REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. In the last experiment, the glass window at the fire end of the boiler blew out with a quick, sharp report, as loud as that of a musket; the fragments of glass, from a hole in the centre of the plate, were projected through a window, about three feet from the boiler, and could not be found. The number for twelve atmospheres is placed opposite to this experiment, as being an approximate result. In the act of observing the gauge, the glass burst, and the mercury at once fell: the Aumber of inches at which the mercury had certainly risen, and above which it was, by an undetermined quantity, not however very considerable, was noted; and from this the pressure given in the table is calculated. Here explosive steam was generated by the injection of water upon red-hot iron, and in a time not exceeding one or two minutes at the most, the interval between the last stroke of the pump and the explosion not having been sufficient to note the height of the gauge; the experimenter being at the pump, which was adjacent to the gauge. By comparing the temperature of the steam in these experiments, with its observed pressure, it will be seen, that in not one of them was water enough injected to give the steam a density even approaching to that corresponding to its temperature: for example, 336° Fahr., should give a pressure of nearly 72 atmospheres, instead of 3.3, the observed pressure; 3880 should give more than 14* atmospheres, instead of 8.2 and 4480; about 27 atmospheres, instead of 10. The violence of the effect was not, therefore, carried as far as it might have been, the metal not having been cooled down as far as it might have been, to produce the greatest effect; and yet within two minutes the pressure was changed from 1 to 12 atmospheres. The rise of temperature shown in the first column serves to prove, that by successive introductions of water the metal was not so far deprived of heat as to be cooled towards the point of maximum vaporisation, but that the results were obtained with metal heated to redness. This A similar experiment to these was made by our countryman, Perkins, but surcharged steam being present in the vessel into which heated water was forced, it was to the action of this that he attributed his result. opinion will be examined subsequently, but the attributed source of action was present only in a very attenuated state, if at all, at the beginning of each experiment made by the Committee. The repetition and extension of the experiments of Klaproth was one of the most laborious of the undertakings of the Committee, * Arago and Dulong. 99 and the results will be found in a future article of the Report. III. To ascertain whether intensely heated and unsaturated steam can, by the projection of water into it, produce highly elastic vapour. The supposition that water, thrown into hot and unsaturated steam, is flashed into highly elastic vapour, forms the basis of the theory of the explosion of steam-boilers of our countryman, Perkins; a theory which has been embraced by many; and which, though shown to be contrary to the deductions from the well-established laws of heat, is not now without its advocates. It seemed to the Committee interesting to appeal to direct experiment upon this point, and thus to ascertain whether any circumstances, not embraced in the general view of the theory, existed; or whether all the circumstance s had been rightly estimate, ar dtke ancia sions drawn from the application of the general laws of heat would be confirmed. Being unwilling to incur any considerable expenditure in this branch of their inquiry, the experiments were rendered uncomfortable beyond any thing which occurred in their other researches. The means of producing the unsaturated steam were these: a row of bricks was removed from the top of the furnace, and near to the boiler, thus laying bare nearly half the convex surface of the latter (five inches from the top). By building on the sides of the top of the furnace, with bricks, loosely arranged, a space was formed for placing fuel, having the boiler for its bottom, and bounded by the bricks on its sides. cap of sheet-iron above served to promote the draught, and toca rry off much of the deleterious gas produced by the charcoal used as fuel. The fusible plate apparatus was removed from the boiler, to prevent injury, and the safety-valve was surrounded by a tin to keep the fuel from contact with the valve. By filling the boiler about half full of water, and applying heat below to raise the water to any required temperature, the upper half of the boiler would be filled with steam of an elasticity due to that temperature, this elasticity being measured by the gauge. Fire being now placed upon the top, would heat the metal of the upper half of the boiler; and this, by communicating its heat to the steam, would surcharge the latter. To measure the temperature thus acquired by the steam, as well as that of the water below it, thermometers were placed in the iron tubes already described; the mercury was removed from the tubes, except enough to cover the bulbs of the thermometers, so that the temperature shown by them might be, as nearly as possible, that of the steam by which the shorter tube was surrounded, and of the A 100 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. water into which the longer tube dipped. The scales of these instruments were proteeted from the fire by surrounding them, at some distance, by tin plates. The scales were of seasoned box-wood, the refinement of a correction for the errors of the instruments was not considered to be at all required by the nature of the research, the results of which errors even a few degrees of temperature would not materially affect. In the final experiments on this subject, the thermometers, with metallic scales, and surrounded by water, were put in place. The apparatus for injecting water consisted of a tube attached to the stop-cock v, fig. 1, on the back head of the boiler, and which communicated with the forcing-pump; the tube terminated in a spherical segment, in which fourteen holes, each of the size of a cambric needle, were made; through these the water was forced in spray. By examination it appeared that the small stream, from the highest hole, struck the top of the boiler near the safetyvalve; that two or three struck the front head; two or three the water in the boiler, near the back head, leaving from seven to nine apertures, the water from which crossed the steam-chamber in an inclined and very effective direction. The effect of the streams from the three or four apertures first spoken of, would be, if they were not taken up by the steam, to vitiate, in degree, the experiments, by striking the top and end of the boiler. With the openings, thus described, the first day's experiments on this subject were made. The heat of the surcharged steam could not, with the arrangements then provided, be raised above 4840. The method of experimenting having been the same as was subsequently used, may as well be stated in this place. The fire having been applied below the boiler, the water was heated to a temperature corresponding to from one and a half to two and a half atmospheres; the coals were then, in part, removed to the top of the boiler, fresh fuel being supplied below: the effect of the heat applied above was soon visible upon the thermometer in the steam, and upon the gauge. When the temperature of the surcharged steam sufficiently surpassed that of the water, as shown by the larger thermometer, the injection of water was commenced, the injection-pipe being carefully kept cool by wet sponges and cloths. The temperature of the thermometers in the water and steam were noticed both before and after the injection by one experimenter, while a second made the requisite number of strokes of the forcing-pump, observed the indications of the steam-gauge, and when the experiment was concluded, gave the quantity of water used. The temperature of the air in the gauge was noted from time to time. The apertures in both the heads were secured with metallic plates, to prevent leakage through them. On the second day, six of the fourteen small holes were plugged up, that the source of error, already mentioned, might not exist. The temperature attained by the surcharged steam was 440o, at, and below which, experiments were made. The general nature of the results, obtained on the first and second days, coincide, allowing for the difference of circumstances, so entirely with those of the final trial, when a satisfactory temperature was obtained in the surcharged steam, that it would be uninteresting to detail them. The same remark may be made of subsequent trials. As the quantity of water thrown in during all these experiments was small, it was considered advisable to increase it, in order that more marked effects might be obtained; this was done by removing the pierced head from the pipe, thus delivering nearly the full capacity of the pump at each stroke. The quantity of water thus injected through the steam by each stroke of the pump was, at a mean, half a fluid ounce. No heating of the injection-water was required, as the heat necessary to raise water from the temperature of the experiments to the boiling point was but a small fraction of that required to convert it into steam. In the last day of trial the heat of the top of the boiler was so great and so long sustained, that the thermometer in the water became, in the course of the experiments, for reasons which will be stated, comparatively useless, as an indicator of the temperature of the water. The following tabular view of the results of the experiments is extracted from the minutes. The first column of the table contains the temperature of the surcharged steam, previous to the injection of water in any experiment; the second column that after the injection: this comparison being made to ascertain whether the heat supplied was, or was not, sufficient to make up for that consumed in vaporising the water thrown in. The third column shows the quantity of water injected; the fourth the height of the gauge before the experiment; the fifth the height after the experiment; the sixth the temperature of the gauge; the seventh and eighth, the pressures in atmospheres, calculated from the height of the gauge, and the temperature of the air within it, before and after each experiment. No notice is taken of the temperature of the scales of the thermometers, it having varied but 10° Fah. namely, from 86 to 96o. The first experiment is introduced, to show the temperature of the water within the boiler, before the long-continued heat had sensibly affected the indications of the thermometer. . REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. 101 6930604 6 21 80 21.70 21.80 21.65 21.80 21.60 21 90 21.90 21.85 21.90 21 70 6.65 6.48 At the close of these experiments, the metal was, in many places, but little short of a red heat, visible in day-light. The precise state of things in a boiler, of which parts are unduly heated, was represented in these experiments; there was the surcharged steam, and heated metal ready to give up its heat to replace what might be absorbed in the conversion of the water injected into steam. This latter circumstance renders the case different from that contemplated in the deductions of theory which have been brought to bear upon the question. The greater or less intensity of the heat afforded by the top and sides of the boiler would necessarily modify the effects observed, by the injection of any given quantity of water; this is observable in the numbers given in the table, where although the greater quantity of water injected does not fail in two consecutive experiments to show a greater depression of the gauge, yet in distant experiments the same is not the case. We see that in no case was an increase of elasticity produced by injecting water into hot and unsaturated steam, but the reverse, and in general that the greater the quantity of water thus introduced, the more considerable was the diminution in the elasticity of the steam. The quantity of water injected was from 3.5 to 24.3 cubic inches. The immediate rise of the gauge after each experiment, shows how rapidly heat was supplied by the sides of the boiler to the steam within. That the steam was highly surcharged with heat, appears by comparing the pressures corresponding to th temperatures with those For comparison, no water injected. Water in the boiler, 318° Fah. Gauge stationary. Temperature of air in gauge noted between two experiments. Gauge fell slightly, then rose to original level. Rose again immediately to 21.90. Fell nearly 2 inch. Note. 533o is, according to the formula of Arago and Dulong, the temperature of saturated steam of more than sixty atmospheres. given by Dulong and Arago for saturated steam. For example, the pressure shown by the gauge when the steam was at 506° Fah. was 6.15 atmospheres, while the table just referred to gives for this temperature a pressure of forty-eight atmospheres. temperature was carried to 533° Fah. when the pressure shown by the gauge was 6.82 atmospheres, while saturated steam at that temperature would have had a pressure of more than sixty atmospheres. The In order to ascertain whether the thermometer relied upon to give the temperature of the steam was affected, if at all, in excess or defect by the conducting power of the metal; the temperature of the boiler just beyond the tubes was taken, as nearly as was practicable, by a thermometer R, fig. 1, dipping into a clay receptacle, upon the top of the boiler. This thermometer did not rise above 4050 Fah.; its distance from this source of heat was ten inches, and that of the iron tube inclosing the thermometer, six and a half inches. Supposing the temperature stationary on top, the temperature of the metal of the top of the boiler near the tube of the thermometer would have been 4790,* show *If we suppose the heat of a small bar of metal, cut from the top of the boiler, to have been derived by the conducting power of the metal alone, the heating effect of the steam within being neglected, and further, that the temperatures of the bar had become constant, then the ratio of the excess of the temperature (y) of any point at a distance (x) above the temperature of the air, to that (y') of any point at a distance (x'), is given by the proportion, y': y: log. x: log. x'. In the case before us, y = 405-80325°, x= 102 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. ing that it tended to carry off heat from the thermometer, which, if at all affected by the metal above it, showed too low a temperature for the steam in contact with it. The temperature of the source of heat would have been from these data, 582° at the extreme end of the part covered with fuel, which was of course at a lower temperature than the middle portion On examining the apparatus after the conclusion of the last day's experiments, it was found that some of the putty used in tightening the lower joint of the thermometer in the water had been softened by the heat, and had flowed into the tube, thus affording a direct communication between the steam and the bulb of this thermometer: this circumstance accounts for the instrument being affected in this day's experiments and not in the preceding ones. IV. The next query may be thus stated: when steam, surcharged with heat, is produced within a boiler by the contact with heated metal, does this steam remain surcharged, or does it take up water from contact with that in the boiler, and become saturated steam? If the latter supposition be correct, at what pressure and temperature with regard to the temperature of the surcharged steam, and to that of the water on which it rests? The answer to this question is given by the experiments just detailed; and as they established the negative in relation to the surcharged steam becoming saturated, there was no necessity for a repetition of the experiments to ascertain the precise temperature of the water in the boiler. When fire was applied to the top of the boiler, the water within was at 318° Fah.; a moderate fire was kept up below, and one so nearly uniform, that great variations from that temperature could not have taken place, and which the results satisfactorily show did not occur. If we assume that during the experiments the temperature was 3084° Fah., a remarkable correspondence will be found in the observed pressures, and in those calculated on the supposition that this steam was expanded by heat, as a gas would have been, without any addition of water. The table below gives the temperatures of the surcharged steam observed at different times during the course of the experiments; the pressure shown by the gauge at that temperature; the pressure which would have been produced by heating steam at 308 to the temperatures given in the first column by the mere effect of expansion; and the pressures of saturated steam at the different temperatures. from expansion of Steam, 308 by heat. REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. 103 acquiring from it the water necessary to V-Inquiry in relation to Plates of It is well known that one of the most scientific nations of Europe relies, particularly, as a means of safety for steam boilers, on the use of plates of fusible metal. The plates are alloys of tin and lead, or of these two metals with bismuth, the proportions of the component metals regulating the point at which they fuse. In France these alloys are prepared at the royal mint, where plates made from them, or ingots of the alloys, may be purchased for use. The examinations which must have been made to determine the proportions of the metals necessary to produce an alloy fusing at a given temperature, and the circumstances of fusion, have not, as far as the committee know, been made public. A table of the fusing points of different alloys of tin, lead, and bismuth, &c., was drawn up by Parke, from experiment, and is contained in his chemical essays, vol. ii. p. 615. This table was made the basis of the investigations* undertaken by the committee, but they soon found it convenient to depart, more or less, entirely from it. The method employed by Parke for determining the fusing point of a metal, or rather the solidifying point of the melted metal, was ingenious. On melting a metal, and allowing it to slowly cool to the point of congelation, and observing a thermometer plunged in it, a rise of temperature, and then a stationary point, is observed; this is a point where a change is going on, by which the heat given out in the change is equal to that of which the metal is robbed by the surrounding medium. This point usually coincides with the passage of the metal to the solid state, from what may be either the liquid state, or a semi-fluid state, similar in aggregation to sand; sometimes the alloy is solid throughout, before the stationary point arrives; and sometimes there is more than one such point. The stationary point is not that at which the alloy, when used as a fusible plate for a boiler, gives way; the plate being covered by a perforated brass disk, to prevent its being pressed outwards before fusion, and so reduced in thickness as to burst, the metal is not forced out through these openings until perfectly fluid; if any part of the metal be At the time these experiments were made, the paper of Rudberg Ann. de Chimie. et de Phys., vel. 48, had not appeared. comes fluid before the rest, and gives way, the rest being in the sandy state, just spoken of, the particles seem to act like those of sand in a similar case, and to oppose an effective resistance to the pressure of the steam; these facts will be further developed in the examination of the application of these plates. The stationary points, when taken with due reference to the state of the metal at the time, afford so many approximate marks by which to compare together the fusibilities of the plates, and to ascertain whether they bear a due relation to each other, when fused, in place upon the boiler; and to study the alloys themselves. In composing alloys of the metals, before referred to, the tin was fused first at as low a temperature as possible, then the bismuth and lead added, the heat being kept up; these metals were readily taken up by the liquid tin, and were thus little exposed to oxidation: the surface of the alloy was always protected by a stratum of oil. The metal was constantly stirred to promote the uniform diffusion of the different metals throughout each other. The alloy being liquid, a thermometer, of which the errors had been carefully ascertained, was plunged into it, and the fall noted until it reached the lowest point; the rise to the stationary point followed, and at this the thermometer usually remained for such a length of time, often some minutes, as to render any error of observation unnecessary. Some of the alloys have no stationary point, properly so called, and the beats of a second's pendulum were used to determine the rate of their loss of heat. When the quantities of metal used were inconsiderable, the heat was observed to be carried off so rapidly as to lower, or entirely to destroy, the stationary point: to avoid this, the crucible containing the alloy was placed in a second one, the edges of the former resting on the middle of the sides of the latter. The quantity of metal used was never less than between five and six ounces, troy. The stationary point being at the passage of the liquid metal to the solid state, or at some interior change of the solid itself, the thermometer was entangled in the metal; and in moving the alloy, to re-melt it, the instrument was endangered.* This was remedied by the use of a small cylinder of very thin sheet-iron, containing mercury. This cylinder was placed in the alloy, and filled up to the surface of the metal with mercury, and the thermometer could now be readily placed Although the instrument was frequently used in determining the stationary points, no permanent changes in the indications of the instrument, such as was noticed by M. Rudberg, took place. |