Single and Dual Input Analyzers for Low Level Conductivity
AX410, AX411, AX413, AX416, AX418, AX450, AX455 & AX456
Appendix A
76
IM/AX4CO Issue 11
A.3.2 Monitoring on AVT Systems
For low conductivity feedwater applications, all volatile chemical
treatment (AVT) is often applied.
Where cation resin columns are used to remove the effects on
the conductivity measurement of volatile ammonia and
hydrazine chemical treatment, it is common practice to measure
both before- and after-cation conductivity. The sensitivity of the
conductivity measurement to chemical treatments is increased
by passing the sample through the cation column.
If it is known that a sample contains only one impurity (e.g.
ammonia), the conductivity measurement now becomes an
indication of the concentration of that impurity and it is then
possible to calculate the pH of the sample from the
concentration data. The result is referred to as 'inferred pH'.
The maximum after-cation conductivity value is programmable
between 0.060 and 10.00
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
25.00
μ
S cm
-1
generate a
BEFORE CAT. HIGH
error message.
The inferred pH range is 7 to 10pH; values above 10pH
generate an
Infr. pH invalid
error message. Refer to Section 8 for
description of error messages.
The inferred pH feature can be used on AVT systems only in the
following circumstances:
1. On steam raising plant.
2. For boiler chemical treatment such as ammonia and/or
hydrazine. In this instance,
A: Temp. Comp.
must be set to
NH3
and
B: Temp. Comp.
must be set to
ACID
- see
Section 5.3.
3. Where the after-cation conductivity value is insignificant
compared to the before-cation value.
A.3.3 Monitoring on AVT Systems with Impurities
Differential conductivity can also give an indication of sample pH
on AVT systems where there are low concentrations of ionic
impurities present in addition to the volatile alkaline agent (e.g.
sodium ch ammonia). In this case, the exchange of
ammonium and sodium ions within the cation column releases
water and hydrochloric acid. The sodium chloride impurity
produces a conductivity after the column that is higher than the
conductivity before. The dual input analyzer, when used to
monitor before- and after-cation conductivities, compensates for
this increase and calculates the inferred pH of the incoming
sample. The user-configurable, after-cation conductivity alarm
can be used to detect unacceptably high levels of impurities in
the sample and inform the user of the validity of the inferred pH
value.
The calculated inferred pH is proportional to:
BC - (AC - 0.055)/R
Where:
BC
= the before column reading
AC
= the after column reading
0.055 = the conductivity of pure water at
25ºC in
μ
S cm
-1
R
= a ratio factor depending on the BC
and AC readings.
The maximum after-cation conductivity value is programmable
between 0.060 and 25.00
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
25.00
μ
S cm
-1
generate a
BEFORE CAT. HIGH
error message.
The inferred pH range is 7 to 10pH; values above 10pH
generate an
Infr. pH invalid
error message. Refer to Section 8 for
description of error messages.
The inferred pH feature can be used on AVT systems with
impurities only in the following circumstances:
1. On steam raising plant.
2. For boiler chemical treatment such as ammonia and/or
hydrazine. In this instance,
A: Temp. Comp.
must be set to
NH3
and
B: Temp. Comp.
must be set to
ACID
- see
Section 5.3.
3. Where the after-cation conductivity value is less than
25.00
μ
S cm
-1
.
Note.
Inferred pH measurement on AVT systems is
inappropriate to chemical treatments such as sodium
phosphate, sodium hydroxide and morpholine.
Note.
Inferred pH measurement on AVT systems with
impurities is inappropriate to chemical treatments such as
sodium phosphate, sodium hydroxide and morpholine.
