COMMERCIAL USES OF WATER

       Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms. 

       Water is an essential resource for both industrial and domestic purposes. In industry water is used for many

processes including washing, manufacturing, etc. The other important use of water is to produce steam in a boiler.

Boiler feed water(water for making steam)

       

           Water used for making steam should be free from dissolved salts and gases, suspended impurities, silica and oil. If used in boilers, these impurities lead to the following problems:

1. Sludge and scale formation in boilers

2.Priming and foaming

3.Caustic embrittlement 

4. Boiler corrosion

1. Sludge and scale formation in boilers

                The concentration of dissolved salts in water increases progressively when water is converted into steam in boilers. When the salt concentrations reach their saturation point, they are thrown out in the form of precipitates in the boilers called sludge or scale.

2. Priming and Foaming

                 Steam sometimes may be associated with small droplets of water. Such steam containing liquid water is called wet steam. These droplets of water carry with them some dissolved salts and sludge material present in water. This phenomenon is called carry over. This occurs mainly due to priming and foaming .

Priming:

        Formation of wet steam by rapid boiling of the water at the heating surfaces is called priming. It may be caused by 

• very high water level
•  high steam velocity sudden steam demands leading to sudden boiling
•  improper boiler design.

Priming can be prevented by:

• keeping the water level lower
• good boiler design with a mechanical steam purifier 
• avoiding rapid changes in steam rate
• using seated waler

Foaming:

   The formation of stable bubbles above the surface of water is called foaming. lt may be caused by: 

•  the presence of oil or grease in water
• fine sludge particles

Foaming can be prevented by:

• removing the foaming stabilising agents such as soluble salts, clay, and organic matter from water antifoaming chemicals like synthetic polyamides.
• adding coagulants such as sodium aluminate, ferrous sulphate, etc. to remove sludge particles.

3. Caustic embrittlement

        Caustic embrittlement is a form of corrosion caused by a high concentration of sodium hydroxide in the boiler

feed water. It is characterized by the formation of irregular intergranular cracks on the boiler metal. particularly

at places of high local stress such as bends and joints.

Causes of caustic embrittlement:

       Boiler water usually contains a small amount of Na, CO, In high

pressure boilers. Na2CO, undergoes hydrolysis to produce NaOH.

          

          Na2CO, + H2O -> 2NaOH+ CO2

This NaOH flows into the minute hairline cracks present on the boiler material by capillary action and dissolves the surrounding area of iron as sodium ferroate. Na2FeO2

            

            Fe + 2NaOH -> Na2FeO2 + H2

This type of electrochemical corrosion occurs when the concentration of NaOH is above 100 ppm. This

causes brittlement of boiler parts, particularly the stressed parts like bends, joints, rivets, etc.

Caustic embrittlement can be prevented by:

• using sodium phosphate as the softening agent instead of sodium carbonate.
• adding chemicals such as tannin, lignin to the boiler water. They block the hairline cracks.
• adjusting the pH of he feed water carefully between 8 and 9.

4. Boiler corrosion

       Corrosion in boilers is due to the presence of

• dissolved oxygen dissolved carbon dioxide
• dissolved salts like magnesium chloride

Dissolved oxygen:

            The presence of dissolved oxygen is responsible for corrosion in boilers. Water containing dissolved oxygen when heated in a boiler, free oxygen is evolved, which corrodes the boiler material.

              4Fe +6H20+302   ->  4Fe(OH)2

Dissolved carbon dioxide :

          When water containing bicarbonates is heated. carbon dioxide is evolved which makes the water acidic. Carbon dioxide that formed dissolved in water forms carbonic acid. This leads to intense local corrosion called pitting corrosion.

            Ca(HCO3)2  -> CaCO3 +H2O + CO2

                CO2 +H2O -> H2CO3

Dissolved magnesium chloride :

          When water containing dissolved magnesium chloride is used in a boiler, hydrochloric acid is produced. HCI attacks the boiler in a chain-like reaction producing hydrochloric acid again and again which corrodes boiler severely.

              MgCl2+ 2H2O  ->  2HCI + Mg(OH)2

                  Fe + 2HCI    -> FeCl2 + H2

               FeC12+ 2H2O  ->  Fe(OH)2 + 2HCl

              Corrosion by HCI can be avoided by the addition ol alkali to the boiler water.