Boiler Feedwater

How Do Chemical Treatments for High-Pressure and Lower-Pressure Boilers Differ?

By Bill Boyd

BOILERS CORROSION HRSG OXYGEN POWER SCALING

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Abstract

There are different categories of high-pressure boilers. Some licensing agencies consider more than 100 pounds per square inch gauge (psig) as high pressure (HP). Utility high-pressure boilers are often from 600 psig drums, up to super-critical once-through (>3,200 psig).

The reaction rates typically increase as you increase temperatures and pressures. Lower-pressure (LP) boilers are generally considered more forgiving than higher-pressure boilers. Typically, they can handle more contamination in the feedwater/drum and are more flexible with the types of treatments than higher-pressure units. Boilers get more restrictive in treatment choices and less tolerant to contamination as pressure increases. Boilers are more susceptible to corrosion at the higher temperatures and there is greater concern for carryover and volatility of solids to the turbine.

High-pressure steam boilers that produce steam for condenser turbines and return condensate for reuse are more restrictive in required purity and treatment requirements than smaller units that produce steam for external uses that may or may not return condensate for reuse. Higher-pressure units are designed to use pure water and treatments to produce pure steam with no carryover of solids to the turbine. Small amounts of contaminated steam can damage turbines. Figure 1 shows a turbine damaged by solids.

The purity is maintained throughout the cycle by the exclusion of contamination and the use of pure volatile treatments that maintain elevated pH throughout the cycle.

After achieving an operating level of chemical treatment, a much reduced but continued feed is required to replace product lost in blowdown, steam losses, product that has decomposed, and neutralization of any organic acids or carbon dioxide in the system. Carbon dioxide can enter the system through air-in-leakage or decomposition products of organics.

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