Engineering Chemistry S Datta

356 ENGINEERING CHEMISTRY
Priming and Foaming
During rapid steam production, some liquid water drops are also carried along with the
steam. This wet-steam formation is called priming.
Priming occurs due to (i) presence of large amount of dissolved solids, (ii) high steam
velocities, (iii) sudden boiling and (iv) sudden increase in steam production. Priming can be
controlled by (i) maintaining low water level in boiler, (ii) avoiding rapid change in steam rate,
(iii) softening of boiler water and (iv) using mechanical device for steam purification.
Foaming is the production of bubbles and foams which do not break easily. Foaming
occurs due to the presence of oil in the water.
Foaming can be reduced by (i) removing oil from boiler-feed water and (ii) adding antifoaming
agents.
Priming and foaming occur together and they are undesirable since they wet other
mechanical parts of the boiler and reduce their efficiency. Actual height of the water column
cannot be judged due to foaming hence creating difficulty in the maintenance.
Highlights:
• Chief sources of water: Sea water, rainwater, ground water and surface water.
• Impurities present in water:
(i) Suspended, (ii) Colloidal, (iii) Dissolved impurities
• Temporary hardness is due to Ca(HCO 3
) 2
and Mg(HCO 3
) 2
and can be removed by
boiling.
• Permanent hardness is due to chlorides and sulfates of Ca 2+ , Mg 2+ , Fe 2+ and other
heavy metals.
• Hardness expressed in equivalent amount of CaCO 3
Mass of hardness producing substances × 50
=
Chemical equivalent of hardness producing substances
• 1 ppm hardness ≡ 1 part of CaCO 3
equivalent hardness in 10 6 parts of water.
• Hard water cannot be used for steam generation in boilers due to the problems
like scale and sludge formation, priming and foaming, boiler corrosion and failure.
Softening of Water
Softening of water means the removal of calcium, magnesium, iron salts and similar
other metallic ions, which would form insoluble metallic soaps. The three important industrial
methods employed for softening of water are:
1. Cold and hot lime-soda process.
2. Permutit or zeolite process.
3. Ion-exchange or demineralization process.
1. Lime-soda process. By this process, soluble calcium and magnesium salts are rendered
insoluble by adding calculated amount of lime [(CaOH) 2
] and soda [Na 2
CO 3
]. The insoluble
precipitates of CaCO 3
and Mg(OH) 2
are removed by filtration. By this method, both temporary
and permanent hardness are removed.
For the removal of temporary hardness the reactions are:
Ca(HCO 3
) 2
+ Ca(OH) 2
⎯⎯→ 2CaCO 3
↓ + 2H 2
O
Mg(HCO 3
) 2
+ Ca(OH) 2
⎯⎯→ 2CaCO 3
↓ + MgCO 3
+ 2H 2
O
MgCO 3
+ Ca(OH) 2
⎯⎯→ Mg(OH) 2
↓ + CaCO 3
↓
Hence, to remove equivalent quantities of Ca and Mg hardnesses the amount of lime
necessary is in the ratio of 1:2.