Scaling in boilers

Boilerscale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent.Evaporationin a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots. Leading to local overheating. Scaling mechanism is the exceeding of the solubility limits of mineral substances due to elevated temperature and solids concentration at the tube/water interface. The deposition of crystalline precipitates on the walls of the boiler interferes with heat transfer and may cause hot spots, leading to local overheating. The less heat they conduct, the more dangerous they are.

Common feed water contaminants that can form boiler deposits includecalcium,magnesium,iron,aluminum, andsilica. Scale is formed by salts that have limited solubility but are not totally insoluble in boiler water. These salts reach the deposit site in a soluble form and precipitate.
The values corresponding to their thermal conductivity are:

Steel 15 kcal/m².h per degree C
CaSO₄1-2 kcal/m².h per degree C
CaCO₃0.5-1 kcal/m².h per degree C
SiO₂0.2-0.5 kcal/m².h per degree C

If unchecked, scaling causes progressive lowering of the boiler efficiency by heat retardation, acting as an insulator. Eventually, scale built-up will cause the tube to overheat and rupture.
Boiler deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.

The firstanti-scalingpreventative measure is to supply good quality demineralised water as make–up feed water. The purer the feed water is, the weaker the driving mechanism to form scale. Scale-forming minerals that do enter the boiler can be rendered harmless by internal chemical treatment. A long-established technique is to detach the hardness cations, magnesium and calcium, from the scale forming minerals and to replace them with sodium ions.

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Presence ofSilica

Silica can vaporize into the steam at operating pressures as low as 28 bars. Its solubility in steam increases with increased temperature; therefore, silica becomes more soluble as steam is superheated. The conditions under which vaporous silica carryover occurs have been thoroughly investigated and documented. Researchers have found that for any given set of boiler conditions using demineralized or evaporated quality make-up water, silica is distribute between the boiler water and the steam in a definite ratio. This ratio depends on two factors: boiler pressure and boiler water pH. The value of the ratio increases almost logarithmically with increasing pressure and decreases with increasing pH.
If the silica enters the boiler water, the usual corrective action is to increase boiler blowdown, to decrease it to acceptable levels and then to correct the condition that caused the silica contamination.

For further information check our web page aboutsilica scaling in boilers.

Find information about the other main problems occurring in boilers:foaming and priming,corrosion.
For a description of thecharacteristics of the perfect boiler water click here.
Check also our web page aboutboiler water treatment, in particular throughdeaeration(deaerating heaters ormembrane contractors).

References
‘Water treatment handbook’Vol. 1-2, Degremont, 1991
‘Industrial water conditioning”,BeltsDearborn, 1991
http://www.thermidaire.on.ca/boiler-feed.html