Boiler scale

The mineral and organic substances present in natural water supplies vary greatly in their relative proportions, but are principally comprised under the carbonates, sulphates and chlorides of lime, magnesia and sodium, iron and aluminum salts, silicates, mineral and organic acids, and the gases oxygen and carbon dioxide.

Boiler scale is formed from the carbonates and sulphates of lime and magnesia, and from the oxides of iron, aluminum and silicon and it will result in:

Reduction in the boiler efficiency because of the decreased rate of heat transfer.
Overheating and burning of tubes resulting in tube failure.

Boiler scales are dangerous long before it reaches this thickness. A very thin scale can cause tube failure due to overheating. Scale has about one-fortyeighth of the heat conductivity of steel.
A scale thickness of about 1 mm (0.04") can be sufficient to reduce the heat transportation to a dangerous point; when the water inside the tube cannot receive and carry away the heat fast enough from the tube metal to keep its temperature below its fusion temperature, resulting in the tubes "burning-out".
Testing the boiler water regularly will help you to avoid boiler water problems.

Accumulation of boiler scale, or incrustation, is objectionable in any and every form of boiler, for irrespective of the wear and tear of the boiler and the repairs which it entails, it is the cause of loss in efficiency. Diligent care should be exercised in preventing scale or incrustation accumulating in the boilers. The cleaning of the boilers internally should consequently receive every care.