EP0267349B1 - Assembly by welding of tube plates in heat exchangers comprising solid titanium tube plates - Google Patents

Abstract

1. Mounting of tube plates at the end of a carbon steel shell of a tubular heat exchanger, each tube plate having, on the side fastened to the shell, a carbon steel part and, on the side exposed to corrosion, a titanium part to which the titanium tubes are rolled in and welded, characterized in that the actual tube plate (2) is produced from solid titanium and is fastened to the carbon steel shell (1) via a steel piece (15) welded by explosion or an equivalent method to the tube plate (2) which is itself welded to the shell (1).

Description

The present invention relates to the assembly of tubular plates at the end of a carbon steel sleeve of a tubular heat exchanger according to the preamble of claim 1.

These exchangers require both perfect sealing of the tube / plate assembly and resistance to corrosion.

To this end, it is known, as will be seen below with reference to FIGS. 1 to 5 of the drawings, to use tubes welded on a tubular plate which can be either solid in titanium or in carbon steel plated or coated with titanium, due to the impossibility of directly welding titanium to steel.

Depending on the case, the tube plate is either welded to the body of the exchanger (Figures 1, 2 and 5), or clamped between flanges (Figures 3 and 4).

Such an exchanger is known from document US-A 2 868 513.

The tubular plate is made of massive steel. To protect it from the side of the water box, a titanium plate is simply applied against the tube plate. The tubes of the exchanger are linked to the steel plate by expansion and at the same time serve as a support for the noble metal armor plate. Therefore, the expansion of the tubes only ensures the seal between the body (inside) of the device and the outside because there is no particular seal between the steel plate and the titanium shielding. . The seal between the water box and the steel plate is provided by welds (designated 11) between the periphery of the tubes and the noble metal shield.

This construction has the disadvantage that it is not possible to locate by pneumatic tests or by helium tests the defective bolts, due to the lack of particular sealing between the steel plate and the noble metal shielding. . This drawback is serious if we consider the fact that the aforementioned checks and locations must take place after complete assembly as well as in operation.

The above controls are part of a series of controls currently imposed on the quality of nuclear constructions.

For this reason alone, the assembly described in this previous publication would not be accepted by the quality control departments of nuclear power plants.

The tightness between the water box and the steel plate being ensured by the sole welds of the tubes to the shielding is constantly threatened by the brittleness of said welds. The shielding plate is linked to the tubes by these weld beads and is thus subjected on one side to the pressure of the water box and on the other side to atmospheric pressure. The deformation of the steel plate is transmitted to the armor plate by said welds which are thus constantly stressed. In addition, if the noble metal plate does not perfectly match the flatness of the armor plate, it also tends to deform under the effect of the thrust due to the pressure in the water box, due to the difference pressure between the body of the device and the outside atmosphere.

• - la figure 1 relative illustre un assemblage de manchette et de plaque tubulaire au niveau de la jonction entre la manchette et le fond de l'échangeur de chaleur;
• - la figure 2 montre un détail de l'assemblage de la figure 1 avec un point de corrosion;
• - la figure 3 illustre une variante d'assemblage de l'art antérieur;
• - la figure 4 donne une vue d'ensemble d'un condenseur de l'art antérieur;
• - la figure 5 illustre une autre variante de l'assemblage de l'art antérieur.

In the drawings relating to the prior art:

• - Figure 1 relating to an assembly of cuff and tube plate at the junction between the cuff and the bottom of the heat exchanger;
• - Figure 2 shows a detail of the assembly of Figure 1 with a point of corrosion;
• - Figure 3 illustrates an alternative assembly of the prior art;
• - Figure 4 gives an overview of a condenser of the prior art;
• - Figure 5 illustrates another variant of the assembly of the prior art.

In these different figures, the same reference notations designate the same elements.

In FIG. 1, the reference notation 1 designates a carbon steel sleeve, 2 designates a tubular plate also made of carbon steel, this plate being traversed by a multitude of tubes 3 made of titanium. These also pass through another tubular plate similar to the other end of the cuff. The cuff and the tubular plates delimit an enclosure B which receives the steam entering from the end coming from a turbine. The titanium tubes 3 are traversed by a fluid such as cold sea water entering through a compartment C. In contact with the tubes 3, the vapor condenses and the condensate leaves at the bottom of the enclosure B. The seawater heated by this same contact goes into the other bottom compartment C of the condenser from where it is discharged, as seen in Figure 4.

The various parts of the assembly are welded together, the welds being designated 4. On the side where it is in contact with seawater, the tube plate 2 is plated with titanium 5, the plating in titanium being produced by explosion. and removed on the periphery of the plate ..

At this periphery, the protection of carbon steel against corrosion accelerated by the galvanic couple between carbon steel and titanium is ensured by a coating 6 which ensures, on the corrosive fluid side (side C), the insulation best possible between the part covered with titanium and carbon steel.

The correct application of a protective coating on carbon steel and up to titanium is a difficult operation, the satisfactory result of which cannot be ensured with perfect certainty and which requires regular inspection.

A covering defect located in the transition zone marked A (see Figure 2) quickly leads to significant and localized corrosion with perforation. This causes pollution of enclosure B by the fluid circulating in the enclosure.

The known technique for overcoming this problem, a technique illustrated in FIG. 3, consists in using a massive titanium plate 2 clamped between flanges 7, 8. This titanium plate 2 cannot in fact be welded to carbon steel. the sleeve 1, the flanges 7, 8 and the bottom of the condenser.

In this way the covering or protective coating 6 relates only to the wall la of the enclosure C and any defect would only result in an outward leak without pollution of the enclosure B. On the other hand, this solution requires the placement of a seal between the flange 7 of the cuff 1 and the solid plate 2 and another seal 10 between the latter and the flange 8 of the bottom 1a of the condenser.

The joint is made of a special material such as braided asbestos or a synthetic resin loaded with glass fibers. The seal 10 is made of a more ordinary material such as synthetic rubber or neoprene.

The seal 9, once in place, and the straight tubes being fixed to the two plates of the apparatus, can no longer be replaced as can be seen in the overview which is in FIG. 4.

In this same FIG. 4, in addition to the elements already described, a entry into force 11, a condensate outlet 12, a seawater inlet 13 and a seawater outlet 14 can be distinguished.

Variations in the sealing of the seals over time can cause either leaks to the outside or air inlets.

Another technique known to remedy the aforementioned difficulties is disclosed in EP-A 0 123 940. It also uses a massive titanium tubular plate and is applicable both in cases where the tubular plate is welded to the cuff and in cases where the same plate is attached to it by flanges. At the connection points between the tube plate and the sleeve or the bottom of the condenser, the connection is made by means of a titanium plate welded to the sleeve or the bottom of the condenser by explosion. The tubular plate is fixed to it by welding on the side of the steam enclosure B or on both sides as illustrated in FIG. 5 where the titanium-titanium welds are designated 5a.

This solution has the drawback that the junction between the cladding 5 and the cuff 1 is stressed when torn off and sheared, and that when the cuff consists of four elements, their connection is difficult to achieve. The invention aims to overcome these drawbacks and to propose an assembly of the tubular plates at the end of a carbon steel sleeve of a tubular heat exchanger ensuring high security.

The invention starts, according to the preamble of claim 1, from said document US-A 2 868 513 and it is characterized in that the tubular plate itself is made of massive titanium and is fixed to the carbon steel sleeve by through a steel part welded to the tube plate by explosion or the equivalent, which is itself welded to the cuff.

Claim 2 describes a particular embodiment of the invention.

• - la figure 6 illustre une forme de réalisation d'un assemblage conforme à l'invention; et
• - la figure 7 donne une vue d'ensemble d'un condenseur comportant un tel assemblage.

The invention will be described below in more detail using a preferred embodiment and Figures 6 and 7 of the drawings where

• - Figure 6 illustrates an embodiment of an assembly according to the invention; and
• - Figure 7 gives an overview of a condenser comprising such an assembly.

As shown in FIG. 6, the massive titanium plate 2 is provided with a carbon steel sheet 15 previously plated by explosion.

The tubes are connected to each plate 2 by swaging and welding designated 20.

Plate 2-15 is used as a flange and is assembled by welding (welds 4i and 4e) at the end of the cuff, as seen in Figure 6.

In this case, the stresses on the veneer are transmitted by the cuff and are consequently lower. Another advantage lies in the fact that no element of the cuff comes into contact with the cooling water and that no special protection is therefore necessary.

Explosion plating methods are known. It is clear that the quality of the assembly must be subject to specific requirements.

The thickness and width of the titanium sheet to be pressed onto the carbon steel sleeve depends on the dimensions of the parts concerned and the forces that can be transmitted. The thickness of the steel sheet is also dependent on the thickness of the titanium.

Thus, when the exchanger considered is a condenser, the thickness of the sleeve can be equal to 20 mm, and the tube plate can be made of a steel sheet plated by explosion of 8 mm, on a massive titanium plate. 25 mm thick.

The pressure prevailing in enclosure B (water vapor) can be 40 mbar while in C (sea water) this pressure can be 2.5 bar.

Claims (2)

1. Mounting of tube plates at the end of a carbon steel shell of a tubular heat exchanger, each tube plate having, on the side fastened to the shell, a carbon steel part and, on the side exposed to corrosion, a titanium part to which the titanium tubes are rolled in and welded, characterized in that the actual tube plate (2) is produced from solid titanium and is fastened to the carbon steel shell (1) via a steel piece (15) welded by explosion or an equivalent method to the tube plate (2) which is itself welded to the shell (1).

2. Mounting according to Claim 1, characterized in that the plate (2-15) is used as a flange.

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