For portable tanks that are intended for use offshore, the dynamic stresses imposed by handling in
open seas shall be taken into account.

A shell which is to be equipped with a vacuum-relief device shall be designed to withstand, without
permanent deformation, an external pressure of not less than 0.21 bar above the internal pressure. The
vacuum-relief device shall be set to relieve at a vacuum setting not greater than minus (-) 0.21 bar
unless the shell is designed for a higher external over pressure, in which case the vacuum-relief
pressure of the device to be fitted shall be not greater than the tank design vacuum pressure. A shell
used for the carriage of solid substances (powdery or granular) of packing groups II or III only, which
do not liquefy during carriage, may be designed for a lower external pressure, subject to the approval
of the competent authority. In this case, the vacuum valve shall be set to relieve at this lower pressure.
A shell that is not to be fitted with a vacuum-relief device shall be designed to withstand, without
permanent deformation an external pressure of not less than 0.4 bar above the internal pressure.

Vacuum-relief devices used on portable tanks intended for the carriage of substances meeting the
flash-point criteria of Class 3, including elevated temperature substances carried at or above their
flash-point, shall prevent the immediate passage of flame into the shell, or the portable tank shall have
a shell capable of withstanding, without leakage an internal explosion resulting from the passage of
flame into the shell.

Portable tanks and their fastenings shall, under the maximum permissible load, be capable of
absorbing the following separately applied static forces:
(a) In the direction of travel: twice the MPGM multiplied by the acceleration due to gravity (g)1;
(b) Horizontally at right angles to the direction of travel: the MPGM (when the direction of travel
is not clearly determined, the forces shall be equal to twice the MPGM) multiplied by the
acceleration due to gravity (g)1;
(c) Vertically upwards: the MPGM multiplied by the acceleration due to gravity (g) 1; and
(d) Vertically downwards: twice the MPGM (total loading including the effect of gravity)
multiplied by the acceleration due to gravity (g)1.

Under each of the forces in 6.7.2.2.12, the safety factor to be observed shall be as follows:
(a) For metals having a clearly defined yield point, a safety factor of 1.5 in relation to the
guaranteed yield strength; or
(b) For metals with no clearly defined yield point, a safety factor of 1.5 in relation to the
guaranteed 0.2% proof strength and, for austenitic steels, the 1% proof strength.

The values of yield strength or proof strength shall be the values according to national or international
material standards. When austenitic steels are used, the specified minimum values of yield strength or
proof strength according to the material standards may be increased by up to 15% when these greater
values are attested in the material inspection certificate. When no material standard exists for the
metal in question, the value of yield strength or proof strength used shall be approved by the
competent authority.

Portable tanks shall be capable of being electrically earthed when intended for the carriage of
substances meeting the flash-point criteria of Class 3 including elevated temperature substances
carried at or above their flash-point. Measures shall be taken to prevent dangerous electrostatic
discharge.

When required for certain substances by 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, portable tanks
shall be provided with additional protection, which may take the form of additional shell thickness or
a higher test pressure, the additional shell thickness or higher test pressure being determined in the
light of the inherent risks associated with the carriage of the substances concerned.

Thermal insulation directly in contact with the shell intended for substances carried at elevated
temperature shall have an ignition temperature at least 50 °C higher than the maximum design
temperature of the tank.
 
For calculation purposes g = 9.81 m/s2.

Design criteria

Shells shall be of a design capable of being stress-analysed mathematically or experimentally by
resistance strain gauges, or by other methods approved by the competent authority.

Shells shall be designed and constructed to withstand a hydraulic test pressure not less than 1.5 times
the design pressure. Specific requirements are laid down for certain substances 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. Attention is drawn to the minimum shell thickness requirements specified in
6.7.2.4.1 to 6.7.2.4.10.

(0.2% proof strength, generally, or 1% proof strength for austenitic steels) the primary membrane
stress σ (sigma) in the shell shall not exceed 0.75 Re or 0.50 Rm, whichever is lower, at the test
pressure, where:
Re = yield strength in N/mm2, or 0.2% proof strength or, for austenitic steels, 1% proof
strength;
Rm = minimum tensile strength in N/mm2.

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.
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