Steam:
Its Generation and Use

particularly the case with “tubular” and “loco- motive ’ ’ boilers. There is good reason for believing that most of the mysterious explosions of boilers which stand the Inspector’s test, and then explode at a much less pressure, are due to the weakening effects of unequal expansions, for a boiler that will stand a hundred pounds test this week can- not explode the next week at fifty pounds press- ure, unless it has suddenly become wonderfully reduced in strength, and no corrosion or other natural cause, with which we are acquainted, save expansion, can produce this result. When we consider that strains from difference of ex- pansion are generally greatest when firing up, and when there is no pressure in the boiler, we can see that the time may arrive when a crack is started or the parts weakened, so as to give way under a moderate pressure just after the test has been made ; and this is the probable reason why so many boilers explode in getting up steam, or so soon after, or upon pumping in cold water, or, even, as in a recent case in England, while cooling off. How to Provide Against Explosions. Very much thought and experiment have been expended on this problem, but though many forms of boilers have been produced, which have attained practical safety from explosion, yet in nearly all of them there have been ignored cer- tain elements necessary at the same time to make them valuable as generators of steam for practi- cal work. Hence, the very7 name of “safety boiler” has unfortunately become, to some per- sons, prima facie evidence of undesirability. But safety is not incompatible with any of the other essentials of a perfect steam generator, and may be secured without detracting from any other desirable feature. The first element of safety is ample strength This can be best attained in connection with thin heating surface, by small diameters of parts ; but this must not be carried so far as to antagonize the equally important features of large capacity and disengaging surface. The second and most important element of safety, is such a structure that the original strength cannot be destroyed by deteriorating strains, from expansion or otherwise. This can be attained in two ways — by rendering unequal expansion impossible, or by providing such elas- ticity that, should it occur, it can produce no deteriorating strain. The third element of safety is such an arrange- ment of parts that when, through gross careless- ness or design, the water becomes low and the boiler overheated, a rupture, if it occur, can pro- duce no serious disaster. No surface which requires to be “stayed” should be permitted in a boiler. It is scarcely possible, and altogether improbable, that such stays are, or can be, so adjusted as to bear equal strains. The one sustaining the heaviest strain gives way, the others follow, as a matter of course, and a disastrous explosion ensues. The photographic view of the boiler which ex- ploded at Washington, January 9, 1888, shows how stay bolts act, and the disastrous explosion at West Chester, Pa., about the same time, was clearly clue to the giving way of the stays which were intended to support the head. Water-tubes an Element of Safety. [From the Manufacturer and Builder, Feb., 1880.] Some recent actual occurrences have a very suggestive bearing upon the relative degree of immunity from violent and disastrous explosions possessed by the water-tube and fire-tube sys- tems of boiler construction respectively. The first case is that of an accident resulting through gross carelessness to a steam boiler on the water-tube system as constructed by Messrs. Babcock & Wilcox. The circumstances of the case were such as to make the test to which the boiler was put a most severe one, and the fact that the result was not a disastrous explosion, scores several points in favor of the water-tube system. The boiler here referred to is located in |he Brooklyn Sugar Refinery, and is rated at 300 horse-power, being one of a set of 1500 IL P. Recently, by one of those oversights that now and then cost scores of lives under the same cir- cumstances, the feed-water was cut off, and not noticed until the water level became so low that the boiler was nearly empty and the tubes were overheated. The result is shown above. One of the tubes burst, and this was the extent of the damage, which was speedily repaired at a cost of $15, and the works were running the next day. The second case is very analogous, but is even more instructive, as the boiler was subjected to a severer ordeal than the other. This boiler is in the Elizabeth (N. J.) jail, and was one of the same kind as that in the foregoing case. It was in charge of one of the convicts, who, after start- ing the fire as usual in the morning, was sur- prised not to observe, after an hour or so of waiting, any signs of activity in his steam gauge. 11