WO2008068550A1

WO2008068550A1 - Method for isolating or sealing a leaky tube

Info

Publication number: WO2008068550A1
Application number: PCT/IB2006/054622
Authority: WO
Inventors: Fausto Foroni
Original assignee: Belleli Energy Cpe S.P.A.
Priority date: 2006-12-06
Filing date: 2006-12-06
Publication date: 2008-06-12

Abstract

The invention concerns a method for isolating a special-steel tube (6) of a tube bundle, anchored to a tube plate (1) of a tube bundle assembly. When leaks are detected in a tube (6), the following repair steps are provided: a) introducing into said tube (6), from the lower side of the tube plate (1), a stopper (9) shaped as a hollow cylindrical body ending above into an ogive, whose outer diameter is substantially equal to the inner diameter of the hole (4) in said tube plate (1); b) pushing said ogival stopper (9) through the thickness of the tube plate (1), and into the tube (6); c) introducing a welding torch (10) into said stopper (9) and performing a welding from within, in a preset position below the leak (8), but in any case in correspondence of said upper layer (3) of anti-corrosion steel of tube plate (1).

Description

METHOD FOR ISOLATING OR SEALING A LEAKY TUBE

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The object of the present invention is a method for isolating a steel tube welded onto a tube plate of a tube bundle assembly, when leaks through said tube and/or the welding thereof are detected.

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

In a previous Italian patent no. 1.200.406, filed on 8.03.1985 by the same Applicant, reference whereto is suggested for a deeper understanding of the invention, it is suggested to manufacture a tube plate consisting of a thick stainless steel plate, provided with a plurality of through- holes, in correspondence of each of which the tubes of the tube bundle are anchored by IBW (Internal Bore Welding) welding. The improvement of manufacturing technologies for heat exchangers draws researchers to find new devices, capable of extending the life of these plants. In this specific case, researchers have focussed on a system capable of preventing highly corrosive substances from penetrating inside the tube plate and encroaching on the integrity thereof.

In this respect, the Applicant, with the technology described in a parallel PCT patent application no. IB2006/054576 of 4.12.06, reference whereto can be made for a deeper understanding, has also devised a process for solving the problems connected to the defective welding lines in tube plates thus manufactured, which provides cutting the defective welding by means of a suitable tool, and subsequently rebuilding the spigot by means of another suitable tool. However, as can be understood by reading the above application, this process may be used only during testing when the tube bundle is not yet mounted in the heat exchanger of the plant it is intended for. However, it is easy to understand that failures may occur also during operation: as a matter of fact, the flow of highly corrosive substances through the tube bundle of high- pressure plants can cause tube corrosion and weaken mechanical resistance, even causing cracks along the individual tubes.

It is equally easy to understand that the entry of corrosive liquid in the tubes is extremely harmful to the tube plate of the bundle, and hence to the entire heat exchanger.

The presence of cracks along the individual tubes, therefore, calls for maintenance operations aimed at protecting the plant from possible corrosion risks, by isolating the individual tubes of the tube plate which display cracks, reducing to a minimum intervention times and invasiveness .

The known art currently provides a method for isolating the cracked tubes in a heat exchanger based whereon widening of the hole in the tube plate where the leak has been detected is carried out, until it is possible to introduce a tube stub of the same size and quality of the tubes of the tube bundle. After having introduced the tube stub in the tube plate hole as far as the original joint between the tube and the tube plate, welding thereof from within is performed by means of a torch. Subsequently, the tube is stopped by a base at the lower end thereof. The base is made of the same quality of material as the tubes of the tube bundle.

However, such a system is complex, difficult to implement - for example due to the fact that the boring is particularly heavy and expensive - and does not allow to establish if the isolating operation will have a satisfactory result over time, since no inspections, X-ray or volumetric checks employing ultrasound can be made.

The object of the present invention is hence to suggest a method to accomplish, both during manufacture and during testing, but especially during an operation halt, the isolation or stopping of the cracked tubes of a tube plate in a fully non invasive and quick manner, allowing to rule out the risk of corrosion of the base structure of the tube plate, and thereby guaranteeing a longer life of the heat exchanger .

A further object is to suggest an isolation or stopping method of a damaged tube of a tube plate, which allows the person in charge of the checks to carry out the inspections required to ensure an accurate check of the isolation operations just executed, so as to guarantee good isolation over time; as a matter of fact a volumetric check of the joint through ultrasound checks can be carried out.

Said objects are obtained through a method for isolating a damaged tube, belonging to the tube plate, which follows the steps set out in claim 1) . Some specific preferred features, in order to better accomplish the above- mentioned method, are described in the dependent claims.

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 shown in the accompanying drawings, wherein: fig. 1 is a section view of a tube welded onto a tube plate, wherein a crack in the lower portion of the tube is highlighted, immediately above the welding line; fig. 2 is an axial-section view of an insert or stopper for the actuation of the method according to the invention; fig. 3 is a section view which shows the first step of the method according to the invention, i.e. the occurred introduction of the stopper into the cracked tube; fig. 4 is an axial-section view similar to that of fig. 3 which shows the second step of the method according to the invention, i.e. the welding of the stopper through a welding lance; fig. 5 is a section view similar to fig. 3 which shows the third and last step of the method according to the invention, i.e. the inspection through a probe for ultrasound check.

In figs. 1 to 4 the detail of an element of a heat exchanger is shown, wherein tube plate 1 undergoes strong stress both in terms of the mechanical stresses and of corrosion, since the same liquids are under great pressure and highly aggressive from a chemical point of view.

Tube plate 1 consists of a thick carbon steel plate 2, whereon a layer 3 of anti-corrosion steel, for example duplex steel, or steel known on the market under the code DPW28

(Toyo - Sumitomo) has been formed. Tube plate 1 provides a plurality of holes 4, at the top of each whereof, i.e. on the side of layer 3, a spigot 5 for the coupling of tube 6 is obtained.

As can be seen in the picture, tube 6 is of such a size that the inner surface thereof is perfectly aligned (exactly the same diameter) with that of hole 4 in tube plate 1.

Moreover, in correspondence of spigot 5, the tube is welded at 7 to tube plate 1.

When a crack 8 is produced in tube 6 due to the heavy working conditions, a liquid having strong corrosive properties could leak through 8 from the outer chamber above the surface I of tube plate 1 inside hole 4, and to the internal chamber, or distributor, below surface I of the tube plate .

According to the invention, stopper 9, obtained from a forged piece of a size capable of being inserted into hole 4 and into tube 6, is manufactured with the same quality of material as the tube bundle and consists of a cylindrical- base, hollow body, which has at the top an ogival closure 9a. The outer size of stopper 9 fully match the diameter of hole 4 and, consequently, that of tube 6; thereby it is possible to snug fit stopper 9 into hole 4 of tube plate 1 and into tube 6.

The method provides some simple steps: once the person in charge of the checks has identified the defective tube, he accesses tube plate 1 from the below and introduces the stopper or stopper 9, forcing slightly, into the tube plate, and partly into tube 6 which is cracked (see fig. 3) up to above crack 8;

- once stopper 9 has been brought into position, the operator introduces into the same a welding lance 10 (fig. 4) to form a welding line 7' similar to welding line 7; lance 10 is preferably equipped with means capable of easing the downward outflow of gases, so as to prevent the protection gas regularly blown in during the welding operation from stagnating in the chamber resulting between the ogival portion 9a of stopper 9 and lance 10; once welding 7' has been completed, in order to facilitate accurate inspection, which is capable of giving certain results, the last step of the method provides that the person in charge of the checks introduces along the hollow portion of stopper 9 a probe 11, which measures the width of welding line 7'. This allows to assess whether the welding line has been actually carried out according to the correct parameters and, in particular, whether a welding at least as wide as the tube thickness has been achieved, or thicker than said tube; this is the parameter to guarantee that the life of the stopper is at least the same as the duration of tube 6.

When all the necessary checks have been carried out, in order to ascertain the correct arrangement of the new welding, and consequently the occurred isolation of the cracked tube from the rest of the tube plate, the apparatus is again fit for use according to the normal protocols, with the exception of the stopped tube 6. As can be easily understood, the desired object has been achieved, in particular an isolation method of a special steel tube on a tube plate of a tube bundle assembly, which is accurate, efficient, quickly made and guaranteed over time . The method proves particularly advantageous in the case of carbon-steel tube plates coated with a coating layer made of a steel highly resistant to corrosion, according to the method described in the already mentioned patent no. 1.200.406 or in PCT application no. IB2006/054514 by the same Applicant.

However, it is intended that the invention is not to be considered limited to the specific arrangement illustrated above, which represents only an example embodiment thereof, but that various changes are possible, all within the reach of a person skilled in the field, without having to depart from the scope of protection of the invention, as defined in the following claims.

Claims

1) Method for isolating a special-steel tube (6) of a tube bundle, anchored to a tube plate (1) of a tube bundle assembly, of the type wherein the tube plate (1) consists of a carbon-steel plate (2) and of an upper layer (3) of steel highly resistant to corrosion, in particular for exchangers intended for processing plants of highly corrosive and highly-pressurised liquids, when leaks have been detected in said tube (6), which cannot be repaired otherwise, characterised in that it provides the steps of:

- introducing into said tube (6), from the lower side of the tube plate (1), a stopper (9) having the shape of a cylindrical hollow body closed at the upper end thereof, whose outer diameter is substantially equal to the internal diameter of the hole (4) in said tube plate;

- pushing said stopper (9) through the thickness of the tube plate (1), as far as the inside of the tube (6);

- introducing into said stopper (9) a welding torch (10), and performing a welding from within, in a preset position below the leak (8), but in any case in correspondence of said upper layer (3) of anti-corrosion steel.

2) Method as claimed in claim 1), characterised in that on said torch (10) means are provided to allow the downward release of the gas. 3) Method as claimed in claim 1) or 2), characterised in that it comprises a step of checking the seal of the stopper by means of a probe (11) to inspect the width of the effected welding.

4) Method as claimed in claim 3), characterised in that said check is accomplished through an ultrasound probe.

5) Stopper (9) to effect the method according to any one of claims 1) to 4), characterised in that it consists of a cylindrical forged piece of the same quality of material as the tube of the tube bundle. 6) Stopper as claimed in claim 5) , characterised in that the outer surface thereof has a shape and size allowing the snug fitting and the introduction thereof in contact with the inner wall of the hole (4) of the tube plate and of the respective tube (6) to be isolated.

7) Stopper as claimed in claim 5) or 6), characterised in that it consists of a cylindrical hollow body ending above into an ogive.