Explain internal treatment methods for removal of scales

Internal treatment (Sequestration):

  • In this method, some chemical reagents are added directly to the boiler water for removing dangerous scale-forming salts.
  • The chemicals convert the scale-forming impurities into sludge or more dissolved compounds.
  • This method is generally followed by a blow-down operation.

Internal treatment consists of unique treatments conducted when boilers operate under moderate or low pressure. In this process, massive amounts of concentrated steam are utilized for the feedwater or in high-quality raw water when accessible. This form of water treatment is utilized to achieve the following:

  • Give corrosion protection
  • Prevent foaming and scaling
  • Eliminate water hardness
  • Conditioning of suspended matter such as iron oxide and sludge

Eradicate oxygen from the feed water and ensure that the water has enough alkalinity to fight corrosion
In the process, softening chemicals like polyphosphates-dispersants are used. These are capable of neutralizing water hardness through the formation of tri-calcium phosphate. Moreover, anti-scaling products such as synthetic and natural polymers are added to mitigate scaling.

Additionally, anti-foaming agents are incorporated to alter the surface tension, eradicate foam and transport fine particles of water into the steam. Oxygen scavengers like hydroxylamine play a vital role in dissolving oxygen and reducing oxides, which prevents corrosion within the boiler system.

With an efficient internal treatment program, a boiler system can operate at its best by preventing scaling, deposition, and corrosion problems.

Important internal conditioning/treatment methods:

Carbonate conditioning:
The reagent added is sodium carbonate.

In low-pressure boilers, scale formation can be avoided by adding sodium carbonate to boiler water
CaSO4 +Na2CO3 -----> CaCO3↓+Na2SO4

Deposition of CaSO4 as the scale doesn’t take place and calcium is precipitated as loose sludge of CaCO3 which can be removed by blow down operation.

Phosphate conditioning:
The reagent added is sodium phosphate.
In high-pressure boilers, scale formation can be avoided by adding sodium phosphate. It reacts with the hardness of water and forms a nonadherent, easily removable soft sludge of Ca / Mg phosphates.
3CaCl2 +2Na3PO4 -----> Ca3 (PO4)2 ↓+ 6NaCl

We can use different phosphates based on the PH of the water
  • Na3PO4 (alkaline in nature) is used when alkalinity is low.
  • Na2HPO4 (weakly alkaline) is used when alkalinity is sufficient.
  • NaH2PO4 (acidic) is used when alkalinity is high

Calgon conditioning:

The reagent added is Calgon.

Calgon = sodium hexa meta phosphate

The process involves the addition of Calgon to the boiler water. It prevents scale and sludge formation by forming highly soluble calcium Hexa meta phosphate complex compound with CaSO4.

 Na2 [Na4(PO3)6] ------>2Na+ + [Na4(PO3)6]2- 
2CaSO4 + [Na4(PO3)6] 2-  -------> [Ca2(PO3)6] 2- + 2 Na2SO4
Highly Soluble
complex ion

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