Q. What are boiler troubles? Discuss their consequences?
Sol. Boiler feed water (water for steam making): water is very important engineering material which is mainly used for the production of steam in boilers .it is essential requirement for industries and power houses. The water thus is called boiler feed water. The boiler feed water should be free from dissolved calcium and magnesium salts. If the boiler feed water contains impurities beyond the prescribed limits they lead to the following problems.
1. (a) priming (b) foaming (c) carry over
2. Boiler corrosion
3. Scale sludge and formation
4. Caustic embitterment
1. Priming Foaming and Carry Over: when steam is produced rapidly in the boiler some small droplets of the liquid water are carried along with the steam. The steam with some small water droplets is called "wet steam ".
(a) Priming: The process of formation of wet steam is called priming. Priming also starts when level of water rises due to foaming. So priming is generally associated with foaming.
(b) Foaming: The formation of foam or bubbles at the water surface in boilers is called foaming which does not break easily.
(c) Carry over: The phenomenon of carrying of water along with impurities by steam is called "carry over". This is mainly due to priming and foaming. Priming and foaming mainly occur together.
2. Boiler corrosion: The "decay" or "disintegration" of boiler material either by a chemical or electrochemical reaction with its environment is known as "Boiler corrosion".
3. Sludge and scale formation: Boilers are used for steam formation. When hard water is continuously evaporated in boilers, the continuous evaporation of water increases the concentration of dissolved salts until the water becomes saturated. Then the salts of Ca++ and Mg++ along with other soluble impurities are precipitated on the inner walls of boilers. When the precipitate formed is soft, slimy and loose, known as sludge and is hard and firmly adhering on the inner walls of boilers, known as scale.
4. Caustic embrittlement: This is the phenomenon during which the boiler material becomes brittle due to the accumulation of caustic substances. It is form of corrosion caused by high concentration of sodium hydroxide in boiler water. It is most likely to occur in boilers operating at high pressures, where NaOH is produced in the boiler by the hydrolysis of some residual Na2CO3 obtained from water softening process like lime soda.
Na2CO3 + H2O = 2NaOH + CO2
The formation of NaOH makes the boiler water caustic. This caustic water penetrates into the minute hair cracks present in the inner side of boiler by capillary action. When the water evaporates and the concentration of dissolved NaOH increases progressively which attracts the surrounding area, thereby dissolving the iron of boiler as sodium ferrate.
Fe + 2NaOH = Na2FeO2 + H2O