The values of Re and Rm to be used shall be the specified minimum values according to national or
international material standards. When austenitic steels are used, the specified minimum values for Re
and Rm according to the material standards may be increased by up to 15% when greater values are
attested in the material inspection certificate. When no material standard exists for the metal in
question, the values of Re and Rm used shall be approved by the competent authority or its authorized
body.

Steels which have a Re/Rm ratio of more than 0.85 are not allowed for the construction of welded
shells. The values of Re and Rm to be used in determining this ratio shall be the values specified in
the material inspection certificate.

Steels used in the construction of shells shall have an elongation at fracture, in %, of not less
than 10 000/Rm with an absolute minimum of 16% for fine grain steels and 20% for other steels.
Aluminium and aluminium alloys used in the construction of shells shall have an elongation at
fracture, in %, of not less than 10 000/6Rm with an absolute minimum of 12%.

For the purpose of determining actual values for materials, it shall be noted that for sheet metal, the
axis of the tensile test specimen shall be at right angles (transversely) to the direction of rolling. The
permanent elongation at fracture shall be measured on test specimens of rectangular cross sections in
accordance with ISO 6892:1998 using a 50 mm gauge length.

Minimum shell thickness

The minimum shell thickness shall be the greater thickness based on:
(a) The minimum thickness determined in accordance with the requirements of 6.7.2.4.2
to 6.7.2.4.10;
(b) The minimum thickness determined in accordance with the recognized pressure vessel code
including the requirements in 6.7.2.3; and
(c) The minimum thickness specified in the applicable portable tank instruction indicated in
Column (10) of Table A of Chapter 3.2 and described in 4.2.5.2.6 or by a portable tank special
provision indicated in Column (11) of Table A of Chapter 3.2 and described in 4.2.5.3.

The cylindrical portions, ends (heads) and manhole covers of shells not more than 1.80 m in diameter
shall be not less than 5 mm thick in the reference steel or of equivalent thickness in the metal to be
used. Shells more than 1.80 m in diameter shall be not less than 6 mm thick in the reference steel or of
equivalent thickness in the metal to be used, except that for powdered or granular solid substances of
packing group II or III the minimum thickness requirement may be reduced to not less than 5 mm
thick in the reference steel or of equivalent thickness in the metal to be used.

When additional protection against shell damage is provided, portable tanks with test pressures less
than 2.65 bar may have the minimum shell thickness reduced, in proportion to the protection provided,
as approved by the competent authority. However, shells not more than 1.80 m in diameter shall be
not less than 3 mm thick in the reference steel or of equivalent thickness in the metal to be used.
Shells more than 1.80 m in diameter shall be not less than 4 mm thick in the reference steel or of
equivalent thickness in the metal to be used.

The cylindrical portions, ends (heads) and manhole covers of all shells shall be not less than 3 mm
thick regardless of the material of construction.

The additional protection referred to in 6.7.2.4.3 may be provided by overall external structural
protection, such as suitable "sandwich" construction with the outer sheathing (jacket) secured to the
shell, double wall construction or by enclosing the shell in a complete framework with longitudinal
and transverse structural members.

In no case shall the wall thickness be less than that prescribed in 6.7.2.4.2, 6.7.2.4.3 and 6.7.2.4.4. All
parts of the shell shall have a minimum thickness as determined by 6.7.2.4.2 to 6.7.2.4.4. This
thickness shall be exclusive of any corrosion allowance.

When mild steel is used (see 6.7.2.1), calculation using the formula in 6.7.2.4.6 is not required.

There shall be no sudden change of plate thickness at the attachment of the ends (heads) to the
cylindrical portion of the shell.
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