Acid conductivity basics and some experiences

Transcription

1 Matarvattenkonferensen 201 Acid conductivity basics and some experiences Karsten Thomsen COWI A/S, Aalborg, Denmark phone: in collaboration with Nordjyllandsværkets Laboratory, Vattenfall, Denmark Outline The basics Species that cause acid conducitivity Analysing increased acid conducitivity Examples Conclusion 1 0 OCTOBER 201

2 Conductivity basics Specific conductivity Column with cation exchanger Acid conductivity Degassing unit e.g.: - thermostating - equilibration - cooling Term Specific conductivity Direct conductivity Acid conductivity Cation conductivity Conductivity after cation exchanger Degassed conductivity Degassed cation conductivity Symbol Comment SC AC CC CACE DC All ionic species contribute In pure water often used as measure of conditioning agent Contributions from conditioning agents like NH₃ and NaOH removed Contributions from salts amplified due to conversion to acid form Primary measure of purity of feed water and steam Contributions from dissolved gasses (CO₂) removed CO₂ is a common contaminant, considered less harmful than salts and organic acid contaminants Degassed conductivity 2 0 OCTOBER 201

3 Acid conductivity - importance Acid conductivity is the most important parameter for supervision of purity of the water-steam cycle International guidelines use acid conductivity as key parameter for feed water and steam for Phosphate treatment (PT) Caustic treatment (CT) All volatile treatment (AVT) and for boiler water for Caustic treatment All volatile treatment Typical specification for feed water and steam at action level 1 is 0.2 µs/cm Feedwater and water for steam attemperation Acid conductivity at AL1 Conditioning Spec. (µs/cm) AVT, CT, PT 0.2 Steam Acid conductivity at AL1 Without measurement of degassed conductivity Spec. (µs/cm) With measurement of degassed conductivity Spec. (µs/cm) 0.2 AC: 0.5 DC: 0.2 Examples from VGB-S-010-T-00 0 OCTOBER 201

4 Example specific and acid conductivity by ammonia conditioning SC = 6.8 µs/cm H 2 O NH₃/NH 4 OH NaCl µs/cm µs/cm 0.1 µs/cm Column with cation exchanger R SOH R SOH NH Na 4 OH Cl SONH SONa Cl 4 OH H H SONH 4 H O 2 AC = µs/cm H 2 O HCl µs/cm 0.5 µs/cm Conditioning with NaOH NaOH neutralised by cation exchanger R SOH R SOH Na Na OH Cl SONa OH SONa Cl H H SONa H O 2 Conditioning with Na PO 4 Phosphoric acid dominates conductivity no removal of contribution from conditioning agent R SOH R SOH Na PO4 Na Cl R SONa H SONa Cl 2 H PO 4 H 4 0 OCTOBER 201

5 Sampling points for acid conductivity drum type boilers & heat recovery steam generators (HRSGs) Sample point AVT CT PT Feed water - attemperator spray - ECO inlet Boiler water Saturated steam Live steam Condensate(s) - main condensate(s) - process condensate Diagnosis Diagnosis Diagnosis Diagnosis No use SC measured Carry-over Diagnosis 5 0 OCTOBER 201

6 Acid conductivity a non-specific measurement Source of acid conductivity Salts/acids with anions like chloride, sulphate Organic matter Carbon dioxide Remark Promotescorrosion in condensate and feed water trains, standstill corrosion and stress corrosion cracking in superheaters and turbine Sources: Primary and secondary cooling systems, water treatment plants Degrades inboiler, small organic acids like formic and acetic acid are formed and contribute to acid conductivity. The acids promote corrosion/stress corrosion cracking. Sources: Numerous, filter elements, surfaces, glycols in cooling water, organic conditioning agents Dissolvesin water, carbonic acid is formed and contributes to acid conducitivity. Considered less harmful with respect to corrosion, but may mask other contamination. Sources: Air in-leakage, carbonates in conditioning chemicals (ammonia, sodium hydroxide) 6 0 OCTOBER 201

7 Acid conductivity from salts/acids Qualitative signs Increased acid conductivity in condensate and feed water (no condensate polishing plant) Main condensate and feed water at about same level Acid conductivity of steam less or not affected due to separation in drum Increasing tendency of acid conductivity (CT, AVT) in boiler water the most sensitive measure because of accumulation of contaminants in boiler water With PT specific conductivity may increase depending on contamination level Supplementary measurements Note Sodium in feed water and condensate use on-line monitors and/or grab samples Contamination with district heating water often accompanied by increased levels of silica also Analyse grab samples of feed water and condensate by ion chromatography specialised analysis however Acid conductivity is "blind" for contamination with sodium hydroxide 7 0 OCTOBER 201

8 Acid conductivity from organic matter Organics CH COOH HCOOH CH COO - HCOO - 2H Qualitative signs Slightly increased acid conductivity in condensate and feed water (no condensate polishing plant) Acid conductivity increase from feed water to saturated and live steam due to degradation of organic matter in the boiler Slightly increasing tendency of acid conductivity (CT, AVT) in boiler water due to accumulation of organic acids Supplementary measurements Oxygen in feed water and condensate use on-line monitors Degassed conductivity in feed water and condensate if available Analyse grab samples of feed water and steam by ion chromatography specialised analysis however Analyse grab samples of feed water and steam for organic matter (TOC/NVOC) specialised analysis however 8 0 OCTOBER 201

9 Acid conductivity from carbon dioxide CO₂ H 2 O H 2 CO HCO - H Qualitative signs Increased acid conductivity in condensate and feed water (no condensate polishing plant) Decrease in acid conductivity from main condensate to feed water due to deaeration in feed water tank Acid conductivity of steam at the same level as feed water No increasing tendency of acid conductivity (CT, AVT) in boiler water often stabilises at a stable low value because carbon dioxide distributes to the steam With PT no increasing tendency in specific conductivity Supplementary measurements Oxygen from air in-leakage in feed water and condensate use on-line monitors Degassed conductivity in feed water and condensate if available Analyse grab samples of feed water and condensate by ion chromatography specialised analysis however 9 0 OCTOBER 201

10 Acid conductivity from carbon dioxide, chloride, and sulphate Acid conductivities 1,4 1,2 Acid cond. (µs/cm) 1 0,8 0,6 0,4 CO2 Cl SO4 0, Concentration () 10 0 OCTOBER 201

11 Example 1 Real life situation from Swedish CHP conditioning: CT Sample AC online µs/cm Feed water Boiler water Sat. steam Condensate 14.8 O₂ online SiO 2 online Na online Na lab , Conclusion: Clear signs of condenser leakage! 11 0 OCTOBER 201

12 Example 2 Real life situation from the same plant Sample AC lab µs/cm Feed water Boiler water Sat. steam Condensate DC lab µs/cm O₂ online SiO 2 online Na lab Cl lab <1 < <1 < <1 <2 Conclusion: Acid conductivity caused by carbon dioxide but no sign of air in-leakage. Uptake of carbonate in ammonia dosing solution is a likely cause OCTOBER 201

13 Conclusions Acid conductivity is the common and widely used measure of purity in the water-steam circuit Acid conductivity is a simple and reliable measurement, relatively easy to operate and maintain Guidelines all specify acid conductivity in several sampling points and for different conditioning modes Acid conductivity is non-specific increased values may be caused by many species capable of dissociation into ions Hints to the cause of increased conductivity may be learned by: Considering all acid conductivities together Considering other on-line measurements: Sodium, oxygen, silica Measuring degassed conductivity permanently or ad hoc Analysing grab samples for anions (chloride, sulphate, acetate ) cations(sodium, calcium ) and organic matter 1 0 OCTOBER 201