7650 Banks St., Justice, Illinois 60458
Water is never pure, except when made so in a laboratory or by distillation; the impurities may be divided into four classes; 1. Mechanical impurities. 2. Gaseous impurities. 3. Dissolved mineral impurities. 4. Organic impurities.
(a) Mechanical impurities may be both mineral and organic. The commonest suspended impurity in water is mud or sand; these may be removed by filtration or by allowing the water to stand long enough to let them settle to the bottom of the tank or cistern and then carefully drawing the water from the top, and without disturbing the bottom.
(b) Gaseous impurities have no effect on water intended for steam boilers.
(c) Dissolved mineral impurities in water are of the most varied description, and are almost always found in it. Among these are found salts of iron, sulphate and carbonates of lime; sulphate and carbonates of magnesia; salt and alkalies, such as soda, potash, etc.; acids, such as sulphuric, phosphoric, and others. All of these are more or less injurious to steam boilers. The most objectionable are the salts of lime and magnesia, which impart to water that property known as hardness. When such water is used in a steam boiler a scale will gradually form, which will, in a short time, become very troublesome.
(d) Organic impurities are present, to a certain extent, in most waters. They are sometimes present in the water in sufficient quantities to give it a very decided color and taste but have little or no bad effect in any water used for steam boilers.
Distilled water for boilers is not to be recommended without some reservation. Chemically pure water, and especially water which has been redistilled several times, has a corrosive action on iron which is often very troublesome. The effect on steel plates by the use of water several times redistilled, such, for example, as that supplied for heating buildings, is well known; information is yet wanting which shall point with certainty to the exact change which the water undergoes and explain why its action on or affinity for steel is so greatly intensified. It has been suggested as a means of neutralizing this corrosive action of the water, to introduce with the feed other water, which shall have the property of forming a scale and continuing it long enough and at such intervals as will permit the formation of a thin scale in the interior of the boiler. However objectionable this may seem at first sight, it is at present the best practical solution of the difficulty.
Scale is a bad conductor of heat and is opposed to economical evaporation. It is estimated that a thickness of half an inch of hard scale firmly attached to a boiler plate will require a temperature of about 700° Fahr. in the boiler plate in order to raise and maintain an ordinary steam pressure of 75 pounds. The mischievous effects of accumulated scale in the boiler, especially in the plates immediately over the fire, are: (1) preventing the water from coming in contact with the plates, and thus directly contributing to the overheating of the latter; and (2) by causing a change of structure in the plates and the consequent weakening brought about by this continual overheating, which would, in a short time, render an iron or a steel plate wholly unfit for use in a steam boiler. The two principal ingredients in boiler scale are lime and magnesia. The lime, when in combination with carbonic acid, forms carbonate of lime; when in combination with sulphuric acid, it then becomes sulphate of lime. This is also true of magnesia.
Sulphate of lime is not so easily gotten rid of, as it is heavier than carbonate of lime and adheres to the plates while the boiler is at work. It is the most troublesome scale steam engineers have to deal with; it is very difficult to remove and by successive layers becomes dangerous, on account of the thickness to which it eventually accumulates.
The carbonates of lime and magnesia may be largely arrested by passing the feed water through a suitable heater and lime extractor. It must be apparent to everyone that any device which will accomplish this is a very desirable attachment to a steam boiler. As it is not possible to eliminate all the foreign matter in the water from it, recourse is often had to the use of solvents and chemical agencies for the prevention of scale. Some of these are very simple and within easy reach; others are surrounded by an atmosphere of uncertainty and the real nature of the compound is hidden under a meaningless trademark. For carbonate of lime, potato has been found to be very serviceable in preventing the formation of scale; its action appears to be that of surrounding the particles of lime with a coating of starch and gelatine, and thus preventing the cohesion of these particles to form a mass. Various astringents have been used for this purpose, such as extracts of oak and hemlock bark, nutgalls, catechu, etc., with varying success.
Carbonate of soda has been used and with very great success in some localities, not only in preventing, but in actually removing scale already formed. It acts on carbonate of lime, not only, but on the sulphate also. It is clean, free from grit, and is quite unobjectionable in the boiler; one or more pounds per day, depending on the size of the boiler, may be admitted through the pump with the feed water; or admitted in the morning before firing up, by simply mixing with water and pouring into the boiler through the safety valve or other opening.
Tannate of soda has been similarly employed and is an excellent scale preventive. It will also act as a solvent for scale already formed in the boiler, acting on sulphate as well as carbonate of lime.
Crude petroleum has been found very beneficial in removing the hard scale composed principally of sulphate of lime.
The employment of zinc in steam boilers, like that of soda, has been adopted for two distinct objects: (1) to prevent corrosion, and (2) to prevent and remove incrustation. To attain the first object, it has been used chiefly in marine boilers, and for the second, chiefly in boilers fed with fresh water. In order that the application of zinc in marine boilers may be effective, it is necessary that the metallic contact should be insured. If galvanic action alone is relied upon for the protection of the plates and tubes, it will doubtless be diminished materially by the coating of oxide that exists between all joints of plates, whether lapped or butted, and also between the rivets and the plates. Assuming the preservative action of zinc to be proved when properly applied, we have now two systems for preventing the internal decay of marine boilers: allowing the plates and tubes to become coated with scale, and employing zinc. It remains to decide which of these two systems is the best with respect to economy and practicability.