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- Fabrication and welding
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Part C Machinery, Electricity, Automation and Fire Protection
Chapter 1 Machinery
Section 3 Pressure Equipments
4 Design and construction - Fabrication and welding
4.2 Welding design
4.2.1 Main welded joints
- All joints of class 1 and 2 pressure parts
of boilers and pressure vessels are to be butt-welded, with the
exception of welding connecting flat heads or tube sheets to shells,
for which partial penetration welds or fillet welds may be accepted.
Fig 26 show
examples of acceptable welding for class 1 and 2 pressure vessels. Figure 26 : Example
of acceptable joints and thickness to be considered
and post-weld heat treatment
1 : Nozzle (set in); 2 : Flange;
3 : Nozzle (set on); 4 : Reinforcing
plate; 5 : Non-pressure part;
6 : Pad (set in); 7 : Pad (set
on); 8 : Manhole frame; 9 : Flat plate.
- Joints of class 3 pressure vessels are also subject to
the requirement in a), however connection of dished heads to shells
by lap welds may be accepted. Fig 27 shows some acceptable details of circumferential
lap welds for class 3 pressure vessels.
4.2.2 Shell longitudinal
and circumferential weldsLongitudinal and circumferential joints are to be welded from
both sides of the plate. Welding from one side may be allowed only
when there is evidence that the welding process permits a complete
penetration and a sound weld root. If a backing strip is used, it
is to be removed after welding and prior to any non-destructive
examination. However, the backing strip may be retained in circumferential
joints of class 2 vessels, having a thickness not exceeding 15 mm, and
of class 3 vessels, provided that the material of the backing strip
is such as not to adversely affect the weld.
4.2.3 Plates of unequal thickness
Figure 27 : Example
of acceptable lap-joints
- If plates of unequal thickness are butt-welded
and the difference between thicknesses is more than 3 mm, the thicker
plate is to be smoothly tapered for a length equal to at least four
times the offset, including the width of the weld. For longitudinal
joints the tapering is to be made symmetrically on both sides of
the plate in order to obtain alignment of middle lines.
- If the joint is to undergo radiographic examination, the thickness
of the thicker plate is to be reduced to that of the thinner plate
next to the joint and for a length of at least 30 mm.
Details (b) and (c) may be used only for pressure vessels
having internal diameter less than 600mm.Figure 28 : Types
of joints for unstayed flat heads (1)
4.2.4 Dished heads
- For connection of a hemispherical end with
a cylindrical shell, the joint is to be arranged in a plane parallel
to that of the largest circle perpendicular to the axis of the shell
and at such a distance from this plane that the tapering of the
shell made as indicated in [2.5.6] is wholly in the hemisphere.
- For torispherical ends made of parts assembled by welding,
no welded joint is normally admitted along a parallel in the knuckle
nor at a distance less than 50 mm from the beginning of the knuckle.
4.2.5 Welding locationThe location of main welded joints is to be chosen so
that these joints are not submitted to appreciable bending stresses.
4.2.6 Accessories and nozzles
- Attachment of accessories by welds crossing
main welds or located near such welds is to be avoided; where this
is impracticable, welds for attachment of accessories are to completely
cross the main welds rather than stop abruptly on or near them.
- Openings crossing main joints or located near main joints
are also to be avoided as far as possible.Figure 29 : Types
of joints for unstayed flat heads (2)
Figure 30 : Types
of joints for nozzles and reinforced rings (1)
Figure 31 : Types
of joints for nozzles and reinforcing rings (2)
- Doubling plates for attachment of accessories such as fixing
lugs or supports are to be of sufficient size to ensure an adequate
distribution of loads on pressure parts; such doubling plates are
to have well rounded corners. Attachment of accessories such as
ladders and platforms directly on the walls of vessels such that
they restrain their free contraction or expansion is to be avoided.
- Welded connections of nozzles and other fittings, either with
or without local compensation, are to be of a suitable type, size
and preparation in accordance with the approved plans.
4.2.7 Connections of stays
to tube platesFigure 34 : Types
of joints for flanges to nozzles
Figure 32 : Types
of joints for nozzles and reinforcing rings (3)
- Where stays are welded, the cross-sectional
area of the weld is to be at least 1,25 times the cross-section
of the stay.
- The cross-sectional area of the end welding of welded stay
tubes is to be not less than 1,25 times the cross-sectional area
of the stay tube.
Figure 33 : Types
of joints for nozzles (4)
Note: Where preparations of Fig 33 are
carried out, the shell is to be carefully inspected to ascertain
the absence of lamination.
Figure 35 : Types
of joints for tubesheets to shell (1)
Figure 36 : Types
of joints for tubesheets to shells (2)
Figure 37 : Type
of joints for stays and stay tubes
4.2.8 Type of weldings Fig
36 and Fig
37 indicate the type and size of weldings of typical pressure
vessel connections. Any alternative type of welding or size is to
be the subject of special consideration by the Society.
Next: 4.3 Miscellaneous requirements
for fabrication and welding