The boiler of the sugar refinery, in Well-close Square, London, was of different thicknesses. At the bottom it was two and a half inches in thick ness; on the two vertical faces, an inch and a half; in the lower part of the dome, not more than seven sixteenths of an inch; and in one place, the thickness was reduced to an eighth of an inch. It was of cast The explosion of the Norwich was in part owing to one of the causes noted. Mr. Tilloch observes that " it was usual to raise the steam to a pressure of 70 lbs. to the inch before starting for the voyage; the end of the boiler was quite incompetent to hazard such a pressure, and the steam was urged beyond all pru dence to beat a rival boat. The wonder is, that it did not explode long before, for it was more common to have steam at from 100 to 120 than even at 70 lbs. to the inch-"t The boiler of the boat Tricolor (low pressure) exploded at Wheeling on the 19th of April 1831, by which eight persons lost their lives, from the imme diate effects of the explosion or by drowning; eight others were scalded very severely. A private account states, that the boiler was 13 years old, the Tricolor being the third boat on board of which it had been placed; and for several hours before the explosion a hot fire had been kept up.
8. Form of the Boiler.
Mr. Evans, from his first attempt to make steam engines, was aware of the superior strength of the circular form for boilers. In his " Steam Engineer's Guide," he says, " a circular form is the strongest possible, and the less the diameter of the circle, the greater the elastic power it will contain. Therefore we make cylindrical boilers not exceeding three feet, and to increase their capacity, their length is ex tended' to 20 or 30 feet, or their number increased." The adoption of the cylindrical form for boilers, evinces the correct philosophical principles upon which Mr. Evans acted. Nothing can be more clear than that by this form the expansive force is equally exerted over the whole of the internal surface of the boiler, and consequently that the steam will have no effect tending to change the form. Experience has proved the justness of his theory.
To ascertain the power exerted by steam to burst one of those boilers, and the thickness of iron neces sary to hold it, Mr. Evans gives a rule, example, and demonstration of the problem of a circle 36 inches di ameter, the result of which is, that 54,000 lbs. weight are required to break the two sides; half of which, 27,000, arc necessary to break one side in any one place. To the solution of this useful problem, which he never met with, he adds a table, showing the pow er exerted to burst each ring of one inch wide of the boilers of different diameters, and the thickness of iron necessary to hold steam of power equal to 1500 lbs. to the inch area.
In another publication lie illustrates the strength of this form by a familiar illustration. " It will bear," he says, " greater elastic power than boilers of other forms, in the proportion that a bar of iron will bear more pulling straight end-wise, than it will on its mid dle, to bend and break it when laid horizontally, sup ported at the ends." Boilers, however, are not al ways made in this form, and, to compensate for the deficient strength of other shapes, stay bolts are used.
The boiler of the London sugar refinery, already re ferred to, was made of flat iron pan, of eight feet dia meter: therefore, extending the bursting surface in proportion of four to sixteen.I Connected with the form of the boilers, is the mode of their construction, and applying the fire. In Corn wall the most common arrangement is to fix one tube within another; the interior one containing the fire place, and the space between it and the exterior con taining water, and in the upper part, steam. The
ends of the boiler fix the tubes together, so that the interior tube is opened at both ends, at one of which is placed the fire grate, and at the other the smoke and flame pass out, and are conveyed to the stack or chimney most commonly by flues passing under and along the sides of the outer case. They are from 20 to 35 feet long, and from 3 to 4 feet in diameter; the outer one from 52 to 61 or 7 feet. Four accidents by collapsing, according to Mr. J. Taylor, occurred in the two or three years preceding 1827, from the use of boilers of this description; the particulars of which he gives.§ The fire is now made under the boiler. Arago mentions several more that occurred in France, in similar boilers. Boilers of this description were formerly very common in the United States, but are now entirely given up.
9. Sediment in Boilers.
This cause has frequently produced bursting of boilers, and the inattention to it by those who own steam-engines, or command steam-boats, is truly sur prising. It is evident that a crust formed within or upon a boiler, offers a non-conducting material to the heat from fire or water; and that the bottom of a boiler, the inside of which is so coated, will require much more fuel to bring the water to the boiling point, and to keep up its temperature, than if this ob struction did not exist. Hence arises additional ex pence. This increased operation of the fire on the metal softens it, and allows the expansive force of the steam to push it out like a bull's-eye glass light, to the extent of several inches in diameter, thus diminishing its substance. Mr. Burr states that the protusion of the bottom of a boiler at Richmond, Virginia,* was as large and as deep as a hat crown, from sediment collected in four or five weeks. The protusion in the bottom of a boiler of an engine in a cotton mill at Kensington, Philadelphia county, from the sediment three inches thick collected on it, was about half that size. The sediment when picked off, will bring scales of the iron with it, and thus tend to lessen the thickness of the bottom plate still more. When the water is over the fire flue, the sediment will descend to the legs, and space in front of the boiler. The effect of a continued application of fire to the metal of boilers thus expanded and weakened, may easily be conceived. The fire and accumulated coals finally burn out the part, the water and steam then rush below, and the pressure from above will cause a collapse of the boiler. This was proba bly the case of the Legislator of New York. Several years since one of the four boilers of the high pressure engine at Fairmount Waterworks, on the Schuylkill, was burnt out from the collection of sediment in one end of it. The deposition of the sediment in that part was promoted by the position of the boiler, which, in order to favour the operation of the fire, was fixed so that the farther end was several inches lower than the door end. The boiler had been cleansed the day before the accident, but, owing to the difficulty of reaching the further end, it was neglected, and a thick hard crust, to which the grease in the water greatly contributed, was permitted to remain. The rent took place under this crust; the water ran into the ash-pit, and followed by that and the steam in the other three boilers, rushed out with a mass of ashes, closing the door of the ash-pit, which opened inwards, and filled it in an instant. Two men were severely scalded, and additionally suffered from the inhalation of ashes and hot steam, in which they were enveloped: they died within two days.