Scale has sometimes been removed by blowing the boiler off at comparatively high pressure, and then filling it with cold water. This causes a severe contraction of the plates, and is likely to loosen the scale; but it will at the same time cause serious injury to the boiler, and is a practice that should not be tolerated.
After the impurities are deposited in the boiler, they may be removed by the blow-out apparatus; and if it is possible to "lay off" the boiler occasionally, it should be allowed to cool down slowly, and then the water may be drawn off and the boiler properly washed out. A considerable amount of heat is abstracted from the boiler by frequent blowing-off, and this is a matter of direct loss, but it is nothing like so much as would be caused by the formation of scale.
Water may be purified to a certain extent by passing it through a purifier before allowing it to enter the boiler. The carbonate and sulphate of lime are precipitated at the same time that the water is heated. The purifier was referred to under the topic of "FeedWater Heaters." The use of soda for the neutralization of sulphate of lime has already been spoken of; but various compounds are on the market for overcoming the evil effects of other solids; and it is possible, by an analysis of the feed water, to prescribe a boiler compound that will give satisfactory results. Cheap compounds, sold without reference to the analysis of the feed-water, should be avoided. Caustic soda may be used instead of the carbonate, but should be used in small quantities. A rapid circulation of the water will prevent the formation of scale, the sediment being swept from the tubes or shell into the mud-drum, whence it may be blown off. This is one of the chief advantages claimed for water-tube boilers.
Zinc plates have frequently been used to prevent corrosion and incrustation. The brass fittings are likely to set up a galvanic action with the steel plates; but if the zinc is put in, it will lie acted upon instead of the iron, which otherwise might be rapidly wasted. It is claimed that this galvanic action prevents the formation of scale by liberating hydrogen at the exposed surfaces. The zinc neutralizes the free acids by combining with them, and takes the place of iron in causing precipitation of copper salts when present.
Kerosene oil is used to a considerable extent to prevent the formation of scale and to assist in its removal. It breaks up and loosens hard scale, and prevents its formation. About one quart a day is sufficient for each 100 horse-power of the boiler.