layer thickness,

- produce a rectified seat in the form of a retaining edge projecting upwards from the outer profile of each of said planar, annular seats, and having an inner diameter larger than the outer diameter of the tube by about 0.15 mm e) the end of each tube is brought in contact with the resting and welding seat of the respective spigot thereof and wedged into said retaining edge, and f) the welding of said tube end is finally performed to said spigot resting seat, with the IBW method.

2) Improved welding process as claimed in claim 1), characterised in that the welding is performed after washing with a degreasing solvent . 3) Improved welding process as claimed in claim 1) or 2), characterised in that the welding is performed in the presence of a protective gas, consisting of a mixture containing at least 90% argon and at least 1% nitrogen

4) Improved welding process as claimed in claim 3), characterised in that the welding is performed in the presence of a protective gas, observing the following gas flow parameters :

Protective gas: 98% argon and 2% nitrogen mixture

Flow (litres/min) : from 6 to 10

Gas outside the tube 99, 99% nitrogen

Flow (litres/min) : from 18 to 23

5) Improved welding process as claimed in any one of the preceding claims, characterised in that it is performed without material input, with a power welding torch observing the following parameters :

Current polarity: D

Current min : A from 50 to 65 max : A from 120 to 140

Tension V from 10 to 13

Starting current A from 35 to 45

Final current A from 8 to 12

Initial rising time sec from 3 to 5 Final dropping time sec from 4 to 6 Pre-flow time sec from 10 to 15

6) Improved welding process as claimed in any one of the preceding claims, characterised in that the welding is performed keeping the torch tip at a distance not below 1 mm from the tube wall.

7) Improved welding process as claimed in claim 6), characterised in that the welding is performed observing the following physical parameters of the welding:

Tungsten type to the torch EW Ce-2

Tungsten size mm 2.4

Torch tip distance from the tube wall mm 1.5

Tungsten distance from the tube resting plane mm 1.0

Cord type RECTILINEAR

Hole radius mm from 10 to 12

Welding time sec. from 55 to 65

No. of pulses per revolution no. from 110 to 130

8) Improved welding process as claimed in any one of the preceding claims, characterised in that, following the welding operation, it comprises a step of checking for the correctness of the welding comprising a radiographic check.

9) Improved welding process as claimed in claims I)-I), characterised in that, following the welding operation, it comprises a step of checking for the correctness of the welding comprising a check with penetrant liquids.

10) Improved welding process as claimed in claims I)-I), characterised in that, following the welding operation, it comprises a step of checking for the correctness of the welding comprising a tensile test and/or a HUEY TEST.

11) Improved welding process as claimed in claims 8) -10), characterised in that, following said step of checking for the correctness of the welding and in case at least one of said welding joints is found to be incorrect, it comprises one or more further welding steps, with refusion.

12) Improved welding process as claimed in claim 7) and 11), taken in combination with one another, characterised in that a further welding step, with refusion, is performed by bringing the welding torch close to the tube inner wall.

13) Improved welding process as claimed in claim 12), characterised in that said close distance is of 1.2 mm.

14) Heat exchanger characterised in that it comprises a tube bundle manufactured with the improved welding process as claimed in any one of the preceding claims.

15) Heat exchanger as claimed in claim 14) , of the type wherein:

- the tube plate consists of a support plate made of carbon-steel or high-yield-point steel, with a duplex-steel welding coating layer placed on top thereof,

- said tube plate has a plurality of holes running across both the carbon-steel plate, and said duplex-steel layer, the inner diameter of the holes being equal to the inner diameter of the tubes to be anchored to the tube plate; - said tube plate further has a plurality of spigots, surrounding each hole, each spigot ending above with a tube resting and welding seat, shaped as a planar, annular surface, characterised in that

- said tube resting and welding seat comprises a retaining edge projecting upwards from the outer periphery of said annular seat of the spigot and having a inner diameter larger than the outer diameter of the tube by about 0.15 mm and

- said tubes are wedged into the respective resting seat thereof and retained there by said edge, before said welding. 16) Heat exchanger as claimed in claim 15) , wherein said retaining edge has, before the welding, a radial thickness of 1.0 mm and an axial height of 1.25 mm.