IX, A, 4 Cox et al. : Water Supplies in the Philippines 373 



form in which it usually exists in water. The carbonate may be precipitated 

 by boiling, and it forms a soft mud in boilers unless sulphates are present 

 when with the other sediments it is cemented together into a very hard and 

 insoluble scale. 



Magnesium carbonate. — Magnesium carbonate is slightly soluble in water, 

 and therefore when held in solution by the presence of carbonic acid will 

 not be entirely precipitated by boiling off the excess of carbon dioxide. 

 Magnesite (natural magnesium carbonate) is used as a pipe covering to 

 reduce radiation. It is, therefore, obvious that it should be avoided as scale. 



Calcium sulphate. — Calcium sulphate is somewhat soluble in both pure 

 and salt water and unlike most salts in inverse ratio to the temperature 

 up to 150°C. This temperature corresponds to a steam pressure of 4.7 

 kilogTams per square centimeter (67 pounds per square inch), and at this 

 pressure all gypsum is deposited before any consequential evaporation has 

 taken place. In boilers it becomes precipitated as fine crystals not so much 

 by concentration as by the elevation of the temperature which when mixed 

 with the mud in the boilers forms a hard undesirable scale so difficult to 

 remove that it can be accomplished only by chipping. This scale may be 

 recognized by its vitreous appearance. 



Magnesium sulphate. — Magnesium sulphate alone does not form any scale 

 in boilers. When present in water with calcium carbonate it reacts with 

 the latter and forms a scale which is generally considered to be a com- 

 bination of calcium sulphate and hydrated magnesium oxide, one of the 

 hardest scales known. 



Silica. — Silica is present in most natural waters as a colloid. It may be 

 precipitated by boiling, and is occasionally found in considerable quantities 

 in scale. The scale formed from it is not excessively hard, and has no very 

 characteristic physical properties. When it is deposited with calcium sul- 

 phate, it forms a very hard scale which is difficult to remove. 



Iron and aluminium. — These metals are found in most natural waters 

 and may be considered as incrustants, appearing in the scale as oxides 

 or hydrated oxides, although neither of them is particularly troublesome 

 especially because they are generally present in feed water in very small 

 amounts. Iron derived from^ corrosion of the boiler may be added to the 

 scale. When too much aluminium sulphate is used as a coagulant for feed 

 water, the excess of the salt may enter the boiler and cause trouble, since, 

 by hydrolysis, sulphuric acid is formed which is highly corrosive. 



Grease. — The action of grease, especially fatty substances, readily de- 

 composed by heat in a boiler is to form a soft deposit similar to scale but 

 with ten times greater resistance to heat which even moi'e seriously inter- 

 feres with the transmission of heat. Mud and sediment must be blown 

 off else they will harden and produce dangerous scale. 



SCALE FORMATION 



Certain dissolved mineral matter remains in solution at low temperatures, 

 but is precipitated when the water is heated or concentrated, and falls to 

 the bottom or is deposited on the boiler tubes and shell.^' With pure water 

 the evaporation efficiency per unit of heating surface will not vary greatly 



" When precipitation takes places inside the water or steam space it is 

 called sediment, but if deposition forms a hard coating on the water-heating 

 surface it is called scale. 



