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Manhole frames, mouthpieces, doors or cover plates
shall be of mild steel. Frames shall be oval and where
of the flanged type shall be formed to bed closely to
the shell and provided a flat jointing surface to the
door. Alternatively, raised circular mouthpieces may
be fitted externally and where of the flanged type shall
be bedded closely to the shell.
Wherever practicable the frame shall be attached to
the inside of the shell with the shorter axis parallel
to the longitudinal centre line of the boiler. In all
cases where the shell thickness exceeds 9/16 inch, oval
frames or circular mouthpieces shall be fitted.
Doors shall be made from mild steel plate formed to
bed closely to the internal surface and shall be fitted
with studs, each provided with an integral collar and
screwed through the plate with a nut on the inside of
the door or fillet welded on the inside, the length
of the leg of the fillet weld being not less than one
quarter of the diameter of the stud or riveted on the
inside of the door. Doors for openings larger than 9
inches x 7 inches shall have two studs, but for doors
of size 9 inches x 7 inches or less only one stud need
be fitted; doors for openings not larger than 5 inches
x 3 ½ inches may have the stud forge solid with
the door. Where the door of a mudhole, cleaning hole
or sight hole is fabricated by welding for boilers not
exceeding a working pressure of 8.8. kg./cm. (125 lbs.
per sq. inch). The stud shall pass through the plate
and be secured to it by strength-welding on the inside
and fillet-welding on the outside to the satisfaction
of the Inspecting Authority. The door shall be stress
relieved by heat treatment after welding.
The cross bars shall be of substantial proportions and
either solid steel forging or cast steel. Lugs may be
satisfactorily welded to manhole doors for the attachment
of the bolts.
The spigots of the door when it is in a central position
in the manhole have a clearance of not more than 1/16
inch all round.
Oval frames and externally raised circular mouthpieces
shall be either:-
| a) |
Formed in one piece without
weld, or |
| b) |
Formed from a suitable
rolled section and forge-welded, or fabricated by
fusion welding, provided they are stress-relieved
by heat-treatment after welding and before attaching
to the boiler unless the whole boiler is to be subjected
to heat-treatment on completion. |
Welds shall be positioned so that they are located on
the transverse centre line of the boiler(see Regulation
556).
The joining flanges of mouthpieces and covers shall be
machined on the face and edges and on the bearing surface
for the bolts. Bolts and nuts shall be machined where
in contact with the flanges and the joints should be formed
inside and outside the pitch line or pitch circle of the
bolts to preclude the possibility of flange distortion.
Cover plates shall be dished outwards to a depth of approximately
one-eighth of the internal diameter of the frame.
All edges shall be machined or flame-cut by machine (see
Regulation 541).
Forms or manhole frames and attachments are shown in Figures
XII/52, XII/53,
XII/54, XII/55 and
XII/56.
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