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
> 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
- EDTA Ammoniated ( 40 % ). : 4 %
- BHEL Max ( inhibitor ). : 0.15-0.2% w/v
- BHEL Sol ( inhibitor ) : 0.15-0.2% w/v
- Hydrazine Hydrate : 1000 ppm
- 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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