Steam:
Its Generation and Use

Importance of Providing Against Explosion. That the ordinary forms of boilers are liable to explode with disastrous effect, is conceded. That they do so explode is witnessed by the sad list of casualties from this cause every year, and almost every day. In the year 1880, there were 170 ex- plosions reported in the United States, with a loss of 259 lives, and 555 persons injured. In 1887 the number of explosions recorded were 198, with 652 persons either killed or badly wounded. The average reported for ten years past has been about the same as the two years given, while doubtless many occur which are not recorded. There is no need to resort to mysterious causes for the destructive energy displayed in a boiler explosion, for there is ample force confined within it to account for all the phenomena. Prof. Thurston* estimates that there is sufficient stored energy in a plain cylinder boiler with 100 lbs. pressure of steam to project it to a height of over three and one-haff miles; a “two-flue” boiler about two and one-half miles ; a “locomotive” at 125 lbs. from one-half to two-thirds of a mile ; and a 60 H. P. return “tubular” at 75 lbs. somewhat over a mile high. He says, “ a cubic foot of heated water under a pressure of 60 to 70 lbs. per square inch, has about the same energy as one pound of gunpowder. At a low, red heat, it has about forty times this amount of energy in a form to be so expended.” Speaking of water- tube boilers he says : “ The stored available en- ergy is usually less than that of any of the other stationary boilers, and not very far from the amount stored, pound for pound, in the plain tubular boiler. It is evident that their admitted safety from destructive explosion does not come from this relation, however, but from the division of the contents into small portions, and especially from those details of construction which make it tolerably certain that any rupture shall be local. A violent explosion can only come of the general disruption of a boiler and the liberation at once of large masses of steam and water. ’ ’ The Hartford Steam Boiler Inspection and In- surance Company report that up to January 1, 1888, they had inspected in all, 799,582 boilers, and had discovered 522,873 defects, of which 93,022 were considered dangerous. If now the above were a fair average of the boilers in ordin- ary use—and who shall say they are not? —we have the startling fact that more than one boiler in nine in common use, is in a “dangerous condition.” That more do not explode, is pro- bably due less to intelligent watchcare than to the fortunate lack of all the necessary conditions existing at one time. * Transactions Am. Soc. Mec. Eng., Vol. 6, page 199. Causes of Explosion. It is now fully established by the experience of Boiler Insurance Associations in this country and England, that all the mystery of boiler ex- plosions consists in a want of sufficient strength to withstand the pressure. This lack of strength may be inherent in the original construction, but is most frequently the effect of weakening of the iron by strains due to unequal expansion caused by unequal heating of different portions of the boiler; or it may be due to corrosion from long use or improper setting. If steam boilers are properly proportioned and constructed, they will, when new, be safe against considerably more pressure than the safety valve is set to; and the hydrostatic test, properly ap- plied, may discover faults in material, or the weakening effects of corrosion ; but, against the danger resulting from unequal expansion, ordin- ary boilers have no protection; a fact not prop- erly appreciated by engineers or the public. In getting up steam many boilers will be very hot in some parts, while other parts will be actu- ally cold; of course, under these conditions, enormous strains must occur in some portions of the boiler, which are thereby weakened; and these strains being repeated every time steam is raised, if at no other time, will eventually so far destroy the strength of the line or point of great- est strain that rupture must result; generally the rupture is small and gradual, but sometimes large and productive of disastrous explosions. In the boilers examined by the Hartford Boiler Insurance Company, up to 18^8, 24,944 fractures in plates were found in, at, or near the seams or through the line of rivets, 11,259 of which, or nearly one-half, had arrived at a dangerous state before discovery. Want of circulation of the water in boilers is a frequent and prolific cause of unequal expan- sion, and deteriorating strains, and little, if any, provision is made for circulation in all ordinary construction of boilers. Another source of dan- ger in all ordinary boilers is low water; and con- stant vigilance is required to keep the water at a proper height. Tn many boilers the fall of only a few inches in the water-line will cause the crown-sheet or some other portion to be exposed to the direct action of the fire, whence it becomes quickly over-heated, and weakened to such an extent that an explosion is likely to occur. Another frequent cause of unequal expansions, and also of weakening by burning and blistering the iron, is the presence of deposit or scale on the heating surface. This is liable to occur in any boiler, but in very many there is no adequate provision for removing it when formed. This is