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67. Strength of Boilers.—In determining the material of which steam boilers should be constructed, the first object to be considered is safety. It is, therefore, necessary to ascertain what excess of strength a boiler should possess in order to ensure safety. On this subject Tredgold observes: "It is clearly a matter of opinion, founded on the experience of past accidents, as to the degree of pressure required, and it has been almost universally allowed, that three times the pressure on the valve in the working state should be borne by the boiler without injury. This degree of excess of power seems to be fully sufficient for the ordinary lowpressure boilers; indeed, I should think twice the pressure a proper allowance; and, were it always provided, there would be little chance of accident, if the valves be properly constructed and attended to. It becomes insufficient in highpressure boilers, because a common low-pressure boiler contains about ten times the volume of steam required for one stroke of the engine, consequently the time of twenty strokes must elapse before the density of the steam could accumulate to three times its working density, supposing the engine to be stopped, and the valve out of order; but if the boiler contains only as much steam as is required for one stroke, the force will be increased to three times in the time the engine would have made two strokes. This rapidity of the increase of force does not leave the necessary time to examine, nor even to open the valves in this extreme case, and the hazard must be greater in consequence. In all cases the time of accumulating power should not be shorter than it is in the common boiler. Besides, in working an engine where the excess of force increases so fast, the loss of steam would be considerable from any variation of the heat of the fire, even were the valve to act properly, and, therefore, there is a temptation to load the valve beyond its regular weight. To render the security on the stoppage of the engine equal in all cases, the excess of strength should be inversely as the space allowed for steam. It is still more important to consider the subject, in relation to the danger arising from

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unequal action of the fire; and, for this, the excess of strength should be inversely as the contents of the boiler expressed in units of the power. Boilers may fail from strains produced by other causes besides the force of the steam, and these may be noticed to guard against the circumstances which produce them." Among these causes may be mentioned the explosion of inflammable gases in flues which are unscientifically constructed; and the formation of hydrogen gas within the boiler, owing to the water being in contact with a part of the boiler which is red hot. In reference to the latter circumstance, Tredgold adds that, although the presence of hydrogen in the boiler would not add to the risk of an explosion, it undoubtedly would render it more destructive if it should take place. The destructive effects of an accident which occurred at the Cyrfartha Iron Works were attributed to the inflaming of hydrogen accumulated within the boiler. The boiler was constructed of the old spherical form, twenty feet in diameter; the thickness of the plates, when new, was, top plates a full quarter of an inch, bottom plates half an inch; load on the safety valve 7 lbs. per circular inch. Many lives were lost by this explosion, and the boiler was thrown to a distance of 150 feet, to a place thirty feet above the level of its former seat. The upper plates were too weak.

68. Materials of Boilers.-The materials generally employed for the construction of boilers, are cast iron, wrought iron, and copper. 1. Cast iron boilers were employed by Smeaton with great success; they possess the advantage of cheapness, and, with careful management, will last for a long time; but, being brittle, they are liable to break from the effect of unequal expansion, or from increased pressure of steam. 2. Wrought iron boilers are much used in this country and in America. Being ductile, they are free from the objections to which the former kind is liable; but they are not free from the effects of incrustation arising from the deposition of foreign matters, which, being combined with the water, are liberated when the water is converted into steam. To obviate this difficulty, Mr. Gurney proposed to inject an

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acid through the tubes, which should combine with the deposited matters, and carry them off. This plan was found to be difficult in practice, and it was afterwards proposed to remove the incrustation by mechanical means. 3. Copper boilers were substituted for those of iron, with the view of preventing the deposition of calcareous and other matters. It has been stated that these matters, instead of forming an incrustation on the copper, would merely be suspended in the water, and would be carried off by the blowing process (p. 41). This subject has been already alluded to at page 6. But Mr. Dinnen is decidedly of opinion, that it is unnecessary to prescribe any specifics for the prevention of deposit or of chemical action in copper boilers employed in marine engines; he states that the same effects occur in copper as in iron boilers, when submitted to the influence of a highly concentrated solution of saline and other matters, in a constantly boiling state, viz. deposition of salts and other deposits, and ultimate destruction of the boilers. But copper boilers possess certain advantages over those made of iron: copper is a better conductor of heat, and thus economises fuel and space; it is much more durable; it is more uniform in its durability; and it is valuable, as a commodity, when old. On the other hand, copper is much more expensive at the first outlay; and it is more flexible than iron at high temperatures.

69. Deposits on the outside of Boilers.-Mr. Dinnen has pointed out a serious evil which occurs in marine boilers, whether made of copper or of iron. This is the accumulation of soot, salt, &c., in the flues, the chemical action of which is very destructive to boilers. Water, he observes, whether cold or boiling, filters through numerous apertures insignificant in dimensions, and almost inseparable from the intricacy of the construction, and mixes with the soot, forming a combination which corrodes the material very rapidly. The heat of the boilers, when employed, evaporates the water, and leaves the salt, &c., in a concrete state; which, when cold, is again dissolved, and recommences its action

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more formidably. In cleaning out the flues of a copper boiler, "the congeries of salt, soot, and water, was thrown out upon the iron plates of the engine room flooring, where it remained for nearly two days; when removed, and the plates well washed, a pretty and novel phenomenon was exhibited. The iron plates had precipitated the copper from the solution which covered them, exhibiting a permanent metallic surface of copper." The leaks from which the salt escaped were found to be so unimportant, that the usual measures to staunch them could not be attempted without the risk of increasing the evil. Mr. Dinnen is of opinion, that copper boilers undergo more injury than those of iron from these deposits. The remedy must be found in sweeping the flues as often as possible, and keeping the boilers perfectly dry when unemployed. As a last remedy he recommends the application of cement to the angles and fillets of the flues, so as to completely enclose the angle pieces, with their defects.*

70. Feeding Apparatus of Boilers.-The loss of water in the boiler by the expenditure of steam, should be supplied uninterruptedly, so as to prevent any considerable variations in the level and temperature of the water. Various contrivances have been adopted, in order to secure the attention of the engine man to this object. 1. In the early engines of Newcomen and Savery, the quantity of water in the boiler was regulated by the use of two gauge cocks. The nature of this apparatus may be seen in the atmospheric engine at page 27, and it is described in page 28. 2. In the early engines of Watt, a self-acting steam whistle was connected with the boiler. It consisted in a tube, one end of which was inserted into the boiler at the lowest level to which the water was to be allowed to fall, while the other end was conducted into the engine-house, where it terminated in a mouth-piece or whistle. When the water had fallen to this level in the boiler, the steam rushed through the tube, and

Dinnen on Marine Boilers. Appendix in Tredgold.

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loudly admonished the attendant to shake off his slumber, or quit his repast, and to feed the boiler. 3. A third contrivance consisted in applying to the boiler a glass tube, bent at right angles at both ends, as represented at T T' in the adjoined figure; one of the ends entering the boiler above, the other below, the required level of the water. From a well-known law of liquids, it is evident, that the water will stand at the same level in the boiler and in the tube, and thus the attendant will be enabled to see, without taking any further trouble, when his services are required. 4. Another contrivance for indicating the periods of replenishing the boiler, is addressed to the eye of the attendant in the apparatus of the following figure. Let the

F

Fig. 37.

Fig. 38.

water in the boiler be supposed to be at its proper level. A weight F may be so prepared as to be nicely balanced, when half immersed in water, by a counter weight, A, connected with it by a flexible string, passing steam-tight through the boiler, and working over a wheel W. When the water falls or rises in the boiler, the balance between the weights is destroyed: if the level of the water rise, the weight F rises also, and the weight A falls; if the level of the water fall, F falls with it, and A rises. By attaching a rod across the wheel, or fixing two pins to its circumference, which shall be in the horizontal position when the water is at its proper level, an index is at once obtained by mere inspection of the wheel. 5. But, after all, the engine man may happen not to look, and the consequence may be serious. Hence it is necessary, that the engine should be enabled to supply its own boiler, by what is called a self-feeding apparatus. A contrivance of this kind has been already described and illustrated at page 86. Other kinds of self-regulating feeders have been employed, on the same principle as that referred to.