After-cation-exchange conductivity (ACC) systems are the universally used method for rapid detection of an-ionic impurity ingress into the steam/water circuit of power plant. In some utilities, ACC is measured after degassing the sample. It is generally considered that carbon dioxide is the least aggressive contaminant of feedwater that is normally present and causes elevation of the measured ACC. The elevation of ACC by carbon dioxide can be particularly noticeable when the oxygen scavenger is carbohydrazide, or the water contains dosed amines or neutral organic compounds in the make-up water. Ingress of carbon dioxide could be considerable in the case of tube leakage in some parts of the boilers of gas-cooled nuclear stations. The elevated ACC then delays unit start-up. There are two widely used techniques for degassed ACC systems: gas stripping with nitrogen and heating to near boiling. Membrane gas-exchange systems are now also beginning to appear on the market. British Nuclear Fuels Pic. and British Energy arranged for an evaluation of these three types of system to determine if they would be sufficiently effective for use in their power stations. The results of the evaluation are summarised here and measured values are compared with theoretical calculations for ACC and degassed ACC.
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