A reference holding time shall be determined for each refrigerated liquefied gas intended for carriage
in a portable tank.

The reference holding time shall be determined by a method recognized by the competent authority on
the basis of the following:
(a) The effectiveness of the insulation system, determined in accordance with 6.7.4.2.8.2;
(b) The lowest set pressure of the pressure limiting device(s);
(c) The initial filling conditions;
(d) An assumed ambient temperature of 30 °C;
(e) The physical properties of the individual refrigerated liquefied gas intended to be carried.

The effectiveness of the insulation system (heat influx in watts) shall be determined by type testing the
portable tank in accordance with a procedure recognized by the competent authority. This test shall
consist of either:
(a) A constant pressure test (for example at atmospheric pressure) when the loss of refrigerated
liquefied gas is measured over a period of time; or
(b) A closed system test when the rise in pressure in the shell is measured over a period of time.
When performing the constant pressure test, variations in atmospheric pressure shall be taken into
account. When performing either tests corrections shall be made for any variation of the ambient
temperature from the assumed ambient temperature reference value of 30 °C.
NOTE: For the determination of the actual holding time before each journey, refer to 4.2.3.7.

 

The jacket of a vacuum-insulated double-wall tank shall have either an external design pressure not
less than 100 kPa (1 bar) (gauge pressure) calculated in accordance with a recognized technical code
or a calculated critical collapsing pressure of not less than 200 kPa (2 bar) (gauge pressure). Internal
and external reinforcements may be included in calculating the ability of the jacket to resist the
external pressure.

Portable tanks shall be designed and constructed with supports to provide a secure base during
carriage and with suitable lifting and tie-down attachments.

Portable tanks shall be designed to withstand, without loss of contents, at least the internal pressure
due to the contents, and the static, dynamic and thermal loads during normal conditions of handling
and carriage. The design shall demonstrate that the effects of fatigue, caused by repeated application
of these loads through the expected life of the portable tank, have been taken into account.

Portable tanks and their fastenings under the maximum permissible load shall 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 6.7.4.2.12, the safety factor to be observed shall be as follows:
(a) For materials having a clearly defined yield point, a safety factor of 1.5 in relation to the
guaranteed yield strength; and
(b) For materials with no clearly defined yield point, a safety factor of 1.5 in relation to the
guaranteed 0.2% proof strength or, in case of 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 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, or when nonmetallic
materials are used the values of yield strength or proof strength shall be approved by the
competent authority.

Portable tanks intended for the carriage of flammable refrigerated liquefied gases shall be capable of
being electrically earthed.

Design criteria

Shells shall be of a circular cross section.

Shells shall be designed and constructed to withstand a test pressure not less than 1.3 times the
MAWP. For shells with vacuum insulation the test pressure shall not be less than 1.3 times the sum of
the MAWP and 100 kPa (1 bar). In no case shall the test pressure be less than 300 kPa (3 bar) (gauge
 
1 For calculation purposes g = 9.81 m/s2.
 
pressure). Attention is drawn to the minimum shell thickness requirements, specified in 6.7.4.4.2
to 6.7.4.4.7.

 

For metals exhibiting a clearly defined yield point or characterized by a guaranteed proof strength
(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.
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