DEVICE FOR THE PROTECTION OF METAL TANKS AGAINST CORROSION, IN PARTICULAR BOILERS

Abstract: This invention falls into the field of devices designed to guarantee the cathodic protection, using a non consumable electrode, of tanks containing liquid, in particular boilers. A tubular element (4), which also has other functional roles within a boiler to be protected against corrosion, is made, at least partially, of titanium or a material with the equivalent anti-corrosion characteristics. The said tubular element fulfils the function of a non-consumable electrode. The surface of the said tubular element may have one or more active and passive sections.

Description: DESCRIPTION Device for the protection of metal tanks against corrosion, in particular boilers. Technical field The invention relates to a device for the protection of metal tanks against corrosion, in particular boilers. Background Art For boilers, both gas and electrical powered kinds, and also for any apparatus with a similar iron-based metallic composition which is designed to contain hot water or water to be heated, the corrosion that occurs on the internal surfaces of the said water tank represents a serious problem. The said corrosion is obviously due to the contact between the water and the ferrous material and is rendered more aggressive, in the aforesaid apparatus, by the high temperature of the water. In the case of boilers, it should be noted that, even though these generally have the internal surface of the water tank covered with porcelain or similar cladding means, corrosion still occurs. For some time, two electrical protection methods have generally been used to prevent, or at least limit, the corrosion: one type of protection called 'anodic' and another called 'cathodic'. The anodic protection is achieved by bringing the structure of the boiler to a potential value which is higher than another electrode until a passivation value is reached and in this case the structure to be protected acts as an anode. The said cathodic protection is obtained by making the potential of the boiler structure more negative than that of the aggressive means, in the case in question being represented by water for sanitary purposes. With this kind of protection, a direct current is made to circulate, via the water stored, from an anode to a cathode constituted by the boiler tank walls. The element functioning as an anode is gradually consumed while the said boiler is in use and, consequently, the said anode is called 'sacrificial'. The current can also be made to circulate by means of an external generator, and in this case, the protection method is referred to as 'impressed current' . The anodic protection system has the drawback that the voltage required to be applied is an extremely precise value and, therefore, if this value is not respected exactly, the walls of the structure to be protected are not protected against passivation and the destructive course of corrosion is rapid. That is why, since the voltage value must be checked so carefully, the anodic protection is not really recommended for heating systems for sanitary water in a domestic environment. With regards to cathodic protection, the so-called 'sacrificial anodic' system has one drawback in exactly that, the anode is consumed, resulting in the said anode needing to be changed on a regular basis. If this replacement is not made, the corrosion will start to set in immediately. As for the cathodic protection in the so-called 'impressed current' method, this kind of protection is becoming more widespread but the solutions proposed until now all have drawbacks generally represented by high costs and a persistent difficulty in guaranteeing the uniform distribution of the impressed current , as well as somewhat laborious electrode assembly operations. The electrode is made of various electrically conductive materials which are unlikely to corrode, e.g. titanium alloy bars, and there are commonly known physical or chemical methods for protecting part of the external surface of the said bars so that the portions of the surface do not emit current. The earlier document EP 0 771 889 has the tubular element constituting a container sheath for the thermostat sensors as a support for the electrode. The functions of the seal and the electrical insulation between the electrode and the tubular element are carried out by a suitable support. The application for the Italian patent n° AN99A000003 has an interesting detail in the electrical connection between the electrode and the boiler tank which is the fact that the connection is made without any electrical cables. This applies, at least, for the section between the said electrode and the external surface of the aforesaid boiler. The said earlier document also envisages a solid, non-corrodible electrode which is inserted and connected electrically to the end of the thermostat sensors container sheath. The sheath, in its turn, is fixed to the flange with a joint made of an insulating material to prevent the electrical contact between the electrode and the tank to be protected. Since the aforesaid tubular support is made of a corrodible metal, it must be protected efficiently along its entire length and even the electrode insertion point in the aforesaid sheath must be protected. This double need for protection against corrosion causes a first drawback of a functional nature, since imperfections can form in the protective covering of both the aforesaid tubular support and also in the electrode insertion area of the support itself. A second drawback regards the construction of the said apparatus, given the technological processes involved in both the connection between the tubular support and the electrode and the production of the aforesaid protective covering. In both the aforesaid patents the -minimum height for the positioning of the electrode is conditioned by the minimum length of the sheath, which is, in its turn, conditioned by the height at which the sensors are positioned. It would therefore be impossible to position the electrode at the tank half s centre of gravity. Disclosure of Invention The aim of this invention is to overcome all of the aforesaid drawbacks. The device for the protection of metal tanks against corrosion, in particular boilers (the said device being of the kind that applies the principle of cathodic protection through a non-consumable electrode and the said device being the subject of this invention) is characterised by the fact that die external surface of a tubular element is used as electrode, the said tubular element having further functional roles within the tank to be protected, such as, for example, containing and/or protection functions for other means present in the tank, and the said tubular element, or at least part of its surface layer, being made of titanium or another material suitable to act as an electrode and which is workable and mechanically resistant, features that are necessary for the possible applications of the invention. These and other characteristics will better emerge in the description that follows of two preferred embodiments of this invention provided in the form of non-limiting examples with accompanying drawings, in which: Figures 1 shows, according to a first possible embodiment of the invention, a section of the metal tank with, going into it, a container sheath for the thermostat sensors; the external surface of the said sheath acts, at least in part, as an electrode; Figure 2 shows, according to a second possible embodiment of the invention, a section of a metal tank with, going into it, a pipe for drawing the hot water covered with a screen-printed electrical resistor, which is, in its turn, protected by an anti-abrasion jacket; the external surface of the said jacket acts, at least in part, as an electrode. With reference to the said drawings, 1 denotes a storage tank of an electrical boiler not illustrated. A flange 2 is fixed to the lower part of the tank and bound indirectly to this flange (by a bushing 3 made of a dielectric material) is a tubular element 4, whose external surface, at least in part, is made of titanium or a material with similar anti-corrosion characteristics. This said external surface acts as an electrode. The flange 2 is also designed to support the electrical heating element not illustrated. The link between the aforesaid tubular element 4 and the aforesaid flange 2 is made in such a way as to guarantee, with a commonly known means, the watertight seal and, as mentioned earlier, to keep the tubular element electrically insulated from the said flange 2 and also, therefore, from the storage tank 1 to be protected. With any commonly known means, such as for example chemical treatments or the application of dielectric materials, a portion 5a of the external surface of the tubular element 4 can be insulated electrically so that it does not emit currents towards the tank 1. Obviously, there can be more than one portion 5a on the same tubular element 4; the upper part of the surface of the tubular element 4, however, which is not covered by treatments or insulating materials, is an active portion 5b of the said tubular element 4 and is therefore capable of emitting a current. Naturally, there can also be more than one active portion 5b. The option of making one or more portions 5a and 5b of the external surface of the tubular element 4 inactive and active can be useful, in certain applications of the invention, for distributing the impressed current better within the interior of the tank 1 to be protected. Shown inside the tubular element 4, which, as mentioned earlier, acts as a sheath for the thermostat sensors, is a temperature sensor 6 (of the bulb kind). A current generator 7 is connected electrically to the tubular element 4 by means of a first conductor 8 and to the flange 2 by means of a second conductor 9; the flange 2 is, in its turn, connected electrically to the tank 1 using any commonly known means (not shown). Alternatively, the conductor 9 could be connected electrically directly to the tank 1. The device in question in the present invention according to the embodiment just illustrated functions works in a very easy way: the electrical connection between the upper part 5b of the surface of the tubular element 4 and the tank 1 made via the said tubular element 4, the first conductor 8, the current generator 7, the second conductor 9 and, potentially, the flange 2, allows a flow of current to be established, through the water contained in the tank, between the upper part 5b of the surface of the tubular element 4 acting as an anode and the wet surfaces of the storage tank 1 acting as a cathode. With reference to figure 2, in addition to the elements already described with reference to figure 1, there is a drawing pipe 10 shown which either draws or brings hot or cold water from or to the tank 1; the said pipe 10 is fixed to the tank 1 with a commonly known means such as, for example, a bushing 11 screwed onto a ring nut 12; the said pipe 10 supports a tubular element 4.1 which constitutes the electrode in this second embodiment of the invention; the composition of the said electrode has already been described and, in this case too, there can be several sections of external surface 5a and 5b, which are respectively passive and active. An insulating layer 13, whose composition can be as described below, can be placed between the pipe 10 and the tubular element 4.1. There is a commonly known technique in which printed resistive electric circuits are manufactured for the production of electrical heating elements. In this technique, one or more first layers of a dielectric material are deposited on a base made of a suitable metal (the said layers being printed using, for example, a screen-printing method) with an electrical resistor, which is then insulated externally with one or more second layers of dielectric material. There are also known means manufactured using this technology which are used for connecting the said printed circuits with electrical cables. A separate patent application has been made in which this technology is used to manufacture electrical heating resistors printed on the external surfaces of piping positioned inside the tank of an electric boiler. According to the said patent application, it may be worthwhile employing an anti-abrasion protective sheath for the external electrical layers. The whole made up of the said first dielectric layers, screen-printed electrical resistor and the second layers of dielectric material may constitute what has been indicated, as a whole, as the insulating layer 13 in figure 2. While the tubular elements 4.1 performs the dual functions of electrode and anti-abrasion protective jacket for the said insulating layer 13, the said insulating layer 13 guaranteeing, in its turn, the electrical insulation between the said tubular element 4.1 and the piping 10. Furthermore, the tubular element 4.1, does not need a direct electrical connection with the tank 1; this connection can be made using any commonly known means, for example, by building the bushing 11 using a dielectric material, while the electrical connection between the said sheath 4.1 and the current generator 7 is made as described earlier for the first possible embodiment of the invention, with the addition, however, that part of the electrical connection (from the interior to the exterior of the tank, up to the first conductor 8) could be produced according to the second technique just mentioned summarily using the screen-printed electrical resistors or, alternatively, the said sheath 4.1 could be extended to reach the exterior of the tank 1 to be connected directly with the first conductor 8. Naturally nothing prevents, according to a third possible, simplified, embodiment of the invention, the tubular element 4.1 fulfilling the sole function of electrode and the insulating layer 13 having the sole role of separation element between the tubular element 4. land the piping 10 or the thermostat sensors sheath, while the functions and characteristics of the remaining elements, such as the insulation elements and the electrical connections, could be exactly as described earlier. A first advantage of the device in question in the invention is that there is no need to manufacture special supports for the tubular elements 4 or 4.1 designed to act as non-consumable electrodes. A second important advantage of the invention, except in the case of the third, simplified, embodiment, is that the said electrodes constituted by the tubular elements 4 or 4.1 are created using mechanical means such as a thermostat sensors sheath or an anti-abrasion protective jacket, which is already an essential part in the tank 1. Another advantage of the invention is that the said tubular elements 4 or 4. lean be extended freely to cover the full length of the tank or that the impressed current can be supplied to sections of the surface 5a in order to distribute the said impressed current better.

Claims: Claims 1. A device for the protection of metal tanks against corrosion, in particular boilers (the said device being of the kind that applies the principle of cathodic protection through a non-consumable electrode), characterised by the fact that a part of the external surface of a tubular element (4) acts as the electrode, the said tubular element also having further functional roles inside the tanks 1 to be protected. 2. A device according to claim 1, characterised by the fact that current is not supplied to one or more parts of the surface layer of the tubular element (4). 3. A device according to claim 1, characterised by the fact that there can be more than one active part (5b) of the surface layer. 4. A device according to claim 1, characterised by the fact that at least one part of the surface layer of the tubular element (4) is made of titanium alloy. 5. A device according to claim 1, characterised by the fact that the tubular element (4) can be substantially extended to cover the entire length of the tank (1). 6. A device according to claim 1, characterised by the fact that the tubular element (4) extends outside the tank in order to be connected to the first electrical connector (8). 7. A device according to claim 1, characterised by the fact that the electrical connection to the first connector (8) is indirect and is made via a section of circuit printed using screen-printed electrical resistors technology. 8. A device according to claim 1, characterised by the fact that the functional role of the tubular element 4 is to act as a containing sheath of thermostat sensors. 9. A device according to claim 8, characterised by the fact that the bushing (3) to fix the sheath in place is made of a dielectric material. 10. A device according to claim 1, characterised by the fact that functional role of the tubular element 4 is to act as a protection jacket for screen-printed resistors. 11. A device according to claim (10), characterised by the fact that the bushing (11) to fix the pipe (10) in place is made of a dielectric material. 12. A device for the protection of metal tanks against corrosion, in particular boilers (the said device being of the kind that applies the principle of cathodic protection through a non-consumable electrode), characterised by the fact that a part of the external surface of a tubular element (4.1) acts as the electrode, the said tubular element being inserted externally into a functional means which is already a necessary part of the boiler (such as the thermostat sheath or piping located inside the tank), the said tubular element (4.1) also being insulated from the said functional means (9) by an insulating layer (13).

Inventors: FIORONI FAUSTO, FIORONI, FAUSTO
Applicants: FIORONI FAUSTO, THERMOWATT S P A, FIORONI, FAUSTO, THERMOWATT S.P.A.
Application year: 2001
Main classification: C23F13
IPC codes: C23F13