A cylindrical boiler 5 feet in diameter and 16 feet long is likely to contain about 6,600 pounds of water and 22 pounds of steam. Neglecting the energy of the steam, which is relatively small, the energy in the water due to its expansion from water at boiler pressure into steam at atmospheric pressure, will be approximately 6,600 X 42,800 = 282,480,000 foot-pounds, or 141,240 foot-tons.
A marine boiler 13 feet in diameter and 12 feet long would develop approximately twice this energy, which would be about equivalent to the energy developed by the explosion of a ton of gunpowder. The explosion of one boiler on a modern battleship would develop sufficient power to lift the ship completely out of the water. Of course it must be realized that a large part of this energy is lost, and considerable is consumed in the destruction of the boiler itself, which leaves but a comparatively small amount to be expended in wrecking the immediate surroundings; but it nevertheless is a fact that the energy developed in the explosion of a large boiler is almost beyond the power of comprehension.
Causes of Explosions. Boiler explosions are usually the result of low water, grease, or scale. The two latter, by preventing the transmission of heat from the water, are likely to cause undue overheating of the furnaces or tubes, which may result in their collapse; these two causes—grease and scale—have been discussed under the subject of "Incrustation." Low water may be caused by failure of the water glass to indicate properly the amount of water in the boiler, or by failure of the feed pump to work properly.
Safety-valves have been known to be rusted to their seats so tightly that they failed to work at the proper time.
It is seldom that a boiler can fail as the result of defective design, for the laws in regard to construction, especially of marine boilers, are very definite. Defective workmanship or material, however, cannot be easily discovered; and it is possible that corrosion or incrustation may take place locally, without being readily detected; and, indeed, boiler-plates may even be tapped, and their thickness calipered, without discovering small local weaknesses which later may cause disaster. Minute fractures which escaped the Inspector's detection have later become serious. The majority of explosions can undoubtedly be traced to mismanagement in either care or operation. Defective Design. If a boiler is improperly set; if the stays are
too small, too few, or cut or bent to clear floats, pipes, etc., danger is likely to result therefrom. All manholes, large handholes, or domes should be strengthened with a reinforcing plate to make up for the material cut out. If the boiler is set too rigidly on its seating, without proper provision for its expansion, trouble is likely to follow. A defective water circulation is likely to cause excessive incrustation and unequal expansion of the plating, which is liable to open seams and produce fractures in the plates.
Deterioration. The strength of a boiler is likely to be impaired by fractures, general corrosion, pitting, or grooving. But external corrosion is the cause of many disasters. It proceeds unnoticed in many cases, and rupture may occur when least expected. In the discussion of "Corrosion," it was shown that improper setting of the boiler would cause or at least aggravate external corrosion; and that, on account of the close setting of the boiler, it was not easy to get at the plates to examine them. The strength of a boiler originally sufficient to sustain high pressure may become suddenly reduced by overheating or over-straining, either of which weakens the plates. Overheating may be caused by poor circulation, lack of water, or the accumulation of sediment or scale. Over-straining is caused by sudden cooling and contraction, or equally by sudden expansion. In starting the fire in a Scotch boiler—or, in fact, in any boiler with a large quantity of water—care must be taken that the fire is started slowly, or the boiler, becoming overheated locally, will develop excessive strains.
Defects of Workmanship. Defective workmanship is not of so frequent occurrence under present conditions as formerly, when many defects used to be produced by careless punching of plates; but for most boilers, and for all marine boilers at present, punching is prohibited; the holes must be drilled, and the plate edges planed and carefully calked. A rigid inspection of material is required, and there seems little danger of unsatisfactory work. Cheap boilers may of course be subject to various defects, but a good boiler should be free from such troubles. Material may be defective and may not be readily detected; but the careful tests now required, especially in marine work, reduce these possibilities to a minimum.