The degassed cation conductivity is a technique for measuring the purity of water used in power plants to precisely detect the concentration of ions that cause corrosion in the water/steam cycle. In order to give a more precise indication of the concentration of sulphate [SO₄] and chloride [Cl], volatile chemicals such as amine, ammonia, volatile organics and CO₂ are removed first from the steam.
Why is it important to measure Degassed Cation Conductivity ?
It helps to monitor trace amounts of corrosive contaminants (anions) like chlorides [Cl] and sulphate [SO₄] present in water/steam, which can cause corrosion of turbine blades.
It provides a more accurate picture of the actual ionic contaminants present by eliminating dissolved CO₂, which may have an impact on the measured conductivity.
The measurement is used to determine whether it is safe to allow steam flow to the turbine, which is crucial during plant startup, when air intrusion can result in higher conductivity. Purity of steam during plant startup is one of the important considerations for healthy and efficient operation of a steam turbine. Chemical analysis of steam during startup and inspection of the turbine give a good indication of steam quality.
How is it measured?
There are three primary steps in the measurement procedure:
1)Degassing:
The degassed cation conductivity measurement employs the same ion exchange technique as the cation conductivity measurement, which involves heating the water, using a reboiler, to a temperature close to boiling. Non-volatile inorganic compounds (such as sulphate [SO₄] and chloride [Cl]) are left behind while the volatile organic compounds and CO₂ are vented to the atmosphere. We term this process as degassing.
2)Cation Exchange:
After the sample has been degassed, it is cooled at 25°C before it is passed through a strong acid cation resin column, which converts all of the positive ions (cations) into hydrogen ions [H+]. Water purity is accurately determined by measuring the resulting conductivity, which represents the combined effect of any anions and the hydrogen ions [H+] from the exchanged cations.
3)Conductivity Measurement:
Finally, the conductivity sensor takes the reading after the sample has been cooled and the gases have been vented. Since the degassing process does not eliminate extremely corrosive inorganic anions like sulphate [SO₄] and chloride [Cl], the residual conductivity is a precise indicator of their concentration.
For instance, pure water at 25°C has a conductivity of 0.055 μS/cm, while pure water saturated with CO₂ can have a conductivity of 1.0 μS/cm. CO₂ in water is converted to carbonic acid, a weak acid, which causes an increase in conductivity. Nitrogen and helium, on the other hand, will dissolve in a sample but not dissociate or react with the water, unlike CO₂. Therefore, there won't be an increase in conductivity in a liquid that has been saturated with inert gas.
Therefore, the best way to determine the amount of inorganic anion (salt) in the steam is to measure the degassed cation conductivity.
Most steam turbine manufacturers recommend cation conductivity limits of 0.2 - 0.3 mS/cm. As stated earlier, the steam turbine manufacturer limits are not realistic unless ammonia and hydrazine are the only products used for feedwater treatment. In practice, well-controlled feedwater chemistry can consistently maintain cation conductivities in the range of 0.4 - 0.8 µS/cm when using organic amine or organic passivators.
Key Applications in a Thermal Power Plant:
Power cycle chemistry monitoring – for ensuring the purity of water in the water/steam cycle.
Plant startup—during starting, air intrusion is frequent and can lead to a rise in the cation conductivity artificially.
Troubleshooting - is useful in determining whether CO₂ or other corrosive species are the cause of high conductivity.