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Reg. 546

Internal Flues

 
a) Sections of internal flues shall each be made from one plate which shall be bent while cold to circular form. The longitudinal seam shall be forge lapwelded, or fusion butt-welded by the metal arc process.
b) The maximum permissible variation in diameter of any cross section shall not exceed ¼ inch or half the thickness of the plate, whichever is greater.
c) Longitudinal welds shall be placed at the lower part of the furnace and shall break joint in successive sections at least by 150 mm.

d) Where the flue sections are flanged for circumferential joints the flanging shall be carried out at one heat by suitable machinery.
e) Edges of all flue flanges shall be machined or flame-cut by machine (See Regulation 541).
f) Flanged circumferential seams shall be arranged so that they do not fall in line with those of the adjacent flue or with circumferential seams of the shell.
g) The minimum mean internal diameter of furnaces shall not be less than those given in Figure XII/20A depending on the not heat input.
gg) Where furnace flue is made up of plain or corrugated sections or both, they may be connected to each other by fusion butt-welded circumferential seams.
ggg)
i) Plain furnaces shall not exceed 3m in length except in the case of reverse flame boilers, which are considered to be inherently flexible or as provided in clause (ii). In all other cases flexibility shall be provided in the furnace by means of corrugations or bowling hoops or stepping of a minimum of 150 mm I  the diameter of the furnace. See figures XII/20B,20C,20D,20E & 20 F.

If corrugations are used to provide flexibility; at least one-third of the furnace length shall be corrugated.

Breathing spaces as per sub-regulation (i) of regulation 590 shall be provided for boilers made as per this clause.
ii) Notwithstanding the provisions of clause (I), plain furnaces longer than 3 m are allowed to be used in a boiler when the length between boiler and plates does not exceed 6.5 metres provided that breathing spaces used in this case are in accordance with sub-regulation (ii) of regulation 590.
h) Where a complete flue is constructed of short plain sections, the ends of each adjoining section shall either be swaged out to a radius to provide an adequate point of support and longitudinal flexibility and butt-welded together or be butt-welded to each side of a bowling hoop. The dimentions shall comply with Figures XII/23, XII/24 or XII/25.
i) Stress relieving by heat-treatment shall be carried out when the construction of a flue involves the use of a circumferential weld, such stress relieving being carried out on the completion of all welding in accordance with regulation 562.
j) The forms of the connections to end plates shall be as shown in Figures XII/13, XII/14, XII/15 and XII/16 and XII/16A or subject to the approval of the Inspecting Authority, be in any other form of welding groove.


 
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