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(a) The following procedure shall be followed when installing riveted patches:
(1) Distorted sheets which require patching shall first be set back straight as nearly as possible before proceeding with the cutting out of the plate so that the patch may be kept as small as possible.
(2) All defective or deteriorated material shall be removed and properly trimmed to provide for caulking and neat workmanship in attaching the patch.
(3) The edges of a patch shall be beveled for calking by planing, chipping, or gas cutting before applying it to the boiler. Rivets should be driven by gun, if at all possible.
(4) All rivet holes shall be drilled full size or the holes may be punched not to exceed one-fourth inch (1/4") less than full size for plates over five-sixteenths inch (5/16"), and one-eighth inch (1/8") less than full size for plates five-sixteenths inch (5/16") or less in thickness, and then reamed to full size with patch in place. Rivet holes shall be not more than one-sixteenth inch (1/16") greater in diameter than the nominal diameter of the rivet.
(5) Seal welding of the patch plate is permissible if a single bead is used with a throat thickness not more than three-sixteenths inch (3/16"). The patch shall be tight under a hydrostatic test pressure equal to the operating pressure before seal welding. Rivets and patch bolts shall not be seal welded.
(6) When three (3) plates are lapped at the corners of a patch, the middle plate shall be scarfed carefully to a feather edge the entire width of the lap as shown in Figure 1.
(7) Upon the completion of repairs, a hydrostatic test of one hundred fifty percent (150%) of the maximum allowable operating pressure shall be applied. All seams shall be tight at this pressure.
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