Thursday, January 24, 2019

EDTA CLEANING PROCESS

Chemical Cleaning of Boilers :

Pre-operative
> Maintaining steam quality at the turbine inlet.
> Minimizing corrosion of the metal surface of boiler.

Post-operative
> Improving heat transfer in the system.
> Enhancing efficiency of the power plant.
> Minimizing under deposit corrosion of the metal surface of boiler.
> Minimizing plant outage due to boiler tube failures.


What is EDTA?
EDTA or Ethylene Di-amine Tetra Acetic Acid is a strong chelating agent having ability to form stable complexes with transition metal ions  present in boiler such as of iron and copper etc.

Alkali Flushing

  • Alkali Flushing 
          > Removes polar organic material such as oil, grease etc. from the metal surface
          > Alkali reacts with polar organic materials to form  soluble glycerols and salts.
          > Loose deposits, dust, etc. are also removed
          > TSP and DSP is used as alkali
  • Hot Water Flushing
  • Cold Water Rinse

ALKALI FLUSHING PROCEDURE
*ALKALI FLUSHING
  • Alkali solution made by  adding TSP ( 01%) & DSP ( 0.05% ) with dm water.
  • Solution is filled into boiler through bottom ring header & economizer drains up to NWL.
  • Boiler is fired. Water temperature is raised to 140 oC and boiler tripped.
  • Solution is allowed to cool till 95 oC and then drained.
* HOT WATER FLUSHING
  • System is filled with DM Water and temperature raised to 1400 oC and allowed to cool to 95 oC and drained subsequently.

* COLD WATER RINSE
  • System is filled with DM Water and drained. The process is repeated till phosphate level is less than 10 ppm.

EDTA Cleaning
  • EDTA pickling& passivation
  • Treated DM water rinsing
PREPARING EDTA SOLUTION
EDTA solution is prepared for total boiler volume by mixing
  1. EDTA Ammoniated ( 40 % ).  : 4 %
  2. BHEL Max ( inhibitor ).          : 0.15-0.2% w/v
  3. BHEL Sol ( inhibitor )             : 0.15-0.2% w/v
  4. Hydrazine Hydrate                  : 1000 ppm
  5. Ammonia.                                : for setting pH as 9.5-9.8


EDTA PICKLING & PASSIVATION
  • The system is filled with EDTA solution upto drum NWL.
  • The boiler is lighted up and temperature raised to 140 oC and maintained for 4 hours and sample solution checked for iron, pH and EDTA concentration every 1/2 an hour.
  • The boiler is tripped on achieving 3 consecutive readings of iron concentration.
  • The solution is allowed to cool till 95 oC and drained.

TREATED DM WATER RINSING
  • A. solution of hydrazine ( 0.01-0.02 % ) and ammonia with DM Water is prepared ( pH 9.5-9.8 ) and the system is filled with this solution.
  • Boiler is fired and temperature raised to 120 oC.
  • Boiler is tripped and allowed to cool. When the temperature comes down to 95 oC, the solution is drained.
SUPER HEATER BACK FLUSHING
Super heater is filled with a solution of hydrazine ( N2H4 . 50 ppm ) & Ammonia ( pH > 9.5 ) in DM Water through SH drain header. When the solution comes through SH openings in the drum, back flushing is continued for 10-30 minutes and drained.


EDTA is a strong chelating agent with transition metal ions and hence has an efficient action as metal remover.

Process advantages
  • Use of nitrogen & its system is eliminated.
  • Auxiliary steam is not required at all.
  • DM Water and fuel consumption is less
  • Shorter cycle tie ( 5 days ).
  • Less temporary piping required
  • Less number of chemicals required
  • Ease of handling
  • Effluent treatment easy
  • Economical
Conclusion
  • EDTA cleaning process ensures efficient cleaning and passivation of boiler internal surfaces.
  • EDTA cleaning has obvious advantages over conventional HCL cleaning with minimum effect on the base metal surface.
Source : by HC Madan, SS Chauhan, Dhruv Garg

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