A common problem in power plants attempting to maintain cation conductivity steam limits is the presence of carbon dioxide resulting in elevated cation conductivities above steam turbine manufacturer requirements for steam purity. CO2 in steam though is not a corrosion concern for most of the steam path in a power plant, it may cause issues for condenser or low pressure heater tubing constructed of copper alloys, but the steam turbine is unlikely to encounter issues due to the presence of CO2 in the steam. This is due to the fact that CO2 preferentially distributes into the gas phase in two phase mixtures, so while CO2 in water can be corrosive, in the steam turbine where liquid water is only likely to be present in the last stages of the LP turbine where it will be present in a two phase mixture (usually referred to as the phase transition zone), there will be virtually no CO2 in the condensing water.
Notwithstanding this, many steam turbine manufacturers only allow the measurement of cation conductivity for evaluating compliance to the prescribed limit as oppose to a degassed cation conductivity, which is the same measurement but with the CO2 contribution removed.
In these cases the power plant operator must find a way to meet the cation conductivity limit by reducing CO2 or end up voiding any steam turbine warranty the manufacturer provided. One method to effectively reduce CO2 in a power plant cycle is to reduce condensate pH. A rough rule of thumb applicable is that CO2 absorption from air ingress into a condenser can be reduced by a factor of 2 for every 0.5 pH unit reduction. So a unit running with a condensate pH of 9.5 with a cation conductivity of 0.3 uS/cm predominantly from CO2 can expect a reduction of cation conductivity down to 0.15 uS/cm by dropping the condensate pH to 9.0.
Adjusting pH to meet CO2 requirements though may cause other issues in the cycle including increasing the potential for iron corrosion and potentially two phase flow accelerated corrosion. Nonetheless the pH of the power plant condensate will be a primary factor affecting the amount of CO2 in the condensate and in cases where the source of air ingress into the condenser can not be eliminated, the pH offers a pathway to reduce CO2 concentrations.
