Corrosion reduction system for power generation plants during shutdown

Abstract

The invention involves a method of reducing corrosion inside power generation cycle equipment during plant shutdown and start-up periods by substantially reducing or totally eliminating internal contamination with corrosion causing acidic gases such as carbon dioxide and oxides of sulfur and nitrogen. Acidic gases are removed at their potential points of entry into the cycle either from ingressing ambient air or dissolved in make-up water in different kinds of filters.

Description

FIELD OF INVENTION[0001]This invention relates generally to corrosion protection of metal vessels containing liquid water and water vapors and more specifically to corrosion protection of internal surfaces of steam and water cycle equipment of both nuclear and conventional fossil fuel fired power generating plants.BACKGROUND OF THE INVENTION[0002]Power generating plants, using either nuclear or conventional fuel, heat water in boilers to generate steam. Steam then powers turbines and electric generators producing electricity. Steam exiting from turbines is condensed in the main steam condenser from which it is pumped back into boilers. The main construction materials of a power generation cycle plant are iron and copper based alloys. Its internal volume may be up to several hundred cubic meters while internal surfaces needing protection against corrosion may be of the order of several tens of thousands m2.[0003]FIG. 1 is the schematic of a typical steam and water cycle in a fossil f...

AI Technical Summary

Benefits of technology

[0016]The object of this invention is a reduction of corrosion inside power generation cycle equipment during plant shutdown and start-up periods by substantially reducing or totally eliminating internal contamination with corrosion causing acidic gases such as carbon dioxide and oxides of sulfur and nitrogen. Acidic gases are removed at their potential points of entry into the cycle either from ingressing ambient air or dissolved in make-up water in two different kinds of filters.

Problems solved by technology

During power plant shutdown periods, some or all parts of the power generating cycle are opened to atmosphere and the quality of water in contact with internal cycle surfaces is typically well outside the recommended control range proportionately increasing system corrosion.

In boilers, reduction of heat transfer due to deposits may lead to overheating tube failures or, if corrosive anions are also present, to under deposit corrosion attack, such as hydrogen damage.

Deposits in turbines may lead both to corrosion failures due to under-deposit corrosion attack and / or plant capacity reduction.

It is also known that low pH water during plant start-ups and shut downs causes initiation and / or propagation of corrosion fatigue cracks on internal surfaces of thermally stressed carbon steel plant components.

Corrosion fatigue failures in boilers are known to be the main cause of plant unreliability but similar type failures in de-aerators and turbines are also important since they may be catastrophic, causing long periods of plant unavailability (EPRI, Palo Alto, Calif., TR-106696, November 1997).

Deposition of corrosion products in nuclear power plant cycles is also known to aggravate health and safety problems due to increased radioactive radiation exposure of station staff.

Further, the necessary periodic removal of excessive levels of corrosion products with chemical solvents generates large quantities of toxic wastes in both the conventional and nuclear power stations.

Field studies indicate that corrosion rate of cycle materials during unprotected shutdown periods does correspond to the inferior quality of water in power cycles.

In summary, it is the corrosion during shutdowns and early start-ups and not the one during closely controlled operating periods, which is the root cause of the majority of reliability and availability problems in power plants.

This technique is the most damaging one to plant equipment integrity yet it is practiced by up to 80% utilities.

Disadvantage of this method is that the protective solutions cannot reach majority of other surfaces around the cycle and its value to the plant equipment protection is thus rather limited.

Another disadvantage is that hydrazine is a known carcinogen and its solutions, which have to be drained from the cycle prior to each plant re-start, pose a serious risk to the environment.

Disadvantage of this art is that draining of system components is ineffective and de-humidification process is time consuming and labor intensive.

Disadvantage of this art is its logistics constraints, technical problems with its comprehensive application and potential safety hazard of asphyxiation to plant personnel (The ASME Handbook on Water Technology for Thermal Power Systems, Chapter 22).

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