Viessmann Boiler Repair Technology; What is scale and its formation process
When impurities contained in the water enter the boiler, after continuous evaporation and concentration, they reach a supersaturated level. At this point, solid-phase precipitates that form on the metal surface of the boiler are referred to as scale. The formation of scale on the boiler's evaporation surface involves two different processes: primary scale (original scale) and secondary scale (derived scale). Primary scale refers to the direct precipitation of scaling salts onto the boiler’s evaporation surface, while secondary scale refers to the reattachment of scaling salts to the evaporation surface after forming sludge. The formation of scale in boilers primarily occurs because the water contains scaling substances such as calcium and magnesium salts, which undergo a series of physical and chemical changes when heated in the boiler.
(1) Some calcium and magnesium salts decompose upon heating, forming insoluble precipitates such as calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). Under conditions where the water pH is high, MgCO3 can further hydrolyze into insoluble magnesium hydroxide (Mg(OH)2).
(2) As water temperature rises, the solubility of certain salts (such as CaSO4 and CaSiO3) decreases. Once these salts reach their saturation point, they directly precipitate out of the water.
(3) Continuous evaporation and concentration of water lead to an increase in salt concentration. Once this concentration exceeds the saturation point, precipitation occurs. The precipitates formed can take two forms: one firmly adheres to the heated surface walls, forming hard and dense scale deposits; the other remains suspended in the water as loose precipitates or sludge (also called water sludge). Whether the precipitate forms scale or becomes sludge depends not only on its chemical composition but also on the conditions under which it is precipitated (such as the pH value of the water). For example, calcium carbonate often forms hard scale in water supply pipes and economizers, but in boilers with turbulent water flow and vigorous boiling, it tends to form sponge-like soft sludge.
Article source: Beijing Viessmann Boiler Repair Center http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3778832.