Question

give the difference sludge impurities and scale impurities in water and 5 difference

Answer 1

In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reach saturation point, they are thrown out of water in the form of precipitates on the inner walls of the boiler. If the precipitation takes place in the form of loose and slimy precipitate, it is called sludge. On the other hand, if the precipitated matter forms a hard, adhering crust/coating on the inner walls of the boiler, it is called scale.

Sludge is a soft, loose and slimy precipitate formed within the comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. Sludge’s are formed by substances which have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc.

11.1 DISADVANTAGES OF SLUDGE FORMATION :

Sludges are poor conductor of heat, so they tend to waste a portion of heat generated.

If sludges are formed along with scales, then former gets entrapped in the latter and both get deposited as scales.

Excessive sludge formation disturbs the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge-glass connection, thereby causing even choking of the pipes.

11.2 PREVENTION OF SLUDGE FORMATION :

(1) By using well softened water, (2) By frequently ‘blow-down operation’, i.e., drawing off a portion of the concentrated water. Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler. Scales are difficult to remove, even with the help of hammer and chisel. Scales are the main source of troubles. Formation of scales may be due to;

(1) Decomposition of calcium bicarbonate

Ca(HCO3)2 → CaCO3  ↓ + H2O + CO2 ↑

                         Scale

However, scale composed chiefly of calcium carbonate is soft and is the main cause of scale formation in low-pressure boilers. But in high-pressure boilers, CaCO3 is soluble.

CaCO3  +  H2O   →  Ca(OH2)2  (soluble) + CO2 ↑

 

11.3 DEPOSITION OF CALCIUM SULPHATE :

The solubility of calcium sulphate in water decreases with rise of temperature. Thus, solubility of CaSO4is 3,200 ppm at 15oC and it reduces to 55 ppm at 230oC and 27 ppm at 320oC. In other words, CaSO4 is soluble in cold water, but almost completely insoluble in super-heated water. Consequently, CaSO4 gets precipitated as hard scale on the heated portions of the boiler. This is the main cause of scales in high-pressure boilers. Calcium sulphate scale is quite adherent and difficult to remove even with the help of hammer and chisel.

(1) Hydrolysis of magnesium salts :Dissolved magnesium salts undergo hydrolysis (at prevailing high temperature inside the boilers) footing magnesium hydroxide precipitate, which forms  a  soft type of scale  e.g.,

 MgCl2  +  2 H2O →  Mg(OH)2 ↓ + 2HCl ↑

(2)  Presence of silica (SiO2), even present in small quantities, deposits as calcium silicate (CaSiO3) and/ or magnesium silicate (MgSiO3). These deposits stick very firmly on the inner side of the boiler surface and are very difficult to remove. One important source of silica in water is  the sand filter

Disadvantages of scale formation :

(1) Wastage of fuel : Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a steady supply of heat to water, excessive or over heating is carried out  and this causes increase in fuel consumption. The wastage depends upon the thickness and the nature of scale : 

thickness of scale (mm)

0.325

0.625

1.25

2.5

12

Wastage of fuel

10%

15%

50%

80%

150%

 

(2) Lowering of boiler safety : Due to scale formation, over-heating of boiler is to be done in order to maintain a constant supply of steam. The over-heating of the boiler tube makes the boiler material softer and weaker and this causes distortion of boiler tube and makes the boiler unsafe to bear the pressure of the steam, especially in high-pressure boilers.

(3) Decrease in efficiency : Scales may sometimes deposit in the valves and condensers of the boiler and choke them partially. Tills results in decrease in efficiency of  boiler.

(4) Danger of explosion :  When thick scales crack, due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes  formation of a large amount of steam suddenly. So sudden high-pressure