Steam:
Its Generation and Use

INCRUSTATION AND SCALE. Nearly all waters contain foreign substances in greater or less degree, and though this may be a small amount in each gallon, it becomes of im- portance where large quantities are evaporated. For instance, a 100 H. P. boiler evaporates 30,- 000 lbs. water in ten hours, or 390 tons per month ; in the comparatively pure Croton water there would be 88 lbs. of solid matter in that quantity, and in many kinds of spring water as much as 2,000 lbs. The nature and hardness of the scale formed of this matter will depend upon the kind of sub- stances held in solution and suspension. Analy- ses of a great variety of incrustations show that carbonate and sulphate of lime form the larger part of all ordinary scale, that from carbonate being soft and granular, and that from sulphate hard and crystalline. Organic substances in con- nection with carbonate of lime, will also make a hard and troublesome scale. The presence of scale or sediment in a boiler results in loss of fuel, burning and cracking of the boiler, predisposes to explosion, and leads to extensive repairs. It is estimated that the pres- ence of i’g- inch of scale causes a loss of 13 per cent, of fuel, | inch 38 per cent., and | inch 60 per cent. The Railway Master Mechanics’ Asso- ciation of the U. S. estimates that the loss of fuel, extra repairs, etc., due to incrustation, amount to an average of $750 per annum for every loco- motive in the Middle and Western States, and it must be nearly the same for the same power in stationary boilers. The most common'and important minerals in boiler scale are carbonate of lime, sulphate of lime, and carbonate of magnesia. Small amounts of alumina and silica are sometimes found, and an oxide of iron not infrequently is present as a col- oring matter. Means of Prevention. It is absolutely essential to the successful use of any boiler, except in pure water, that it be ac- cessible for the removal of scale, for though a rapid circulation of water will delay the deposit, and certain chemicals will change its character, yet the most certain cure is periodical inspection and mechanical cleaning. This may, however, be rendered less frequently necessary, and the use of very bad water more practical by the em- ployment of some preventives. The following are a fair sample of those in use, with their results : M. Bidard’s observations show that “anti- incrustators ” containing organic matter help rather than hinder incrustations, and are there- fore to be avoided. Oak, hemlock, and other barks and woods, sumac, catechu, logwood, etc., are effective in waters containing carbonates of lime or magne- sia, by reason of their tannic acid, but are injuri- ous to the iron, and not to be recommended. Molasses, cane juice, vinegar, fruits, distillery slops, etc., have been used with success so far as scale is concerned, by reason of the acetic acid which they contain, but this is even more injuri- ous to the iron than tannic acid, while the organic matter forms a scale with sulphate of lime when it is present. Milk of lime and metallic zinc have been used with success in waters charged with bicarbonate of lime, reducing the bicarbonate to the insoluble carbonate. Barium chloride and milk of lime are said to be used with good effect at Krupp’s Works, in Prussia, for waters impregnated with gypsum. Soda ash and other alkalies are very useful in waters containing sulphate of lime, by converting it into a carbonate, and so forming a soft scale easily cleaned. But when used in excess they cause foaming, particularly where there is oil coming from the engine, with which they form soap. All soapy substances are objectionable for the same reason. Petroleum has been much used of late years. It acts best in waters in which sulphate of lime predominates. As crude petroleum, however, sometimes helps in forming a very injurious crust, the refined only should be used. Tannate of soda is a good preparation for gen- eral use, but in waters containing much sulphate, it should be supplemented by a portion of car- bonate of soda or soda ash. A decoction from the leaves of the eucalyptus is found to work well in some waters, in Cali- fornia. For muddy water, particularly if it contain salts of lime, no preventive of incrustation will prevail except filtration, and in almost every instance the use of a filter, either alone or in connection with some means of precipitating the solid matter from solution, will be found very desirable. In all cases where impure or hard waters are used, frequent ‘ ‘ blowing ’ ’ from the mud-drum is necessary to carry off the accumulated matter, which if allowed to remain would form scale. When boilers are coated with a hard scale diffi- cult to remove, it will be found that the addition of X lb. caustic soda per horse-power, and steam- ing for some hours, according to the thickness of the scale, just before cleaning, will greatly facili- tate that operation, rendering the scale soft and loose. This should be done, if possible, when the boilers are not otherwise in use.