Boiler Feedwater

Part 1: Controlling Condensate and Feedwater Dissolved Oxygen and Air Inleakage at the Source

By Robert D. Bartholomew, P.E., and Gary H. Roberts, P.E.

CONDENSATE POLISHING CONDUCTIVITY DISSOLVED GASES OXYGEN POWER GENERATION STEAM

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Abstract

Air is comprised of 20.9% by volume oxygen (23.1% by weight) and ~0.04% by volume of carbon dioxide. Air is the ultimate source of all dissolved oxygen contamination of the condensate and feedwater. Deionized water in equilibrium with air can have ~8 to 14 parts per million (ppm) (~8,000 to 14,000 parts per billion [ppb]) as O2 of dissolved oxygen (DO), depending on the temperature and based on oxygen solubility. Carbon dioxide has a higher solubility and it undergoes equilibrium reactions with water. Therefore, even though carbon dioxide is a small proportion of air, substantial amounts can be absorbed in water. Based on experience, cation conductivities due to carbon dioxide absorption from the air can reach ~0.8-1.0 ᄉS/cm in demineralized makeup water. These conductivities theoretically correspond to about 0.50-0.75 ppm as CO2 of total inorganic carbon or about 0.41 to 0.64 ppm as CO2 of free carbon dioxide, which is in equilibrium with bicarbonate.

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