Even utilities that are very diligent about treating and monitoring their boiler water, demineralized water makeup, and cooling water may know little about treating one of the most critical water systems in the plant: stator cooling water.
Stator cooling water is contained in a closed loop system that cools the copper stator bars in water-cooled generators. The holes though which the water flows are narrow. Unimpeded flow through all stator bar openings is critical to operation of the generator. Overheating of stator bars can result in reduced generating capacity or even catastrophic failure of the generator.
Stator cooling systems contain only deionized water. The cooling loop removes heat from the stator coolers and conveys it away through heat exchangers. The water is continuously passed through a mixed bed polisher that removes any soluble ionic contaminants that enter the water. The stator ion exchange resins often also act as a filter for particulates in the water, though some systems have a separate filter. The ion exchange resin will eventually become exhausted, but in many systems, it is common for the pressure differential across the resin bed (created by accumulated particulates) to require that the resins be replaced before the ion exchange capacity is reached.
Copper contamination
The stator cooling water system’s heat transfer surfaces are typically copper, though some are stainless steel. The chemistry of copper in oxidizing and reducing conditions has been the subject of a great deal of recent research. One area of intense focus has been copper corrosion in feedwater systems and corrosion product transport into the boiler and on to the HP turbine. Some of the general metallurgical principles learned in studying copper in feedwater systems can also be applied to stator cooling systems. As with copper in feedwater systems, we know that dissolved oxygen and pH play a major role in determining the corrosion product formation rate and transportation rate through the stator cooling system.
It is important to remember that the major cause of problems in stator cooling systems has not been corrosion per se but, rather, deposit accumulation in critical areas. These deposits are copper oxides released from one area of the stator coolers and deposited in another. The amount of dissolved oxygen in the system, and particularly variations in that oxygen concentration, determines when copper oxides are released.
Copper forms cuprous oxide (Cu2O) under reducing (low-oxygen) conditions and cupric oxide (CuO) when dissolved oxygen is high. Either of these oxides can be stable and create a passive oxide layer on the channels in the stator bars. A slightly alkaline pH increases the stability of the oxide layer.
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