WO2007010335A2

WO2007010335A2 - Accumulator water heater with adjustable cathodic protection

Info

Publication number: WO2007010335A2
Application number: PCT/IB2006/000463
Authority: WO
Inventors: Lucio Latini; Angelo Mancini; Roberto Sampaolesi; Alessandro Stopponi
Original assignee: Merloni Termosanitari S.P.A.
Priority date: 2005-07-20
Filing date: 2006-02-23
Publication date: 2007-01-25
Also published as: CN101374977B; WO2007010335A3; CN101374977A; EP1904667A2; ITAN20050037A1; RU2365681C2; RU2007140698A

Abstract

The present invention relates to an accumulator water heater (1) fitted with impressed current corrosion protection devices (5, 6, SC). The present invention also describes a method to regularly measure the value of the impressed current (Id) to be applied. Said accumulator water heater (1) can be additionally fitted with a cathodic protection device with sacrificing anode (4). The invention relates also to the alternative operating methods for the protection of the sacrificing anode (4) or of the impressed current, and to the methods designed to control the presence and degree of wear of said sacrificing anode (4).

Description

ACCUMULATOR WATER HEATER WITH ADJUSTABLE CATHODIC PROTECTION

DE S C RIPTI ON

The present invention relates to an accumulator water heater, preferably electrical, with dual cathodic protection and to the devices required to attain said dual cathodic protection.

The present invention also relates to a method for the protection of a water heater tank in ferrous material from galvanic corrosion.

The tank of accumulator water heaters, whether gas or electric, is generally made of ferrous material coated with a vitreous glazed, zinc-plated or synthetic material layer; the inside of the tank generally contains the heating element (the flue or electric resistances) and a sheath with one or more temperature sensors controlled by the temperature regulation and protection thermostats .

The inside of the tank also frequently contains a cathodic protection device designed to prevent the corrosion of the metal tank.

The cathodic protection device can be a "sacrificing anode" composed of an anode (generally in magnesium) designed to protect the tank in ferrous material. Cathodic protection can alternatively be attained by using an "impressed-current" cathodic protection system, which consists of an electrode fitted inside the tank (acting as anode) and requires applying a higher voltage between the tank (acting as cathode) and the internal electrode in order to reduce the electric potential of the tank (i.e. to achieve a sufficiently negative and a sufficiently high absolute value) by the amount required to prevent corrosion. Document EP 1 426 467 states that the impressed current should be appropriately adjusted according to the operating status of the water heater and recommends the most suitable methods to achieve this result; more specifically the document states that the current applied to active electric resistances is greater than that required for inactive resistances.

It is however known that impressed current causes the formation of hydrogen due to water hydrolysis, though this possibility is remote because the absolute value of the potential required to guarantee a protection against corrosion is generally, but not always, far smaller than that required to produce significant amounts of hydrogen; in algebraic terms this means that the negative potential which guarantees the required protection is generally far higher than the one that causes significant hydrolysis. All these devices, and especially those of water heaters, are preferably but not necessarily fitted on a single closing flange of the tank for practical purposes, as described in the embodiment referred to in the description of this invention. Currently known corrosion protection devices exhibit several problems and limitations in terms of effectiveness.

Despite their simplicity and low initial cost, cathodic protection devices based on sacrificing anodes present a problem in that they require regular wear control and prompt replacement of badly worn sacrificing anodes. Impressed-current cathodic protection systems present a problem in that they guarantee water heater protection only when it is on-line, but do not provide any form of protection when no power supply is present (typically when the water heater is not powered, as occurs when it is installed and filled with water in completed but not yet occupied apartments). The aim of this invention is to offer a solution to at least part of the known problems of accumulator water heaters and more specifically to solve the problems described above.

This is achieved by means of a water heater compliant with the description in claim 1 and by a method for protecting the ferrous tank of the water heater from corrosion with methods as described in claims 15, 18, 19 and 20. Further advantages can also be obtained by means of the supplementary characteristics described in the subordinate claims.

A possible embodiment of the invention according to the claims of the patent is described below with reference to the appended drawings wherein: - Figure 1 shows a schematic view of a cross-section of the water heater according to the first embodiment;

Figure 2 shows a schematic view of a cross-section of the water heater according to the second embodiment;

Figure 3 shows a flow diagram according to the first embodiment; - Figure 4 shows a flow diagram according to the second embodiment.

In the appended drawings, the numeral 1 refers throughout to an accumulator water heater comprising a tank 2 in ferrous material and equipped with a metal flange 3 bolted to it.

Tank 2 comprises corrosion protection devices 4, 5 designed to lower the electric potential of the tank 2 and protect it from galvanic corrosion.

Said corrosion protection devices 4, 5 are composed of a sacrificing anode 4, electrically connected to the tank 2 and of an impressed current anode 5, which is electrically insulated from the flange 3 by means of insulation devices 10 and controlled by a power supply 6 that supplies the required electric energy to ensure the cathodic protection of said tank 2.

Also envisaged is a thermostat T with a temperature sensor 7 fitted inside the sheath 8, which enables and disables the heating element (in this case, the electric resistance

9).

According to the embodiments described, the water contained in the tank 2 is heated by an electric resistance 9 fitted inside the tank.

Those skilled in the art will realize that this invention also applies to gas accumulator water heaters.

A control system SC controls the corrosion protection devices.

More specifically this control system SC is able to store the values of the impressed current Id that must be applied when the heating element (in particular electric resistance 9) is disabled; the control system also provides the memory and processing functions required to at least change the said value of impressed current Id according to the methods in this invention, described further on.

Said control system SC may also possess additional processing functions suited to carry out the operations described in EP 1 426 467 and more specifically to calculate the impressed current value Ia that must be applied when the heating element (and in particular the electric resistance 9) is enabled.

The combination of the sacrificing anode 4 and of the impressed current anode 5 guarantees a cathodic protection of the tank 2 both when the water heater 1 is powered and when it is disconnected from the power supply, and simultaneously reduces the frequency of maintenance operations on the cathodic protection devices, thus increasing the life of the tank 2.

The sacrificing anode 4 is made in a less noble metal than that of the tank 2 in order to allow the continuous flow of current from the sacrificing anode 4 to the tank 2 through the water in the tank and to guarantee the galvanic cathodic protection of the tank.

Given that the tank 2 is in ferrous material, the sacrificing anode 4 can be composed of an anode in magnesium (Mg).

The power supply 6 creates a difference in potential between the impressed current anode 5 and the tank 2, acting as cathode, in order to produce a direct current between the impressed current anode 5 and the tank 2 through the water contained in the tank 2 itself.

The impressed current anode 5 is obviously electrically insulated from the tank 2 by means of insulating devices 10 to permit the application of different potential. The impressed current anode 5 should be preferably made in titanium (Ti).

It would be even more preferable to use a titanium coated anode 5 according to prior art, with a layer of mixed metal oxides designed to improve the electric conductivity of the anode.

The water heater 1, designed according to this invention, is protected against corrosion, in normal operating conditions, thanks to the impressed current anode 5 appropriately powered by power supply 6.

When the power supply is not available, the tank 2 is protected by the sacrificing anode 4.

In the first embodiment shown in Figure 1, the sacrificing anode 4 is connected by a direct electrical connection to tank 2: this connection is represented in Figure 1 by the power cable 11 but should be preferably ensured by the metal device that fixes the sacrificing anode 4 to the flange 3 and by the device that fixes the flange 3 to the tank 2.

In this case the sacrificing anode 4 can contribute, although to a limited degree, towards cathodic protection even when the impressed current anode 5 is operating; however if the latter applies an electric potential higher than that of the electrochemical potential of the sacrificing anode 4, it limits the consumption of the latter.

In the second embodiment shown in Figure 2, the sacrificing anode 4 is fitted on the flange 3 that is electrically insulated from the tank 2 by means of the electrical insulation 12, and the electric connection between the sacrificing anode 4 and the tank 2 is achieved by means of the power cable 11 and a switch 13 that can be controlled by the control system SC.

In this way it is possible to disable the electric connection between the tank 2 and the magnesium anode 4 (when the generator 6 applies the electric potential between the impressed current anode 5 and the tank 2) by interrupting the consumption of the sacrificing anode 4 during the operation of the impressed current anode 5.

The water heater should be preferably fitted with devices suitable for measuring the consumption of the sacrificing anode 4. According to the first embodiment of this measuring method, the water heater may comprise, as shown in Figure 1, a voltage meter to compare the voltage of the tank 2 with a reference electrode when the power supply 6 is off.

The reference electrode should preferably be the impressed current anode 5.

In the first embodiment shown in Figure 1, the potential V between the tank 2 and the impressed current anode 5 is measured. For electric water heaters it is preferable to perform the measurement every time the thermostat is disconnected (i.e. technically in the interval of time between the thermostat disconnections commonly referred to as "thermostating" cycle), that is when the electric resistances are off-line, to prevent interference with the readings. The potential Vs is measured by the control system SC after the power supply 6 has been turned off by means of the switch 15, the voltage meter 14 has been enabled by means of the related switch 16, and after the preset interval of time m from the last disabling of the heating element (electric resistance 9 in the examples shown) has expired. The potential Vs is then compared with the reference value V₀ and saved in the memory.

If the potential Vs is higher than the reference value Vo, the event is saved in the control system SC and the procedure is completed.

If the potential Vs is higher than the reference value V₀ for n subsequent procedures, alarms are activated to signal that the sacrificing anode 4 needs to be replaced.

The reference value V₀ should be preferably equivalent to -1.5 V.

The preset interval m should preferably be equivalent to at least 15 minutes.

This procedure is described in the left section of the flow diagram of Figure 3.

In the second embodiment shown in Figure 2, the voltage meter 14 fitted on the electric connection 11 between the sacrificing anode 4 and the tank 2 can be replaced or integrated with a current meter 17 able to measure the electric currents in the specified range (approximately 0 - 20 mA).

Electric current Is is measured by the meter 15 while the sacrificing anode 4 is operating to provide information on the degree of wear of the sacrificing anode. To check the degree of wear of the sacrificing anode, it is necessary to disable the power supply 6 of the impressed current anode 5 by opening the switch 15.

The current flowing from the sacrificing anode 4 to the tank 2 should be preferably measured after a preset interval of time m equivalent to at least 15 minutes from the disabling of the impressed current anode. After the expiry of said interval of time m, the current Is between the tank 2 and the sacrificing anode 4 is measured with the current meter 15.

The current Is is then compared with a reference value Is_ref and saved in the memory.

If the current is lower than the reference value Is_ref, the above-described steps are repeated n times. If the current is lower than the reference value Is_ref n times, alarms are activated to signal that the sacrificing anode 4 needs to be replaced.

The reference value Is_ref should preferably be equivalent to a value ranging from 0 to

3 mA.

The preset interval m should preferably be equivalent to 15 minutes. This procedure is described in the left section of the flow diagram shown in Figure 4.

In both embodiments of the method used to control the wear of the sacrificing anode

4, the alarms signaling the need to replace the sacrificing anode 4 remain active until the anode is replaced and also signal the need to maintain the power supply on-line in order to guarantee the protection of the tank 2 with impressed currents. The alarms are disabled when the anode is replaced.

It is also preferably to foresee regular control and adjustment setting of the impressed current Id by applying an electric potential between the tank 2 and the impressed current anode 5.

This control and adjustment setting of the impressed current Id can also be useful when the sacrificing anode is absent.

First of all, the control and adjustment setting of the impressed current Id may include the assignment of the most appropriate operating current value Id to the impressed current anode 5.

Said control and adjustment operations are carried out when the electric resistance is disabled and can be started immediately after the thermostat T has been disconnected, that is at the end of the thermostating cycle.

The potential meter 14 is used to measure that potential Vi between the impressed current anode 5 and the tank 2.

The potential measured V is then saved and compared with the reference value V₁. If the potential Vi is greater (in algebraic terms) than the reference value V₁, a preset increase Δld is applied to the operating impressed current value Id foreseen with the disabled heating element or the potential Vi is reduced by a preset value ΔVi, which is technically equivalent.

The increase of the impressed current Id is repeated until the potential Vi is lower than V₁.

When the potential Vi is lower than the reference value V₁, the potential Vi is compared with a second reference value V_H, which represents the limit below which there is a risk of forming a significant amount of hydrogen If the potential Vi is higher than the second reference value V_H (i-e. there is no risk of hydrogen being formed), the operating current value Id is maintained equivalent to the last assigned value; the control and adjustment setting of the impressed current Id is completed, and is re-activated by the control system SC after a preset period of time (i.e. after p thermostating operations) that is considered sufficiently short in comparison to the possible changes of the required protection conditions. If potential Vi is lower than the second reference value V_H (i.e. there is a risk of hydrogen being formed), the operating current value Id is reset to the second before last assigned value; the control and adjustment setting of the impressed current Id is completed and is re-activated, as explained above, after a preset interval of time, hi the latter case, value Id is sufficient to guarantee good protection because potential Vi will be almost equivalent to the optimum value.

This becomes even more effective as time passes, because the lime deposit that forms in the water heater in time reduces the current value required to assess this condition.

For tanks with vitreous glazed coating, the operating current value Id, with disabled heating element, should be preferably equivalent to 6 mA while the operating current value Ia for the enabled heating element is 9 - 12 mA.

For tanks with vitreous glazed coating, the reference value V₁ of the potential should be preferably equivalent to -1.8 V while the second reference value V_H of the potential is equivalent to -2.5 V. It is also possible to use an electronic logic means to decide when to enable the voltage generator 6 and the potential meter 14, for example every p thermostating operations.

This procedure is illustrated in identical terms in the right section of the flow diagrams of Figures 3 and 4.

Claims

1. Accumulator water heater ( 1 ) comprising:

- a metal tank (2),

- a heating element (9) to heat the water inside said metal tank (2), characterized in that said heating element (9) is composed of an electric resistance (9),

- thermostating devices (T) to enable/disable said heating element (9),

- impressed current cathodic protection devices (5, 6, SC) to protect the tank (2) from electrolytic corrosion, characterized in that said devices (5, 6, SC) are composed of an impressed current anode (5), a power supply (6) supplying direct current to said impressed current anode (5) and a control system (SC) suitable to regulate said DC power supply (6), characterized in that there are additional devices (5, 14, SC) to regularly adjust the current (Id) of said power supplies (6) so that the potential (Vi) of the tank (2) is maintained within a pre-determined range.

2. Accumulator water heater (1) according to the previous claim, characterized in that said devices (5, 14, SC) used to regularly adjust said current (Id) comprise

- an electrode (5) immersed in the tank (2)

- and potential meter (14) designed to measure the differential potential Vi between the tank (2) and the impressed current anode (5) applied by said DC power supply (6).

3. Accumulator water heater (1) according to the previous claim, characterized in that said devices (5, 14, SC) used to regularly adjust said current (Id) further include

- timers (SC) to perform the measurement of the tank potential (2) during the interval of time during which the heating element (9) is disabled, - devices (SC) to change the current (I) of said power supply (6) according to the potential measured.

4. Accumulator water heater (1) according at least to claim 2, characterized in that said electrode (5) immersed in the tank (2) corresponds to said impressed current anode (5).

5. Accumulator water heater (1) according to any of the claims above, characterized in that it also includes a sacrificing anode (4).

6. Accumulator water heater (1) according to the previous claim, characterized in that said sacrificing anode (4) is electrically connected to said tank (2) by means of a switch (13) designed to interrupt the electric connection when said power supply (6) supplies power to said impressed current anode (5).

7. Accumulator water heater (1) according to claim 5 or 6, characterized in that it is fitted with devices (14, 17, 15, T, SC) to monitor the wear of said sacrificing anode (4).

8. Accumulator water heater (1) according to claim 7, in particular having a configuration where said devices (9) used to heat the water comprise at least one electric resistance (9), characterized in that said devices (14, 17, 15, T, SC) used to monitor the wear of said sacrificing anode (4) comprise

- devices (14, 17, 15, T, SC) to regularly interrupt the operation of said impressed current anode (5) and said resistance (9), - timers (SC) to perform at least one measurement of said potential (Vs) after a preset interval of time (m), immediately after the disconnection of said impressed current anode (5) and said resistance (9).

9. Water heater according to at least the previous claim, characterized in that said interval of time (m) is equivalent to at least 15 minutes.

10. Accumulator water heater (1) according to claim 8 or 9, characterized in that it comprises the following additional components:

- a voltage meter (14) designed to measure the difference in potential (Vs) between a reference electrode (5) and said tank (2),

- devices to signal when said difference in potential (Vs) exceeds a reference value (V₀).

11. Accumulator water heater (1) according to the previous claim, characterized in that said reference value (Vo) for the said difference in potential (Vs) is equivalent to at least -1.5 V.

12. Accumulator water heater (1) according to claim 8 or 9, characterized in that it further comprises:

- a current meter (17) designed to measure the electric current (Is) between said sacrificing anode (5) and said tank (2), devices (SC) to signal when said current (Is) between said sacrificing current (4) and said tank (2) falls below a preset threshold (Is_ref).

13. Accumulator water heater (1) according to the previous claim, characterized in that said preset threshold (Is_ref) for said current (Is) ranges from 0 and 3 mA.

14. Accumulator water heater (1) according to any of the previous claims, characterized in that said power supplies (6) can be adjusted depending on whether said heating element (9) has been enabled or disabled by said thermostat (T).

15. Method to protect a tank (2) in ferrous material of an accumulator water heater (1), according to claim 1, characterized in that it comprises the steps to: a) measure the potential of the tank (2) during the interval of time in which said heating element (9) is disabled, b) store the measured potential (Vi), c) compare the measured potential (Vi) with a first reference value (V₁), d) increase the input current Id of the impressed current value by a preset value Δld and consequently reduce said potential (Vi), if said potential (Vi) is higher than said reference value (V₁), e) repeat step d) until said potential (Vi) is lower than said reference value (V₁), f) compare the measured potential (Vi) with a second reference value (V_H) and g) maintain said last operating current (Id) obtained through subsequent increases in Δld, if said potential (Vi) is higher than said reference value (V_H), and terminate the procedure, otherwise, to return to the second before last operating current value (Id) obtained through the Δld reduction and terminate the procedure.

16. Method to protect a tank (2) according to the previous claim, characterized in that said procedure is repeated for a preset interval of time.

17. Method to protect a tank (2) according to the previous claim, characterized in that said preset intervals of time are equivalent to the intervals of a preset number of thermostating cycles (p).

18. Method to protect a tank (2) in ferrous material of an accumulator water heater (1), according to claim 5, characterized in that it includes the steps to

- protect said tank (2) by means of a sacrificing anode (4)

- and, if the water heater is powered by the mains, to simultaneously protect said tank (2) by means of an impressed current anode (5).

19. Method to protect a tank (2) in ferrous material of an accumulator water heater (1), according to claim 6, characterized in that it includes the steps to:

- allow the protection device of said tank (2) to be disabled by the sacrificing anode (4),

- and simultaneously protect said tank (2) by means of an impressed current anode (5).

20. Method to protect a tank (2) in ferrous material of an accumulator water heater (1), according to claim 5, characterized in that it regularly includes the step that monitors the wear of said sacrificing anode (4).

21. Method according to claim 20, characterized in that said periodical monitoring of the wear of said sacrificing anode (4) is carried out

- within a preset interval of time (m) after completion of the thermostating operation, - by disabling the power supply of impressed currents (6).

22. Method according to claim 21 , characterized in that said periodical monitoring of the wear of said sacrificing anode (4) further comprises the steps to a) measure the potential (Vs) between the tank (2) and a reference electrode (5), b) compare said measured potential (Vs) with a reference value (Vo)₅ c) save the event if said potential (Vs) is lower than said reference value

(Vo).

23. Method according to the previous claim, characterized in that if said potential (Vs) is lower than said reference value (V₀) for n consecutive monitoring operations, an alarm is generated to signal the wear of the sacrificing anode (4) and that said alarm ceases only after the next periodical monitoring operation that confirms the good working order of the sacrificing anode (4).

24. Method according to claim 21, characterized in that said periodical monitoring of the wear of said sacrificing anode (4) further includes the steps to: a) measure the current (Is) that flows between said sacrificing anode (4) and said tank (2), b) compare said current (Is) with a reference value (Is_ref), c) save the event if said current (Is) is lower than said reference value

(ISref).

25. Method according to the previous claim, characterized in that if the current (Is) is lower than said reference value (Is_ref) during n consecutive periodical monitoring operations, an alarm is generated to signal the wear of the sacrificing anode (4) and that said alarm ceases only after the next periodical monitoring operation that confirms the good working order of the sacrificing anode (4).

26. Method according to claim 21, characterized in that said preset interval of time (m) after the completion of a thermostating operation is equivalent to 15 minutes.

27. Method according to at least claim 15, characterized in that the potential reference value (Vl) for tanks with vitreous glazing is equivalent to - 1.8

V and that the second potential reference value (VH) is equivalent to — 2.5 V.

28. Method according to claim 23 or 25, characterized in that the cathodic protection is carried out automatically using the method based on impressed currents as long as the alarm is active.