Scale formation in boiler due to the presence of various salts, which comes out of solution and deposited because of the effects of temperature and density.
The salts of calcium and magnesium are the major sources of scale formation.
Typical constituents of scale and deposits in boiler are:
- Calcium Carbonate : CaCO3
- Calcium Sulphate : CaSO4
- Calcium Phosphate : Ca3(PO4)2
- Magnesium Hydroxide :Mg(OH)2
- Magnesium Phosphate :Mg3 (PO4)2
- Iron and Copper Oxides:Fe2O3,CuO
- Complex Silicates of Magnesium, Iron, Sodium, and Aluminium
Dissolved solids in water, which can lead to the formation of deposits can be divided in 3 hardness groups:
1. Alkaline (temporary) Hardness Salts
- These are bi-carbides of Ca and Mg, which are slightly alkaline in nature.
- These decompose upon heating, formation CO2 and corresponding carbonates, which then deposits as a soft scales or sludge.
2. Non-Alkaline (permanent) Hardness salts:
- These are permanent hardness salts due to the presence of Sulphate, Chlorides, Nitrates and Silicates of Calcium and Magnesium.
- With the exception of silicates and caso4 the permanent hardness salts are all very soluble in water and do not normally produce scale but favour corrosion by galvanic action.
- Caso4 is the worst scale forming agent in the water depositing as thin, hard gray scale at temperature above 140C or at densities above 96,000ppm.
- MgCl2 is soluble under normal boiler condition but can do some extent break down inside the boiler to form Mg(OH)2 and HCL –this can set up an active corrosion with in the boiler metal.
MgCl2 + H2O → Mg(OH)2 + HCl
- CaCO3 is an alkaline hardness salt, it deposits as a white sludge but with CaSO4 forms composite sale of carbonate and sulphate.
- Greater percentage of carbonate makes the scale progressively softer.
- Silica is found in most waters, casting sand and welding flux. In low-pressure boiler Si03 combines with ca and mg to form calcium and magnesium silicates, which can precipitate and form hard scale. In high-pressure boiler silica volatilizes with steam and deposits in turbine blades causing severe loss in turbine efficiency. The silica scales are glassy, extremely hard and difficult to remove.
3. Non-Hardness Salts:
- These consist mainly of Na salts, with remain in solution and do not deposit under normal boiler density.
- It can come out of solution at very high densities above 225,000 ppm and deposits as a soft incrustation.
- Heavy concentration of NaCl salts can cause foaming and priming of boiler water.
In next article we can see the prevention of scale formation in boilers.