In general all waters contain the same substances in solution, in all sorts of amounts and proportions. Besides dissolved salts, waters have certain gases in solution, most commonly Carbon Dioxide and air. The air is usually spoken of as dissolved Oxygen because of the inertness of Nitrogen which causes no trouble. In the case of dissolved hydrogen sulphide, sulphur water results. In addition to the dissolved minerals most river or surface waters carry mud, clay and vegetable matter in suspension. All of these play a part in causing boiler troubles. And while the gases can be gotten rid of in open feed water heaters, and the mud settled and filtered out, the dissolved solids that cause scaling and corrosion cannot be eliminated or changed so that their harmful effects are neutralised except by chemical treatment.

The common salts in Boiler Waters are Calcium Carbonate and Bicarbonate which are present as temporary hardness, and which forms scale when the Carbon Dioxide in the water is driven out. Magnesium Carbonate and Bicarbonate, which are also present in amounts depending on the nature of the rock over which the water flows, are not so easy to remove and must be changed to the Hydrate before it can be removed. There is also present Calcium and Magnesium Sulphates which can only be eliminated by chemical treatment. In special cases waters contain Sodium Chloride Sulphate and Carbonate, and in these cases special consideration should be given before they are considered as boiler feed waters.

The chief producers of boiler scale are salts of Calcium and Magnesium and silt, the latter being carried into the boiler in suspension in the water. These constitute two classes of hardness. The first class consists of Bicarbonates of Calcium, which is precipated in some preheaters as Calcium Carbonate when the Carbon Dioxide dissolved in the water is driven off, but as time is necessary this reaction is seldom completed. The second class of hardness is known as permanent hardness, consisting of Sulphates of Calcium and Magnesium, and ordinary boiling does not affect them, their removal being possible by Chemical treatment only. All hard waters contain these two classes of scale forming compounds, and it is impossible to predict whether the scale will be hard and adhering or not, as the type of boiler, the pressure, the rate of operation, the degree of concentration allowed before blowing off and the load maintained are all factors in governing the nature and amount of scale. Another phase of the scaling problem not often mentioned is the effect of oil. The insulating effect of a thin film of oil is more than an average thickness of 1/16 of an inch, and is much worse as it will cause corrosion as well as loss of heat.

The presence of scale in the tubes of a boiler makes a very considerable difference in its capacity by reducing the circumference of the tubes. An 1/8 of an inch of scale will make a difference of so much per cent., 1/4in. a difference of twice, and § of an inch a difference of three times. This simply means the boiler capacity has shrunk this amount.

The physical condition of the scale is the big factor in heat insulation, which causes with hard scale a very considerable loss as shown in the following table :-

These losses do not occur in full in the boiler, because the whole boiler surface does not usually become covered. Still the loss is always serious apart from the stresses set up in the plate.

Any water that contains free acid will cause corrosion. Magnesium salts under boiler conditions will re-act with water, liberating acid which if not counteracted will cause pitting and corrosion. This may also be caused by galvanic action in the boiler itself, or by the indiscriminate use of such coagulants as alum, or of oil as before mentioned. Another danger in the indiscriminate use of substances in the boiler is clearly set out in Technical Paper No. 218 of the United States Bureau of Mines, where it says :- " As an instance of the serious danger of an unintelligent dosing of boiler waters may be cited the results of a long series of investigations which have apparently shown that Carbonate of Soda in solution produces brittleness in Boiler Steel. Carbonate of Soda (Soda Ash) is used in most water treating processes and its unintelligent use may readily lead to a very dangerous condition in the boiler."

This is further supported by the University of Texas bulletin No. 1752, which points out that this embrittlement seems to manifest itself in small hair lines starting from a joint, and in many cases so small that they could not be discovered without removing the rivets. The metals of the boilers so affected seem to lose their toughness, becoming quite brittle in the parts that are so affected.

Where the water is hard and the installation of sufficient size a water softening system is undoubtedly the best method of prevention. This is frequently not practical and treatment in the boiler may be used satisfactorily.

The crystallising of the calcium and magnesium salts on the tubes and plates of the boiler causing scale may be prevented if the salts are changed to a flocculent or jellylike form. The proper grade of Silicate of Soda is one of the best Chemicals for this purpose, consequently is the basis of many of liquid boiler compounds on the market. These have the soda and silica in the right ratio to furnish the necessary soda to break up the lime and magnesia salts in the water along with the right amount of silica to combine with the lime and magnesia and form the flocculent calcium and magnesium silicates.

The important point is to change the substances that form crystalline deposits into precipitates that are soft and flocculent. The result is a larger amount of sludge or mud in the boiler which can readily be blown out.

Water Testing

The following tests for various impurities in water are given by Messrs. D. P. Battery Co., Ltd., Bakewell.

Chloride Test

Carefully clean a test tube with the water to be tested ; half fill the tube with water, add a few drops of pure nitric acid and 5 drops of silver nitrate. The water should remain clear, or show a very faint apales-cence only.

Ammonia Test

Carefully clean a test tube with the water to be tested ; thereafter fill the tube with water, and add 5 drops of Nessler's Solution ; shake the mixture. The water should remain colourless.

Nitrate Test

To half a test tube of the water add 3 to 4 drops of Brucine Sulphuric Solution. The water, on shaking, must remain colourless.

Nitrite Test

To half test tube of water add 5 c.c. of Metapheny-lene-diamine Sulphuric Solution. Water must not show a brown discolouration.

Iron Test

Clean a porcelain evaporating basin with the water to be tested, and pour 100 c.c. of the water into the clean basin. Evaporate the water down to about 20 c.c. Allow to cool, then pour it into a clean test tube. Add one drop of pure concentrated sulphuric acid and one drop of pure concentrated nitric acid, then two drops of potassium sulpho-cyanide solution. If it remains clear and colourless, it may be used, but not otherwise.

Lime Test

Clean a test tube with the water to be tested, half fill the tube with the water and add 10 drops of ammonium oxlate solution ; shake the mixture. Hold the test tube in a Bunsen or spirit lamp flame for about half a minute.

Allow the tube to cool for five minutes. The water should remain clear.