BOILER ACCESSORIES: BOILER EXPLOSIONS Safety is one of the first requisites in a steam boiler, and must be assured not only by proper design in the beginning, but by subsequent care and proper maintenance. The evil effects of corrosion and incrustation have been clearly shown; and it is apparent that a boiler which has suffered materially from either cause is not in condition to stand full steam pressure. Since the explosion of a boiler, especially in a city or a factory, is likely to prove fatal to many people and to cause the destruction of considerable property, not only by the explosion itself but also by fire, which almost invariably follows such an occurrence, it is impossible to lay too great emphasis on the necessity of seeing that the boiler is in proper working condition.
All boilers must be carefully tested—land boilers, by the State Inspectors; marine boilers, by the United States Inspectors. The boilers are carefully examined inside and outside, and subjected to a hydraulic pressure test 50 per cent greater than the designed pressure of steam; and if there is the slightest sign of pitting or corrosion, the boiler-plates may be drilled and the thickness calipered, the hole being refilled by a proper plug. If a boiler passes inspection, a subsequent explosion will probably be the result of mismanagement, although inspection is not infallible.
The owner of the boiler is usually held liable in case of explosion; but may protect himself from financial loss by insurance against accident in any of the boiler insurance companies. If so insured, the Insurance Inspector, as well as the State Inspector, examines the boiler; and there is consequently less likelihood of an explosion, for an insurance inspector will naturally be exceedingly careful in the interests of his company.
The damage done by an explosion is due to the energy stored in the hot water, which energy can be calculated by thermodynamic methods. If a boiler contains a large quantity of water at high pressure, and that pressure is suddenly relieved, as would happen in case of rupture, a considerable portion of this large volume of water will be turned instantly into steam, and the resulting explosion will ensue.
When a fracture starts in a boiler-plate, the steam escaping through the rent or opening tends to diminish the pressure rapidly within the boiler; and this causes the rapid formation of a large amount of steam. It must be remembered that the water in the boiler
at high pressure is held in the form of water only because of the high pressure exerted on it. If this pressure is relieved, large quantities of water will evaporate into steam at once, without the application of further heat. This almost instantaneous formation of a large quantity of steam prevents the boiler pressure from dropping, and the fracture naturally widens. The larger the body of hot water, the greater the disaster. This accounts for the relative safety of water-tube boilers. The division of the water in such a boiler into small masses in different sections, prevents a violent explosion. Should a water tube burn out, probably nothing more serious would happen than the rapid escape of a considerable quantity of steam, which might fill the boilerroom, drive out the attendants, and ultimately cause the destruction of the boiler because of the absence of water together with a hot fire. It would be necessary for several water tubes to burst at once in order that there should be serious damage from such an accident.
Energy. The available energy in one pound of hot water at 150 lbs. absolute pressure and 358° F., is about 42,800 foot-pounds; that is, it is sufficient to move one pound nearly eight miles; and if at 250 lbs. pressure, it has sufficient energy to move, it nearly twelve miles. This energy may be determined somewhat as follows: From the table of the properties of saturated steam, given in the back of the book, it is seen that at 150 lbs. absolute pressure (approximately 135 gauge), the temperature is 358.26°F. The heat contained in a pound of hot water at this temperature will be 330 equivalent to 330 X 778 = 256,740 foot-pounds. This represents the total heat energy in one pound of hot water at boiler pressure; but since one pound of steam at atmospheric pressure contains very many more heat units than a pound of water at 150 lbs. pressure, it is apparent that only a portion of this water can evaporate into steam, the balance remaining as hot water. About 17 per cent of the total energy will be thus available in vaporizing the water into steam; or, approximately, 42,800 foot-pounds per pound of water will be developed. The remaining heat is in the form of hot water.