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A3 Inferred pH Derived from
Differential Conductivity
A3.1 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
NH
+NaCl or NaOH dosed system.
3
Notes.
• If the analyzer is used with a cation resin column,
Sensor A
be installed
must
Sensor B
the column for the correct calculation
after
of inferred pH.
• Both conductivity inputs must be configured as
S cm
–1
in order to calculate inferred pH.
,
3
the column and
before
Warning.
The calculation of inferred pH relies on the strict control
of chemical conditions within the NH
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
include:
• Boiler start-up. CO
can be present in the sample
2
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
contamination. Organic compound contamination
may be caused by break-through from the water
treatment plant or from condenser leaks. Formates
are
also
formed
when
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
Independant pH readings are necessary to confirm that
the correct chemical conditions prevail for the accurate
calculation of inferred pH.
APPENDIX A...
, NH
+NaCl or
3
3
contamination
2
a
source
of
CO
2
organic
compounds
.
2
75
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