WO2002103255A1

WO2002103255A1 - Perfected hot-water boiler

Info

Publication number: WO2002103255A1
Application number: PCT/IB2002/002056
Authority: WO
Inventors: Ruggero Marchetti
Original assignee: Merloni Termosanitari S.P.A.
Priority date: 2001-06-18
Filing date: 2002-06-05
Publication date: 2002-12-27
Also published as: ITAN20010027A1; EP1397619A1

Abstract

In a hot-water boiler equipped with means (3, 20) for electrochemical protection, a tank (1) made of corrodible metal and a flanged metal cap (5) which requires electrochemical protection but prevents an adequate supply of protection current to the other metal components (1) to be protected owing to the vicinity of a feed electrode, said metal cap (5) is fastened to the tank (1) so that it is connected electrically to it only by way of a resistor (11). In the near vicinity of said metal component (5) and electrically isolated from it, a grounding device (9) is envisaged which intercepts any flashovers generated by failures.

Description

PERFECTED HOT-WATER BOILER

DESCRIPTION

This invention relates to a grounding device for tanks containing electrically conductive liquids; in particular, the device can be applied with profit to electric hot-water boilers with a metal tank subject to corrosion and provided with a cathode-type anti-corrosion protection also called "impressed current cathodic protection". Its use, however, is advantageous also together with sacrificial anode cathodic type or even anode type protections.

These types of protection taken as a whole are referred to hereinafter as electrochemical protections. For the sake of simplicity, special reference will be made herein to the use of the hot-water boiler with "impressed current" cathodic protection. The aforementioned protection methods are well known. The cathodic protection is obtained by increasing the negativity of the potential of the metal structure to be protected compared to that of the aggressive environment, i.e. the water to which the metal structure is exposed. The drop in the potential is obtained by making direct current circulate from one electrode (anode) to the metal structure to be protected which acts as the second electrode (cathode) through the aggressive environment which is electrically conductive.

The current can be generated galvanically between a metal less noble than the one to be protected (e.g. Mg, Zn), which acts as the anode, and the metal to be protected, which acts as the cathodic electrode; this is the case of what is called sacrificial or consumable anode cathodic protection. The circulation of the current can be obtained also by applying current by means of an external generator; this is the case of "impressed current" cathodic protection. Anodic protection is possible only for those metals called active-passive in which passivation phenomena can be obtained at specific potentials; it is obtained by increasing the potential of the structure to be protected to a level greater than that of the electrode until the passivation value is reached; in this case the structure to be protected acts as anode; the required voltage is usually obtained by means of a voltage generator.

Sacrificial anode cathodic protection is quite simple and has low initial costs, but its main disadvantage is the wearing of the anode which hence needs to be replaced on a regular basis by a technician; if said operation is discarded, the tank will rapidly corrode. Anodic protection offers the advantage of having extremely low levels of current consumption, but it requires the application of a more or less precise voltage value which needs to be controlled to be effective. If the precise value is exceeded or not reached, the metal structure is not passivated and hence is subject to rapid corrosion; for these reasons it is the most suitable method for the protection of industrial systems but not for consumer products; in any case the device forming the subject matter of this invention is suitable for these types of protection.

Permanent anode cathodic protection, owing to the simplicity of its implementation, has been spreading widely over the past few years also in consumer products and is rapidly supplanting the sacrificial anode; the fact that the electrode acting as anode must be made of costly materials to avoid its corrosion has led to the need of optimizing the use of the materials and the assembly methods especially for large scale production.

Henceforth, unless otherwise specified, specific reference will be made to impressed current cathodic protection and the term electrode is to be understood as the permanent anode protecting the metal structure. In order to provide the entire surface to be protected with the required current, several electrodes may be required, but for small tanks, that is to say up to a few hundreds of liters, a single electrode positioned at the center is enough; in any case it is important that the current supplied to the surface to be protected is as uniform as possible since some areas would risk not receiving an adequate amount while in others the current density would be too great and hence likely cause the electrolysis of the water and the production of hydrogen.

However, it is not always that easy and convenient to position the electrode in a symmetric position inside the tank.

An extremely simple form of electrode is composed of a bar of titanium or another suitable material fastened with dielectric means, for instance a plastic bushing, to the metal cap closing the boiler's tank in the lower part. It is usually shaped like a flange and it acts also as a support for the resistors and for the sheath containing the thermostat's sensor. In more economic versions, the entire surface of said electrode is active, that is to say it supplies the impressed current from the relevant generator to the part of its surface immersed in the water. Therefore, while the upper end can be positioned close enough to the tank's dome to supply it with a current density basically equal to that of the vertical walls, the lower part, located close to the metal cap, which for safety reasons must be grounded and which in any case could not benefit from the electrochemical protection if it were electrically insulated, discharges towards it an excessive amount of current which is hence not supplied to the other metal surfaces; as a matter of fact, the electric resistance of the electrode - water - metal cap route or, worse, the route involving the electrode - water - metal components connected electrically to the metal cap (i.e. the resistors and the thermostat sheath) is much lower than that of the electrode itself towards the remaining metal walls. This makes it impossible to provide for the uniform protection of the surfaces.

This shortcoming could be eliminated by mounting the flange so that it is isolated electrically from the rest of the structure thus preventing it from drawing to itself the protection current. But, without any additional contrivance, this would give rise to two other problems: the flange would be left without any anti-corrosion protection and, above all, it is not possible to ground it as required for safety reasons in case there is a failure of the resistors and these supply voltage to the metal cap. The purpose of the invention in the case of metal tanks using anodic or cathodic protection devices against corrosion and in particular electrical hot-water boilers with impressed current cathodic protection is that of providing means capable of preventing an excessive absorption of current by the metal cap without the risk, among other things, that the resistors deliver voltage to the metal cap in case of an anomalous distribution of current following the failure of the resistors themselves. Another purpose of this invention is that of avoiding impressed current density values or voltage gradients so high that these may cause locally the production of hydrogen. These and other purposes are attained by a grounding device as per the invention in question which makes it possible to limit the current absorbed by the metal cap without prejudice to safety as described hereinafter and in the attached claims which form an integral part of the description and as illustrated in the drawings hereto attached.

Fig. 1 shows a cross-section view of a part of a hot- water boiler tank using a grounding device as per this invention.

Fig. 2 shows a cross-section and a detailed view of the grounding device already illustrated in the fig. 1.

With regard to figures 1 and 2, these show: the tank 1, made of corrodible metal, of a hot- water boiler; a resistor 2 for the heating of the water; an electrode 3 for impressed current composed, for instance, by a titanium bar; the dielectric support 4 of electrode 3; the metal cap 5 which acts also as a support for one or more resistors 2 and for electrode 3 and which in this particular case is of the flanged type; the bolts 6 fastening the metal cap 5 to the metal tank 1; the dielectric bushings 7; the seal 8, made of isolating material, on the tank 1; the entire assembly of the grounding device 9; the grounding connection 10; the resistor 11 of the grounding connection 10; the grounding 12 of the tank 1; the grounding bar 13; the draining tube 14 with the relevant draining cap 15; the seal 16, made of isolating material, screwed on and tightened on the draining cap 15; the dielectric bushing 17; the fastening nuts 18 on the bar 13 and the grounding connection 19; the impressed current generator 20 connected to the tank 1 and the electrode 3. The stop collar 13.1 and the threaded stem 13.2 on the bar 13 are shown in detail.

As per the invention, the metal cap 5, as illustrated in the figures, is not connected electrically in a direct manner to the tank 1 owing both to the dielectric bushings 7 surrounding the bolts 6 and to the seal 8; it is connected however indirectly by means of the grounding connection 10 which comprises the resistor 11. The latter can be chosen so that the electrochemical potentials of both the tank 1 and the metal cap 5 can reach values assuring cathodic protection. The protection current circuit supplied both to the tank 1 and to the metal cap 5 closes by means of the impressed current generator 20 by using a known method. The foregoing is enough to guarantee an adequate degree of electrochemical protection for all of the metal surfaces. Of course, as stated above, the resistor 11 must have an adequate resistance value which depends on various factors like the shape of the tank 1 and the distances of the various components and finally the water's electric resistivity. As a consequence, the resistor 11 is specific for each type of hot- water boiler and/or environment in which the hot- water boiler is used. Therefore, either several types of resistor or a model with adjustable ohmic resistance can be used. By way of example, it has been seen that a resistance of 500 - 600 Ω meets the purpose for the most common hot-water boilers. With the aforementioned electric connection it is hence possible to assure an adequate degree of electrochemical protection both for the tank 1 and for the metal cap 5 but not against the flashovers coming from the electric resistor 2 in case of its failure; in this case, in fact, the resistor 11 would keep the metal cap 5 charged also because it may have been blown out by the flashover and the water in the tubes connected to the hot-water boiler right up to the cocks would be electrified as well.

The grounding device 9 provides for the prevention of the situation above. As illustrated in the figures, the bar 13 found inside it is immersed in the water and its upper end is positioned close to the resistors 2; instead, the other end provided with the threaded stem 13.2 is grounded directly by means of the grounding connection 19 while the connection to the metal cap 5 by means of the stop collar 13.1, the seal 16, the dielectric bushing 17, the draining cap 15 and finally the draining tube 14 provides for the electrical isolation between the bar 13 and said metal cap 5. In normal operating conditions, the bar 13, although discharging to the ground, does not cause imbalances in the supply of the impressed current on the part of the electrode 3. In fact, the extension of its surface is completely negligible compared to the metal surfaces which need to be protected so that the bar 13 absorbs a minimum part of the protection current; instead, in case of a failure of the resistors 2 that puts the internal electric filament in contact with the water; the bar 13, owing to the vicinity of its end to the resistors 2, would hence establish a preferential route for the current from the resistors 2 to the ground thus avoiding that the metal cap 2 and the water contained in the tubes are charged. As regards the tank 1, it is grounded as usual for instance with the grounding circuit 12.

Of course, the bar 13 is made of metal. Since it is likely that it can be scarcely protected by the impressed currents depending on its embodiment and particularly in the case of the example illustrated in the figures, it is recommended to use stainless steel for example of the AISI 303 type.

In the description above, the bar 13 is supported by the draining tube 14, the training cap 15 and the seal 16; as a matter of fact, it was decided to provide an example of an advantageous embodiment of the grounding device 9 using the draining device already present in many hot- water boiler models by duly modifying its components. However, nothing prevents the use of means similar to those specified above. The only condition that is absolutely necessary is that the bar 13 or another equivalent conductor be mounted near the resistors 2, directly grounded and isolated from the hot-water boiler's metal parts.

Also the metal cap 5 which is illustrated in the figures in its preferable and more usual embodiment as a flange can be of a different type. In this case the fastening and isolating means as embodied in the example by the bolts 6, the dielectric bushings 7 and by the seal 8 are to be replaced by equivalent means which must nevertheless assure the water-tightness and electric isolation between the metal cap 5 and the tank 1.

Finally, this invention can be applied usefully to any tank 1 containing a conductive and corrosive liquid in which there is a metal component similar to the metal cap 5 as per the example above

- that requires electrochemical protection;

- that can be fastened to the tank in an electrically isolated manner,

- that is close enough to an electrode 3 so as to prevent an adequate supply of protection current towards the other metal components to be protected, - that is likely to be charged in case of failures in the system.

In this general case, in fact, the current absorbed by said metal component can be regulated by means of a resistor 11, while any flashovers are dispelled and grounded by means of a grounded bar 13 isolated as per the example of said metal component.

Claims

Riv. 1. Hot- water boiler equipped with means (3, 20) for its electrochemical protection composed of

- a tank (1) made of corrodible metal and of

- a metal component (5) which - requires electrochemical protection,

- can be fastened to the tank in an electrically isolated manner,

- prevents, owing to the vicinity of an electrode (3) for electrochemical protection, an adequate supply of protection current towards the other metal components (1) to be protected, - is likely to be charged in case of failures in the system, characterized by the fact that said metal component (5)

- is fastened to the tank (1) to prevent any direct electrical connection between said metal component (5) and said tank (1), - is equipped with electrical connection means (10) providing for the electrical connection to said tank (1) only by means of a resistor (11), and by the fact that in the near vicinity of said metal component (5) and electrically isolated from it, a grounding device (9) is envisaged, comprising - suitable fastening means (13.1, 14, 15, 18),

- suitable electrical isolation and seal means (16, 17),

- finally, a metal element (13) connected to a grounding connection (19) by means of suitable fastening means (13.2, 18), intending to mean by "near vicinity" that the distance is adequate to assure that said grounding device (9) can intercept the flashovers generated by failures.

Riv. 2. Hot- water boiler as per the previous claim characterized by the fact that the aforesaid metal component (5) is fastened to the tank (1) by way of separate fastening means (6) and electrical isolation means (7, 8). Riv. 3. Hot- water boiler as per any one of the previous claims characterized by the fact that said metal component (5) is composed of a metal cap (5) supporting at least one or more resistors (2) and at least one electrode (3). Riv. 4. Hot-water boiler as per the previous claim characterized by the fact that said metal cap (5) is of the flanged type while said fastening means (6) and electric isolation means (7, 8) are composed respectively of bolts (6), dielectric bushings (7) and a seal (8). Riv. 5. Hot- water boiler as per either claims 3 or 4 above and characterized by the fact that said grounding device (9) is fastened directly to said metal cap (5). Riv. 6. Hot- water boiler as per any one of the previous claims characterized by the fact that some of said appropriate fastening means (13.1, 14, 15, 18) as well as electrical isolation and seal means (16, 17) are composed of the draining tube (14) with the relevant draining cap (15) and the seal (16) already envisaged in hot- water boilers and modified accordingly. Riv. 7. Hot- water boiler as per any one of the previous claims characterized by the fact that the resistance value of the most suitable resistor (11) depends on the type of hot- water boiler and on the usage conditions. Riv. 8. Hot- water boiler as per any one of the previous claims characterized by the fact that the resistor (11) can of the variable resistivity type. Riv. 9. Hot- water boiler as per any one of the previous claims characterized by the fact that the resistor (11) for the most common models of hot- water boilers has a resistance of500 - 600 Ω. Riv. 10. Hot- water boiler as per any one of the previous claims characterized by the fact that the electrochemical protection can be either of the cathodic or anode type and that in the former case it can be of what is called the "impressed current" type or of the sacrificial anode type while, accordingly, the aforesaid electrode (3) can either be an electrode supplying the current generated by a generator (20) or a sacrificial anode.