Degas conductivity is used as an indicator of water quality in the water/steam cycle of power stations. Excessive conductivity values often indicate high corrosion potential, especially with certain ions such as chloride and acetate ions. These can be particularly damaging to the blades in the steam turbine. Degas conductivity is measured after the water sample has flowed through a resin and has had carbon dioxide removed by a degassing process. Specific conductivity and Cation conductivity are the other main types of measurement.
Application of conductivity in steam analysis
Conductivity measurements in the water/steam cycle of power stations are commonly used as indicators of the quality of the water used in the process. Excessive conductivity values often indicate a high corrosion potential, especially in the case of certain ions such as chloride and acetate ions. These can be particularly damaging to the blades in the steam turbine.
Typically, there are three major types of conductivity measurements used: Specific conductivity, a measurement that indicates the total dissolved solids in an aqueous solution Cation conductivity, a measurement taken after the water sample has flowed through a resin bed (known as a cation exchanger) Degas conductivity, a measurement taken after the water sample has flowed through a resin and has had carbon dioxide removed by a degassing process
Generally, degas conductivity is measured from condensed and cooled samples of primary steam. It may also be relevant for analyzing condensate return, especially in cases where the condensate is returned from a separate plant that used the steam in another process.
Above three conductivity measurement gives one of the most robust measurement. Also with three measurement it is possible to calculate:
(1) pH of condensate/ Steam and Boiler feed water. (Refer VGB-S-010-T-00;2011-12.EN-ebook)
(2) Calculated values (Refer ASTM D4519 Standard)
Methodology
After the ions have been removed from the conditioning of the circulating water (e.g. ammonium NH4+) in the cation exchanger, ions resulting from gaseous components must be removed in order to determine degas conductivity. These are typically gases from the atmosphere which have penetrated into the system through leaks in the water-steam circuit. Of all gases occurring in the atmosphere, typically only carbon dioxide (CO2) dissolves chemically into ions in circulating water. The remaining gases (oxygen, nitrogen, etc.) dissolve physically and do not form ions, and thus do not contribute to conductivity. The chemical reactions of carbon dioxide in water proceed according to the following reaction equation (mass action law):
A) CO2 + 2 H2O <--> HCO3 + H3O+ pK = 6.3
B) HCO3− + 2 H2O <--> CO32− pK = 10.3
See the graph showing relative CO2 concentration. After the cation exchanger, the sample pH value is generally 5.5–6, so that means almost only CO2 is present as gas, and only about 6% is carbon carbonate ion CO32−. The bicarbonate ion (HCO3−) is practically absent.
However, ionic components of carbon dioxide are far less dangerous to corrosion than the ions of the saline components, e.g. Cl−. In order to obtain a selective conductivity value for these saline-containing ions (with the maximum potential for corrosion), all remaining carbon dioxide must be removed from the sample in order to accurately determine the presence of corrosive ions.
There are generally two methods for removing carbon dioxide from the water sample: use of a reboiler to heat the sample and expel the CO2, and use of inert gasses. In the latter method, an inert gas which does not contain CO2 is passed through the sample water, whereby the gas components in the sample water are displaced by the gas components of the inert gas. Use of bottled inert gases can be problematic in some industrial applications. Reboilers are very efficient degassing with results over 92%, but they typically require anywhere from 20 to 45 minutes to achieve useful results. Manufacturers of reboiler systems include Swan Analytical, Forbes Marshall, Mettler Toledo, and Sentry Equipment Corp.