G. E. Davis, inspector of chemical works, draws attention to the fact that the incrustation of the inner surface of a boiler is frequently caused by the use of water which would not be suspected of any such power, and points out the danger of using water which has been condensed from the boiling down of organic substances. Many nostrums have been brought forward, and claims made on their behalf that they would prevent incrustation, bat most are absolutely worthless, if not injurious. Boiler "scales " nearly everywhere are principally composed of sulphate of lime, and he made a number of experiments to try and find some substance which should neutralize the action of that substance on the boiler plates. After many trials, he came to the conclusion that tribasic phosphate of soda, known to commerce as "tripsa," is the best of all preparations. It absorbs the free carbonic acid in the water, and, acting upon the sulphate of lime, precipitates it with the mud to the bottom of the boiler. In one case where this preparation was used, the boiler was worked for 5 months without being cleaned, and only the very slightest deposit, which could be easily displaced by a touch, was formed.
J. Waugh asserts that neither compounds, glycerine, nor galvanic action will prevent incrustation, and that the best course is to have a separate vessel in which to heat the water before it passes into the boiler. In this way the feed water can be raised to a temperature of 212° F. (100° C), and afterwards to 300°'F. (149° C); and while the carbonates of lime and magnesia will be precipitated in the one vessel, the sulphate of lime will be precipitated in the other - the result being that, however impure the water, there will be no incrustation.
Dingier's Polyt. Journal, says, A boiler with clean plates yielded with 1 lb. coal 7.5 lb. steam, after two months only 6.4 lb. steam, or a decrease of 17 per cent. At the same time the boiler had suffered by continual working. Suppose a boiler free from inside crust would yield a saving of only 5 per cent, in fuel (and this figure is taken very low compared with practical experiments), it would be at the same time a saving of 1 1/2d per cub. yd. of water. If the cleaning of 1 cub. yd. of water therefore costs less than l 1/2d., this alone would be an advantage. For a long time, efforts have been made to find some means for this purpose, and we have reached good results with lime and chloride of barium, as well as with magnesia preparations. But these preparations have many disadvantages. Corrosion of the boiler iron and muriatic acid gas have been detected. Chloride of calcium, which is formed by using chloride of barium, increases the boiling-point considerably, and diminishes the elasticity of steam; while the sulphate of soda, resulting from the use of carbonate of soda, is completely ineffectual against the boiler iron.
It increases the boiling-point of water less than all other salts, and diminishes likewise the elasticity:of steam. (Wullner.)
In using magnesia preparations tin precipitation is only very slowly and incompletely effected - one part of the magnesia will be covered by the slush and the formed carbonate of magnesia in such a way, that it can no more dissolve in water and have any effect (Dingler's Polyt. Journal, 1877-78). The use of carbonate of soda is also cheaper than all other above-mentioned substances. One milligramme equivalent sulphate of lime in 1 litre = 68 grm. sulphate of lime in 1 cub. m. requiring for decomposition: 120 gr. (86-88 per cent.) chloride of barium of commerce - cost, 0.6d.
Or 50 gr. magnesia preparation - cost, 1/2d.
Or 55 gr. (96-98 per cent.) carbonate - cost, 0. 41d. The proportions of cost by using chloride of barium, magnesia preparation, carbonate of soda will be 6:5:4.
By far the most important recent addition to existing information on the subject of boiler-incrustations is contained in a paper read by W. Ivison Macadam, before the Society of Chemical Industry, on the results of examinations of the chemical composition of steam-raising waters, and of the incrustations formed from them, with notes on the action of the materials commonly used as anti-incrustators, and on the various processes for softening water for steam purposes. Following is an abstract of the main facts.
Waters used for steam-raising purposes generally contain much solid matter in solution, and not unfrequently insoluble materials in suspension. These solid constituents, whether in solution or suspension, consist partly of salts and are partly organic in nature. The waters are derived from many sources, springs or rivers, pit discharges, sewage, sea water and surface water. The following analyses show the chemical composition of some samples.
From these results it is seen that the waters usually hold in solution much saline matter, and are divisible into (1) those containing calcium carbonate, and (2) those having a proportion of calcic sulphate in solution. The pit waters, especially those obtained from shale and coal seams, frequently contain large amounts of the acid sulphates of iron and alumina, and prove most hurtful to iron plates. Sewage impregnated waters have a considerable proportion of oily and fatty matters, with soda salts in solution, and are not advisable for boiler supply. At the same time it should be mentioned that such waters are in much repute with practical engineers, on account of their scouring properties, and it is by no means uncommon where these waters can be obtained to regularly employ them for cleaning purposes. Were the use of the waters restricted in this way, little damage might accrue, but their systematic' employment should not be allowed, on account of the deleterious effect of the oily constituents.