ABB AX410, User Manual

Page: 79 / 84

Single and Dual Input Analyzers for Low Level Conductivity
AX410, AX411, AX413, AX416, AX418, AX450, AX455 & AX456 Appendix A
IM/AX4CO Issue 11 77
A.3.4 Monitoring on Solid Alkaline Treated Systems
Generally, boiler waters treated with solid alkaline chemicals, for
example, sodium hydroxide, have relatively high conductivities.
The dual input conductivity analyzer, in conjunction with a cation
resin column, can be used to indicate sample pH. If the sample
also contains salts (e.g. sodium chloride), the after-cation
conductivity reading reflects the acid conductivity released by
the salts; the reading is typically three times higher than normal
owing to the acid. Hence to derive the concentration and pH of
the alkaline agent, one third of the after-cation conductivity
increase must be subtracted from the before-column reading. In
addition, a factor must be applied for the molar conductivity
change of the alkaline agent. The analyzer software applies the
following equation:
Where: BC = the before column reading
AC = the after column reading
F = molar conductivity change for the alkaline
agent (243 μ S cm -1 per mmol/l for sodium
hydroxide*)
The maximum after-cation conductivity value is programmable
between 1.00 and 100.0
μ S cm -1 dependent on local conditions.
After-cation values above this level generate an
AFTER CAT.
HIGH
error message and before-cation values above 100.0 μ S
cm -1 generate a
BEFORE CAT. HIGH
error message. The inferred
pH range is 7 to 11pH; values above 11pH generate an
Infr. pH
invalid
error message. Refer to Section 8 for description of error
messages.
The inferred pH feature can be used on solid alkali treated
systems only in the following circumstances:
1. On steam raising plant.
2. For boiler chemical treatment such as sodium hydroxide.
In this instance,
A: Temp. Comp.
must be set to
NaOH
and
B: Temp. Comp.
must be set to
ACID
- see section 5.3,
page 21.
3. Where the after-cation conductivity value is less than
100.0 μ S cm -1 .
* Refer to Appendix to VGB Guideline VGB-R 450 L.
Note.
Inferred pH measurement on solid alkaline treated
systems is inappropriate to samples containing sodium
phosphate, ammonia or morpholine.
Inferred pH = 11 + log(BC –
1 /
3
AC)
F