The present invention concerns a method for purifying the cooling circuit of an alternator stator operating in ventilated mode. It also concerns a device for implementing this method.
Alternators in power stations are subjected to the passage of high-density currents which cause strong heating effects. To combat this heating, alternator stators comprise networks of stator bars which have hollow conductors made of copper in which the cooling water circulates.
In order to achieve the lowest possible conductivity of the cooling water, about 1% to 7% of the nominal flow of the water circulating in the stator is diverted towards a mixed-bed demineralisation resin.
A problem frequently encountered with this cooling system is the formation of deposits on the internal walls of the hollow conductors, which obstruct these conductors. Such obstructions lead to heating and, in the longer term, to frequent down-time of the alternator or to reductions in power load.
An attempted solution consists in conditioning the cooling circuits under air. However, this has not proved satisfactory in the long run as the phenomenon of obstruction persists.
To combat these obstructions, alternators have therefore been periodically stopped for chemical cleaning of the hollow copper conductors using acidic and/or complexing solutions. However, these cleaning operations are protracted, cause stopping of the machine and are often corrosive for the hollow conductors.
Thus a real need exists for a method of combating the formation of, and/or allowing the elimination of, the deposits responsible for obstructing the hollow copper conductors of alternator stators in a cooling circuit operating in ventilated mode. Such a method should not cause stopping of the alternator, should not cause high conductivity in the cooling water and should not cause major corrosion of the hollow copper conductors.
After lengthy research, the inventors have succeeded in developing such a method.
This method enables the formation of these deposits to be combated and, at the same time, allows the destruction of any deposits formed inside the hollow copper conductors, without stopping the alternator and without generating high conductivity, while causing minimal corrosion.
Thus, the invention relates to a method of purifying the water circulating in the hollow copper conductors of an alternator stator the cooling circuit of which is conditioned in ventilated mode, according to which method the water of the closed cooling circuit of the alternator stator is drawn from the expansion tank by a pump, cooled and, if necessary, filtered in order to be fed to the hollow copper conductors of the alternator stator, characterised in that part of the water is diverted upstream of the alternator to a deionisation circuit feeding into the expansion tank, said deionisation circuit comprising a demineralisation circuit formed by a mixed-bed resin and a treatment circuit placed parallel to the demineralisation circuit, and characterised in that:
when the inflow rate into the deionisation circuit is fixed at a value less than 10%, and preferably less than about 20% of the nominal flow passing through the stator, the treatment circuit consists of a cation-exchanging resin,
when the inflow rate into the deionisation circuit is fixed at a value greater than 10%, and preferably greater than or equal to about 20% of the nominal flow passing through the stator, either the treatment circuit is interrupted, or the treatment circuit consists of a cation-exchanging resin or of a mixed-bed resin.
Such a method can be used for all types of alternators cooled by a circulation of aerated water in hollow copper conductors in the stator bars forming the stator. As examples of such alternators one can cite all alternators of fossil-fuel fired thermal power stations or nuclear power stations with a power of at least 250 MW.
In the method according to the invention, the water diverted into the demineralisation circuit passes through a mixed-bed column capable of retaining both HCO3xe2x88x92 ions and Cu2+ ions. Using such a column, the Cu2+ ions resulting from the normal degradation of the internal wall of the copper conductors, which have a slightly alkaline character, are retained, as are the HCO3xe2x88x92 ions resulting from the dissolving of carbon dioxide from the air in the water, which have a slightly acidic character. Thus, the pH of the cooling water is kept neutral. Notable examples of mixed-bed columns are ARM 9882 of the ROHM and HAAS company and NRW 354 of the PUROLITE company.
When a cationic resin is employed, a cationic resin capable of retaining Cu2+ ions is used. This resin is selected especially from resins of the NRW 160 type of the PUROLITE company and ARC 9652 of the ROHM and HAAS company.
According to the invention, when the water of the treatment circuit passes through a cation-exchanging resin, the process comprises a cycle of stages consisting of:
a) interrupting the demineralisation circuit while maintaining circulation in the treatment circuit;
b) restoring the operation of the demineralisation circuit while maintaining circulation in the treatment circuit.
Thus, during stage a), the Cu2+ ions are trapped. The medium therefore becomes acidified as a result of the relative increase in the concentration of HCO3xe2x88x92. Now, the Applicant has demonstrated that the deposits responsible for the obstruction are deposits of CuO, which are formed by precipitation of the cupric ions and which dissolve more readily in an acidic milieu. The dissolving of the CuO deposits is therefore favoured during stage a).
However, if the concentration of Cu2+ is too high, there is a risk that CuO will again be precipitated, and if the pH is too weak there is a risk that the copper will be depassivated and that there will be high rates of corrosion of the copper. This is why it is necessary to restore the operation of the demineralisation circuit. During stage b) the HCO3xe2x88x92 ions are trapped, so that the pH of the water will increase and, the concentration of dissolved copper being high, a large part of the copper will be precipitated. The pH will immediately become neutral again and the concentration of dissolved copper will diminish. When normal operating conditions are restored, stage a) is started again. Globally, more CuO is dissolved than is precipitated; this treatment therefore permits the elimination of the deposits responsible for the phenomenon of the obstruction of the hollow conductors.
The transition from stage a) to stage b) is therefore determined by the concentration of copper dissolved in the water upstream of the treatment circuit constituted by the cation-exchanging resin and by the pH of the water.
During the cationic purification, that is, when the treatment circuit consisting of a cationic resin is operating, the maximum permitted concentration of dissolved copper upstream of the treatment circuit is such that it does not allow precipitation of CuO, and the minimum permitted pH is such that it allows excessive corrosion of the hollow copper conductors to be avoided.
Thus, the maximum permitted concentration of dissolved copper is less than 200 ppb, and preferably below 180 ppb, and still more preferably below about 160 ppb.
The pH is controlled so as not to be less than 6.
The cycle of stages a) and b) is stopped when all the deposits responsible for the obstruction have been dissolved, and then the cation-exchanging resin of the treatment circuit is replaced by a mixed-bed resin so chosen that the two mixed-bed resins arranged parallel are capable of treating a flow of more that 10%, and preferably greater than or equal to about 20% of the nominal flow of water passing through the stator. With such a flow treated on the mixed bed the risk of formation of obstruction-causing deposits is substantially limited.
The mixed-bed resin introduced into the treatment circuit is preferably capable of treating a flow greater than 10%, and preferably greater than or equal to about 20% of the nominal flow passing through the stator, so that the demineralisation circuit can be interrupted to change a defective mixed bed if necessary. This circuit is then open and the demineralisation circuit closed. The defective mixed bed is replaced by a new mixed bed.
During a treatment according to the invention on an industrial site, it is considered that the deposits causing obstructions are dissolved about 4 months after the stator has recovered its original technical characteristics. It is considered that the stator has recovered its technical characteristics when the contamination coefficient Ke defined below has attained its limit value, a value corresponding to that calculated when the device is new.
Ke=temperature of water in the barxe2x88x92temperature of inflow into the stator/(active power)2+(reactive power)2
In the procedure according to the invention, when the treatment circuit consists of a mixed-bed resin,
either the inflow rate into the demineralisation circuit has a value greater than 10%, and preferably greater than or equal to about 20% of the nominal flow passing through the stator, and the treatment circuit is interrupted, or inversely;
or both circuits are opened so that the water passes through both circuits.
In the latter case, the inflow rate into the deionisation circuit is greater than 10%, and preferably greater than or equal to about 20% of the nominal flow of water passing through the stator.
Thus, when the treatment circuit consists of a mixed-bed resin, it can be used for demineralisation jointly with the demineralisation circuit, or it can be used to replace any mixed-bed resin in the demineralisation circuit which has become defective. In the first case, therefore, the water passes through both circuits, while in the second case one of the circuits is interrupted so that the defective resin can be replaced.
Of course, the concentration of dissolved copper upstream of the deionisation circuit is far below the limiting threshold of 160 ppb determined above. Likewise, the pH is always above 6.
The present invention also relates to a device enabling the method described above to be implemented.
This device consists of a closed circuit for cooling water in the hollow copper conductors of the alternator stator comprising a pump, a cooling device, a filter if necessary, the alternator, an expansion tank and a deionisation circuit situated upstream of the alternator and feeding into the expansion tank, said deionisation circuit comprising a mixed-bed demineralisation circuit and a treatment circuit parallel to the demineralisation circuit, characterised in that:
when the inflow rate into the deionisation circuit is less than 10%, and preferably less than about 20% of the nominal flow of water passing through the stator, the treatment circuit consists of a cation-exchanging resin,
when the inflow rate into the deionisation circuit is greater than 10%, and preferably greater than or equal to about 20% of the nominal flow of water passing through the stator, the treatment circuit consists of a mixed-bed resin or a cation-exchanging resin.
Given that the cooling circuit operates in ventilated mode, the device is such that level with the expansion tank there is a mixture of air and water, and especially if the water is well aerated, the oxygen content is between 4 and 8 ppm, in which case there is a partial air pressure significantly greater than 0.2 atm. Below this pressure blockages can again appear.
This device includes a means enabling the demineralisation circuit to be closed or opened while keeping open the parallel treatment circuit, and inversely.
Advantageously, this device also includes a means for monitoring the concentration of dissolved copper upstream and downstream of the demineralisation circuit, and means for monitoring the pH of the cooling water. In practice, these monitoring means consist of a valve for bleeding water from the cooling circuit, the measuring of concentration and pH of the water being carried out by conventional means.
In the device according to the invention, the mixed-bed column is able to retain both HCO3xe2x88x92 ions and Cu2+ ions, and the cationic resin is able to retain, in particular, Cu2+ ions.
The mixed-bed column can be a type ARM 9882 column of the ROHM and HAAS company or NRW 354 of the PUROLITE company.
The cationic resin can be of type NRW 160 of the PUROLITE company or ARC 9652 of the ROHM and HAAS company.
The present invention will now be described in more detail with reference to the drawings and to the example given for purely illustrative purposes.