AW T4 20 |
U N I V ER S A L 4 - W I R E , D UA L- I N P U T T R A N SM I T T ER | O I/AW T4 2 0 - EN R E V. B
101
Appendix C Dual 2-electrode conductivity calculations
Inferred pH derived from differential
conductivity
Monitoring on steam-raising plant
For many years, it has been standard practice in power plants
to use inferred pH, calculated from before- and after-cation
conductivity measurements, to confirm values obtained by
direct laboratory or on-line pH measurement.
According to EPRI, IEC and VGB Guidelines, feedwater and
boiler water quality can be assessed by measuring the
conductivity of samples before and after a cation ion-exchange
resin column. Depending on the type of plant and chemical
treatment applied, differential conductivity can also give an
indication of the pH of the sample.
Both before and after measurements can be made on one
dual input conductivity analyzer.
The choice of inferred pH calculation depends on controlled
chemical conditions, i.e. whether or not the system is an NH
3
,
NH
3
+NaCl or NaOH dosed system.
NOTICE
• If the analyzer is used with a cation resin column, Sensor A
must be installed before the column and Sensor B after
the column for the correct calculation of inferred pH.
• Both conductivity inputs must be configured as µS cm
-1
to calculate inferred pH.
WARNING
The calculation of inferred pH relies on the strict control of
chemical conditions within the NH3, NH3+NaCl or NaOH
dosed sample. Contamination with chemical substances
other than those with which the sample is dosed introduces
significant errors in the inferred pH calculated value and, in
the worse case, invalidates the calculation completely.
Carbon dioxide in particular has a very adverse affect.
Sources of CO2 contamination include:
• Boiler start-up.
– CO
2
can be present in the sample for several hours or
even days immediately after boiler start-up.
Note
. This also applies to 'two shifting' or 'cycling'
boilers, i.e., boilers whose full output is required only
during peak demand periods.
• Organic compound contamination.
– Decomposing organic compounds are a source of CO
2
contamination.
– Organic compound contamination may be caused by
break-through from the water treatment plant or from
condenser leaks.
– Formates are also formed when organic compounds
decompose, these further increase errors in inferred pH
calculation.
• Carbon compound contamination.
– The use of carbon compound chemical treatments such
as carbohydrazide (used as an oxygen scavenger) can
contaminate the sample with CO
2
.
Independent pH readings are necessary to confirm that the
correct chemical conditions prevail for the accurate
calculation of inferred pH.
