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(a) Riveted patch seams exposed to the products of combustion may be double-riveted or constructed similar to the original seams of the boiler.
(b) Patches exceeding twenty-four inches (24") in length shall have the proper width as determined by these orders. Patches exceeding fifteen inches (15") to and including twenty-four inches (24") in length shall be triangular, crescent, diamond, or oval in shape and width, W, in the circumferential direction, shall be at least twice the length, L, in the longitudinal direction. Patches fifteen inches (15") or less in longitudinal direction may be circular in shape.
(c) If it is found that a patch will extend extremely high, and also to avoid calking in sharp corners, it may be shortened in width to the extent that not more than four (4) rivets will be in a longitudinal line, as shown in Figure 2.
(d) If it is found that a patch will have to be sixty inches (60") or more in length, consideration should be given to the use of a sheet having a width equivalent to five-eighths (5/8) of the circumference of the boiler, and the longitudinal seam shall be of a design similar to the design of the original seam of the boiler.
(e) In laying out new patches over twenty-four inches (24") long, it is recommended that they be triangular or diamond in shape as may be required for the particular job; with definite straight-line sides but with corners properly rounded to permit adequate caulking, as illustrated in Figures 1, 2, 3, and 4. The length designated as "L" and the width designated as "W" are also shown in Figures 1, 2, 3, and 4.
(f) Rivets shall be used in "riveted" patch seams unless their use is difficult or impossible, in which case, patch bolts may be used. Rivet holes may be countersunk in patches on shells that have the heads supported by tubes or braces, providing the angle of the chamfer with the center line of the rivet hole does not exceed forty-five degrees (45°), and the depth does not exceed half the thickness of the plate.
(g) Patch material shall be of fire box or flange steel depending upon the plate it replaces. Tank steel shall not be used. The repair shop shall be required to produce a copy of the manufacturer's mill test report for the material to be used.
(h) The patch plate material shall contain the steelmaker's brand. If only a part of a plate is required and this part does not contain the brand, the brand shall be transferred to the patch plate in the presence of a qualified inspector or a representative of the plate manufacturer before the plate is cut. Replacement rivets, patch bolts, and staybolts shall be of material approved for new construction by the code.
(i) All patch plates except patch plates containing blowoff connections shall be placed inside of the boiler shell or drum if exposed to the products of combustion and if installed where there is a possibility that scale deposits may be pocketed. Patch plates containing blowoff connections shall be placed on the outside of the boiler shell.