Introduction
11
Waltron User Manual 101-046-B.3
9096 Degassed Cation Conductivity Analyzer
2
INTRODUCTION
2.1
PRELIMINARY REMARKS
The primary purpose of power plant chemistry programs is the protection of major capital
components by the prevention of corrosion. Online instrumentation is used to monitor the
presence of potentially corrosive chemicals in the water and steam cycle, thereby giving the
operator the opportunity to take corrective actions to prevent corrosion or depostion of deposits
on the major components in the steam generator or turbine.
Conductivity is frequently used as a water quality measurement. However, the presence of
commonly used chemicals such as ammonia, hydrazine, or caustic can mask the effects of
corrosive species in the water if conductivity is used without further conditioning. Cation or acid
conductivity is used to remove the cationic species from the water by passing the sample through
a resin column containing hydrogen form cation resin.
The presence of dissolved carbon dioxide increases the cation conductivity but does not have any
negative influence on corrosion. For this reason, removing the carbon dioxide from the cation
resin treated sample makes it possible to obtain the cation conductivity value due to the presence
of corrosive species, especially sulfate and chloride. Especially during turbine start-up when there
are large amounts of carbon dioxide present in the steam and condensate, accurate cation
conductivity readings are needed to minimize startup times and meet production goals.
The conductivity measurements in power plants provide indispensable information about the state
of the water-steam circuit. Specific conductivity can be used to infer the pH and Ammonia
concentration. Passing a water sample first through a cation exchange column removes the high
conductivity cations such as NH
4
+
to provide a conductivity value based to the total anion load in
the sample. Frequently, there is enough dissolved carbon dioxide in a sample to provide a false
“high” cation conductivity indication, so removing the carbon dioxide becomes the final step in
providing an accurate cation conductivity value.
The new 9096 Degassed Cation Conductivity Analyzer from Waltron removes carbon dioxide
using a desorption column. Using this method, 93.5% of the CO
2
is removed in the 45 seconds
the sample is in the desorption column. Another advantage of this approach is that there is no
requirement for either a Nitrogen source nor for a heater to boil off the CO
2
as was done with the
original reboiler units.
2.2
WORKING PRINCIPLE
As the sample passes through the cation exchanger, the ions from the process treated sample
water (eg. Ammonium NH
4
+
) are removed. Next, the ions present due to gaseous components
must be removed. These gaseous components from the atmosphere are introduced into the
sample during the process and through leaks in the water - steam cycle. Of the atmospheric
gases, only carbon dioxide (CO
2
) dissolves in the water sample to chemically form ions. 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 are shown by the
following equations:
A) CO
2
+ 2 H
2
O
HCO
3
-
+ H
2
O + H
+
pK = 6,3
