Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2000-10-06
2003-06-03
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S739000, C210S097000, C210S253000, C210S263000
Reexamination Certificate
active
06572777
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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.
OBJECTS AND SUMMARY OF THE INVENTION
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.
MORE DETAILED DESCRIPTION
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 HCO
3
−
ions and Cu
2+
ions. Using such a column, the Cu
2+
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 HCO
3
−
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 & HAAS company and NRW 354 of the PUROLITE company.
When a cationic resin is employed, a cationic resin capable of retaining Cu
2+
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 & 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 Cu
2+
ions are trapped. The medium therefore becomes acidified as a result of the relative increase in the concentration of HCO
3
−
. 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 Cu
2+
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 HCO
3
−
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 tr
Electricite de France (Service National
Hoey Betsey Morrison
Piper Rudnick
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