Requirement of Boiler Feed Water

January 26, 2024

Boiler feed water

A boiler is a device used to produce steam. One of the major uses of water in industry is generation of steam by boilers.

The water used to feed in boiler for the production of steam is called boiler feed water.

Requirement of boiler feed water

The specification of water required for boiler use is as follows.

S. No Water specification Volume (ppm)
1. Hardness <0.02
2. Soda alkalinity 0.15 – 1
3. Caustic alkalinity 0.15 – 0.45
4. Excess soda ash 0.3 – 0.55

The boiler feed water should be free from dissolved salts, gases, suspended impurities, silica and oil. When water from natural sources having hardness is fed directly to boilers, then the following problems or troubles may arise.

(i) Scale and sludge formation;
(ii) Priming and foaming;
(iii) Caustic embrittlement;
(iv) Boiler corrosion.

Scale and sludge formation in boilers

When water is converted into steam in boiler, the suspended and dissolved matters may get deposited within the boiler.

If these substances form hard or adherent deposits on the walls of the containers, they are called scales.

If these substances form soft or non-adherent coatings, then they are called sludge or mud.

Scale and sludge in boilers

In boilers, water evaporates continuously and the concentration of the dissolved salts progressively increases. When the salt concentration reaches a saturation point, they are thrown out in the form of precipitates on the inner walls of the boiler. The less soluble one gets precipitated first.

Sludge

Nature of Sludge

  1. The principle sludge formers are MgCO3 , MgCl2 , CaCl2, MgSO4  etc.
  2. It is a soft, loose and slimy precipitate.
  3. It is formed at the cooler portions of the boiler.
  4. Sludge can be easily scrapped off with a wire brush.

Disadvantages of sludge formation

  1. Excessive sludge formation disturbs the normal functioning of the boiler.
  2. Sludge is a poor conductor of heat.
  3. Sludge does not allow the water to come in contact with plates and tubes of the boiler.

Prevention of sludge formation

  1. It can be prevented by using pure water.
  2. By ‘blow down operation’. That is, forced ejection of suspensions, dissolved solids and sludge from the boiler at regular intervals.

Scale

Nature of Scale

  1. The principle sludge formers are Ca(HCO3)2 , CaSO4 ,Mg(OH)2  etc.
  2. They are hard deposits which stick firmly to the inner surface of the boiler.
  3. Scales are the main source of boiler troubles, since it is difficult to remove.

Disadvantages of Scale formation

  1. Danger of boiler explosion.
  2. Wastage of heat energy and fuel.
  3. Decrease in boiler efficiency.
Prevention of Sludge Formation
  1. It can be prevented by external and internal treatments.
  2. They can be removed by supplying thermal shock.
  3. They can be dissolved by using suitable chemicals such as EDTA and HCl.

Difference between sludge and scale

S. No Sludge Scales
1. Sludges are soft and             non-adherent deposits. Scales are hard deposits which stick to the inner surface of boiler.
2. They can be removed easily. They are very difficult to remove.
3. They can transfer heat to some extent and are less dangerous. They are bad conductors of heat and are more dangerous.
4. They are formed by  substances like MgCl2 and CaCl2. Scales are formed by Substances like CaSO4, and Mg(OH)2

Priming and foaming in boilers

When a boiler produces steam rapidly, the steam containing small droplets of water is called ‘wet steam’. Such carrying over of water by steam along with the suspended and dissolved solids is called ‘carry over’. This is mainly due to priming and foaming.

Priming  Foaming
The process of production of wet steam formation is called priming. Foaming is the formation of small but permanent bubbles above the surface of the water, which do not break easily.

Priming is caused due to:

  1. presence of large amount of           dissolved salts;
  2. high steam velocities;
  3. improper boiler design;
  4. sudden increase in the steam       production rate;
  5. sudden boiling.

 Foaming is caused due to 

  1. presence of impurities like alkali metal salts and finely divided suspended matter;
  2. presence of oily and grease substances in water.

Priming can be avoided by:

  1. maintaining low water levels in the boiler;
  2. using softened water;
  3. good boiler design;
  4. steady rate in steam production.

Foaming can be eliminated by:

  1. removing oily materials from the boiler water by adding suitable chemicals like sodium aluminate or alum and soda;
  2. adding anti-foaming agents like synthetic polyamides.

Disadvantages of priming and forming

(i) The life of machinery is decreased.
(ii) Maintenance of pressure in the boiler becomes difficult as the exact height of the water level cannot be judged properly.
(iii) Priming and forming occur together. They are harmful because efficiency of the super heater and the turbine blades decreases as the dissolved salts carried by the wet steam get deposited on them.

Caustic embrittlement

This is a type of corrosion which may attack boilers. This type of corrosion is caused by high alkalinity (concentrate NaOH) of the boiler feed water. This type of corrosion is observed in high-pressure boilers.

Causes

Presence of sodium carbonate (Na2CO3) is responsible for caustic embrittlement. In high-pressure boilers,  Na2CO3is hydrolysed to produce sodium hydroxide, and this makes the boiler water “caustic”.

Na2CO3 + H2O  →  2NAOH + CO2

            Sodium carbonate + water           Sodium hydroxide + Carbon dioxide

Sodium hydroxide (NaOH), thus formed flows into the minute, irregular, intergranular hair-line cracks which are present in the inner sides of the boiler and induces caustic embrittlement.

Water evaporates slowly and the concentration of caustic NaOH increases. This NaOH attacks the surrounding area and dissolves iron of boiler as sodium ferroate, where the metal is stressed. This causes embrittlement of boiler parts such as joints, rivets, bends etc., causing even failure of the boilers.

Fe + 2NaOH → Na2FeO2 + H2 ↑
Sodium ferroate

Prevention

Caustic embrittlement can be avoided by using the following methods:
(i) sodium phosphate can be used instead of sodium carbonate;
(ii) by adding tannin or lignin to the boiler water;
(iv) by adjusting the pH of the solution between 8 and 9;
(iii) by adding sodium sulphate (Na2SO4) to the boiler water, preventing the infiltration of sodium hydroxide (NaOH) into the cracks.

Boiler corrosion

Boiler corrosion is due to the decay of boiler material by a chemical or electrochemical attack from its environment. It is caused by using the unsuitable water in the boilers.

Main reasons for the boiler corrosion are:
(i) dissolved oxygen;
(ii) dissolved carbon dioxide;
(iii) dissolved salts.

(i) Boiler corrosion due to dissolved oxygen

Water contains about 8ml of dissolved oxygen per litre at the room temperature. At high temperatures, this dissolved oxygen attacks the boiler material.

2Fe + 2H2O + O2 → 2Fe(OH)2 ↑
4Fe(OH)2↑ + O2   → 2[Fe2 O3.2H2 O] ↑

Ferrous hydroxide              Rust

Removal of O2

In order to avoid boiler corrosion, the dissolved oxygen in the water should be removed. This can be carried out by chemical or mechanical methods.

(a) Chemical methods:
(a) By using sodium sulphite
Sodium sulphite converts the dissolved oxygen into sodium sulphate.
2Na2SO3 + O2  → 2Na2SO4
                                     Sodium sulphite            Sodium sulphate
(b) By using hydrazine

Hydrazine converts the dissolved oxygen into water (H2O).

N2H4  + O2    →   N2 + 2H2O

                                      Hydrazine             Nitrogen    Water

(c) By using sodium sulphide
Sodium sulphide converts the dissolved oxygen into sodium sulphate.
Na2S      +     2O2      →       Na2SO4
                       Sodium sulphide                            Sodium sulphate

(b) Mechanical method:

By mechanical deaeration method dissolved O2 can be removed from water.

Deaerator is a tower filled with perforated plates. Water is allowed to fall through the plates from the top and deaerator is heated from outside, which is connected to a vaccum pump (to reduce pressure inside the chamber). Availability of high temperature, low pressure and large exposed surface reduces the dissolved oxygen content in water.

Mechanical deaerator

Fig 1.2 Mechanical deaerator

(ii) Boiler corrosion due to dissolved carbon dioxide (CO2)

Presence of carbon dioxide in the boiler water also induces boiler corrosion.

Carbon dioxide in the water gets converted into carbonic acid [H2CO3]. Carbonic acid (H2CO3) thus formed has a slow corrosive effect on the boiler material.

CO2    +    H2O  →   H2CO3

                              Carbon dioxide  + Water      Carbonic acid

Removal of CO2

Carbonic oxide present in the boiler water can be removed by treating it with aqueous ammonia [NH4OH].

2NH4OH  +  CO2  →  (NH4)2CO3 + H2O

    • It can be removed by filtration through limestone.
    • It can be removed by mechanical deaeration process.

(iii) Boiler corrosion due to dissolved salt

Dissolved salts like magnesium chloride (MgCl2) undergo hydrolysis and liberate acids. The liberated acid reacts with iron which attacks boiler in a chain like reactions producing HCl again and again that leads to corrosion.

MgCl2  + 2H2O  →  Mg(OH)2 +2HCl

  Fe  +  2HCl   → FeCl2 +H2 ↑

          FeCl2 + 2H2O    → Fe(OH)2 ↑ + 2HCl

Removal of acids

Corrosion by acids can be neutralised with suitable alkaline solution in the boiler water.

Read More Topics
Introduction of water technology
Dalton’s law of partial pressures
Chemical vapour deposition (CVD)

About the author

Nandhini Sathish

View all posts

Leave a Reply