METHOD AND CUTTING TOOL FOR CUTTING A TUBE BUTT-WELDED TO A TUBE PLATE AND FOR RECONSTRUCTING THE WELDING SEAT

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The present invention concerns a method for cutting a special steel tube butt-welded to a tube plate of a tube bundle assembly, in particular for exchangers intended for plants processing highly corrosive and highly-pressurised liquids. The method also comprises the reconstruction of the tube welding seat on the tube plate. Tube-bundle heat exchangers 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.

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

In the previous Italian patent No. 1,200,406, filed on 8.03.1985 by the same Applicant, reference whereto can be made for a 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 tube bundle tubes are anchored by IBW welding (Internal Bore Welding) . More precisely, according to what has been set forth in this patent, in order to manufacture 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, as well as other technical and economic advantages over stainless steel - and tubes made of ferritic, stainless steel which have better anti-corrosion features. Welding the ferritic, stainless-steel tubes to the carbon-steel tube plate carries serious joint safety problems, due to the different nature of these materials, if not even the impossibility of carrying out said welding; in order to overcome this problem, a welding procedure providing to build a ferritic, stainless- steel welding coating on the surface of the tube plate was suggested in the above-mentioned patent no. 1,200,406., such

procedure performing the machine-drilling of the tube plate and of said coating layer at the same time, and subsequently machining the spigots provided by the IBW method for anchoring the tubes; and finally to weld by the IBW method the tube ends to the seats of such spigots. Welding between same-type materials - welding coating and tubes, both of ferritic, stainless steel - is thereby obtained and hence with no material incompatibility problems.

Starting from the new invention by the Japanese companies Toyo Engineering Corporation and Sumitomo Metal Industries, Ltd, which allowed to devise a duplex stainless steel described in document EP-I, 340, 829 and marketed as DPW28 - displaying resistance features to mechanical stresses and to corrosion which are remarkably better than those of the previously known materials, the Applicant, with the technique described in a PCT patent application no. IB2006/054514 of 29.11.06, parallel to the present application, and whereto reference can be made for further details, has completed a process capable of allowing the perfect welding of DPW28 steel tubes to a carbon-steel tube plate, equipped with a welding coating layer equally made of DPW28.

However, in order for this process to give the desired result, it is firstly necessary to achieve a high mounting precision of the tubes on the plate, and secondly a perfectly even welding, as free as possible from imperfections which may allow corrosive liquids to seep in from one chamber to the other of the tube bundle. The two requirements are met by suitably shaping the seats on the anchoring spigots and by accurately defining the welding parameters. In addition thereto, the spigots are obtained by machining affecting only a part of the thickness of the DPW28 duplex steel which makes up the welding coating layer, without said machining hence affecting the underlying thickness of the carbon-steel tube plate . Now, during tube bundle testing, after thousands of welding operations have been carried out, and before the installation of said tube bundle testing in the plant, it is possible to detect that the welding is not perfect in 100% of

cases. Since there is sometimes the risk of seepage of even small amounts of the liquids in the plant, it is necessary to correct welding imperfections. It is therefore the object of the present invention to suggest a repair method which - by cutting the welding - allows to replace the incorrectly welded tube. The method according to the invention allows to perform this repair possibly even after a previous repair attempt by refusion, as reported in the above-mentioned PCT application no. IB2006/054514 of 29.11.06 parallel to the present application.

This object is achieved through the features described in claim 1, according whereto it is possible to cut the welding between the tube and the tube plate, to remove the incorrectly welded tube and to subsequently work on the tube plate to rebuild a suitable resting seat of a new tube and to make it integral with the tube plate through a new welding.

A further object of the invention is to provide a cutting tool capable of suitably executing the above-mentioned cutting method and of redefining the anchoring seat of a tube on the tube plate which has undergone the above-mentioned cutting method. This further object is achieved through features reported in claim 9.

Further features and advantages of the invention are in any case more evident from the following detailed description of a preferred embodiment, given purely by way of a non- limiting example and illustrated in the accompanying drawings, wherein : fig. 1 is a side view, partly in axial section, of the cutting tool used with the method according to the invention; fig. 2 is a diagram of the cutting tool, which shows in plan the eccentric position of the cutting mill displacement; fig. 3 is a diagram of the cutting tool, which shows in plan the position of the cutting mill centred with respect to the mandrel axis; fig. 4 is a perspective view intended to show the defective welding between a tube and the main body of a tube plate; fig. 5 is similar to the view of fig. 4, wherein a tube

being cut by the cutting tool is shown in the initial cutting stages; fig. 6 is a view similar to those of figs. 4 and 5, at the end of the cutting operation, with the cutting tool still arranged inside the anchoring spigot of the tube plate; fig. 7 is a side view of the tool for the reconstruction of the tube anchoring seat, protruding from the anchoring spigot of the cut tube; and fig. 8 is a perspective view, which illustrates the reconstruction operation of the tube anchoring seat.

As can be observed in fig. 1, the cutting tool consists of an essentially tubular mandrel 1 snugly-guided into hole 2 of tube plate P, in correspondence whereof the welding cutting and separation operation of respective tube 3 must be performed. Mandrel 1 has its axis coinciding with axis X-X of hole 2 and tube 3, and is snugly-guided both axially and rotationally within said hole 2.

Inside mandrel 1 there is formed a hole Ia having its axis

Y-Y parallel but eccentric to axis X-X of hole 2; inside such hole Ia a cylinder 4 is rotatably, snugly-housed. Said cylinder

4 houses in turn an axial through-hole 4a having an axis Z-Z parallel but eccentric to axis Y-Y. Inside hole 4a there is rotatably housed a shaft 5 which bears a mill 6 at the top thereof protruding from mandrel 1. This construction practically represents an eccentric mounting, with variable eccentricity, of mill 6 on mandrel 1, as better described in the following.

During operation, as can be easily observed in figs. 1 to

3, mill 6 and its shaft 5 initially lie in a position wherein the axis of shaft 5 coincides with the axis X-X of the mandrel; this position (fig. 3) is such that the mill contour lies entirely inside the contour of mandrel 1. The mandrel can therefore be snugly-fitted, upwards, into hole 2 of tube plate

P, without the mill contour interfering with the walls of hole 2.

Mandrel 1 can then be pushed into hole 2 until mill 6 arrives at the welding area 7 of tube 3 on anchoring spigot 8 of tube plate P (fig. 1) . As can be understood, the arrangement

is such that, in this initial home position, whenever shaft 5 of mill 6 is brought into rotation, said mill rotates freely inside tube 3, with no interference with the inner wall thereof . In order to perform the cutting of tube 3 (see figs. 1, 2, 5 and 6) , tubular body 4 is caused to rotate, for example by 180°, about the axis Y-Y thereof. In this rotation the axis of shaft 5, due to the effect of the eccentric position thereof with respect to Y-Y, progressively moves - with a radial movement component - from the original position A, centred on axis X-X (fig. 3), to and into position B (fig. 2 and, shown by a discontinued line in fig. 3) wherein the axis thereof coincides with axis Z-Z shown in figs. 1 and 2; in this radial movement, the sharp edge of mill 6 progressively interferes with the wall of tube 3 (as can be seen in fig. 1) , until it cuts it, producing an initial slit (see figs. 5 and 6)

The 180° displacement of body 4 is driven by a wheel 9 integral with body 4; the run is limited due to the engagement between a semi-circular, slot-like slit 10, formed on the same wheel 9, and a stopping pin 11, borne by a second wheel 12.

When the mill has reached position B and carried out the initial cutting, it is then possible to drive the rotation of mandrel 1 within hole 2, by acting on wheel 12. This rotation occurs about axis X-X and mill 6, always rotating about itself, also moves circumferentially on a path of about 360°, consequently causing the slit formed in the inner wall of tube 3 to advance evenly; this occurs until it allows the full detachment of tube 3 from tube plate P (see figs. 5, 6) . In actual fact, mill 6 lies, instant by instant, in contact, with constant force, with subsequent points of the wall of tube 3. The distributed cutting action thereby gives an accurate result in a short time.

Once the operator was able to remove tube 3 from the tube plate (fig. 6), he removes the cutting tool from hole 2 in the plate P. The tube plate is now free from tube 3, while coupling spigot 8, despite displaying a sharp cut, does not display instead the surface arranged as a joining step, as in the original arrangement, allowing the introduction of tube 3 in

the welding seat.

In order to allow the formation of a step-shaped joining seat, capable of allowing the tube introduction and welding operations, the present invention therefore provides the use of another tool; as a matter of fact, the operator introduces into the same hole 2 of plate P, as a replacement of cutting tool 1, 4, 5, 6, the tool for the reconstruction of the welding seat, consisting of elements 11, 14, 16, as shown in figs. 7 and 8.

Similarly to the mill cutting tool 6, the reconstruction tool provides a guiding body 11, snugly engaged with hole 2, within which a mandrel shaft 15 rotates, carrying a knife 16.

The mounting of these element occurs as follows: firstly, the snugly guiding body 11 is introduced upwards into hole 2, causing it to protrude from the side of spigot 8 together with mandrel shaft 15 ending with a head 15a supporting knife 16. Mandrel shaft 15, with corresponding head 15a, when devoid of knife 16, is of such a size that it can be introduced into mandrel body 11 either after or before the introduction of mandrel 11 into hole 2; it is introduced so as to rotate freely, but snugly on body 11. It must be appreciated that, unlike the eccentric mill tool 5, 6, this mandrel 15 pivots centred on the same axis of guiding body 11.

On supporting head 15a of mandrel 15, projecting from hole 2, knife 16 is mounted, oriented perpendicularly to the mandrel axis; through two subsequent positioning operations, this knife then allows to reconstruct the step seat at the top of the spigot, fully identical to the original seat. More precisely, during a first step knife 16 is arranged in a widely- protruding, first working position, wherein it engages with the entire radial width of the edge of spigot 8, in order to rectify the spigot surface and create an annular resting plane. During a subsequent step, knife 16 is then mounted in a recessed, second working position in order to engage only with the innermost portion of the radial width of the spigot, in order to obtain a step-shaped seat on the edge of spigot 8, said step-shaped seat being capable of allowing the subsequent snug coupling of the tube with the tube plate.

At the end of the operations, after having released knife

16 from the tool and having removed the tool from hole 2, the operator is again capable of snugly arranging a new tube 3 on the welding seat shaped stepwise at the top of spigot 8 of tube plate and of then providing to the relevant welding. Such a method implies the reduction of the axial height of the anchoring spigot with respect to the plane of plate P, by a value comprised between 0 and 3 mm, and usually not above 2 mm. Such values are in any case fully sufficient to allow the welder to perform a good welding and to the controller to complete the necessary joint checks, such as X-rays, seeping liquids, inner and outer visual tests of the joint, such as in the case of the original welding.

The desired objects have thus been achieved, i.e. a quick, accurate and inexpensive method has been obtained for cutting a welding and for reconstructing the anchoring seat of a tube 3, in a heat exchanger, such that - at the end of the reconstruction process - a fully working tube plate can be obtained.

A tool for the cutting of a tube welded to a tube plate has further been manufactured, which is capable of allowing the accurate and quick cutting of an individual tube on the tube plate, and with a modest economic effort. The tool thus manufactured further allows accurate and easy cutting, without the operator being forced to prise the tube or the tube plate. The method described is usually applicable during the only manufacturing step or final test step of the exchanger; however, it can be applied also on the building site, during a step in which the exchanger is stationary, although more rarely and in particular on the outer rows of the tubes, the only ones granting easy access.

It is intended, however, that the invention is not to be considered limited to the particular arrangement illustrated above, which represents only an example embodiment thereof, but that different variants 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 .