1. Conductivity in power plants
Introduction
A power plant with its steam/water cycle, raw water Due to the fact that the conductivity of the H+ is several
treatment plant and, if used, district heating water system, is times higher than the conductivity of all other cations, the
an evident field for conductivity measurements. sensitivity is enlarged several times (approx. 3.5 times).
Thus values of 1-5 mg/NaCl/ton can be detected.
Applications such as control of the water quality are nearby,
but also the ion-exchangers exhaust or wash-out after The diagram shown above is just an example showing
regeneration is easily controlled. possible applications of conductivity measurements. Not all
the points of measurement are always needed, but a suitable
Conductivity measurements are useful as supplement to selection can be made to meet individual requirements. The
other kinds of measurements. If a correlation exists between conductivity measurements are the most reliable quality
the pH-value and the ion-concentration, it is expected that a assurance regarding system purity and pH.
correlation also exists between the conductivity and the pH-
value. That is the case of the boiler feedwater when The raw water deionization plant shown is a rather complex
alkalized with ammonia. type with its weak acid and strong acid cation exchangers as
well as weak base and strong base anion exchangers. The
A peculiar application is the detection of impurity anions, advantage is an excellent utilization of the regeneration
where the water passes through a cation exchanger before reagents.
the conductivity measurement. The district heating water loop shown uses relatively pure
water. Common practice is often more simple concerning
What happens is: Cations from free base (NH+) are the water conditioning, the result of which is some quite
exchanged to H+, which reacts with OH- from the base, different conductivity values of the district heating water.
giving pure water of very low conductivity. Cations from
salts are also exchanged to H+, which form corresponding In the following, the single measurements are mentioned
amounts of free acids with anions from the salt. more detailed.
Application note A1.6E -1-

2. Indication of cooling water break-in Dosing of chemicals
After having passed the turbine and produced the power, The conductivity at 8 is used for direct control of the NH3
the steam condenses in the condenser. In coast regions, the or N2H4 dosage pump.
condenser is often cooled by seawater, the break-in of
which has serious effects due to its contents of salt. A Control of the boiler feed water:
leakage of the condenser (break-in of cooling water) will Quality, pH control, and salt
give a rise of the conductivity at 1 and 2. Measurement of contents
specific conductivity 1 gives a response very fast, but the Immediately after the mixed bed filters, the water is
sensitivity is not very high. When the cation exchanger alkalized to a pH value at 8-9.5. In addition, the water is
removes cations and increases the sensitivity, the preheated and deaerated. A sample of the boiler feed water
measurement of conductivity 2 will give an excellent is cooled, and the conductivity is measured by the
sensitivity. The cation filter will delay the response of instrument at 9 expressing pH-value. If no considerable
measurement. Thus, measurement 1 very fast indicates any amounts of impurity salts are present in the water, a
big in leakage and salt water and measurement 2 indicates correlation between the conductivity and the pH-value is
very small in leakage of salt water, though delayed for a found. In order to detect also small amounts of impurities,
few minutes. The cation exchanger has to remove the the conductivity is also measured after a cation exchanger,
conductivity from the alkalinity of the raw condensate and which removes the alkalinity and increases the sensitivity of
to increase the sensitivity of the measurement. the measurement. The meter at 10 makes the measurement
in question.
Control of district heating
condensate Control of the salt content in boiler
The steam, which is to heat the district heating supply water (drum boiler)
water, comes from the bleeding turbine. After a In the case of a drum boiler, it is of interest to control the
measurement of the conductivity at 3, the condensate is purity of the boiler water. A sample of the water from drum
diverted to the main turbine condensate if the conductivity is cooled, and the conductivity before and after a cation
is to high.If the conductivity is low, i.e. the condensate is exchanger is measured. The conductivity at 11 indicates the
pure enough, the condensate is pumped into the deaerator sum of dosed chemicals and accumulated salts; the
storage tank. conductivity at 12 indicates the amount of accumulated
salts. If NaOH is used as the alkalizing agent, pH can be
estimated from measurements 11 and 12. The control of
Control of the cation exchanger dosage can be based on these very reliable measurements.
The cation exchangers of the two parallel condensate The measurement of conductivity at 12 can be used for
polishers have to remove the cations as well as the control of blow-down from the drum.
ammonia. Under normal conditions, a rise of the
conductivity at 4 (or 5) indicates and exhausted ion Control of the water in the start
exchanger. A cooling water break-in also gives a rise of cyclone
conductivity, but that is detected by a corresponding rise of As previously mentioned, the cation exchanger removes the
the conductivity at 2. alkalinity from the N2H4 and NH3 and gains the sensitivity
of the salt measurement. In case of a once through boiler,
Control of the condensate polisher the measurement at 12 is made of the water in the start
effluent, control of the ion cyclone during start-up or low load operation.
exchangers
The mixed bed ion exchangers have to secure the purity of
the boiler feedwater. The water is now ultra pure water,
which is practically free of impurity ions. The conductivity
meter 6 (or 7) controls the purity of the water prior to the
dosage of chemicals e.g. NH3 and N2H4. In plants with full
flow condensate polishing, this measurement is often
considered as the most important criterion for assurance of
pure steam/water cycle. The meter also indicates the end of
rinse after regeneration of the mixed bed exchanger to
determine the point of putting into operation.
Application note A1.6E -2-

3. Steady control of the boiler steam
Control of the make-up water
salt content
Small quantities of salt are carried with the steam. To The make-up water is normally stored in a tank before
control that, a conductivity measurement can be made on a injection in the condenser, which acts as a deaerator. Under
sample of condensed steam. This measurement is also made certain circumstances, the water absorbs carbondioxide or
after a cation exchanger, which removes the ammonia impurity ions which increases the conductivity. The
always present in the steam due to the boiler feedwater conductivity at 17 controls the purity of the make-up prior
dosage and thus increases the sensitivity of the salt to injection into the condenser.
conductivity measurement.
Control of district heating return
Control of ion exchanger and rinse water
after regeneration Several possibilities of contaminating the district heating
The conductivity measurement at 14 and 15 can be used to water exist. The measurements of the district heating return
control the proper functioning of the ion exchangers as well water conductivity is the best quality control and can also
as the end of operation cycle and the end of rinse after be used to localize a leak section.
regeneration. If recorded, the conductivity can be used to
predict the rate of regeneration of the mixed bed filter. The Control of district heating water
weak base anion exchanger A1 absorbs mineral acids and before heat exchanger
the bulk of organic content of raw water. The break through The district heating return water is conditioned to pH 9 or
of mineral acids is giving an increased conductivity after more to protect the whole water system including the heat
the weak base anion exchanger bed. The mineral acids start exchangers. Conductivity at 20 is used to control the
to replace previously absorbed organics, which can result in conditioning.
fouling of the strong acid anion exchanger. The strong base
anion exchanger A2 normally absorbs the silicate and the
Control of the district heating
rest of the carbon dioxide. Sodium leakage of the cation
conditioning loop
exchangers will produce NaOH at the output of the A2 ion
exchanger, giving a rise of the conductivity as well as of the In the district heating water-conditioning loop cation
pH-value. exchanger removes the alkalinity, and the anion exchanger
purifies the water. The conductivities at 21 and 22 can be
used to control the proper functioning of the ion exchangers
Control of the make-up water and of
as well as the exhaust and the end of rinse after
the ion exchanger
regeneration.
After the mixed bed filter, the water is practically free of
impurity ions SiO2 and CO2, and ready for use as make-up
water. The mixed bed filter absorbs the impurity ions
passing through the precedent ion exchangers. The
conductivity at 16 controls the purity of the make-up water
when leaving the deionization plant; the measurement also
determines the exhaust of the mixed bed filter as well as the
end of the rinse after regeneration.
Application note A1.6E -3-

4. The conductivity of high purity water
Ultra pure water is water from which all impurity ions have This cable compensator is used in monitor types 3213 and
been removed. The high purity water from the raw water 3214. The conductivity changes with the temperature in a
treatment plant and from the condensate polisher of modern way, which depends on the ions present in the water. For
power plants is close to the ultra pure water level. that reason the temperature is measured by a platinum
resistance thermometer and used for temperature
Water in this range has an odd conductivity/ temperature compensation.
relationship, the curves of which can be seen at figure 1 for
different levels of purity. By an A/D converter, a microprocessor reads the
conductivity and the temperature. The microprocessor
Big problems arise about the temperature compensation of recalculates the measured conductivity at the actual
the high purity water conductivity, since the curves change temperature to its value at the reference temperature. The
according to the purity. However, the problems can be temperature compensation algorithms take into account the
solved. conductivity originating from the auto-proteolysis of the
water, the so-called background conductivity.
The conductivity of high purity water is contributed by the
thermic dissociation of the water molecule itself H2O ï H+ First the background conductivity at the actual temperature
+ OH-, called the background conductivity, and by the is calculated and subtracted from the measured
impurity ions present, e.g. NaCl → Na+ + Cl-, called the conductivity. Then the remaining conductivity is
salt conductivity. compensated for according to the selected reference
temperature and temperature compensation (neutral, acidic,
alkaline, or ammonia environments). Finally, the
background conductivity is calculated at the reference
temperature and added.
Fig. 2
The principle of measurements
Conductivity measurement by means of the 4-electrode The necessity of the way of compensating used is illustrated
system is based upon separate measurements of current and in figure 2. The upper curve shows the reading of a
voltage. Hence electrode overvoltage due to polarization conductivity meter (e.g. type 2822) having a single TC
and scaling is compensated for. compensating according to the salt conductivity
temperature curve only. The error (mS/m) is independent of
A separate channel measures the potential of the voltage the range of the conductivity meter.
electrodes. The signal is rectified and compared to a
reference voltage in a regulator, which controls the current The lower curves show the reading of a conductivity meter
electrode current to provide a constant potential across the type 3213 having a dual TC, compensating according to
voltage electrodes. The current flowing is now linear to the both the salt conductivity temperature curve and the
conductivity. background conductivity curve.
The 4-electrode sensors can have very small electrodes. The If the background conductivity is not compensated for, it is
electrode capacitance will thus be very low and no errors evident that troubles arise when conductivities close to ultra
are seen due to the electrode capacitance. The cable pure water conductivity (0.055 µS/cm at 25°C) are
capacitance is compensated for by a unique built-in cable detected, especially when the temperature varies.
compensator, for all cable lengths up to at least 50 meters.
Application note A1.6E www.kemotron.com