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 ↑
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.
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.
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