1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



151 



minute particular of matters of this nature, we are led to explain, 

 wliat, in our opinion, is one primary cause of such explosions. 



Fig. 2. Fig. 1. 



In the preceding woodcuts, fig. 1, represents a vertical section, 

 and fig. 9, a plan of the underneath part of a circular-shaped 

 boiler, concave at the bottom, and hemispherical or domed over at 

 tlie top; not uncommon in the mining districts of tlie liingdom. 

 Boilers of this kind, from having been extensively adopted by the 

 eminent engineer, are not unfrequently called, the ^^ Snieatnn- 

 boiler." By other persons, the " egy-bniler," from its appearance, 

 when rising above the brickwork, assimilating to that of an egg in 

 its cup. 



It has fallen to our lot to witness, during our professional prac- 

 tice, the destructive effects of explosion, as produced by two boilers 

 of this peculiar construction : one in Lancashire, the other 

 in Staffordsliire. In both instances, the boilers, though of great 

 weight, were lifted from their seats, and blown to almost incredible 

 distances. Yet, the boilers were employed ordinarily in generating 

 low-pressure steam; and, so far as could be ascertained, there was 

 no reason to doubt that, at the time, either of them was acting 

 otherwise than in the usual manner. Numerous opinions, entirely 

 of a speculative character, were advanced as to tlie causes of these 

 explosions. Most of them hinged, as is usual in such cases, eitlier 

 on the supposition that the safety-valve was defective, which 

 allowed of an undue augmentation of steam in the boiler, until it 

 attained to a pressure tliat could not l)e resisted ; or, to the non- 

 effective working of the hot-water pump, whieli, by not supplying 

 the boiler with water to compensate for that vapourised, allowed 

 the metal of the boiler to become so heated by the action of the 

 fire, as eventually to absorb the oxygen from a portion of the 

 water, and thereby liberate its other constituent, the hydvogen, — 

 w hereby, in the opinions of such persons, ex])losions do take place. 

 AVe, from our own examinations, entertained very difl'erent 

 tlioughts at the time, although we had not occasion publicly to 

 avow them. Since those periods, the personal inspection of 

 numerous boilers have confirmed the impressions we then enter- 

 tained. 



We shall now endeavour to elucidate, by familiar exposition, the 

 causes of such explosions ; and we do so tlie more willingly, as we 

 are in the hope that much good may be educed, by eliciting the 

 attention of engine proprietors and engine-tenters to the matter. 

 "W'e must state, however, in the first place, that as we have not 

 got by us, convenient for reference, the dimensions of the two 

 boilers, to whose explosions we have referred, we shall, for the ar- 

 gument, take supposititious dimensions. 



Suppose the diameter of the circular part of each boiler, at o, 6, 

 or c, (I, to liave been 12 feet, and that, for sake of simplification, 

 the curved top and bottom parts of the boiler, though conve.x and 

 concave, be considered to have been ilat,^each presenting the 

 same diameter of 12 feet ; under such circumstances, the area of 

 tlie top and bottom plates, respectively, would have been 1(),'286 

 inches. If, tlierefore, the presure of the steam within the boiler 

 ranged no higlier than 12 lb. beyond the atmosphere, the total 

 amount of pressure on the top and bottom plates would not have 

 been less than 195,4-32 lb., or about 87^ tons each. 



Now, if we examine attentively the nature of this pressure, or 

 force, we shall perceive that, so long as the boiler remains sound, 



or is in good condition, this enormous amount of power acts 

 equally, mid internally, both against tlie bottom of the boiler with 

 a tendency to force it the more firmly on its seating of brick-work 

 and against the top of tlie boiler with an inverse tendency to pro- 

 ject it into the air on the principle of the sky-rocket. Both forces 

 being equal, and acting in opposite directions, balance one another. 

 Hence, so long as the boiler remains sound, these conditions are 

 undisturbed, and the action of the force is equivalent to that of 

 statical equilibrium. The boiler, therefore, has no tendency to 

 ascend or descend, by virtue of that pressure ; but is retained on 

 its seat by the weight of the metal of which it is composed, and the 

 weight of the water within it. 



Suppose, however, on the other hand, that from long usage, and 

 consequent w eakening of the boiler by the action of the fire upon 

 it, a rent, or considerable fracture of the metal, takes place below, 

 so as to allow of a sudden and comparatively large escape of heated 

 water into the flue, or space </, A, and on and against the red-hot 

 brick-work. The consequences then become frightful. The pres- 

 sure on the top and bottom of the boiler, internally, still balance 

 one another, minus the less amount of pressure on the bottom, 

 caused by removal of that portion of the metal displaced by the 

 fracture. But underneath the boiler, between it and the brick- 

 work, the destructive efl'ect of the pressure — caused by instanta- 

 neous evolutions of large bodies of steam from the heated water 

 and heated brick-work — becomes alarmingly great. It is of itself 

 amply sufficient, without extraneous aid, to account for all those 

 devastating and painful casualties we are accustomed to witness at 

 such times. 



We repeat that, just before, and immediately subsequent to the 

 fracture, the pressures in the interior of the boiler are equal, minus 

 the less amount of pressure on the bottom, subducted by the open- 

 ing made by the rent. But beneath the boiler, the pressure acts 

 equally against the brick-work of the flue, g, li, and the under-side 

 of the'bottom part of the boiler; and as this latter is unconnected 

 with its seating of brick-work excepting by its weight, which in a 

 boiler of that construction, does not, with its complement of water, 

 often exceed twenty tcuis, the projection of the boiler into the at- 

 mosphere is the inevitable result. 



It is not possible to determine what the amount of that projec- 

 tile force may be. For, when the fracture takes place, and the 

 water, by escape, occupies a greater space, the pressure of the 

 steam is most probably diminished in the interior of the boiler, 

 although acting with equal intensity of force as regards that pres- 

 sure, both against the top and bottom of the boiler internally. 

 But beneaththe boiler the pressure is augmented to an enormous 

 extent, partly by large bodies of the heated water — already at the 

 temperature 24o° — flashing, when liberated, instantaneously into 

 steam; and partly by other quantities of such water being pro- 

 jected against the red-hot brickwork of the flue, and on the large 

 mass of ignited coal on the fire-grate — and, by suddenly absorbing 

 from such sources other and large quantities of caloric, being as 

 instantaneously flashed into steam. It should also be borne in 

 mind, that the additional quantities of steam thus generated exert 

 no force whatever inside the boiler, or comparatively none; and 

 that the vvhole amount of the pressui-e is directed against the 

 exterior of the boiler, increasing, largely, the projectile power. 



Suppose, therefore, that the amount of pressure lost by the 

 escape of the water into a larger space, at the time of the fracture, 

 to be reinstated by the additional quantities of steam thus suddenly 

 evolved (and we think it quite possible), it will he perceived that, 

 in such case, the pressure exerted against the under-side of the 

 boiler by the newlv-evolved steam, is'ST^tons; that the resistance 

 to that force is derived only from the weight of the boiler, and the 

 water within it, together amounting, probably, to 17^ tons; and 

 that the projectile force is equal to 70 tons. Hence, the boiler 

 must be blown from its seating, and projected through the air, and 

 the brickwork be scattered in every direction. But even if we 

 admit the force, as exerted by the pressure, to be equal only to one- 

 half of that amount, or 35 tons, still being derived frum an elastic 

 agent, it is amply sufficient to produce all those devastating effects, 

 vvhich, under such circumstances, we are accustomed to see recorded. 



The direction that an exploded boiler may take in its flight most 

 probably is influenced by the position of the fracture. This will 

 be better understood by reference to the subjoined diagram {see 

 next jxige). 



In the preceding section let fig. 3 represent a wagon-shaped 

 steam-engine boiler, set in the usual manner, in brickwork. By 

 the arrangement, as thus exhibited, the flame, and heat, and gaseous 

 products of combustion, pass from the fire-grate over the bridge, 

 along the bricked flues beneath the boiler, and at the back of it; 



