62.What are the chemical cleanings for a boiler and end point for them?

Alkali boil out - It uses Alkali such as NAOH, CACO3, etc. The purpose is to remove grease in the tubes. The boiler will be pressurized to 20 Kg/cm2 and kept for 24 to 48 hours. Blow-down will be opened for 1 min for every 1 hour. End point is no traces of grease in the effluent.

Acid Cleaning – It uses diluted Sulphuric acid. The purpose is to remove loose mill scales in the boiler tubes. The end point is the saturation of iron in the mixture or 2 hours, which one is early. Sometime acid cleaning at very mild concentration will be applied for old boiler to remove any scaling in the water wall tubes.                                                                                 

Passivation – The reaction of acid shall be cleared after acid cleaning. For this purpose, the mixture Ammonia phosphate at 200 ppm and Hydrazine at 200 ppm will be left to evaporate at 20 Kg/cm2 for 24 hours.

The effectiveness will be confirmed by the colour on the coating tube inside.
All chemical cleaning are applied for water wall tubes. SH shall be protected during Acid cleaning.