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to be fitted. After this operation the plate should have the appearance shown by fig. 309, and the edges can then be flanged with great accuracy.
Hand Flanging.–When flanging by hand stops are sometimes used, though in some works it is deemed sufficient to make a few chalk marks on the flanging mould, and some smiths content themselves with marking the line of flange with deep centre punch marks.
The combustion chamber sides are seldom flanged. Now and then designs are met with where the back edge is turned in (fig. 310).
There is no advantage in this plan, and the flanging operation is a more difficult one than usual.
Another arrangement is to leave the back tube plate and combustion chamber plate flat, and to flange out the back and front edges of the top and side plates (figs. 311, 312). Unless very much rounded at the corners the flanges grow thin, or tear, and must be welded. Boilers with these combustion chambers are said to be difficult to clean, and objections have been raised against the caulking liners (fig. 312), but they do not seem to give trouble. Some combustion chambers have rounded backs (fig. 313). These must, of course, be bent in the rolls before flanging. Some combustion chamber tops are flanged to meet the girder plates (see fig. 375, p. 292). Instead of the furnaces, the lower ends of the back tube plates may be flanged (fig. 314) like the
forward outer plate of the fire-box of a locomotive. It is better not to let the tube plate extend much below the centre line of the furnace, because it is next to impossible to make the two a good fit. Besides, an extra seam across the combustion chamber saves a seam round the furnace bottom. If the tube plate is made to end at A, the top part can be fitted quite close, and there will be less chance of leakage, a danger to which this arrangement is specially liable.
Reference has occasionally been made to the practice of flanging the furnace saddles round the tube plate corners-i.e. placing the back tube plate on the fire side of the furnace flange (fig. 315). It has the advantage over the ordinary style (fig. 316) of allowing both seams to be caulked efficiently; nor can steam-bubbles lodge under the landing, and it permits of the radius of the tube plate flange being made smaller than that of the furnace flange. It is asserted that the flame impinging on the caulked edge will do harm, but no trouble has ever been noticed at this point. In the one case the side corners will have to be shaped as shown in end view (figs. 288-291, p. 271); in the other, as in fig. 67, p. 36. In the one case the tube plate corners, in the other
the furnace saddle corners, will have to be drawn out or chipped taper. As these corners have often given trouble by leaking, some works have adopted the plan of welding them.
Before concluding the remarks on this subject it is necessary to mention an isolated practice of flanging the shell plates, instead of the
boiler end plate (fig. 317). There does not seem to be any
advantage, except that the upper end plates of boilers need not be annealed. The shell plate being in tension, any injury caused to these parts by the flanging operation would be doubly dangerous, as it cannot be removed by annealing.
The work is carried out as
follows: The ends of the longiFIG. 317
tudinal seams are welded, the
seams riveted, and the edge of the shell heated and flanged in a Tweddell's flanging press, which, to suit the requirements of the case, has to be placed on its back.
Another interesting subject is cold flanging. The results of Messrs. Easton and Anderson's experience will be found in the Journal of the Iron and Steel Institute' for 1882, p. 528. They produced annular discs of the shape shown in fig. 318 with plates in. thick, but the results cannot be said to have been satisfactory, for out of sixty plates, Landore SS quality, nine cracked their inner flanges, and one its outer flange; five were not annealed, and of these three cracked. Out of fourteen which had been twice annealed two cracked. The forty-one remaining were annealed only once, but some of them as long as forty-eight hours and in ashes. Five of these cracked, although one-half of the lot were flanged slowly, requiring about three to four minutes instead of a quarter of a minute.
No mention is made as to whether any of the plates cracked later on, though it is to be expected, for nearly all bent test pieces crack at their inner radius some time after leaving the press or hammer.
Fitting Together of Plates.-Having completed the flanging and annealing, the plates are fitted together and secured to each other by means of a few bolts. Whenever possible, the various seams are closed up cold by hammering. Troublesome parts are warmed by heaters, but, as they often reach a blue heat, it would be better to heat them properly. On this point practices differ. Some works prefer heating only one plate, bolting it to the other, and hammering it, so as to fit the cold one. This plan requires very great care in the flanging of the plate which remains cold, as it cannot be made to alter its shape, and the danger exists that it will be made blue hot. Other works heat both plates while bolted together; then, of course, it is easy to correct any slight defects of form in both. Hardly any works re-anneal these pieces, which may account for some cracks. Less risk would be run if the corners at least were reheated.
Particular care should be taken with the flanged corners of the furnaces where they meet the back tube plate and the combustion chamber sides. Not only will injudicious treatment increase the liability of these parts to crack after the boiler has been put in use, but also, on account of the impossibility or difficulty of caulking this seam at both edges, there is a greater chance of the water forcing its way through here than through any other seam of the boiler (see fig. 413, p. 300).
Fitting Combustion Chamber Plates. The fitting together of the various internal parts is done as follows: The tube plates are bolted to the furnaces and hammered up close, the rivet holes drilled, and the riveting carried out at once or postponed till the other parts have been prepared. The combustion chamber back plate is then bolted to the furnace and tube plate by means of strips of iron. The sides, top and bottom plates are then successively fitted. The order in which this is done depends on the position of the seams. Representations of the various plans will be found in figs. 319-327. If the whole of the riveting is to be done by hydraulic power, the arrangement shown in fig. 325 must be adopted, the top plate being riveted before the sides are put in position ; and these again are riveted before the bottom plate is secured. But this can only be riveted if the depth of the riveter is sufficiently long to reach from the furnace mouth to the back plate. If the head of the machine is a clumsy one, the flanges will have to be made sufficiently deep so that the rivets can be reached. By removing the back plate after all the side plates have been fitted it is possible to rivet at least the tube plate flange by machinery. If the riveting of all the combustion chamber seams is to be done by hand, it is immaterial in which order the plates are put on, and preference will naturally be given to arrangements like those in
figs. 319, 320. When the two plates are of equal thickness the seams can be placed higher up, as in fig. 321. Should the lengths of the plates be found too great, or, what is more likely, should the back combustion chamber plate not be shaped exactly like the back tube plate and
furnace, which would lead to trouble in fitting the circumferential ones in one or two lengths, it may be safer to use three plates, as in figs. 321, 322, 323, or even four plates (figs. 324, 325, 326, 327). The arrangements shown in figs. 319, 320, 321, 322, 324, 325 require that
one or the other of the plates should be bent at two points, which, for fitting, is more troublesome than if each plate has got to be bent at only one point, as in fig. 327. The arrangement shown in fig. 322, and on a large scale in fig. 328, doubles the bending operations. The