“Points” Relating to the Scaling of Steam Boilers.
The peculiarity about the sulphate of lime is that the colder the water the more of it will be held in solution. Water of ordinary temperature may hold as high as 7 per cent. of lime sulphate in solution, but when the temperature of the water is raised to the boiling point a portion of it is precipitated, leaving about .5 of one per cent. still in solution. Then as the temperature of the water is raised, still more of the substance is precipitated and this continues until a gauge pressure of 41 pounds has been reached which gives a temperature of about 200 degrees; at this point all the sulphate of lime has been precipitated. Many other scale forming substances act in a similar manner. This shows quite plainly that any temperature that can be produced by the use of exhaust steam would not be sufficient to cause the precipitation of all the substances which might be contained in the water.
That boiler incrustations are the immediate causes of the majority of steam boiler explosions is no longer a doubtable question.
Nearly all foreign matter held in solution in water, on first becoming separated by boiling, rises to the top in the form of what is commonly called scum, in which condition much of it may be removed by the surface blow-off. If not removed, however, the heavier particles will be attracted to each other until they have become sufficiently dense to fall to the bottom, where they will be deposited in the form of scale, covering the whole internal surface of the boiler below the water line, with a more or less perfect non-conductor of heat.
It is recorded that the engineer of the French ocean steamer St. Laurent omitted to remove a bar of zinc when repairing and cleaning out his boilers. On opening the boilers at the end of the voyage to his great surprise he found that the zinc had disappeared, but his boilers were entirely free from scale and the boiler plates not injured in the least.
It has been recently determined by some German experimenters that sugar effects a strong action upon boilers. It has an acid reaction upon the iron which dissolves it with a disengagement of hydrogen. The amount of damage done increases with the amount of sugar in the water. These results are worthy of note in sugar refineries and places where sugar sometimes finds its way into the boilers by means of the water supplied. The experimenters in question also find that zinc is strongly attacked by sugar; copper, tin, lead and aluminium are not attacked.
Two reasons, relating to incrustations, for not blowing out a boiler while under steam pressure may be given as follows: One is, that the foreign matter floating on top of the water will be deposited on the shell of the boiler as the water gradually subsides, and, second, the heated walls of the furnace will communicate a sufficiently high temperature to the boiler to dry and flake the sediment that would otherwise remain in the boiler in the shape of mud, which could easily be washed out were it not for the baking process.
Bark, such as is used by tanners, has an excellent effect on boiler incrustations. It may be used as follows: Throw into the tank or reservoir from which the boilers are fed a quantity of bark in the piece, in sufficient quantity to turn the water to a light brown color. Repeat this operation every month at least, using only half the quantity after the first month. Add a very small quantity of the muriate of ammonia, about one pound for every 2,000 gallons of water used. This will have the effect of softening as well as disintegrating the carbonate of lime and other impurities deposited by the action of evaporation.
Note.—Care must be exercised in keeping the bark, as it becomes broken up, from the pump valves and blow-off valves. This may be accomplished by throwing it into the reservoir confined in a sack.
Among the best samples of boiler compounds ever sent to the laboratory for analysis was found to be composed of:
| Pounds | |
|---|---|
| Sal soda | 40 |
| Catechu | 5 |
| Sal ammoniac | 5 |
This solution was formerly sold at a good round figure, but since its nature became more generally known, it is not found in market, but is largely used, consumers putting it up in lots sufficient to last a year or so at a time.
The above is strongly recommended by those who have used it, one pound of the mixture being added to each barrel of water used but after the scale is once thoroughly removed from the boiler, the use of sal soda alone is all that is necessary. By the use of ten pounds per week a boiler 26 feet long and 40 inches in diameter in one of the iron mills of New Albany, Ind., has been kept clean of scale equal to a new boiler.
There are other evils sometimes inherent in hard waters over and above the mere production of a crust. Some waters contain a great deal of soluble magnesia salts, together with common salt. When this is the case there is a great chance of corrosion, for the former is acted on by steam at high pressure in such a way that muriatic acid fumes are produced, which seriously corrodes the boiler, and, what is far worse, passes with the steam into the engine, and produces corrosion in the cylinders and other delicate fittings into contact with which the steam passes. All this can, however, be obviated by the removal of the magnesia from the water.
There has not been, and never can be, made a mechanical device which will precipitate all the ingredients contained in a water taken from a natural source of supply, and if it were possible to do so it would be the most ruinous thing one could do for the boilers, as water is the greatest solvent known to chemistry, and its nature is to hold in solution and be impregnated with the different elements it comes in contact with, to a certain per cent., and if its lime, magnesia, and the mineral salts are taken away, and the pure water is pumped into the boilers, it will take up the iron, causing pitting and grooving of the boilers. It is better to let nature take its course, to a certain extent, and neutralize what little mineral deposit forms in the boilers with as small an amount of vegetable matter as possible.
It is well to note that different waters require different treatment; what will be of benefit in one instance will be of no value whatever in a different water, many of the “compounds” sold to prevent and remove scale will certainly destroy a boiler if they are used persistently, because they are composed of the exact opposite chemicals which should be used; as an example it is stated that at one establishment one thousand dollars were expended annually for a mixture which it is said resulted in the reduction of the life and usefulness of the boilers of 50 per cent.
ENGINEERS’ TESTS
FOR IMPURITIES IN FEED WATER.
Much expense can be saved in fuel and boiler repairs by a little preliminary expenditure of money in securing a supply of good water for the steam boilers of a new establishment. Well water is nearly always inferior to the running water of streams; water from mines is especially hurtful, containing, as they do, large quantities of free sulphuric acid. Wells along the sea shore or on the banks of rivers affected by the tides, are likely to be saturated with chloride of magnesium. It is in determining these points that these ready tests of feed water are most useful.
A thorough and really scientific analysis of feed water is a costly and tedious process, but a simple and perhaps sufficiently accurate test may be made as follows: take a large (or tall) clear glass vessel and fill it with the water to be tested; add a few drops of water of ammonia, until the water is distinctly alkaline; next add a little phosphate of soda; the action of this is to change the lime, magnesia, etc., into phosphates, in which form they are deposited in the bottom of the glass. The amount of the matter thus collected gives a crude idea of the relative quality of sediment and scale-making material in the water.
Water turning blue litmus paper red, before boiling, contains an acid, and if the blue color can be restored by heating, the water contains carbonic acid. Litmus paper is sold by druggists.
If the water has a foul odor, giving a black precipitate with acetate of lead, it is sulphurous.
An experiment may be tried by dissolving common white or other pure soap in a glass of water, and then stirring into the glasses of water to be tested a few teaspoonsful of the solution; the matter which will be deposited will show the comparative amount of the scale-making material contained in the feed water.
In order to ascertain the proportion of soda to the feed water the following method is recommended:
1. Add 1⁄16th part of an ounce of the soda to a gallon of the feed water and boil it. 2. When the sediment thrown down by the boiling has settled to the bottom of the kettle, pour the clear water off, and 3, add 1⁄2 drachm of soda. Now, if the water remains clear, the soda, which was first put in, has removed the lime, but if it becomes muddy, the second addition of soda is necessary.
In this way a sufficiently accurate estimate of the quantity of soda required to eliminate the impurities of the feed water can be made and the due proportion added to the feed water.
By exercising a little judgment, the use of pure chemicals, with well cleaned vessels, test tubes, etc., the following reagents will determine the character of the most important elements which injure the iron surfaces of a steam boiler.
Carbonic acid is indicated by baryta water.
Sulphates are indicated by chloride of barium.
Chlorides are indicated by nitrate of silver.
Lime salts are indicated by oxalate of ammonia.
Organic matter is indicated by chloride of mercury.
The “base” of the better class of the various patented boiler compounds is tannin (whence tannic acid) and some form of alkali, and if the compounds were to be deprived of these two elements they would be absolutely worthless.
Where they contain, as some certainly do, sal-ammoniac, muriatic, hydrochloric and sulphuric acids, they cannot but act as boiler destroying agents.
Tannin or tannic acid is the principal ingredient used in preparing leather. It is found in a great variety of plants—sassafras root has it in large proportion, the gall nut and the bark of various trees, especially the oak produce it.
It is the presence of this acid that gives their only value to very many “compounds,” tan bark, gum catechu (which sometimes contains one-half part of tannic acid), etc. The acid seems to have but little effect where large quantities of sulphate of lime are present, but in waters where carbonate of lime predominates its detersive qualities are more marked.
The records of the Patent Office show that one boiler compound contains 23 per cent. of catechu, and others, 60, 81, 5, respectively, by which may be inferred the large quantity of this agent, which has been sold in combination with other chemicals, principally soda.