Single and Dual Input Analyzers for Low Level Conductivity
AX410, AX411, AX413, AX416, AX418, AX450, AX455 & AX456
Appendix A
IM/AX4CO Issue 11
75
A.3 Inferred pH Derived from Differential Conductivity
A.3.1 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.
Note.
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
μ
S
cm
-1
in order to calculate inferred pH.
Warning.
The calculation of inferred pH relies on the strict control of
chemical conditions within the NH
3
, NH
3
+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 CO
2
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 teatments such as
carbohydrazide (used as an oxygen scavenger) can
contaminate the sample with CO
2
.
Independant pH readings are necessary to confirm
that the correct chemical conditions prevail for the
accurate calculation of inferred pH.
