CHAPTER XII.
BOILERS AND FIRE-BOXES.
CARE OF LOCOMOTIVE BOILERS.
The present tendency of steam engineering, in the effort to increase the work performed in return for every pound of fuel consumed, is to employ steam of very high pressure. The greater the initial pressure of the steam, the greater are the advantages to be derived from its expansive principle. To resist successfully the enormous aggregate of pressure to which locomotive boilers are subjected, a well-constructed strong boiler is absolutely necessary; and the various railroad companies throughout the country meet the required conditions in an admirable manner, as is evidenced by the remarkable exemption of such boilers from serious accidents. Although the locomotive is the most intensely pressed boiler in common use, that supreme disaster, an explosion, is of rare occurrence, considering the vast number of boilers doing service all over the States. This result is due to constant care in the construction, in the maintenance, and in the management, of the locomotive boiler. Like the conservation of liberty, eternal vigilance is the price of safety.
FACTOR OF SAFETY.
There is perfect safety in using a boiler so long as a good margin of resisting power is maintained above the tendency within to tear the sheets asunder. This margin is very low for locomotive boilers generally, hence the greater necessity for care in maintenance and management. Years ago the mechanical world established by practice a rule making one-fifth of the ultimate strength of a boiler its safe working-pressure. That is, a boiler carrying 140 pounds working-pressure should be capable of withstanding a tension of 700 pounds to the square inch before rupture ensues. Locomotive practice in this country does not provide much more than half of that margin of safety. When deterioration or accident reduces this margin, danger begins.
BOILER EXPLOSIONS.
Certain mechanical empirics and impractical quasi-scientists have at various times attempted to surround the cause of boiler explosions with a halo of mystery. But our most accomplished scientists who have made the subject a special study, and our best mechanical experts who have devoted years of patient experiment and research to the investigation of boiler explosion, attribute the terrible phenomenon to intelligible causes alone. The conclusions of the practical part of the mechanical world are well summed in one sentence in one of the annual reports of the Master Mechanics’ Association. It says, “Explosions originate from over-pressure: it matters not whether the whole boiler, or a portion of it, is too weak to resist the pressure.”