Scaling of Jacket Water System
Scale and deposit formation
In areas of deposit formation, dissolved solids, specifically Calcium and magnesium hardness constituents can precipitate from cooling water as the temperature increases. Deposits accumulate on the heat transfer surfaces as sulphates and carbonates, the magnitude of which is dependent on the water hardness, the dissolved solid content, local temperatures and local flow characteristics.
Temperature solubility curves for CaSO4
Scales can reduce heat transfer rates and lead to loss of mechanical strength of component parts, this can be exacerbated by the presence of oils and metal oxides.
The degree and type of scaling in a cooling water circuit are dtermined by;
- System temperatures
- Amount of leakage/makeup
- quality of make up
- quality of treatment
Calcium Carbonate
Appears as a pale cream, yellow deposit formed by the thermal decomposition of calcium bi-carbonate
Ca(HCO3)2 + Heat becomes CaCO3 + H2O + CO2
Magnessium Silicate
A rought textured off white deposit found where sufficient amounts of Magnessium are present in conjunction with adequate amounts of silicate ions with a deficiency onh OH alkalinity
Mg2+ + OH- becomes MgOH+
H2SiO3 becomes H+ + HSiO3-
MgOH+ + HSiO3- becomes MgSiO3 + H2SO4
Silicate deposit is a particular problem for systems which utilise silicate additives for corrosion protection. Thi sis typical of systesm with aluminium metal in teh cooling system. The silicate forms a protective barrier on the metal surface. A high pH (9.5 - 10.5) is required to keep the silicate in solution. In the event of sea water contaimination or some other mechanism that reduces the pH the silicate is rapidly precipitated and gross fouling can occur.
Copper
The prescence of copper within a cooling system is very serious ast it can lead to agressive corrosion through galvanic action. Specific corrosion inhibitors are contained with cooling water system corrosion inhibitors.
Effects of scale deposition
The effects of scale deposition can be both direct or indirect,typically but not specifically
Insulates cooling surfaces leading to;
- increased material temperatures as the temperature gradient must increase to ensure maintain heat flow.
- Loss of efficiency as exhaust gas temperatures form cylinders increases
- Increased wear due to lubrication problems on overheated surfaces
Indirectly;
- Lead to caustic attack be increasing the OH- ion concentration
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