Sidebilder
PDF
ePub

EXPLOSION OF THE AMERICAN STEAMER

MEDORA."

11

metal between the rivet holes and the edge the utmost strain that this boiler could have of the sheets, gave way by tearing out, leav borne, without giving way at this, the weakess ing the rivets and intermediate metal unin place. The pressure here spoken of, is of jured, and this consists with the preceding course the effective pressure, or the excess of estimate which makes this element of the that of the steam over that of the atmosphere. joint the weakest of the three. Inasmuch, Let us now proceed to estimate the preshowever, as they approach so nearly an equa sure which could have been produced upon lity of strength, and each may occasionally the boiler of the Medora by loading the give way before the others, it may be as well safety valve to the utmost with the weights to take the average of their resistances for which were attached to it, and intended so an expression of the strength of the joint, to be used, when occasion should require the

1:125 x 0.925 +0.936 maxiraum pressure, considered by the engine and this should be

3

builder to be safe. The diameter of the 1.015, or say one square inch of metal for valve was 154 inches at the bottom, with a every 12 inches (6 inches of strip and 6 in. mitre of 14 inches, making its top diameter of space) in the length of the boiler, i th of

17 inches. The levers of the valve were of a square inch, per running inch of the same;

the second order, and two in number. The and this is the measure of the strength of primary lever, operating immediately on the that part of it over the legs. Now the ele valve, had a total length of 35 inches, and ments of the calculation to determine the

from the fulcrum to the centre of the valve strain that a cylindrical vessel will bear, fron disk, was 12 inches, making a ratio of 28. the outward pressure of an elastic fluid, con

The secondary lever, operating on the end sists of the diameter of the cylinder, the

of the primary one, had a total length of 67 thickness of the material composing it, and

inches, and from the rod connecting the two, the modulus of the strength of that material.

to the fulcrum of the former, the distance Thus, if D be the diameter of the cylinder,

was 10 inches. There were two weights of t the thickness of the iron (both in inches,)

cast iron, on the secondary lever, the largest, P the average force in pounds that will tear nearest to the end of it, weighing (by estiasunder a square inch of boiler, iron, and X mate) 200 pounds, and the smaller 56 pounds. the steam pressure per square inch on the

When these two weights, which slid as usual boiler sufficient to burst it, we have the

on the arm of the lever, which they were 2 Pt

perforated to receive, were in contact, and equation Dx 2 P t, where x =

pushed out to the end of the lever, the dist. D

ance of their centre of gravity from the fulNow D=132 inches - P= 55,000 lbs. t=1 crum of the long lever, would be 576 of an inch.

Consequently x = 2081 lbs., inches ; so that the ratio of this lever would which would be the pressure of the steam

1

be and the ratio compounded of those per square inch required to burst the boiler, 5.76 if it were a continuous hollow cylinder with

1
1

1 out seams or joints to reduce the quantity

of the two levers

5.76 2.8 16:128" and resistance of the metal composing it.

If the area of the valve be calculated, from But this is not the case in any boiler.

The

its mean diameter, viz : joinings of the plates, if there were no

15.25 + 17 holes in the boiler, would necessarily re

16į, it will be found = 204

2 duce the strength to about two-thirds of the entire strength of the sheets

inches. square

Then the extreme pres

were it not for the support they yield each other at

sure which the attached weights could prothe laps, where they are doubled upon each

duce upon the safety valve would be equiother. In the Mezora's boiler, it is seen

valent to 256 lbs. * 16.128 = 4128160 lb. above, that for every 12 inches in the length or, per square inch of the valve

20. of the cylinder over each leg, there is but

This is exclusive of what is due to the weight one square inch of metal resisting rupture,

of the valve, and its rod and lever, which instead of three square inches, which there

may be estimated as follows: for the valve would be if there were no rivet holes, or

itself, without leverage, 100 lbs.; for the spaces, between the strips of iron. So the

short or primary lever, 20 lbs. x (the strength of the boiler over the legs is reduced

12.5 to the one-third of it full strength in other leverage of its centre of gravity)=28 lbs. ; places, where there are so seams, or perfo

35

for the connecting rod, 10 lbs. x rations ; consequently, we must divide the

12.5 value of x, as above obtained, by 3, to get 28 lbs.; for the long or secondary lever, the real pressure, per square inch, that the ,

35 33.5 boiler was capable of sustaining. Then 56 lbs. *

= 536 lbs., being in

12.5 10 208 - 33 69.44 pounds per square inch,

all 692 lbs, at the seat of the valve ; and

17.5

Х

men.

*

12

EXPLOSION OF THE AMERICAN STEAMER MEDORA," this will increase the whole pressure there the bursting of the boiler-were so situated to 4820,75 lbs., or to 2375 lbs. per square that the fire could not have been at all in inch of the valve and boiler.

contact with them had the boiler been dry, This appears to be the highest pressure as they occupied the spaces over the legs, which could be brought upon the boiler, by and, consequently, must have been always the system of weights and levers, belonging immersed either in water or steam. There properly to the valve.

is, however, no evidence that the water was The strength of the boiler, in its weakest deficient in quantity at the moment of explaces, being then estimated, as above, at plosion; on the contrary, many witnesses 69 44 lbs. per square inch, amounted to declared that the gauge cocks showed a full just three times the extreme pressure which supply, and, considering the vast size of the the engine builder appears to have intended boiler, the shortness of the time between it should be called on to bear, and in pro the lighting of the fire and the occurrence of portioning the strength of his work to the the explosion, it seems not likely that the duty it was to perform, he would seem to evaporation could have sunk the water to a have been sufficiently prudent. How, then, dangerously low level, if, as is to be supdid the boiler explode, when guarded by a posed, it was properly filled at first. I safety valve of such ample dimensions, de could, indeed, discover no trace of injury to signed to give way at one third of the burst any part of the boiler by burning of the ing strain?

metal, and my examination of the upperThat the free action of the valve must most tubes, and the top of the back smoke have been interfered with, would be a natu chamber, was very careful, and if these parts ral conclusion, and is corroborated by the of the boiler were overheated, they, nevertestimony of one of the assistant engine. theless, stood firm, and left the rupture to

take place elsewhere. No incrustations

likely to impede the transmission of the caBut the boiler may have been in fact loric to the water, and thus render the iron weaker than it has been estimated, from a liable to burn, anywhere appeared, and were deficiency of strength in the iron of which not to be expected in a perfectly new boiler. it was made, and which has been assumed The degree of heat imparted to the parts at 55,000 lbs. per square inch. The nu of the boiler covered with water, would not, merous experiments that have been made at all events, have exceeded the temperature upon the strength of this metal (of which due to the effective pressure of about 45 those recorded in the Journal of the Frank. atmospheres, or 69.44 lbs. per square inch, lin Institute, vols. xix. and

2nd series, which has been above estimated as sufficient are the most extensive and satisfactory that to burst the vessel. This temperature (see I have seen, and also the most applicable to Journal of the Franklin Institute, vol. xvii., the present case, as they were made upon page 291, 2nd series,) is not more than 300 boiler iron,) show that its cohesive power degrees of Fahrenheit, and the experiments as often exceeds as it falls short of that recorded in the same journal (vol. xx., pages measure, which

may

be taken as a fair aver 24 to 31, 2nd series, and curve traced in age. If the iron of the Medora's boiler was, plate X.,) show that the tenacity of iron is in fact, of bad quality, it may, however, increase l by heat, until a temperature of at have possessed a tenacity far within that least 400° Fahrenheit is surpassed, when it just given ; but it scarcely could have de begins to diminish. The boiler, then, could scended as low as the one-third of 55,000 not, in my opinion, have burst from overlbs., or to 18,000 or 19,000 lbs. per square heating the metal in the parts where the inch, which it must have done to have yield rupture actually took place to wit, in the ed to the steam pressure of 23 lbs. per strips over the legs, uniting the segments square inch, supposing the preceding esti of the cylinder between them. In estimatmates of strength and pressure to be cor ing the strength of these strips, I have suprect. My examination of the iron, as it posed, from inspection and measurement appeared on the torn edges of the sheets, of those that remain, that they contained did not impress me with an unfavourable one-half of the original quantity of metal in opinion of its quality, although it showed the sheets, which was further reduced to the distinctly laminous structure which most one-third of that quantity by the rivet holes. sheet iron exhibits. Its strength in the In this estimate there is room for mistake, ruptured parts could not have been dimin and, possibly, I may have in this assigned ished by over heating, for those parts were more strength to the boiler than it in fact far under the lowest level to which it is in possessed. In both the quality of the iron the least degree probable that the water and the amount of metal in the strips, then, could have fallen, even had it declined be there may be room for considerable reduclow a safe and proper height. Moreover, tions upon the preceding calculations of the the strips--the giving way of which caused ability of the boiler to withstand the pres

XX.,

EXPLOSION OF THE AMERICAN STEAMER

MEDORA.”

13

sure of the steam. Again, although the metal did not, on account of its greater strength than that of the strips, give way where, if at all, it must have overheated-viz. in the top of the smoke box, or in the upper flues- there is a possibility that these parts may have been laid dry by the falling of the water, and have become so hot as to generate suddenly a larger body of vapour (or, who knows, an explosive gas,) than the safety valve could vent with sufficient rapidity to save a rupture, even if that had been loaded to less than the strength of the boiler. This supposition is, however, in the face of the evidence, though, unfortunately, too little credit is due to such testimony in cases of this kind.

It is stated, by several witnesses, that the safety-valve did not blow at all while the steam was getting up, and that from the time of making the fire, up to the moment of the explosion, not more than two or three short puffs proceeded from it. These would, indeed, suffice to show that it could not have stuck fast in its seat from the action of some adhesive force-an instance of which kind is on record. But it also proves that if, after all, the valve was not overloaded, at least the engineman and his assistants, &c., were strangely indifferent to the unusual absence of that audible evidence of its free action, which always attends the starting of a steamboat.

Although doubt must continue to rest upon the true and special cause of the explosion of the boiler of the Medora, and I am not prepared, in relation to that cause, to offer more than the preceding facts and inferences, for the judgment of others, yet some general conclusions may, I submit, be satisfactorily derived from the circumstances of this catastrophe. First—the boiler was too large in its diameter for the strength of metal employed, looking to the risk of bad material and workmanship. The thickness of the sheets was a quarter of an inchthickness, by the way, almost universally employed in the construction of steamboilers of all diameters, as if there were some magic in this particular dimension, which made it most pliantly applicable to all cases, however varying. Referring to

2 Pt the formula x= we find that iron one

D quarter of an inch thick would require a pressure of 573 lbs. per square inch to rupture it in a cylinder of three feet diameter, (a diameter of usual occurrence in locomotive and other high-pressure boilers,) if the modulus of its strength be 55,000 lbs. per square inch, and a deduction be made for the seams, of twenty-five per cent. While

the Medora's boiler, of eleven feet diameter, and the same thickness and strength of sheet iron, would have borne but 1564 lbs. per square inch, with a proportional reduction of one-quarter for the joints, (saying nothing of the still greater subtraction of strength due to the apertures over the legs,) can there be any propriety in using the same thickness of plate for each of these widely differing diameters? Yet it is done under the influence of the apparently prescriptive right of the quarter inch iron to be employed in all cases whatever. It is true that the small diameter boilers are high-pressure, and subjected to the greater strain, but their excess of strength is, at the same time, vastly greater than that of the large low-pressure boilers. The usual high-pressure strain is, perhaps, about 100 lbs., and the low-pressure strain, 25 lbs. per square inch. The excess of strength in the three feet highpressure boiler is, then, 473 lbs. per square inch, and in the eleven feet low-pressure boiler of the Medora it would be but 1264 lbs. per square inch. It may be said, indeed, that the ultimate strength of the former is but 5•73 times its usual strain, while that of the latter is 6.25 times its ordinary stress; also, that at the high temperatures accompanying high pressures, a given increase in the temperature causes a more rapid rise in the pressure than at the low temperatures of the lower pressures, so that there is occasion for more excess of strength in the former than in the latter cases, to guard against accidental augmentations of temperature. Also, that the consequences of explosion at high pressures are more disastrous than at those of a lower grade, and should, there. fore, be more carefully guarded against. There may be some propriety in the two first of these suggestions, but not so much in the last, as some of the most fatal explosions have occurred in low-pressure boilers ; and still I think that low-pressure boilers, whose diameters are generally from eight to nine feet, are usually too weak when made of quarter inch iron; and this opinion is held à fortiori in rcgard to the Medora, with her boiler of eleven feet across. Braces are indeed used in these large boilers, but with often only partial effect, and in the Medora, as has been already remarked, they could not be applied in the vertical, or radial direction, in which they would have done most good, on account of the positions of the tubes.

Second-a boiler of the colossal size of that of the Medora, presents a bold and striking aspect, and seems fitted for the generation of a vast supply of steam; and so, doubtless, was the boiler in question, the fire surface and steam room of which

-a

14

EXPLOSION OF THE AMERICAN STEAMER "MEDORA." was of unusually ample extent. But the occupy any other position than that of a same fire surface will be as effectual if dis perpendicular to the surface supported by tributed among two or three boilers, and the tie or brace. Here they are diagonal with great increase of security against ex to the ends and roof, and as chords, less plosion. The one large boiler will, per than diameters, across the cylinder. haps, cost less in the manufacture, and Some other obvious remarks suggest occupy less room in the boat, but it will be themselves, in conclusion, and are generally much more difficult to move, in placing and applicable to steamboats when under trial, displacing it; and if accident happens to it, on such occasions as this ill-fated vessel was the supply of steam is wholly cut off ; while about to begin. with more than a single boiler, each of Most of the persons on board the Medora, which can be insulated from the other, it at the time of her explosion, were, as before may be kept up, and the engine worked stated, workmen who had been engaged in at a lower speed, till the injured boiler is her construction. It was not safe to leave repaired.

the boat as it was left, in the hands of these Third, the design of a boiler resembling the men, most of them, probably, reckless Medora's, is deficient in strength at the junc of danger by character, and fired with the tion of the belly with the legs which support false ambition congenial to the occasion. it, and form the sides of the fire-place. The The safety valve and mercurial gauge should perforations of the cylinder must be large have been constantly under the eye of the enough to permit the water and steam bub engine builder, whose machine was being bles to pass freely up and down, in the submitted to proof. The valve lever and necessary circulation of them through the its attached weights, and the means of movboiler. Small holes over the legs would ing them, should have been so constructed, not allow this; the circulation would be and surrounded by guards, as to make it checked, less steam would be generated, impossible that more than the extreme and the legs of the boiler become unduly pressure designed to be put upon the valve, heated for want of the constantly required could be applied without doing violence to supply of cold water, which, in a boiler with the defences of the apparatus. The boiler no obstruction to circulation, is constantly (as should all steamboat boilers) should descending towards the fire, from the upper have been provided with a small pumping and cooler parts of the vessel — towards steam engine, to keep up the water, as at which last, the steam bubbles are simul such times, especially, the boat has often to taneously rising to the steam chamber; the wait a good while before starting, the water two currents thus running contrary to each, gets low, and the engine man and his assistwithout mutual interference, as their very ants (viz., the crowd around him) are too different specific gravities maintain an easy excited, and anxious for a quick and favourseparation between them. Thus the vertical able commencement of the trip, to go to has an advantage, in regard to circulation, work at the drudgery of pumping up by over the horizontal tubular boiler ; the tubes hand. Frequent trials of the state of the of the former interfering much less with the safety valve should be made, and it should passage of the two counter currents, and its not be left to blow of its own accord, at a fire-place being at the bottom of the boiler, safe pressure, but should be raised by force, instead of at one end of it, horizontally, and the surplus steam permitted to escape. gravitation gives more assistance to circula This would prevent the valve from adhering tion. The maker of the Medora's boiler to the seat, as it has been known to do, in was right, therefore, in giving wide passages consequence of the rusting of the iron, or for water into the legs, but, as to do this the introduction of some glutinous or conecessarily weakened his boiler so much, it hesive matter thereto. The propriety of shows a faultiness in its plan. It is not, these and similar precautions, need not be indeed, easy to see why he departed from enlarged upon. It is remarkable, and yet a the usual mode of building boilers of this clear consequence of the laws of mechanical character by arching the fire-places, and so momentum, that, in all these explosions, the arranging his flues as to permit numerous rents made in the vessel should be so much ties in a radial direction across the boiler, greater than necessary to vent the steam connecting the outer shell with the fire and water with the rapidity that one would arches, as well as the flues. Here no cut suppose far more than suficient to relieve ting of holes would have been required, and the pressure upon all other parts of the any degree of strength given without inter boiler than the part first ruptured, so much ference with circulation. The ties introduced as to save them from injury. While the into the Medora's boiler, are not in the confined fluid is quiescent, however power- . most effective position. A tie, or brace, in ful its effort to escape, it does no harm, a cylindrical boiler, should never, if possible, except in the preparation it is making to

RECENT AMERICAN PATENTS.

15

com-

force an outlet, by gradually increasing its oil alone, would have the same effect with
strain on the metal, until an equilibrium is the compound above indicated. We have
attained between the two forces of impact been in the habit of hardening steel, and
and resistance. But the instant that equili know of many devices which have been re-
brium is passed, upon a single square inch sorted to for the purpose of communicating
of the vessel, and the imprisoned fluid be toughness, but believe them all to be founded
gins to put itself in motion, it breaks its in error. The less rapid the cooling, the
way out, in a hundred directions instead of less will be the hardness, and, of course, the
one, and we see parts of the vessel, of ex greater the toughness of the article operated
tremely unequal strength, giving way at upon.
the same moment. Thus in the Medora's IMPROVEMENT IN PROPELLING BOATS.
boiler, several pieces were blown out at the Meredith Mallary.There is to be
back, which were unquestionably far stronger mon water-wheel, such as is used for driving
than the strips of metal over the legs. The mills, on each side of the vessel, or boat,
sudden escape of the steam and water, also, immediately in the rear of the ordinary pad.
acting on the sides and bottom of the boat, dle wheel, and the shafts of the two sets of
give to the boiler that projectile force which wheels are to be geared together; the water
throws it, en masse, into the air, and inflicts which is thrown up by the common paddle-
other great injuries in its fall. In all con wheels is to fall into the buckets of the
siderable explosions, indeed, the boiler has water-wheels, and is there to lend its aid in
been more or less displaced.

the business of propelling.
Thus is demonstrated the unsoundness of Claim." Now what I claim as my in-
the opinion that simple pressure, steadily vention is the applying the water lifted or
increasing, within a steam boiler, ought to thrown up by the paddle-wheels of steam-
open the seams, as being the weakest part boats, so as to produce an auxiliary pro-
of the vessel, and thus provide a safety valve, pelling power, in the manner described, or
for the escape of the vapour. To account in any way analogous thereto."
for the fact that boilers never do give way That these auxiliary wheels will exert some
in this manner, the hypothesis of explosive influence we have no doubt, but we are
gases has been paraded, with, as I humbly much mistaken if they do not prove to be-
conceive, very little foundation in fact, al. long to the class of consumers only, and
though rare instances of that description that of the worst kind, as they will not pay
may have occurred under particular circum for what they devour.
stances.

IMPROVEMENTS IN THE CONDENSER AND
APPARATUS FOR SUPPLYING STEAMBOIL-

ERS. Joseph Echols.—The apparatus, as
ABSTRACTS OF RECENT AMERICAN

described, for supplying boilers, consists of
LICATIONS.

a receptacle, communicating with a cistern
(Selected and abridged from the Franklin Journal and with the boiler, by means of pipes
for October and November 1842.]

governed by a four-way cock, which alter-
IMPROVEMENTS IN

nately forms a connexion with the cistern,
HARDENING

-Perry Davis.--The from which to receive the water, and then
method of hardening steel as practised by with the boiler to supply it with the water
the patentee is as follows : :-" Instead of thus received. The pipes forming these
plunging the steel to be hardened into cold connexions are narrow and high, and the
water, as is usual, it is plunged into a com one leading to the boiler has an inclination
position of borax, oil, and charcoal, to towards and opens into it above the water
harden it, which mode of hardening renders line. Thus, when the communication be.
the metal malleable with the same degree of tween the boiler and receptacle is open, the
hardness that was obtained by the old me steam from the former passes into the latter,
thod; so that the metal, after being har and from this the water rups down the lower
dened in this way, can be straightened or inclined surface of the tube into the former.
bent without any danger of being broken." The cock being then turned, cuts off this

Claim." What I claim as my invention, communication and opens it with the re-
and which I desire to secure by letters pa. servoir, which allows the water to run into
tent, is the mode of hardening steel so as to the receptacle and condense the steam by
render it flexible by means of the composi which the water was expelled in the previous
tion of oil, cbarcoal, and borax, as set part of the operation. For low pressure
forth."

engines, it is said that the top of the reser-
What influence the above described mix. voir must be closed, and a connexion formed
ture can exert upon the part to be hardened, between it and the condenser.
we are at a loss to perceive, and apprehend The condenser is divided into two com-
that the method frequently practised of using partments, separated by a diaphragm pierced

PUB-

THE

PROCESS

OF

STEEL

« ForrigeFortsett »