As a makeshift ferrules are driven into the ends of boiler tubes (figs. 51 and 52), but they soon burn away. Better results are said to have been obtained by making the ferrules of cast instead of wrought iron, and leaving an air space between them and the tube (see fig. 53 and A. J. Durston's paper).

44

FIG. 51

FIG. 52

FIG. 53

Cleaning Boilers.-From the foregoing remarks it is clear that many troubles are caused by the accumulation of scale, grease, or even salt, and where this cannot be prevented, great care has to be taken that the boiler shall be cleaned out and scaled as often as possible. This work is never done well unless the various parts of the boiler are easily accessible, not only to boys, but to the engineer, who has to see that the work is properly done. None of the water spaces between the tubes should, therefore, be less than 10 ins., and if that amount of space cannot be provided over the wing surfaces, it ought to exist between the tubes and the shell, or a manhole should be fitted in the two wings of each boiler. They should not be made smaller than 10 x 14 ins., but 12 x 16 ins. is ample. A sufficiently large manhole should also be fitted between and under the furnaces, either at the front or back end of the boiler.

Removal of Scale.-There are several methods of removing scale. The general one is to chip it off. This work is very laborious on account of the hardness of the scale. Scale can be softened by adding a sufficient quantity of soda to the boiler a few days before reaching port. The soda converts the sulphate of lime into carbonate of lime and the change has a loosening effect on the scale. After a time, however, this scale would harden again and further additions of soda would have no effect. When the boiler is empty, the scale has to be removed as rapidly as possible because air hardens it. Other methods are based on this very fact, therefore boilers should be allowed to cool with all their water in them. The scale can then be removed with a hose pipe, brushes, and scrapers, and without chipping. The men should have oil-skins, and the water in the boilers should only be lowered as the cleaning progresses. Even old scale will yield to this treatment if repeated. Scale grows hard in about three hours after exposure to the atmosphere. Another method is to blow the boiler down while steam is up, care being taken that the brickwork of the firebridges is sufficiently cooled down, so as not to injure the furnace plates. The boiler is then allowed to cool, no doors being removed until the men are ready to enter the boiler. They will find the scale fairly soft and so well detached from the plates that it can be scraped off.

Removal of Grease.-The most effective method is to allow scale to accumulate and to remove it and the grease together. Naphtha and similar oils are often recommended with which the boiler surfaces have to be rubbed down. Of course naked lights may not be used, and as this plan is not very effective and said to be injurious to health, it should not be adopted.

Corrosion. This subject is fully dealt with in a separate chapter, but here it will be necessary to make a few remarks on the external wasting away of plates. This never occurs except in the presence of moisture, and is found chiefly near manholes, near leaky joints, at the boiler bottoms and ends, where they come in contact with bilge water, and above all near the furnace fronts and combustion chamber bottoms. These are exposed to the very injurious action of moist ashes, containing large percentages of sulphuric and other acids, which are partly liberated by the ash-cock water. By keeping the boilers perfectly tight, and protecting the front plate from moist ashes, all these troubles can be avoided. Naturally also leaky tubes and seams cause external corrosion, for then a combination exists there of heat, moisture, salt, and noxious acids.

The leakage from valve and other flanges, the drainage from test cocks, and the moisture which collects in the wood of platforms adjoining the boiler backs, all tend to shorten the life of a boiler, and should be prevented as much as possible.

FIG. 54

As regards manholes, an excellent practice is gaining ground of flanging them and facing the edges and grooving the door, as shown in fig. 54, which permits of their being faced up again when

worn out.

Boiler Repairs.-Sooner or later even the best managed boilers will need repairs. If badly done they lead to further troubles, and may prematurely necessitate the renewal of the entire boiler. Care should therefore be taken that the workmanship is of the best, and that no structural defects are introduced, which would either prevent circulation or hinder the removal of scale.

Bulged Furnaces.-The most serious troubles with new boilers are the furnaces. A careless engineer, or other causes, which have been already mentioned, may suddenly bring them down. If the deformation is not great, i.e. only a few inches, and spread over a considerable area, the best remedy is to press the plates out again. Attempts have repeatedly been made to do this cold, but the deformation can never be perfectly removed in this way, and soon returns. Besides, other parts of the furnaces are thereby strained, and it has happened that

while pressing up a furnace crown the welded seam at the bottom cracked and had to be patched. Good results are obtained by heating the furnaces locally, and then pressing them out, but there is the danger of making the plates brittle by this treatment. As a safeguard such furnaces should be heavily hammered twenty-four hours after they have cooled; if very brittle they will crack. In order to guard against such injurious effects it is well to reheat the plates after they have been pressed out hot, for local heating seems to be injurious only when accompanied by straining of those parts. (See p. 259.)

Satisfactory results have been obtained by using a cast-iron mould for a head to the press. It must be made of a substantial thickness, heated to redness and then applied, and left in position till cold. The correct shape of the furnace is very soon regained, and while slowly cooling the hot cast iron anneals the plate. Only screw jacks should be used for this work, because the hydraulic ones can never be kept perfectly tight and have to be pumped up occasionally, whereby strains are produced during the period of cooling, which it is the object of this proceeding to avoid. Furnaces which have once come down very often do so again unless they are strengthened by rings.

Collapsed Furnaces. If the furnaces have collapsed thoroughly they must be renewed, and it depends very much upon the boiler design whether this can be done efficiently. If the furnace back end was flanged inside of the tube plate, as is now generally the case, there is hardly any other remedy than to cut the back end and to insert an unflanged furnace (figs. 55, 58). Sufficient width should be left for a double row of rivets, for such a joint seems to give better results than a single-riveted one. Chiefly for cheapness' sake, but partly for facilitating the above-mentioned repairs, the tube plates,

one.

instead of the furnaces, are sometimes flanged (fig. 56). Should the furnace collapse, it is then only necessary to draw it and to fit a new Under any circumstances no expense or trouble should be spared on these seams to bring the two plates into metallic contact. It is well to wash the two surfaces in sal ammoniac, so as to remove the scale; also,.when all the holes have been drilled, which should be done in place, the furnace ought to be partly withdrawn, the burrs removed, as well as every trace of oil, because this is a very bad conductor of heat. The riveting should not be started from the bottom

of the seam; otherwise, before the top is reached the plates at the crown will be far apart (see fig. 57). It is then practically impossible

to make the seam per

manently tight, and after being in use a short time it is sure to leak again. Sometimes it only opens on the fire side, and then, though the blade of a pocket knife can be inserted up to the other caulked edge, no leakage seems

FIG. 57

to take place. In order to be quite safe, it is best to fit and rivet the furnace crown into position before the furnace bottom is introduced.

FIG. 58

The longitudinal seam should be slightly inclined, as shown in fig. 58, and then it is an easy matter to make it a very good fit. Another

FIG. 59

way of repairing a collapsed furnace is shown in fig. 59. The furnace crown is cut in two, and the forward end is renewed, while the saddle

is taken out, flanged, as shown, and refitted with an Adamson's ring.

With a furnace whose back end is not flanged, repairs may sometimes be effected by removing all the rivets and turning it round, so that the saddle seam is placed in the ashpit. The weld which was at the bottom of the furnace is now somewhere near the crown. Central

FIG. 60

furnaces can, under certain conditions of shape, be renewed by first removing the combustion chamber bottom plate.

If the furnace saddle is flanged over the tube plate, and if the furnace front plate is outside the front tube plate (see fig. 60), then both it and the furnace can be removed and the one renewed. But if the furnace front plate is placed inside the front tube plate, and has

FIG. 61

to be withdrawn, it is necessary to cut away its two corners (fig. 61), so as to be able to tilt it out of and into position. The amount of metal to be removed depends on the depth of the flange. This plan cannot be carried out on boilers with three furnaces, unless the horizontal seam intersects the central bundle of tubes.

The arrangement shown in fig. 60, which facilitates these repairs, has the further advantage that both the back and the front seams are very easily caulked from the outside. Its only drawback is that one loses about 1 in. of tube length.

Flanged furnaces can occasionally be removed and replaced intact by cutting away the lower part of their back ends (fig. 62). The wing combustion chambers would have to be specially constructed, so that no part of the furnace flange is wider than the front ends of the furnaces (fig. 63).