Specific Conductivity Vs. Cation Conductivity
What are specific conductivity and cation conductivity?
The overall ability of water to conduct electricity is reflected by its specific conductivity (commonly known as conductivity), which quantifies the total ionic contamination or total dissolved solids (TDS) in water. It detects both - the positive ions (cations) and the negative ions (anions) in the water. It is less susceptible to contamination at very low levels.
In high-purity water systems such as power plants, cation conductivity, also known as cation-exchange conductivity or after-cation conductivity (ACC), is a more sensitive measurement. It involves passing the water through a strong acid cation exchange resin, which replaces all positive ions (cations) with [H+] ions. This process converts neutral or less conductive anions into highly conductive acid anions, amplifying the contribution of contaminants like chloride and sulphate and thus detecting very low-level ionic impurities.
Actually, cation conductivity is used to detect chloride and sulphate contamination. Early detection of this contamination gives the plant an opportunity to take action before deposits and corrosion build up on the turbine.
Specific conductivity is a general measure of all ions, while cation conductivity is a specialized technique that transforms anions into their extremely conductive acid counterparts in order to isolate their influence, providing a far more accurate way to detect ionic contamination in high-purity water systems.
Why it is important to measure cation conductivity:
Cation exchange resin eliminates positively charged ions (cations) and substitutes them with [H+] ions, as was previously mentioned.
Cations are found in extremely low concentrations (in ppb) in high-purity water. When the water is passed through the cation column; for example, if NaCl is present, the [Na⁺] ion is removed and the [Cl⁻] ion reacts with the [H⁺] ion to produce HCl (hydrochloric acid), which has a higher conductivity than mineral salt solutions. The generally accepted lowest detection limit (LDL) is 0.05 μS/cm. With such sensitivity, cation conductivity can be a very useful measurement to detect condenser tube leaks.
Cation conductivity measures the anions in water indirectly, primarily the chloride and sulphate found in steam samples. Stated differently, cation conductivity indicates the purity of steam and magnifies the anion present in water. For boiler drum units under 200 MW, the majority of steam turbine manufacturers recommend a cation conductivity limit of less than 0.3 µs/cm.
Please remember that water does not contain a specific cation or anion on its own. Their presence is always in combination with one another. such as MgCl₂, NaCl, etc.
High conductivity is observed in the water due to the below points:
1) A high level of anion contamination is present in water.
2) Exhausted cation exchange column.
3) Overfeed of organic amine & organic oxygen scavenger chemicals.
4) Total organic carbon or organic decomposition products (acetate, formate) present in high level.
Crucial measurement locations of cation conductivity in power plants:
It is frequently measured in power plants' water and steam cycles on:
Boiler drum (the part of a boiler that stores water and produces steam).
Feedwater (DM water supplied to the boiler).
Condensate (water that passes through a turbine and condenses from steam).
Reheat steam (returned steam from a turbine is heated again in a reheater before being sent back to the turbine).
Main steam (after going through the superheater, the primary steam exits the boiler).
In practice, well-controlled and maintained feedwater chemistry can consistently maintain cation conductivity below 0.5 µs/cm when using organic amine and organic oxygenscavenger chemicals.