ABB AWT420 Operating Instruction

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ABB AWT420 Operating Instruction Manual pdf 101 page image

AW T4 2 0 | U N I V E R S A L 4 - W I R E , D U A L- I N P U T T R A N S M I T T E R | O I/A W T4 2 0 - E N R E V. B101Appendix C Dual 2-electrode conductivity calculationsInferred pH derived from differentialconductivityMonitoring on steam-raising plantFor many years, it has been standard practice in power plantsto use inferred pH, calculated from before- and after-cationconductivity measurements, to confirm values obtained bydirect laboratory or on-line pH measurement.According to EPRI, IEC and VGB Guidelines, feedwater andboiler water quality can be assessed by measuring theconductivity of samples before and after a cation ion-exchangeresin column. Depending on the type of plant and chemicaltreatment applied, differential conductivity can also give anindication of the pH of the sample.Both before and after measurements can be made on onedual input conductivity analyzer.The choice of inferred pH calculation depends on controlledchemical conditions, i.e. whether or not the system is an NH3,NH3+NaCl or NaOH dosed system.NOTICE• If the analyzer is used with a cation resin column, Sensor Amust be installed before the column and Sensor B afterthe column for the correct calculation of inferred pH.• Both conductivity inputs must be configured as μS cm-1to calculate inferred pH.WARNINGThe calculation of inferred pH relies on the strict control ofchemical conditions within the NH3, NH3+NaCl or NaOHdosed sample. Contamination with chemical substancesother than those with which the sample is dosed introducessignificant errors in the inferred pH calculated value and, inthe worse case, invalidates the calculation completely.Carbon dioxide in particular has a very adverse affect.Sources of CO2 contamination include:• Boiler start-up.– CO2 can be present in the sample for several hours oreven days immediately after boiler start-up.Note. This also applies to 'two shifting' or 'cycling'boilers, i.e., boilers whose full output is required onlyduring peak demand periods.• Organic compound contamination.– Decomposing organic compounds are a source of CO2contamination.– Organic compound contamination may be caused bybreak-through from the water treatment plant or fromcondenser leaks.– Formates are also formed when organic compoundsdecompose, these further increase errors in inferred pHcalculation.• Carbon compound contamination.– The use of carbon compound chemical treatments suchas carbohydrazide (used as an oxygen scavenger) cancontaminate the sample with CO2.Independent pH readings are necessary to confirm that thecorrect chemical conditions prevail for the accuratecalculation of inferred pH.