Conductivity in power plantsIntroductionA power plant with its steam/water cycle, raw water               Due to the fact ...
Indication of cooling water break-in                             Dosing of chemicalsAfter having passed the turbine and pr...
Steady control of the boiler steam                                                                                 Control...
The conductivity of high purity waterUltra pure water is water from which all impurity ions have     This cable compensato...
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Conductivity in power plants

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Conductivity in power plants

  1. 1. Conductivity in power plantsIntroductionA power plant with its steam/water cycle, raw water Due to the fact that the conductivity of the H+ is severaltreatment plant and, if used, district heating water system, is times higher than the conductivity of all other cations, thean 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 showingregeneration is easily controlled. possible applications of conductivity measurements. Not all the points of measurement are always needed, but a suitableConductivity measurements are useful as supplement to selection can be made to meet individual requirements. Theother kinds of measurements. If a correlation exists between conductivity measurements are the most reliable qualitythe 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 complexalkalized with ammonia. type with its weak acid and strong acid cation exchangers as well as weak base and strong base anion exchangers. TheA peculiar application is the detection of impurity anions, advantage is an excellent utilization of the regenerationwhere 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 concerningWhat happens is: Cations from free base (NH+) are the water conditioning, the result of which is some quiteexchanged 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 fromsalts are also exchanged to H+, which form corresponding In the following, the single measurements are mentionedamounts of free acids with anions from the salt. more detailed.Application note A1.6E -1-
  2. 2. Indication of cooling water break-in Dosing of chemicalsAfter having passed the turbine and produced the power, The conductivity at 8 is used for direct control of the NH3the steam condenses in the condenser. In coast regions, the or N2H4 dosage pump.condenser is often cooled by seawater, the break-in ofwhich 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 saltgive a rise of the conductivity at 1 and 2. Measurement of contentsspecific conductivity 1 gives a response very fast, but the Immediately after the mixed bed filters, the water issensitivity is not very high. When the cation exchanger alkalized to a pH value at 8-9.5. In addition, the water isremoves cations and increases the sensitivity, the preheated and deaerated. A sample of the boiler feed watermeasurement of conductivity 2 will give an excellent is cooled, and the conductivity is measured by thesensitivity. The cation filter will delay the response of instrument at 9 expressing pH-value. If no considerablemeasurement. Thus, measurement 1 very fast indicates any amounts of impurity salts are present in the water, abig in leakage and salt water and measurement 2 indicates correlation between the conductivity and the pH-value isvery 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 ofto 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 boilerThe 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 themeasurement of the conductivity at 3, the condensate is purity of the boiler water. A sample of the water from drumdiverted to the main turbine condensate if the conductivity is cooled, and the conductivity before and after a cationis to high.If the conductivity is low, i.e. the condensate is exchanger is measured. The conductivity at 11 indicates thepure enough, the condensate is pumped into the deaerator sum of dosed chemicals and accumulated salts; thestorage 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 forpolishers have to remove the cations as well as the control of blow-down from the drum.ammonia. Under normal conditions, a rise of theconductivity at 4 (or 5) indicates and exhausted ion Control of the water in the startexchanger. A cooling water break-in also gives a rise of cycloneconductivity, but that is detected by a corresponding rise of As previously mentioned, the cation exchanger removes thethe 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. exchangersThe mixed bed ion exchangers have to secure the purity ofthe boiler feedwater. The water is now ultra pure water,which is practically free of impurity ions. The conductivitymeter 6 (or 7) controls the purity of the water prior to thedosage of chemicals e.g. NH3 and N2H4. In plants with fullflow condensate polishing, this measurement is oftenconsidered as the most important criterion for assurance ofpure steam/water cycle. The meter also indicates the end ofrinse after regeneration of the mixed bed exchanger todetermine the point of putting into operation.Application note A1.6E -2-
  3. 3. Steady control of the boiler steam Control of the make-up water salt contentSmall quantities of salt are carried with the steam. To The make-up water is normally stored in a tank beforecontrol that, a conductivity measurement can be made on a injection in the condenser, which acts as a deaerator. Undersample of condensed steam. This measurement is also made certain circumstances, the water absorbs carbondioxide orafter a cation exchanger, which removes the ammonia impurity ions which increases the conductivity. Thealways present in the steam due to the boiler feedwater conductivity at 17 controls the purity of the make-up priordosage 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 heatingThe conductivity measurement at 14 and 15 can be used to water exist. The measurements of the district heating returncontrol the proper functioning of the ion exchangers as well water conductivity is the best quality control and can alsoas 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 topredict the rate of regeneration of the mixed bed filter. The Control of district heating waterweak base anion exchanger A1 absorbs mineral acids and before heat exchangerthe bulk of organic content of raw water. The break through The district heating return water is conditioned to pH 9 orof mineral acids is giving an increased conductivity after more to protect the whole water system including the heatthe weak base anion exchanger bed. The mineral acids start exchangers. Conductivity at 20 is used to control theto replace previously absorbed organics, which can result in conditioning.fouling of the strong acid anion exchanger. The strong baseanion exchanger A2 normally absorbs the silicate and the Control of the district heatingrest of the carbon dioxide. Sodium leakage of the cation conditioning loopexchangers will produce NaOH at the output of the A2 ionexchanger, giving a rise of the conductivity as well as of the In the district heating water-conditioning loop cationpH-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 ofimpurity ions SiO2 and CO2, and ready for use as make-upwater. The mixed bed filter absorbs the impurity ionspassing through the precedent ion exchangers. Theconductivity at 16 controls the purity of the make-up waterwhen leaving the deionization plant; the measurement alsodetermines the exhaust of the mixed bed filter as well as theend of the rinse after regeneration.Application note A1.6E -3-
  4. 4. The conductivity of high purity waterUltra pure water is water from which all impurity ions have This cable compensator is used in monitor types 3213 andbeen removed. The high purity water from the raw water 3214. The conductivity changes with the temperature in atreatment plant and from the condensate polisher of modern way, which depends on the ions present in the water. Forpower plants is close to the ultra pure water level. that reason the temperature is measured by a platinum resistance thermometer and used for temperatureWater in this range has an odd conductivity/ temperature compensation.relationship, the curves of which can be seen at figure 1 fordifferent levels of purity. By an A/D converter, a microprocessor reads the conductivity and the temperature. The microprocessorBig problems arise about the temperature compensation of recalculates the measured conductivity at the actualthe high purity water conductivity, since the curves change temperature to its value at the reference temperature. Theaccording to the purity. However, the problems can be temperature compensation algorithms take into account thesolved. conductivity originating from the auto-proteolysis of the water, the so-called background conductivity.The conductivity of high purity water is contributed by thethermic 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 measuredimpurity ions present, e.g. NaCl → Na+ + Cl-, called the conductivity. Then the remaining conductivity issalt 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. 2The principle of measurementsConductivity measurement by means of the 4-electrode The necessity of the way of compensating used is illustratedsystem is based upon separate measurements of current and in figure 2. The upper curve shows the reading of avoltage. Hence electrode overvoltage due to polarization conductivity meter (e.g. type 2822) having a single TCand scaling is compensated for. compensating according to the salt conductivity temperature curve only. The error (mS/m) is independent ofA separate channel measures the potential of the voltage the range of the conductivity meter.electrodes. The signal is rectified and compared to areference voltage in a regulator, which controls the current The lower curves show the reading of a conductivity meterelectrode current to provide a constant potential across the type 3213 having a dual TC, compensating according tovoltage electrodes. The current flowing is now linear to the both the salt conductivity temperature curve and theconductivity. background conductivity curve.The 4-electrode sensors can have very small electrodes. The If the background conductivity is not compensated for, it iselectrode capacitance will thus be very low and no errors evident that troubles arise when conductivities close to ultraare seen due to the electrode capacitance. The cable pure water conductivity (0.055 µS/cm at 25°C) arecapacitance 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

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