Lapas attēli
PDF
ePub

were cast from it; and one of these being placed in the opening in the slide of the apparatus already described, was covered with the pierced disk, and the slide moved so as to bring the plate over the opening in the boiler. The steam was now raised, the temperature being noted from time to time, until the plate gave way; steam was then let off to keep the temperature from rising; the plate, which had just fused, removed, and one of an alloy, fusing at a higher tempe rature, substituted. The steam was again retained, and allowed to rise in temperature, the new plate pushed to its place, and the operation renewed. This course was continued until the alloy, fusing at the highest point of those prepared, had been used, or until the limit of the elasticity of steam, which could be produced in the actual condition of the boiler and state of the fire, was attained; steam was then let off, water thrown into the boiler, and a new series commenced. The tables which will be given, required many days of trial, and of close attention.

To try the effect of thickness on the fusion of the plates, three different thicknesses were cast of each of the alloys used; the first, or thickest, was fifteen-hundredths of an inch thick; the second, eight-hundredths; and the third, four-hundredths. There were five different alloys of tin, lead, and bismuth, composed; the stationary points of which, and the points at which they gave way in the boiler, appear below.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

The plates of experiments 1, 2, and 3, were exposed to pressures tending to render them of less than one atmosphere; 1 and 3, the two extremes of thickness, show a great uniformity in the point at which they give way, and render it probable that some flaw, in casting the plate number two, caused its fusion at a lower point than that of either of the others; we see too, that at

these low pressures the fusing point in the boiler coincided, very nearly, with the point at which the alloy was a soft solid in the crucible. In this case the thinnest plates, when properly cast, were probably thick enough to withstand the small pressure to which they were exposed, and therefore did not give way at lower temperatures than the thickest, each attaining the temperature at which they were soft solids.

The next series, Nos. 4, 5, and 6, with a less fusible alloy, show, first, that the thinnest plate was too feeble to resist the pressure of steam, and gave way before the metal lost its solidity; second, that the plate, eight-hundredths of an inch, was probably defective, as it gave way at a lower temperature than the thinner plates of No. 4. No. 6 presents a curious fact; the point of yielding of the plate, given by four experiments, is actually above the point at which the alloy from which it was composed became liquid: this would appear inexplicable to one who had not attentively observed the mode of fusion of these thick plates, and would lead to the suspicion of error. The observation of the point at which the alloy became liquid, was however, deduced from three trials; and four experiments, with the plates in place, are given, the extremes differing but three degrees. The explanation is to be found in the mode of fusion already spoken of; the more liquid parts of the alloy are forced out, the less fusible remain, and if strong enough to resist the pressure, the process goes on; this takes place, unequally, in different alloys. The importance of attending to such indications is obvious.

In the next series, the first thickness seems to have been decidedly too weak, and the second to have been hardly sufficient, while the third exhibits a point of fusion when the metal was in a softened state.

In the remaining experiments, both thicknesses were too inconsiderable to sustain the pressure, as appears by comparing the points at which the plates gave way, with the stationary points. Something of this kind seems to have been deduced from practice in the use of these plates in France, for the last royal ordinance, in relation to the means of safety to steam-boilers, prescribes for the plates a thickness of not less than nine-sixteenths of an inch, making of them fusible plugs rather than fusible plates.

Experiments were subsequently made on plates of greater thickness, the use of which led to an interesting termination to this series of experiments. Before, however, stating the results thus obtained, some further experiments with the plates just considered, will be described. These inquiries were directed to the effect which would be produced by the mode of casting the plates, upon their fusibility; it being not improbable that rapid cooling might so modify the physical properties of the alloy, as to change the fusing point from that of the same alloy when slowly congealed.

As low pressures afforded the most easy means of determining this point, plates were cast from the alloys of series No. 1, No. 2, and No. 3, and tried, in place, upon the boiler. Some of the plates were cast from metal at a high temperature, and the mould as cold as the perfect casting permitted; others, of the same alloy, in a heated mould allowed to cool slowly; and others from metal heated to a temperature as little above the point of fusion as possible. In the case of the higher temperatures, care was taken not to raise the heat so far as to oxidate rapidly either of the constituents of the alloys, thus changing the fusing point. A comparison of the results obtained, showed no greater differences than those which have been seen to occur between plates similarly cast, and from the same alloy; and the conclusion derived was, that the mode of casting has no effect on the fusing point, which is appreciable in this mode of applying them. The French instructions expressly recommend to those

who make or use boilers, to obtain plates in preference to the fusible metal in ingots; on the ground that it will be found difficult to procure plates of the same fusibility with the ingot, from that form of the alloy. This remark led to the experiments just referred to, and the explanation of it which they give, refers to the undue heating of the alloy in the casting of the plates, since they show that the particular mode of casting produces no difference worthy to be regarded in a practical point of view.

Plates were now cast quite thick, viz.-one-fourth of an inch, of the alloy of eight parts of bismuth, eight of tin, and seven of lead; this alloy being intended to give way at a temperature corresponding to about one atmosphere of bursting pressure. The alloy was completely liquid at 275° Fah. and solid at 254° F.,* when examined in the crucible. The heat was raised as slowly as possible, so as to permit the full effect of the temperature indicated by the thermometer; the observations recorded are as follows:

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

Plate 4th of an inch. Metal stands fused in the holes of the brass disk covering the fusible plate.

Steam issues in a very small stream through chinks between the fused metal and an unmelted part within.

Steam issues as before; no clear passage through the plate. Steam kept for a long time at this temperature. Six minutes elapsed in raising temperature 44 degrees. Plate gave way, affording a free passage to steam. A second plate of the same composition and thickness, put in place; fluid metal stands in the holes of the cap. Metal which has oozed out remains in a fluid state upon the sliding plate of the apparatus.

Plate gives way, torn in a thin part.

The thermometer on top of the boiler dipped into the mercury in a small cistern, made by inclosing a space on the top of the boiler, with clay; so that the top of the boiler formed the bottom of the cistern. The first plate having failed to give way until the temperature within the boiler was twenty-four degrees above that at which the alloy, of which it had originally been composed, had been fluid, was examined with great care. The plate had decidedly given way to pressure, and not by fusion; it had lost its metallic lustre at the side where it was torn; yielded readily to the nail, which scraped off small particles. A piece of the plate being cut off and laid upon the top of the boiler, remained solid, though the portion which had oozed out, was perfectly fluid, near the same spot. The same remarks apply, generally, to the second plate. They afford a solution of the perplexing circumstances, which had occurred throughout these experiments, and which had led to so many trials to discover their cause. The portions of the metal which oozed out from these two plates had their fusing points taken, by gradually raising their temperature in a bath of oil, while the alloy rested on a small shelf of copper, wholly immersed in the oil. The first portions of fluid metal which had oozed from both the first and second plates, melted at betwen 221 and 223 degrees, Fah., being solid at the lower, and perfectly fluid at the higher, of these two temperatures. The

*This alloy had no stationary point in passing from the liquid to the solid state; but some internal change in the solid at about 2060, produced a rise and stationary point at 208°.

second portion which oozed out from the first plate, fused at between 230° and 233°, Fah., and a portion of that from the second plate was fluid at about 2351 Fah. The parts which were left, of the first plate, were a soft solid, at 2991, fluid at one edge, at 312°, and entirely fluid at 345°.

The portions left, of the second plate, lost their cohesion, and could be broken. by pounding, into small particles or grains, at 3004°; and the whole was fluid at 356° Fah. A comparison of these results appears in the annexed table.

[blocks in formation]

To pursue this subject further, by the clue thus obtained, only freeing the different oozes from accidental admixture, a small iron cylinder was made, closed at one end, and perforated near the closed end with a number of minute holes, not larger than the stem of a common pin. Into the cylinder was fitted, nearly tight, an iron piston, with a rod, to apply pressure. An alloy having been made and introduced into the cylinder, the whole could be heated in an oil bath to any desired temperature; and pressure being applied to the piston, the liquid parts would ooze out, through the small apertures near the end of the tube. The first alloy experimented upon was the same in composition with that just referred to; being composed of eight of bismuth, eight of lead, and seven of tin, by weight. This alloy was fluid at 2544° Fah. At a temperature of 229°, some drops of fluid metal were forced out by pressure, and at about 2394° other portions were forced out. Both melted at 227° Fah. The portion left was a soft solid at 2763° Fah.; fluid at 2901°. The alloy of one atom of lead, one of tin, and one of bismuth, is fluid at 2734° Fah., and that of one atom of lead, one of tin, and two of bismuth, at 219° Fah.

These experiments, the committee deem conclusive in regard to the use of fusible plates in the ordinary way, and they conceive that substituting fusible plugs of greater thickness, say half an inch, as has been directed by a recent ordinance in France, would not serve as a remedy to the defect thus exposed. The true remedy is to be sought in inclosing the fusible metal in a case, in which it shall not be exposed to the pressure of the steam, but only to its heating effect: the more fluid parts of the metal will then not be exposed to be forced out of the mass; and the whole will become fluid as if exposed to heat in a crucible. With this view of the subject, trial was made of an apparatus described by Professor A. D. Bache, in the Journal of the Franklin Institute, for October, 1832, under the title of an "alarm to be applied to the interior flues of steam-boilers." This apparatus is obviously as applicable to a common boiler as to one with interior flues; the following description of it is given in the journal.

[ocr errors]

"A tube of iron, or copper," according to the material of the boiler, "closed "at the lower end, passes through the top of the boiler, its closed end reaching "the flue to which it is attached." "This tube, it will be observed, affords a "ready access to the flue, to ascertain its temperature, without any restraint "from packing." "A mass of fusible metal placed at the bottom of the tube," "will become fluid very nearly as soon as the flue takes the temperature of "fusion of the alloy." "To show when the metal at the bottom of the tube "becomes fluid, a stem is attached with a cord and weight," "or with a lever "and weight." "The weight and longer arm of the lever, descending, may be

This paper was published in 1832, and the experiments of the Committee were made in 1833-4.

"made to ring a bell, or, by appropriate attachments, to turn a cock, permit"ting just enough steam to issue to give the alarm, and then to be closed at 66 once. A projection on the lower end of the rod prevents it from being "drawn from the metal until this latter is fused, and by widening the lower "part of the tube, making it slightly tapering, the metal is kept from being "drawn out by the rod."

P

M

R

K

[ocr errors]

In the annexed figure "AB is a section through the top of the boiler; CD, a corresponding section of its flue, EH represents a tube closed at the lower end, which is attached to the upper side of the flue. The mode of attachment by a projection on the tube and a ring screwed to the flue, is shown in the figure, as also the stuffing box, RS, through which the upper end of the tube passes. The lower part HI, of the tube, is made tapering, to retain the fusible metal. KL is the stem, the lower part being inclosed by the fusible metal, the upper part attached by a chain to a lever KP. The weight M, draws the rod KL upwards, and on the fusion the alloy HI, carries the lever below the bell N, which, being attached to a spring, rings an alarm."

H

D

The form of this apparatus, which was subjected to trial by the committee, was essentially the same with that described. One of the tubes in which the thermometers were ordinarily placed, was used to contain the fusible metal, and as giving the more severe test, the short one entering only into the steam, was selected. For the convenience of removing the metal, it was placed in a metallic case, fitting loosely into the iron tube, and having a wire attached, by which it could be drawn out of the tube. This certainly diminished the sensibility of the apparatus, particularly, as the case was quite as thick as the inclosing tube, and as there was a small space between its convex surface and that of the tube; it was required, however, for the convenience of the experiments.

The results of the several trials are contained in the following table. The temperature was registered by the adjoining thermometer dipping into the water of the boiler, and already often referred to; it was raised as rapidly as possible in all the experiments except the first. The first four trials were made on an occasion specially devoted to this purpose, the last two were made incidentally when upon another subject.

Tempera

Number of ture.

Trial.

REMARKS.

Fah.

1

23

268 Stem rises. No particular attention paid to raising the temperature rapidly.

270

Stem rises. Steam raised rapidly.

Metal drawn out and suffered to cool, re-deposited cold in tube.]

Steam at 258°, and raised to 274° in 2 minutes.

Stem rises.

Metal drawn out and cooled. Steam at 250°, when metal was replaced. Steam raised to 274° in 3 minutes.

[blocks in formation]

Stem rises.

[blocks in formation]

Metal had not become solid again. Steam let off rapidly.

Melted below this temperature.

Stem rises. Metal remains a soft solid, so that the stem can be drawn out, until 240°.

« iepriekšējāTurpināt »