Cracks in the crown-sheet or side of a fire-box boiler, or top head of the upright boiler can be temporarily repaired by a row of holes drilled and tapped touching one another, with 3⁄8 or 1⁄2 inch copper plugs or bolts, screwed into the plates and afterwards all hammered together.
For a permanent job, cut out the defect and rivet on a patch. This had better be put on the inside, so as to avoid a “pocket” for holding the dirt. In putting on all patches, the defective part must be entirely removed to the solid iron, especially when exposed to the fire.
Note.—When fire comes to two surfaces of any considerable extent, the plate next to the fire becomes red-hot and weakens, hence the inside plate, in repairs, must be removed.
The application of steel patches to iron boilers is injudicious. Steel and iron differ structurally and in every other particular, and their expansion and contraction under the influence of changing temperatures, is such that trouble is sure to result from their combination.
DEFECTS AND NECESSARY REPAIRS.
Fig. 68.
[Fig. 68] represents a patch called a “spectacle piece.” This is used to repair a crack situated between the tube ends. These are usually caused (if the metal is not of bad quality) by allowing incrustation to collect on the plate inside the boiler, or by opening the furnace and smoke doors, thus allowing a current of cold air to contract the metal of the plates round the heated and expanded tubes.
The “spectacle piece” is bored out to encircle the tubes adjacent to the crack, or in other words, to be a duplicate of a portion of the tube plate cracked. These plates are then pinned on to the tube covering the crack.