« AnteriorContinuar »
almost unknown: the case of one which exploded at Barking, January 6, 1899 (Board of Trade Report, No. 1173), due to excessive pressure, is therefore of special interest, and gives some indications as to permissible working stresses.
The boiler was built of iron in 1878 by Messrs. R. and W. Hawthorn, Leslie & Co. Dimensions : Diameter 10 ft., length 9 ft. 10 ins. ; one dome; two furnaces 34 ins. diameter; two separate combustion chambers. Thicknesses : Shell & in., joints welded ; furnaces je in. (single butt straps), reduced by corrosion to in. along firebars; combustion chambers in., reduced generally to in., except at bottom of back, where thicknesses of it in. were general; these parts had also been patched. Screwed stays 11 in., riveted over ends except 21 in the back, which were new and nutted. Marginal stays 13 in. diameter, nutted. One screwed stay had a cover patch (see fig. 71). Pitches 8 ins. square, and also 8 ins. x 12 ins. Front end plates, top id in., apparently not reduced. Stays 2 in. diameter, with plus threaded ends welded on; diameter reduced to about 13 in. Washers 6 in. diameter, } in. thick ; pitch 14 ins. square, and also 15 ins. X 12 ins. Tube plate i in.; pitches of stay tubes 13} ins. x 9 ins. Back plate & in. Maximum pitch of stays 8 ins. x 12 ins.
The accident was due to two successive pressure gauges being out of order, whereby the man who was setting the safety valves screwed them down to 240 lbs., at which pressure the boiler exploded.
The shell tore along the top, through the dome hole, at 8.1 tons per square inch, but not through any weld; it is, however, more than probable, there being two reports, that the first to rupture were the welds of the steam space stays, say at 7.6 tons per square inch; then the end plates tore, and then the shell. The furnaces do not appear to have collapsed. The stress in these plates was 4.2 tons at the thickest uncorroded parts, and 5:8 tons near the firebars. The tube plate does
t2 not appear to have bulged. Using the formula W.P.=C where the thickness is expressed in sixteenths of an inch, we get C= 265. The steam space plates were bulged, but it is not certain that this happened before the explosion. In this case we get C= 450. The combustion-chamber plates do not appear to have bulged, nor were many stays drawn out. Here we have C = 310 for the & in. plates, and C = 620 for the fin. plates. Many of the screwed stays were wasted.
The experience gained in this case is directly applicable to boiler strengths, except as regards the furnaces, for in this
case the fires were only very light at the time of the explosion, and therefore not likely to produce stresses and deformations previously discussed. The questions to be decided are, therefore : what extra allowance can be made for the use of steel instead of welded iron, and what factor of safety is desirable? For here, of course, as the boiler was under working conditionsi.e. high pressure and high temperature-our uncertainty as to what happens when we calculate boiler strengths with the help of experimental data of cold steel falls away.
A very instructive case as regards strengths of boiler girders is the explosion (Board of Trade Report, 1900, No. 1254) of a stationary loco-boiler. The girders were 74 ins. long, 7 ins. deep, 1in. thick, placed 5 ins. apart, and each one carrying 14 stays, pitched 41 ins., and supported, but at the wrong points, by slings. "It was assumed that the explosion took place at a pressure of 140 lbs., but the boiler had recently been tested to 200 lbs. At this pressure, neglecting the influence of the slings, the stress was 33 tons per square inch, or if, as appears probable, the girders gave way when hot at a pressure of 140 lbs., then the stress under these conditions was 23 tons per square inch.
List of some Boiler Tests and Explosions. R, H. Thurston, 1872, · Frankl. Inst.,' ii. vol. lxiii. p. 89. Tested boiler to 82 lbs. cold, then burst at 90 lbs. hot. p. 93. Stayed flat plates. Tested cold to 138 lbs., then burst at 165 lbs. hot.
p. 95. Tested box boiler to 60 lbs. cold. Vertical braces gave way; repaired these and tested hot. Reports were heard at 50 lbs. pressure; boiler burst at 532 lbs. pressure.
p.. 99. Tested boiler to 200 lbs. at 100° F. Several braces broke under 115 lbs. steam pressure.
p. 99. U.S.A. steamer Algonquin' tested cold to 150 lbs. Some braces broke under 100 lbs. steam pressure.
R. H. Thurston, 1887, ' Explosions.' Tested boiler to 300 lbs. steam; burst at 235 lbs. on opening valve.
P. Carmichael, Eng. Scot.,' 1869-70, vol. xiii.; also 1878–9, vol. xxii. Experimental bursting of a boiler.
L. E. Fletcher, 1876, “M. E.,' p. 59. Experimental bursting of a boiler (cold).
W. Siemens, .M. E.,' 1878.
D. Greig and Max Eyth, ‘M. E.,' 1879. Experimental bursting of three boiler shells.
W. Parker, ‘N. A.,' 1881, vol. xxii. p. 12. “Livadia's' boiler burst under cold water test above working pressure.
Ibid., N. A.,' 1885, vol. xxvi. p. 253. Shell boiler burst under hydraulic test. Ibid., ‘N. A.,' 1889, vol. xxx. p. 290. Experimental bursting of a boiler (cold).
J. Scott, · N. A.,' 1889, vol. xxx. p. 285. Experimental bursting of a Navy boiler.
Parliamentary Committee on Boiler Explosions, 1817.
Parliamentary Committee on Boiler Explosions, 1871. Parliamentary Reports No. 186, vol. lxvi. p. 43, No. 378, vol. lxvi. p. 85; 1877, No. 361, vol. Ixviii. p. 373.
Martens, Rep. U.S.A.,' various dates.
* Board of Trade Reports on Boiler Explosions.' Amongst these latter the following are of interest:
1861, part iv. Locomotive boiler tested to 196 lbs. at 162° F.; burst seven months later under 120 lbs.
• Acrefair,' Dec. 10, 1880. Boiler locally weakened to 61 lbs. permissible working pressure; burst at 32 lbs.
No. 228. Drying cylinder burst. Factor of safety about 20-3.
No. 243. Boiler shell burst through manhole at 100 lbs., after having been recently tested to 150 lbs. steam.
No. 249. Boiler shell exploded. Factor of safety 2.75.
No. 265. Boiler shell exploded at 83 lbs. It had recently been tested cold to 92 lbs.
No. 314. Boiler shell exploded. Factor of safety 3.5.
No. 346. Furnace collapsed at 50 lbs. It had been tested to 95 lbs. only three days previously.
Other interesting cases are--Explosion on the steamer · Mülheim No. 5,' 1866 ; explosion on the steamer 'Parana,' 1869; explosion on the steamer
America,' 1871; explosion of a locomotive boiler, ‘Engineering,' 1890, vol. 1.
T. W. Trail (C. E.,' 1884, vol. li. p. 31) mentions the bursting of two boilers whose factors of safety were 3:9.
L. E. Fletcher (C. E.,'1884, vol. Ixxx. pp. 136 and 139) mentions several curious explosions.
J. J. Platt, ‘M. E.,' 1878, p. 260. A steel fire-box plate cracked after being in use five months.
MacFarlane Gray ("N. A.,' 1877, vol. xviii. p. 326) mentions that a boiler intended for a working pressure of 30 lbs. was tested to 400, repaired, and then worked all right.
J. A. Rowe, 1884, p. 7. A boiler burst at 30 lbs. working pressure after a cold water test to 59 lbs.
In the following pages the various workshop practices of boiler construction will be dealt with as briefly as the subject will allow. The question of cost cannot, of course, be entered upon, but occasional reference will be made to the time required for the various operations; this necessarily varies in different works, depending not only on the perfection of the machinery, but also on the skill and energy of the men. More attention has been paid to the different practices and tools for obtaining the same object, and occasionally methods have been mentioned which are practically obsolete or not in use, but which may have done good service or must be looked upon as warning examples. Naturally, every modern device has not been discussed, but it is hoped that none except unimportant ones have been neglected.
The order in which boiler construction will be taken is to deal firstly with the various operations to be performed on the boiler shell, then with the internal parts, and then with the boiler as a whole.
The plates as they arrive from the rolling mills are never exactly of the specified sizes, being usually about į in. larger in each dimension. The thickness is kept within reasonable limits by the condition that any excess weight beyond, say, 5 % margin will not be paid for; but difficulties are sometimes occasioned by stipulations that certain weights shall not be exceeded which, when compared with the plate thicknesses, are too low. The weight of 1 sq. foot of iron is equal to about 40 lbs. per inch of thickness, and 41 lbs. for steel.
The edges of plates are nearly always thinner than the centres. This is almost unavoidable, for as soon as the rolls are set so that the conditions are reversed the plates are seriously puckered. It is not possible to give a full explanation without entering into unnecessary details, but there is no difficulty in understanding that if during the last pass through the rolls the thickness of a plate is reduced in., while its thickness is } in. at the edges and is at the centre, then these parts will stretch relatively to and 14 of their length, and the centre must pucker. Should the rolls have been made quite parallel, then as soon as they get heated their centres swell and produce the above result. If they are made slightly hollow, the plates which are rolled first will be thin at the edges, and perhaps frilled; but the greatest trouble is experienced when the widths of the plates to be rolled vary much, for an enormous pressure (about 500 to 1,000 tons, has to be exerted, and the spring in the rolls is very appreciable.
Shearing Operations. In a boiler shop large shearing machines are not required, but certain plates, such as those for the boiler ends and for the combustion chambers, are often ordered with a comparatively large margin, on account of flanging, and this excess is then most easily removed by shearing. A small but powerful machine, capable of cutting steel plates up to 1} in, thick, will be found a very handy tool. The levers and handles for working it should not project in front; otherwise they may come in contact with such of the plates whose flanges have to be sheared while standing on end. The arrangements for replacing the shearing blades should be such that this can be done quickly, so that, as occasion arises, curved or cornered shears may be substituted for straight ones.
The question as to how much material ought to be planed off sheared edges in order to remove the injurious effect is, and probably will remain, an unsettled one. Some engineers look on shearing and punching as being perfectly harmless; others insist on subsequently planing away at least 1 in. Probably the experiences which led to these diverging views are due to the use of various qualities of material. Remarks on this subject will be found in the chapter on 'Strength of Materials,' and the conclusions arrived at there are that good material is not injured, while bad material grows brittle, and the more so the thicker the plates are. Evidence will also be found there in support of the view that the brittleness caused by shearing gradually extends into the plates. Under any circumstances it is well to guard against possible failings of this sort by insisting on a cold bending test of samples with sheared edges, or of samples with punched holes. In this latter case a standard punch and bolster should be determined upon, as their relative diameters influence the results. If a sample which has been punched or sheared bends well when cold, and particularly if it does so after having been put aside for a week or more, then there will be little fear that the material can satisfactorily withstand this and the less severe workshop treatment.
Punching Operations.--The effects of punching are so very similar to those produced by shearing that nothing need here be said about them, except, perhaps, that it would be better if no punching machines were used in a boiler shop, so that all holes could only be drilled. However that is rarely, if ever, the case, and therefore it is as well to fit as strong and accurate a machine as can be obtained, and to insist on carefulness in setting the bolster. If carelessly placed the punched hole will be a very irregular one (see fig. 197). This is particularly the case if the guides of the punching press slide are a loose fit, for then it will happen that one hole is quite fair, while the next has a slanting form, or even a worse one (as in fig. 198), due to the punch having started penetrating while its edge was actually overlapping the circumference of the die. The breaking of punches under such conditions is not to be wondered at.
The clearance, or rather the difference, between the diameters of the punch and of the die hole is usually about 10 to 15%. This agrees fairly well with the following published recommendations :
W. H. Shock (1880, p. 166) gives 15 to 20%. J. H. Wicksteed