Welded Steel Tanks for Oil Storage

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3-2 API STANDARD 650
3.1.5.4 Lap-Welded Bottom Joints
Lap-welded bottom plates shall be reasonably rectangular
and square edged. Three-plate laps in tank bottoms shall be at
least 300 mm (12 in.) from each other, from the tank shell,
from butt-welded annular-plate joints, and from joints
Single-V butt joint
between annular plates and the bottom. Lapping of two bottom
plates on the butt-welded annular plates does not consti-
;; ;; Single-U butt joint
tute a three-plate lap weld. When annular plates are used or
are required by 3.5.1, they shall be butt-welded and shall have
a radial width that provides at least 600 mm (24 in.) between
the inside of the shell and any lap-welded joint in the remainder
of the bottom. Bottom plates need to be welded on the top
side only, with a continuous full-fillet weld on all seams.
Double-V butt joint
Unless annular bottom plates are used, the bottom plates
under the bottom shell ring shall have the outer ends of ;;
the
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joints fitted and lap-welded to form a smooth bearing for the
shell plates, as shown in Figure 3-3B.
3.1.5.5 Butt-Welded Bottom Joints
Butt-welded bottom plates shall have their parallel edges
prepared for butt welding with either square or V grooves. Square-groove butt joint
Double-U butt joint
Butt-welds shall be made using an appropriate weld joint configuration
that yields a complete penetration weld. Typical Note: See 3.1.5.2 for specific requirements for vertical shell joints.
permissible bottom butt-welds without a backing strip are the
same as those shown in Figure 3-1. The use of a backing strip
at least 3 mm ( 1 /8 in.) thick tack welded to the underside of the
Figure 3-1—Typical Vertical Shell Joints
plate is permitted. Butt-welds using a backing strip are shown
in Figure 3-3A. If square grooves are employed, the root openings
shall not be less than 6 mm ( 1 /4 in.). A metal spacer shall
Optional
be used to maintain the root opening between the adjoining
outside angle
plate edges unless the manufacturer submits another method
of butt-welding the bottom for the purchaser’s approval.
Three-plate joints in the tank bottom shall be at least 300 mm
(12 in.) from each other and from the tank shell.
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3.1.5.6 Bottom Annular-Plate Joints
Bottom annular-plate radial joints shall be butt-welded in
accordance with 3.1.5.5 and shall have complete penetration
and complete fusion. The backing strip, if used, shall be compatible
for welding the annular plates together.
3.1.5.7 Shell-to-Bottom Fillet Welds
a. For bottom and annular plates with a nominal thickness
12.5 mm ( 1 /2 in.), and less, the attachment between the bottom
edge of the lowest course shell plate and the bottom plate
shall be a continuous fillet weld laid on each side of the shell
plate. The size of each weld shall not be more than 12.5 mm
( 1 /2 in.) and shall not be less than the nominal thickness of the
thinner of the two plates joined (that is, the shell plate or the
bottom plate immediately under the shell) or less than the following
values:
Nominal Thickness
of Shell Plate
Minimum Size of
Fillet Weld
(mm) (in.) (mm) (in.)
5 0.1875 5 3 /16
> 5 to 20 > 0.1875 to 0.75 6 1 /4
> 20 to 32 > 0.75 to 1.25 8 5 /16
> 32 to 45 > 1.25 to 1.75 10 3 /8
Angle-to-shell
butt joint
complete penetration
Single-bevel
butt joint
complete penetration
Alternative
angle-to-shell joint
Square-groove
butt joint
complete penetration
Double-bevel
butt joint
complete penetration
Note: See 3.1.5.3 for specific requirements for horizontal
shell joints.
Figure 3-2—Typical Horizontal Shell Joints