By Brad Buecker, Corresponding Editor
Water treatment recommendations
More and more power generation in the United States will come from Independent Power Producers (IPPs) and utilities or industries that install combined-cycle or cogeneration power plants. The boilers or heat recovery steam generators on such plants are often designed to operate at pressures of 700 psi to 1,200 psi. While this may not seem excessive when compared to utility boilers that operate at pressures up to 3,400 psi, water chemistry is still very important. Here are some of the latest water chemistry treatment trends for IPP and cogeneration plant operators.
Oxygen is often the prime corrodent of feedwater systems and boilers. It usually enters feedwater systems via air leaks at the condenser and in the makeup water. Unless oxygen is quickly removed from the condensate, it will attack feedwater and boiler components. Furthermore, the corrosion products can then be transported to the boiler where they will deposit on the boiler tubes and cause additional problems. In low pressure boilers (
Unfortunately, at pressures above 900 psi (and in some cases at pressures no greater than 600 psi), sodium sulfite breaks down to produce sulfur dioxide and hydrogen sulfide, two acidic, highly corrosive compounds. For many years, therefore, the preferred oxygen scavenger for medium and high pressure boilers has been hydrazine (N2H4). Hydrazine reacts with oxygen to produce nitrogen and water, and it passivates oxidized steel and copper surfaces. Should any hydrazine reach the boiler, it decomposes into ammonia, an alkaline compound. Furthermore, hydrazine does not add dissolved solids to boiler systems as does sodium sulfite. However, increasing evidence suggests that hydrazine may be a carcinogen, and its use is being restricted or eliminated at many utilities.
New plant operators should consider some of the hydrazine substitutes on the market. The most common are carbohydrazide, hydroquinone and methyl ethyl ketoxime. Carbohydrazide breaks down to hydrazine after it enters the feedwater system. Hydroquinone and methyl ethyl ketoxime react with oxygen directly. In many cases, these substitutes react with oxygen and/or passivate metal surfaces at lower temperatures than hydrazine, and thus can be added at the condensate pump discharge. This offers protection for the entire condensate/feedwater system. These chemicals are supplied by the various water treatment companies.
Boiler water chemistry and monitoring
For medium-pressure units, phosphate treatment of the boiler water is still the preferred choice. The detailed chemistry of phosphate treatment programs has been well documented and does not need to be recounted here. However, the following discussion illustrates the importance of good water chemistry monitoring. Plant managers need to be aware that boiler water chemistry can change quickly.
Sodium phosphate compounds are added to boiler water to control the pH and to react with any hardness compounds that might enter due to condenser leaks, demineralizer upsets, etc. But, phosphate is not a catch-all for any problems that arise. Even a moderate condenser tube leak can introduce enough contaminants to quickly consume the available phosphate. Once this happens, the heat from the boiler will cause reactions between the contaminants and water to produce acidic or basic conditions depending on the nature of the contaminants.
Furthermore, the untreated hardness compounds will deposit on the boiler tubes and cause conditions which can lead to tube overheating or underdeposit corrosion. Severe tube damage has been known to occur within hours following a major chemistry upset. Accordingly, many utilities have installed continuous, on-line chemistry analyzers that sample strategic locations in the feedwater/boiler water/steam circuit. The sophistication of such systems varies from plant to plant, but at a minimum, continuous monitoring of the condensate conductivity or sodium and the boiler water pH should be considered. Catastrophic damage to boiler tubes usually is caused by a condenser leak and subsequent consumption of the boiler water phosphate. These conditions will first be detected by the analyzers listed above. Plant chemists and operators must be alerted whenever such conditions occur.
Chemical treatment of cooling water is an issue that is heating up because of the Environmental Protection Agency?s (EPA) concern with chlorine. Chlorine reacts with organics in water to produce halogenated compounds (e.g., trihalomethane) which are suspected carcinogens. Some environmental groups are calling for an all-out ban on chlorine. While this may seem to be excessively restrictive, it certainly appears that the EPA will lower the concentrations of chlorine allowed in utility and industrial cooling waters. The limits may become so low as to make chlorine ineffective against microbiological fouling. Plant owners and managers should keep this in mind when applying or reapplying for operating permits.
For new plant managers, and for current managers, environmental regulations and other factors are changing the ways in which plants may be operated. It is wise to prepare for these changes well in advance. END