KEPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. 81 ducing equal measures of air from the point of a sliding-rod gas-measure (Hare's); this operation was performed repeatedly, and by multiple measures to verify the results, until the marks made for the equal volumes, on a paper scale attached to the tube, coincided, in the various trials. The lengths of the spaces occupied by the equal volumes were then carefully measured upon the brass scale to be used with the gauge. The slight differences between the lengths given by adjacent parts of the tube, showed that it might be considered as divided into so many small portions of uniform diameter. The mercury rising into the gauge-tube from the cistern when pressure is applied, the level of the cistern is necessarily depressed; the amount of the correction for this depends upon the relation between the areas of the cistern and tube, supposed uniform. The areas of the cistern were found to be, within the limits of its use, sensibly the same; those of the tube might be so assumed for such a purpose: the ratio was, therefore, found by filling the gaugetube with mercury, and pouring this into the cistern, noting the rise produced; comparing this with the mean length of the tube, the ratio of depression in the gauge for elevation in the tube was found to be as '01 to 1. The air within the tube was next carefully dried by the introduction of a receptacle of chloride of calcium, of the same length with the tube; the air having been in contact with this substance for a sufficient time, the receptacle was withdrawn through the mercury over which the drying had been effected; the tube was next placed over a dish of mercury, in the receiver of an air-pump, and the air withdrawn until on re-admitting air to the receiver the mercury rose in the tube above the iron ferule. The gauge-tube was next introduced into the cistern, the level of which corresponding to the zero of the brass scale was then arranged, and the point of the scale at which the mercury stood was ascertained, the barometer and thermometer being noted. It was intended in the experiments to keep the pipe from the gauge to the boiler cool, so that it might contain water, and thus give a nearly constant pressure upon the mercury of the cistern, besides preventing the exposure By this method, each volume of air in the tube was in contact with nearly a twelfth of its bulk of the chloride. This and very many of the other precautions to insure accuracy, are borrowed from the able memoir of Dulong and Arago on the elastic force of steam at different temperatures; the result of their labours as members of a Committee of the French Academy. Those who have engaged in questions of research will know that too great care cannot be taken to prevent the i troduction of error, even in researches where great nicety may not be considered essential. of the apparatus to heat; the height of this column, above the level of the cistern, was therefore ascertained, after the gauge was put in place by screwing the cistern i to the stand. All the elements for calculating the elasticity of the steam within the boiler, from the height of the mercury of the gauge, were thus known; the temperature of the apparatus being supposed constant. The elastic force of the steam within the boiler, together with the column of water in the steam-pipe, balances the elasticity of the compressed air within the gauge, together with the column of mercury above the level of that in the cistern. This level is not the original zero, but lower than that by the depression produced by the rise of mercury in the gauge-tube. The depression of the inercury changes the level above which the pressure of the column of water in the steam-pipe is measured, but the change in the pressure by the column of water is altogether inconsiderable. The law of the elastic force of dry air, which has been recently shown, by Dulong and Arago, to be accurate, at pressures from one to fifty atmospheres, was made use of in determining the elasticity of the air in the gauge: this elasticity is inversely as the space occupied by the air. From the data already obtained, and upon the principles just stated, a table was calculated by which the observed heights of the gauge were converted into the corresponding pressures in inches of mercury or in atmospheres. The calculations were rendered rather tedious by the unequal diameter of the bore of the tube, on account of which equal lengths did not correspond to equal volumes. The usual method of calculation was resorted to, namely, to determine, by rigid calculation, the pressures, for points sufficiently near each other, and then to interpolate for intermediate heights. The foregoing remarks take for granted that the temperature of the air in the gauge, as well as that of the mercury, remains constant; to secure this, an arrangement was adopted similar to that employed by Dulong and Arago for the same purpose. The gauge and scale were surrounded by a glass tube, figs. 1 and 3, cemented below into a brass cap m, fig. 1, which had an opening in the side, communicating with a discharge-pipe n, figs. 1 and 3. The tube was attached above by an air-tight juncture to a tin vessel P, of considerable capacity compared with the tube. Water being introduced into the glass tube surrounding the gauge, the flow through this tube was regulated by a stop-cock o, placed at the end of the discharge-pipe, the cistern above being filled with water. To ascertain the temperature of the column of water surrounding the gauge, a ther REPORT OF EXPERIMENTS ON THE mometer p, fig. 3, with a very small bulb, was attached to the scale at the middle of its height: by this instrument, the flow of water through the casing of the gauge was regulated so as to keep the temperature nearly constant, and any deviations from a constant temperature were ascertained and noted, that the proper correction might be applied. The correction for the expansion of the air in the gauge, by a rise in its temperature during the progress of the experiments, was made according to the rules furnished by the rate of expansion of the gases, as determined by Gay Lussac, extended to compressed air by the experiments of Davy.* The correction for the changes of height of the mercurial column, within the range to which the tem Let e represent the elastic force of the air within the gauge-tube, expressed in inches of mercury; let h be the height of the mercurial column above the original zero; h', the height of the column above the new level; a, the height of the column of water in the steam-pipe above the zero; 8, the specific gravity of mercury; t, the tension of the steam within the boiler, in inches of mercury. Then h'-h is the depression in the cistern caused by the rise of mercury in the gauge, and a+h'—h, the height of the column of water in the steampipe above the new level in the cistern. then, e+h+h'~h~a + h' — h 8 We have =t. To introduce the correction for temperature, since the elasticity produced by an increase of tempera. ture correspond with the expansion produced, and since the expansion of condensed air follows the same law as that of air of ordinary density, expandingth of its bulk at 32°. for each additional degree of Fahr. above this point, or 'h of its bulk at 48°; calling e" the elastic force of the heated air, e' that of the same air at 48°, n being the number of degrees of heat above 48°. EXPLOSIONS OF STEAM-BOILERS, 85 perature was suffered to increase, could not have been appreciable if acting entirely, and the counteracting effect of the expansion of the glass further justified its being neglected. For similar reasons, no reference was made to the effects of heat on the mercury in the cistern i, on the cistern itself, and on the water within the pipe communicating with the boiler. On the Thermometers. In most of the researches of the Committee, refinements in the mode of using the common thermometer would have been out of place. Results which might be obtained with little additional labour, and which would be interesting in both a practical and scientific point of view, were not to be neglected, and to some of them great accuracy was essential. In the questions of the first class the thermometers were provided with wooden scales, and were graduated by immersion up to the point at which the scale commenced, the scale and upper part of the tube being exposed to the air; this was proper, as they were intended to be immersed in mercury nearly up to the scale. These instruments were examined after coming from the makers' hands, and the instrumental error ascertained. The tubes in which the thermometers were placed, and which contained mercury, were at first placed horizontally in one of the heads of the boiler; this had the advantage of rendering the tube for indicating the temperature of the water entirely independent of the steam, and thus any differ. ence between the temperature of one and the other might be more effectually ascertained than when the tube giving the temperature of the water passed through the steam. The position of these instruments interfered so much with other parts of the apparatus, and so much inconvenience and danger of error was experienced from the separation of the column of mercury in the thermometer, that these tubes were not used after the first weeks of experiment, and two vertical tubes, placed as already shown, were substituted for them. The thermometers used, when the relation between the temperature of the steam and water, and the elasticity of the steam were to be observed in conjunction with some of the subjects more directly under the cognizance of the committee, had much pains bestowed upon them. The scales (M and N, fig. 1,*) were metallic, and surrounded by glass tubes, fitting into a cup, a', through the bottom of which the stem of the thermometer passed watertight; a pipe, b'c', fig. 2, from the side of In fig. 2, thermometer N, to render it conspicuous, is shown, as if the scale were turned to the front of the boiler. 86 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. each cup, and provided with a stop-cock, d', At no The other parts of the apparatus, less general in their use, as the water-gauge, safetyvalve, fusible plate apparatus, &c., will be more conveniently described in connexion with the experiments for which they were devised. Subjects of Investigation. The queries originally proposed, together with the new matters, which were made the subjects of experiment, will be treated in the following order: I. To ascertain whether, on relieving water heated to, or above, the boiling point from pressure, any commotion is produced in the fluid. Including the examination of the efficacy of the common gauge-cocks, of the glass gauge, and of Ewbank's proposed gauge cocks. Upon the scale of one of these instruments there were 314° in 6 inches. Brass expandsds of its length, from 32° to 212°. These 6 inches, at 32o, would become 6'0113 at 212°. Ten degrees upon the scale would become 9.99 by a variation of temperature from 32° to 2129, a diminution of only '01 of a degree for a variation of 180° in the temperature of the scale. In practice, the variation never exceeded thirty degrees. The investigation of the question whether the elasticity of steam within a boiler may be increased by the projection of foam upon the heated sides, more than it is diminished by the opening made. II. To repeat the experiments of Klaproth on the conversion of water into steam by highly heated metal, and to make others, calculated to show whether, under any cir cumstances, intensely heated metal can produce, suddenly, great quantities of highly elastic steam. First, The direct experiment in relation to the production of highly elastic steam in a boiler heated to a high temperature. Not to interrupt the general train of inves tigation which follows a well known theory of explosions of steam-boilers, the results of the experiments on the former part of this query are inserted in another place. III. To ascertain whether intensely heated and unsaturated steam can, by the projection of water into it, produce highly elastic vapour. IV. When steam, surcharged with heat, is produced in a boiler, and is in contact with water, does it remain surcharged, or change its density and temperature? V. To test, by experiment, the efficacy of plates, &c., of fusible metal, as a means of preventing the undue heating of a boiler, or its contents. 1. Ordinary fusible plates and plugs. 2. Fusible metal, inclosed in tubes. 3. Tables of the fusing points of certain alloys. VI. To repeat the experiments of Klaproth, &c. 1. Temperature of maximum vaporization for copper and iron under different circum stances. 2. The extension to practice, by the introduction of different quantities of water, under different circumstances of the metals. VII. To determine, by actual experiment, whether any permanently elastic fluids are produced within a boiler when the metal becomes intensely heated. VIII. To observe accurately the sort of bursting produced by a gradual increase of pressure, within cylinders of iron and copper. IX. To repeat Perkins' experiment, and ascertain whether the repulsion stated by him to exist between the particles of intensely heated iron and steam be general, and to measure, if possible, the extent of this repulsion, with a view to determine the influence it may have on safety-valves. X. To ascertain whether cases may really occur when the safety-valve, loaded with a certain weight, remains stationary, while the confined steam acquires a higher elastic force than that which would, from calculation, ap REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. pear necessary to overcome the weight on the Palve. XI. To ascertain by experiment the effect of deposits in boilers. XII. Investigation of the relation of the temperature and pressure of steam, at ordinary working pressures. Table from 1 to 10 atmospheres. 1. To ascertain, by direct experiment, whether on relieving water heated to, or above, the boiling point, from pressure, any commotion is produced in the fluid. The first experiments on the effect of relieving water in ebullition from pressure, were made in a glass boiler; here the fire was under the whole length of the boiler, which was a cylinder of fourteen and a quarter inches in length, and seven and a half inches in diameter. The steam within, being at a pressure of less than two atmospheres, by opening a cock at the end of the boiler, or the safety-valve, also at the end, large bubbles were formed through the whole extent of the boiler. The inquiry was prosecuted in the iron boiler already described, a distinct view of the interior being had through the glass windows placed in the heads. The greatest intensity of the fire was in front of the middle of the boiler, and extended through about one-third of its length: the part immediately near the flue, was next to this band in temperature. With this boiler experiments were made, which showed, that on making an opening in the boiler, even when the pressure did not exceed two atmospheres, a local foaming commenced at the point of escape, followed soon by a general foaming throughout the boiler, the more violent in proportion as the opening was increased. This small boiler was completely filled with foam by opening the safety-valve (nearly two-tenths of an inch in area), which was placed on the middle of the top, and the water violently dis charged through the opening of the valve. The area of the valve bears to the horizontal section of the boiler, at the water line, the ratio of one to two thousand and fifty-five nearly. The boiler was half full of water in these experiments The gauge fell always on making the opening. The foaming, which was so repeatedly observed, must be produced in a greater or less degree every time that steam is drawn from a boiler to supply the engine; every time that a gauge-cock is opened, or the safety valve raised. It is interesting in two points of view; first, in its effects upon the apparatus designed to show the level of the water within the boiler; second, by its throwing the water against the heated sides of the beilor. Gauge-cocks and Glass Water-gaige. 87 The apparatus most commonly used in our country for determining the level of the water within a boiler, consists of three gauge-cocks attached to the boiler head, one of them being at the water level, and the others equally distant above and below that level. These cocks in the experimental boiler, a bc, figs. 1 and 2, were 1.95 inches and 18 inches apart, measuring from the centre of the opening of the middle one, to the one above and to the one below. The steam in the boiler being not higher than two atmospheres, the following experi ment was made:-The level of the water was reduced until it stood just below the lowest gauge-cock. On opening this cock, steam at first flowed out, then water and steam; on opening the second cock, in addition, water flowed freely from the lowest, which was above the hydrostatic level; the foaming within the boiler, which was produced by thus relieving the pressure, was distinctly seen through the glass windows. On opening the third cock, steam and water issued from the second, which was two inches above the water level; and on partially raising the safety-valve, water flowed freely from the second cock. A further rise of the valve filled the boiler with foam, water flowed freely out of the third cock, more than three inches and three-quarters above the water level, and finally through the opening of the safety-valve itself. In these experiments, an opening of 03 of a square inch in area, the lowest cock, which, to the area of the water surface, was as one to thirteen thousand seven hundred, cause water and steam to issue through a cock, below which the water was known to be. A further opening of '03 of a square inch, making, with the first, '06 inch, or one six thousand eight hundred and fiftieth the area of the water surface, brought water from the lowest cock; a total opening of '09 inch (th of the area of the water surface), brought water and steam from the middle cock, indicating that the level of the water was nearly two inches higher than it really was. A first apparatus, which was contrived for applying fusible plates to the boiler, suddenly opened an aperture of 95ths of an inch in diameter. Even at low pressures, the scalding contents of the boiler were violently discharged, through this opening, against the roof of the experiment house. It is time now to speak of the glass gaugetube, as a means of indicating the level of the water within a boiler, in connexion with which an experiment bearing upon the per formance of the gauge-cocks will be stated. The form given to the water-gauge, on its 88 REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. first trials, was that described to the Committee by Mr. Hartshorne, of Cincinnati. This was a prismatic box of brass, of suitable dimensions; one face of which was supplied by a glass plate; this box, being put in communication with the boiler, by two pipes, one entering from the steam, the other from the water, the level of the water was seen through the glass plate. This apparatus was attached to the experimental boiler, and its indications compared with those of the gauge-cocks in the experiments already detailed. On relieving the water from pressure, the water within the gauge was agitated; during the further foaming its oscillations did not amount to half an inch, so that the hydrostatic level was truly shown by it; and further, on closing the openings, the fluid in the gauge became tranquil at the mean level of its oscillations, showing that it had fallen with the fall of the water within, caused by the escape of steam. An instructive experiment to the same purport was made on the occasion of a fracture in one of the glass windows, described as placed in the ends of the boiler. The account taken from the minutes of the day's experiments is as follows: The temperature being at 292° Fahr., and the pressure indicated by the gauge four atmospheres, the north window of the boiler, which had a flaw in it, cracked across the middle, and nearly horizontally; steam issued slowly through the crack; on looking into the boiler a foaming at the end where the steam was escaping was observed. The crack rapidly enlarging, the steam issued in quantities through it; the water was in general agitation throughout the boiler, running out at the crack, though its hydrostatic level was at the bottom of the window, about one inch and a quarter below the crack, and being distinctly seen at the opposite window, foaming near the top of the glass; the water-level gauge began to fall, oscillating not half an inch in its fall. The safety-valve was now opened by hand, so as to waste the water with great rapidity. The water still issued through the crack, the water-level gauge falling. On closing the valve the water settled down, becoming comparatively tranquil; the water-gauge remained at the same level: it had, therefore, indicated constantly the true level unaffected by the foaming, except in slight oscillations. In fact, this gauge shows truly the height of the water within the boiler, until the foam rises so high as to pour over through the upper connecting tube. The idea was suggested that by placing the gauge-cocks in a prism, connected above with the steam, and below with the water in the boiler, the true level of the water would be indicated. Such a cock was, therefore, applied to the box of the water-gauge; its opening produced a local foaming in the gauge, which brought water through the cock, although the true level was much below it. The area of this cock was nearly equal to the area of that which opened into the steam chamber of the boiler. In relation to the form of the water-gauge, as already described, it does not seem to offer as many advantages as the tube which has been adapted to the boilers of some of the English locomotive engines.* The glass plate requires the support of horizontal bars, which are objectionable, or it must be reduced so much in breadth that the level is obscurely seen through it; the strain upon the plate being unequal, it is very liable to fracture; such fractures repeatedly took place near the centre of the plates in the gauge used by the Committee. To the use of the glass gauge for the highpressure engine, an objection occurs, from the effect produced by high steam upon the glass, anparently by its action on the alkali; by which the transparency of the glass is gradually destroyed. A similar effect was recorded by Cagniard de Latour, in his experiments on liquids at high temperatures, confined in glass tubes.+ As far as the experiments of the Committee have gone, they show that green glass is not so readily injured; and as it is easily procured in tubes, a further reason appears for preferring the tube in practice, to the plate. An attempt which was made to substitute windows of mica in the boiler for those of glass, bears upon the use of that mineral for the plate of the water-gauge; as does also another attempt which was made to protect the glass plates by a lamina of that mineral. The mica exfoliated under the action of the steam which insinuated itself between the laminæ through cracks which were invisible, if existing, before the experiment, or which may have been produced by the steam itself; the laminæ were separated, and thus the steam quickly found a more or less direct passage through the plate. The tube-gauge which was substituted for the prism is shown in figs. 1 and 2. w x is the tube of green glass passing into the The application of such a gauge to a locomotive-engine can give but little idea of its use for a stationary engine. The jarring in the locomotive must constantly expose the gauge to fracture, and perhaps may prevent its use. The glass water-gauge has been adopted in at least one of the boats plying between New York and Amboy, New Jersey, belonging to Messrs. Stevens; and the Committee understands, are also in use upon their locomotiveengines. + See also recent experiments on the action of water at high temperatures upon glass, by Professor Turner, of the University of London. Royal So ciety's Trans. for 1834. |