Power
Application Note
Medium Purity Condensate Monitoring
Background
In high pressure boilers, it is necessary to monitor the return
condensate before it goes back to the boiler to ensure that it is
not contaminated beyond acceptable limits. In smaller boilers,
the return condensate can have a fairly high dissolved solids
concentration, which can be measured with a standard conductivity
meter. In the case of large boilers, such as those used to generate
electricity, the allowable level of solids is in the low parts-permillion range. The conductivity caused by the dissolved solids
is often masked by the larger conductivities of water treatment
chemicals that have been added to protect the boiler. In a case like
this, the Larson Lane Condensate Monitor Model CH16D is used to
separate the conductivity caused by the dissolved solids from the
conductivity caused by the water treatment chemicals.
Process
In high purity boilers, the Total Dissolved Solids (TDS) level of
the water ought to be limited to a maximum of 1 to 2 ppm and
sometimes less than 0.1 ppm. In order to do this, it is necessary
to start with deionized water, which will have a conductivity in
the range of 0.055–0.1 μS/cm (μS/cm). Various water treatment
chemicals are then added to the deionized water to prevent deposits
from forming, to regulate the pH, and in other ways to improve
boiler life and performance.
When the water is boiled, most of the solids remain in the boiler
with a few of them being carried over into the steam. In time, some
steam will be lost and require make-up water to be added. This will
eventually cause a build-up of undesirable dissolved solids in the
water such as chlorides or sulfates, gases such as carbon dioxide or
oxygen, and other harmful materials.
A simple conductivity measurement of this water would measure
the total conductivity of the “good stuff,” such as the water
treatment chemicals, and the “bad stuff,” such as the chlorides,
sulfates, etc. In a smaller boiler, the total conductivity could be
between 1 and 20 μS/cm (or in some plants even higher), and it
will be necessary to detect contamination in the order of 0.1 to 2
ppm, which would add 0.2 to 5 μS/cm. This is not a very sensitive
technique.
One of the simplest methods of detecting small amounts of
contamination in return condensate is through the use of a cation
conductivity measurement. In this type of measurement, the
condensate must first be cooled to under 122 °F (50 °C) – preferably
under 86 °F (30 °C) to prolong resin life. The sample pressure must
be reduced to under 5 psi in most systems before the condensate
enters the cation column.
When the water passes through the cation resin bed, the resin
exchanges any positive ions in the solution for hydrogen ions. If a
salt, such as NaCl, passes through the resin, the Na+ ion is held in
the bed, and an H+ ion is released. In so doing, 1 ppm of NaCl salt
(which has a conductivity of approximately 2.2 μS/cm) is converted
to 1 ppm of HCl acid (which has a conductivity of approximately
11.7 μS/cm). In a like manner, other salts are converted to their
corresponding acids, which have 3 to 6 times the conductivity of the
corresponding salt. This is shown in Figure 1.
Chemicals that are alkaline in nature, including most water
treatment chemicals, also have their positive ion portion bound
in the resin column with a corresponding release of hydrogen ion.
Thus, a chemical such as ammonia (which would be NH4OH in
water), which has a conductivity of approximately 6.6 μS/cm for
1 ppm, has the NH4+ ion bound up in the resin bed and an H+ ion
released making HOH, or water, with no added conductivity. In this
way, most water treatment chemicals become bound in the column
and release water, reducing the conductivity contributed by these
chemicals to close to zero.
Chemicals that are acidic, such as H2CO3 (which is formed when CO2
dissolves in water), pass through the resin unchanged.
When the return condensate passes through the cation column,
the conductivity of the “bad stuff,” such as an acid, would be
unchanged, while that of dissolved salts would increase 3 to 6
times. The conductivity of the “good stuff,” such as water treatment
chemicals, will generally decrease to near zero. Thus, the cation
conductivity coming out of the resin column is caused by the
undesirable materials in the water. In this type of a system, we could
detect as little as 10–20 ppb of dissolved solids in condensate that
has a total conductivity of 5 μS/cm.
Power
Figure 1
Inlet
INLET
Neutral Salts
Na++ Cl- 2.2 μS/ppm
2Na++ Cl- 2.1μS/ppm
Alkaline
NH4+ OH- 6.6 μS/ppm
(Ammonia in Water)
Flow
Valve
Cation
Column
+
H+ H
H+
H+
H+
H
+
+
H+ H
H+
Acid
2H+ + CO3-2 5μS/ppm
(Carbon Dioxide in Water)
Outlet (drain)
H+
H+
H
H+
H+
H+
H+ H+
+
Flow
Meter
H+
Conductivity
Cell
Outlet
Changes to Acid
H+ + Cl- 11.7 μS/ppm
2H+ + SO4-2
8.8 μS/ppm
Changes to Water
H++ OH- 0 μS/ppm
Remains Unchanged
2H++ CO3-2 5 μS/ppm
Instrumentation
To make this measurement, the customer will need to reduce the
return condensate to a maximum of 122 °F (50 °C) and 5 psi. The
cation column would be the Model CH16D, with standard resin, or
the Model CH16DE, which has a resin that changes color to indicate
when it needs replacement.
An Endurance™ 404 conductivity flow cell with a1056 cation
conductivity analyzer is used to measure the outlet of the column.
Since this water is coming from a cation column, it is acidic and
requires an instrument with a temperature compensator designed
to track ultrapure cation water. In some cases, the customer may
also want to measure the condensate water before it goes into the
column, which will give an indication of the condition of the cation
column and the water treatment system. In this case, an additional
404 flow cell and a1056 dual input conductivity analyzer would be
used because this analyzer offers the flexibility of one analyzer to
measure two cell inputs.
1056 Dual Input Intelligent Analyzer
ƒƒ
ƒƒ
ƒƒ
ƒƒ
Multi-parameter – single or dual input
Easy to install with modular boards,
removable connectors, easy to wire power,
sensors, and outputs.
Intuitive menus screens with advanced
diagnostics and help screens save
maintenance time.
Exclusive Quick Start screens appear and
the instrument auto-recognizes each
measurement board and prompts the user
for immediate deployment
404 Flow-Through
Contacting Conductivity Sensor
ƒƒ
ƒƒ
ƒƒ
ƒƒ
Page 2
Small hold-up volume to ensure fast
response to changing conductivity.
PVC construction capable up to 60 °C
(two required).
316SST construction up to 100 °C
(two required).
Housing protects sample from air contamination.
Power
Notes
Page 3
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