46 C.F.R. § 59.15-1 – Furnace repairs.

(a) Where corrugated or plain furnaces or flues are distorted by 1.5 inches or more, they must be repaired by either of the following methods:
1. (1) The furnace must be forced back to a true circular shape, and the Officer in Charge, Marine Inspection, may require strongbacks or other acceptable means of support to hold the furnace; or
2. (2) The furnace must be adequately stayed as determined by the Officer in Charge, Marine Inspection.
(b) Distortion means the difference between any single measured diameter of the furnace and the diameter of a true circle at the same location.
(c) Where the distortion does not exceed 1.5 inches it will not be necessary to force the furnace back to a true circle if the allowable pressure is reduced in the ratio of 1.5 percent for each one-tenth of an inch of distortion. However, if the maximum distortion does not exceed 1 inch, the repairs or reduction in pressure will not be required unless considered necessary by the marine inspector.
(d) When it becomes necessary to rivet a patch to a furnace or other part of the heating surface, the riveted patch must be placed on the waterside of the plate in order not to form a pocket in which sediment may collect.
(e) Furnace crowns which have become distorted, not in excess of the limitations provided in paragraph (c) of this section, may be repaired by forcing back the distorted section to as nearly a true circle as possible and reinforcing the same by means of a ring, arc- or gas-welded to the distorted corrugation as shown in figure 1 to § 59.15-1, the welding to be done by welders and welding processors qualified in accordance with part 57 of this subchapter.
Figure 1 to § 59.15-1—Approved Method of Reinforcing Furnaces by Means of Arc or Gas Welding