ABB AX410 User Manual

Also see for AX410: Manual User guide User guide

Contents

ABB AX410 User Manual Manual pdf 78 page image

76A3.2 Monitoring on AVT SystemsFor low conductivity feedwater applications, all volatile chemicaltreatment (AVT) is often applied.Where cation resin columns are used to remove the effects onthe conductivity measurement of volatile ammonia andhydrazine chemical treatment, it is common practice to measureboth before- and after-cation conductivity. The sensitivity of theconductivity measurement to chemical treatments is increasedby 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 anindication of the concentration of that impurity and it is thenpossible to calculate the pH of the sample from theconcentration data. The result is referred to as 'inferred pH'.The maximum after-cation conductivity value is programmablebetween 0.060 and 10.00S cm –1 dependent on localconditions. After-cation values above this level generate anAFTER CAT. HIGH error message and before-cation valuesabove 25.00S cm –1 generate a BEFORE CAT. HIGH errormessage. The inferred pH range is 7 to 10pH; values above10pH generate an Infr. pH invalid error message. Refer toSection 8 for description of error messages.The inferred pH feature can be used on AVT systems only in thefollowing circumstances:a) On steam raising plant.b) For boiler chemical treatment such as ammonia and/orhydrazine. In this instance, A: Temp. Comp. must be set toNH3 and B: Temp. Comp. must be set to ACID – see Section5.3.c) Where the after-cation conductivity value is insignificantcompared to the before-cation value.Note. Inferred pH measurement on AVT systems isinappropriate to chemical treatments such as sodiumphosphate, sodium hydroxide and morpholine.A3.3 Monitoring on AVT Systems with ImpuritiesDifferential conductivity can also give an indication of sample pHon AVT systems where there are low concentrations of ionicimpurities present in addition to the volatile alkaline agent (e.g.sodium chloride + ammonia). In this case, the exchange ofammonium and sodium ions within the cation column releaseswater and hydrochloric acid. The sodium chloride impurityproduces a conductivity after the column that is higher than theconductivity before. The dual input analyzer, when used tomonitor before- and after-cation conductivities, compensatesfor this increase and calculates the inferred pH of the incomingsample. The user-configurable, after-cation conductivity alarmcan be used to detect unacceptably high levels of impurities inthe sample and inform the user of the validity of the inferred pHvalue.The calculated inferred pH is proportional to:BC – (AC – 0.055)/RWhere: BC = the before column readingAC = the after column reading0.055 = the conductivity of pure water at 25C inS cm –1R = a ratio factor depending on the BC andAC readings.The maximum after-cation conductivity value is programmablebetween 0.060 and 25.00S cm –1 dependent on localconditions. After-cation values above this level generate anAFTER CAT. HIGH error message and before-cation valuesabove 25.00S cm –1 generate a BEFORE CAT. HIGH errormessage. The inferred pH range is 7 to 10pH; values above10pH generate an Infr. pH invalid error message. Refer toSection 8 for description of error messages.The inferred pH feature can be used on AVT systems withimpurities only in the following circumstances:a) On steam raising plant.b) For boiler chemical treatment such as ammonia and/orhydrazine. In this instance, A: Temp. Comp. must be set toNH3 and B: Temp. Comp. must be set to ACID – see Section5.3.c) Where the after-cation conductivity value is less than25.00S cm –1.Note. Inferred pH measurement on AVT systems withimpurities is inappropriate to chemical treatments suchas sodium phosphate, sodium hydroxide andmorpholine.…APPENDIX A