3 December 2022
WTP

FEED WATER QUALITY | BOILER WATER QUALITY | WATER CHEMISTRY

FEED WATER QUALITY AND BOILER WATER QUALITY

FEED WATER & BOILER WATER QUALITY

  1. The DM water is recommended to have TDS less than 0.1 PPM.
  2. It is targeted to have DM water with no impurities.
  3. Pure water has TDS = 0 and pH = 7.
  4. For all practical purposes DM water can best be produced from 0.02 to 0.1 TDS.
  5. This indicates an impurity level of 0.1 PPM continuously going to boiler Drum.
  6. The water converts to steam and this impurity elements start getting concentrated in the drum.
  7. To control the TDS level below 100 PPM, blow-downs are required to be given (CBD or IBD)
  8. DM water can best be produced in the range of 6.8 to 7.2 pH. But the boiler feed water requirement is 8.5 to 9.2 pH.
  9. To correct the pH chemicals like ammonia, mophylene or Cyclohexylamine are used.
  10. All these chemicals are volatile and depending on Liquid-Vapour ratio will get evaporated in the drum resulting to reduction in pH of the drum.
  11. The evaporated ammines will be carried over with steam and will improve the pH of steam and condensate hence protecting the downstream system.
  12. To maintain the pH of the drum, a non volatile alkaline chemical is required to be added in HP dosing system.
  13. Tri-sodium phosphate can be used for this purpose.
  14. Phosphate addition has an added advantage as phosphate precipitates out and hardness creating elements in form of sludge which can be removed by blow-downs.
  15. It should be understood that neither DM water nor the steam after volatile treatment can be maintained to a level of 0.1 PPM TDS.
  16. The Feed water purity is required to be checked and controlled at the outlet of mixed bed in an online meter only but in this case the pH can best be in the range of 6.8 to 7.2.
  17. Steam TDS can be maintained to 0.1 PPM only if a complete non volatile treatment is used measured on an online meter but this will have a pH from 6.8 to 7.2 risking the downstream system with threats of corrosion and therefore not recommended.
  18. Pure water is highly reactive and very hungry for ions and will absorb the impurities from the atmosphere nearby.
  19. Therefore the purity level can not be checked to a high level of accuracy by taking sample and sending it to a laboratory (both feed water and steam).
  20. It is to be checked and controlled using online meter at outlet of Mixed bed in the DM plant. For accurate measurement of impurity level of steam and feed water, an online meter after a highly acidic cation exchanger may be installed
  21. Mechanical and thermal deaeration can remove oxygen best to a level about 4 to 7 PPB.
  22. But even small traces of oxygen in water will lead to disintegration of magnetite layer and corrosion of pressure parts.
  23. Hydrazine is used as oxygen scavenger to remove the residual oxygen.
  24. It is normally difficult to measure dissolved oxygen in the laboratory on regular basis.
  25. Since Hydrazine and oxygen can not remain together, presence of traces of hydrazine in water ensures that there is no dissolved oxygen present.
  26. However a test of dissolved oxygen on a regular interval of time is always recommended.
  27. Hydrazine is best reactive at saturation temperature of water (Or oxygen is best reactive at saturation temperature of water when oxygen solubility in water becomes very low and it comes in un-dissolved stage)
  28. Therefore the hydrazine dosing is to be done at the outlet of a charged deaerator so as to attain the best scavenging results.
  29. It must be clearly understood that corrosion and scaling are two different and opposite phenomenon and the remedy of one could aggravate the other and both of them can seriously damage the boiler.

So, a boiler operation engineer has to be very vigilant in maintaining the water chemistry parameters.

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