BOILER TROUBLES-----WATER TECHNOLOGY---CY2111 183101 –ENGINEERING CHEMISTRY –I Lecture Notes

2. Priming and Foaming:

Due to rapid boiling, the steam may carry some water droplets along with it. This is called wet steam .The process of wet steam production is called Priming. It can reduce the heat of the steam and cause corrosion in the pipelines.

Priming is due to: 

a)   Improper design of boiler

b)     High water level

c)      High velocity of steam

d)     Uneven boiling

     Priming can be controlled by

i)                    Proper boiler design

ii)                  Maintaining proper water level

iii)                Proper boiling

If oils and greases are present, they produce bubbles on the water surface.  This will increase the wet steam production.  This is known as “Foaming”.

Foaming is prevented by adding

i)                    Anti foaming agents (e.g.) synthetic poly amides , castor oil

ii)                  Coagulants (e.g.) Aluminium hydroxide

Foaming and priming are collectively known as ‘ Carry over”.

3. Caustic Embrittlement:

It is the intercrystalline cracking of boiler due to NaOH.  NaOH content is increased due to the dissolved salts like Na₂CO₃ which is added during internal treatment.

Na₂CO₃    +   H₂O  à  2 NaOH  + CO₂

This NaOH occupies the hair line cracks of boiler metal and converts the insoluble Fe into soluble Sodium Ferroate.  Thus it makes the cracks bigger in bents, joints and crevices.

Fe + 2 NaOH  à  Na₂FeO₂   + H₂ ↑

(Insoluble)                   (Soluble)

Prevention of caustic embrittlement:

1.      As softening agent, we can use sodium phosphate instead of sodium carbonate.

2.      The hair line cracks can be sealed by waxy materials like Tannin and Lignin.

4.Boiler Corrosion

  It may be due to three major reasons:

i) Dissolved Oxygen    ii) Dissolved CO₂      iii) Dissolved salts like MgCl₂

i)Corrosion Due to dissolved oxygen:

Dissolved oxygen in presence of water, causes corrosion.

4Fe + 6 H₂O + 3O₂  à  4 Fe (OH)₃  (Rust)

Prevention from oxygen:

a) Chemical method - 

Adding Sodium Sulphite:  2 Na₂SO₃  + O₂   à  2 Na₂SO₄

This method results in other precipitates which can have some side effects.  So this method is less preferred.

Adding Hydrazine:   N₂H₄  + O₂  à  N₂  +  2 H₂O

This method results in inert gas and pure water, it is preferred.

b) Mechanical deaeration method:                                             

This is based on the principle that at high temperature and low pressure and high

exposed area, the solubility of gases in water is decreased.  So, the water is fed into the

mechanical deaerator which is provided with vaccum pump, heaters and perforated

plates.  The out coming water will be free from dissolved gases.

DIAGRAM

ii)Corrosion due to Carbon – di – oxide:

Salts like Calcium bicarbonate on heating produces CO₂ .  CO₂ dissolves in water to form carbonic acid which corrodes the boiler metal.

                  Ca (HCO₃)₂   ∆à     CaCO₃ + H₂O + CO₂

H₂O + CO₂  à  H₂CO₃

Prevention from CO₂

1.      Chemical method:  By adding calculated amount of ammonium hydroxide

2NH₄OH  +  CO₂  à  (NH₄)₂CO₃  +  H₂O

2.      Mechanical deaeration method  ( similar to oxygen method)

iii)Corrosion due to Dissolved salts like MgCl₂

Dissolved salts like MgCl₂ cause acid formation. This will be prevented by alkali       neutralisation.

                   MgCl₂ +  2 H₂O  à  Mg(OH)₂  +  2 HCl  (Corrosive acid)

 Neutralisation:   HCl  +  NaOH  à  NaCl  +  H₂O