Chemical Terms and Explanations Relating to Feed Waters
Chemistry is a science which investigates the composition and properties of material substances.
Nature is composed of elementary elements; knowledge of these bodies, of their mutual combinations, of the forces by which these combinations are brought about, and the laws in accordance with which these forces act, constitute chemistry, and the chemistry of steam engineering largely deals with the foreign bodies contained in the feed water of steam boilers.
Element. In general, the word element is applied to any substance which has as yet never been decomposed into constituents or transmuted to any other substance, and which differs in some essential property from every other known body. The term simple or undecomposed substance is often used synonymously with element.
There are about 70 simple elements, three-quarters of which are to be met with only in minute quantities and are called rare elements. Copper, silver, gold, iron, and sulphur are simple elements— the metal irridium, for example, is a rare element—it is the metal which tips the ends of gold pens—it is heavier than gold and much more valuable. Probably there are not two tons of it in existence.
A Re-agent is a chemical used to investigate the qualities of some other chemical—example, hydrochloric acid is a re-agent in finding carbonic acid in limestone, or carbonate of lime, which when treated by it will give up its free carbonic acid gas, which is the same as the gas in soda water.
An Oxide is any element, such as iron, aluminium, lime, magnesia, etc., combined with oxygen. To be an oxide it must pass through the state of oxidization. Iron after it is rusted is the oxide of iron, etc.
A Carbonate is an element, such as iron, sodium, etc., which forms a union with carbonic acid—the latter is a mixture of carbon and oxygen in the proportion of 1 part of carbon to 2 of oxygen. Carbonic acid, as is well known, does not support combustion and is one of the gases which come from perfect combustion. This acid, or what may be better termed a gas, is plentifully distributed by nature and is found principally combined with lime and magnesia, and in this state (i.e., carbonate of lime and carbonate of magnesia) is one of the worst enemies to a boiler.
An Acid is a liquid which contains both hydrogen and oxygen combined with some simple element such as chlorine, sulphur, etc. It will always turn blue litmus red, and has that peculiar taste known as acidity; acids range in their power from the corrosive oil of vitriol to the pleasant picric acid which gives its flavor to fruits.
Alkalies are the opposite to an acid; they are principally potash, soda and ammonia—these combined with carbonic acid form carbonates. Sal-soda is carbonate of soda.
A Chloride is an element combined with hydro chloric acid—common salt is a good example of a chloride—being sodium united with the element chlorine, which is the basis of hydro chloric acid. Chlorides are not abundant in nature but all waters contain traces of them more or less and they are not particularly dangerous to a boiler.
Sulphates are formed by the action of sulphuric acid (commercially known as the oil of vitriol) upon an element, such as sodium, magnesia, etc. The union of sodium and sulphuric acid is the well-known Glauber salts—this is nothing more than sulphate of soda; sulphate of lime is nothing more than gypsum. Sulphates are dangerous to boilers, if in large quantities should they give up their free acid—the action of the latter being to corrode the metal.
Silica is the gritty part of sand—it is also the basis of all fibrous vegetable matter—a familiar example of this is the ash which shows in packing, which has been burnt by the heat in steam; by a peculiar chemical treatment silica has been made into soluble glass—a liquid. 65 per cent. of the earth’s crust is composed of silica—it is the principal part of rock—pure white sand is silica itself—it is composed of an element called silicum combined with the oxygen of the air. Owing to its abundance in nature and its peculiar solubility it is found largely in all waters that come from the earth and is present in all boiler scale.
In water analysis the term insoluble matter, is silica. This is one of the least dangerous of all the impurities that are in feed water.
Magnesia is a fine, light, white powder, having neither taste nor smell, almost insoluble in boiling, but less so in cold water. Magnesia as found in feed water exists in two states, oxide and a carbonate, when in the latter form and free from the traces of iron, tends to give the yellow coloring matter to scale—in R. R. work, yellow scale is called magnesia scale.
Carbonate of Magnesia is somewhat more soluble in cold than in hot water, but still requires to dissolve it 9,000 parts of the latter and 2,493 of former.
Magnesia, in combination with silica, enters largely into the composition of many rocks and minerals, such as soapstone, asbestos, etc.
Lime, whose chemical name is calcium, is a white alkaline earthy powder obtained from the native carbonates of lime, such as the different calcerous stones and sea shells, by driving off the carbonic acid in the process of calcination or burning.
Lime is procured on a large scale by burning the stone in furnaces called kilns, either mixed with the fuel or exposed to the heated air and flames that proceed from side fires through the central cavity of the furnace in which the stones are collected.
The calcined stones may retain their original form or crumble in part to powder; if protected from air and moisture they can afterwards be preserved without change.
Soda is a grayish white solid, fusing at a red heat, volatile with difficulty, and having an intense affinity for water, with which it combines with great evolution of heat.
The only reagent which is available for distinguishing its salts from those of the other alkalies is a solution of antimoniate of potash, which gives a white precipitate even in diluted solutions.
Sodium is the metallic base of soda. It is silver white with a high lustre; crystallizes in cubes; of the consistence of wax at ordinary temperatures, and completely liquid at 194°, and volatilizes at a bright red heat. It is very generally diffused throughout nature though apparently somewhat less abundantly than potassium in the solid crust of the globe.
Salt, the chloride of sodium, a natural compound of one atom of chloride and one of sodium. It occurs as a rock inter-stratified with marl, and sandstones, and gypsum, and as an element of salt springs, sea water, and salt water lakes.
The proportions of its elements are 60.4 per cent. of chlorine and 39.6 per cent. of sodium.
In salt made of sea water the salts of magnesia with a little sulphate of lime are the principal impurities.
The above mentioned chemical substances can be classified into two distinct classes, i.e., incrusting and non-incrusting.
Of the incrusting salts, carbonate of magnesia is the most objectionable, and any feed water that contains a dozen grains per gallon of magnesia can be expected to have a most injurious effect on the boiler, causing corrosion and pitting. Carbonate of lime, while not as bad as the magnesia carbonate, yet has a very destructive action on a boiler and 20 grains per gallon of this is considered bad water. All silicates, oxides of iron, and aluminium, and sulphate of lime are also incrusting. The non-incrusting substances are three, viz., chloride of sodium (common salt), and sulphate and carbonate of soda.
Note
In view of the increasing importance laid upon a knowledge of the chemical formation of feed water, these chapters of Chemical Terms and Analysis of Feed Waters are given to indicate the direction in which the advanced engineer must push his inquiries. There are more millions of treasure to be made by properly “treating” the water which enters the steam generators of the world than can be extracted from its gold mines.
An important “point” is to make sure, before adopting any permanent system for purifying the waters of a steam plant, that it is always the same in its ingredients, i.e., that the impurities contained in the water are the same at all times.