INIEANS OF PREVENTING EXPLOSIONS Several of the causes which have been ascertained to produce explosions, having been mentioned in the pages, it follows, that the means of pre venting these fatal occurrences, are to guard against their causes. It is a fact, that the greater part of these explosions have originated from the wanton Lolly, neglect, or inattention on the part of those, to whom the management of the fire is committed.
I. Metal for Boilers. The first point to be at tended to is the material of which the boiler is made. The propriety of rejecting cast iron, and substituting wrought iron, has already been men tioned. 'Where salt water is employed, copper is preferable, as it does not decompose water at any temperature, and hence an incrustation of oxide will be less liable to be formed internally, in a boiler of copper, than in one of iron; for this metal, at a high temperature, rapidly decomposes water per se, and with the aid of various salts or acids, decomposes it at the ordinary temperature of the air, and still more rapidly, at, or above boiling heat.
2. The form of the Boiler. No other than that of a cylinder should be permitted, for reasons before given. • 3. Strength of the Boiler. The third point would seem to be, the strength of the boiler.
It has long since been proposed, that every boiler intended to be used under steam of high pressure should be previously proved by injecting, by means of forcing pump, cold water into it, while the safety valve is loaded with a much greater weight than is to be employed when working with steam; or Bramah's hydraulic press may be employed.
The waterproof having been performed, the boiler should be next subjected to a similar trial by steam, of twice the force that is usually to be gene rated in the boiler, without causing the safety-valves to act. In France it is required by law, that all high pressure boilers be subjected to a proof Eve times as great as the boiler is intended to bear, when in ser vice. Arago says that this pressure is reduced to three times for boilers of rolled or hammered iron, but that even these do not afford entire security. Ile
judiciously remarks that " these trials only show what a boiler can bear when new, not what it will be able to sustain after some weeks, or some months use; after inequalities of temperature have strained the ni.Aal in every direction; or after rust has acted upon it," &c. A boiler should therefore be proved from time to time, and occasionally examined to detect any weak part.
Mr. Bramah in confirmation of this advice may be quoted. He says, if a boiler was prepared to sustain one hundred pounds, and strained with a force of two hundred pounds, it might afterwards break at forty, the straining having injured it.* The extreme proof required in France is injudicious. A proof of double the pressure intended to be used would seem suffi cient; more might defeat the object of the trial. Mr. Vivian adds, that explosions may be easily pre vented by proving the boiler every time it is cleansed, which should be at least every month.I- John Taylor says in proof of this, that a cast iron boiler was proved to one hundred pounds to a square inch, by the water proof, and commonly used with about forty pounds pressure, but it broke one day with less than twenty pounds pressure; the fracture being caused by the heat expanding unequally4 Mr. Donkin has known a boiler wear out in six months; and another used for fourteen years.§ Mr. Dodd says, that when a wrought iron boiler will not support the cold water test of twelve pounds to an inch, it is time to have a new boiler. II 4. Size of the Boiler. Mr. Evans never exceeded the diameter of three feet, and to increase capacity extended their length to twenty or thirty feet, or more; or increased the number. Messrs. Bush and Muhlenberg, his successors, now make their high pressure boilers of thirty inches diameter. The Engineer," before quoted, restricts their diameters to five feet; from which it would seem that in England high pressure boilers arc sometimes of a greater diameter.