Requirements for the design, construction, inspection and testing of portable tanks intended for the carriage of non-refrigerated liquefied gases

NOTE: These requirements also apply to portable tanks intended for the carriage of chemicals under
pressure (UN Nos. 3500, 3501, 3502, 3503, 3504 and 3505).

Definitions

For the purposes of this section:
Alternative arrangement means an approval granted by the competent authority for a portable tank or
MEGC that has been designed, constructed or tested to technical requirements or testing methods
other than those specified in this Chapter;
Portable tank means a multimodal tank having a capacity of more than 450 litres used for the carriage
of non-refrigerated liquefied gases of Class 2. The portable tank includes a shell fitted with service
equipment and structural equipment necessary for the carriage of gases. The portable tank shall be
capable of being filled and discharged without the removal of its structural equipment. It shall possess
stabilizing members external to the shell, and shall be capable of being lifted when full. It shall be
designed primarily to be loaded onto a vehicle, wagon or sea-going or inland navigation vessel and
shall be equipped with skids, mountings or accessories to facilitate mechanical handling. Tankvehicles,
tank-wagons, non-metallic tanks, intermediate bulk containers (IBCs), gas cylinders and
large receptacles are not considered to fall within the definition for portable tanks;
Shell means the part of the portable tank which retains the non-refrigerated liquefied gas intended for
carriage (tank proper), including openings and their closures, but does not include service equipment
or external structural equipment;
Service equipment means measuring instruments and filling, discharge, venting, safety and insulating
devices;
Structural equipment means the reinforcing, fastening, protective and stabilizing members external to
the shell;
Maximum allowable working pressure (MAWP) means a pressure that shall be not less than the
highest of the following pressures measured at the top of the shell while in operating position, but in
no case less than 7 bar:
(a) The maximum effective gauge pressure allowed in the shell during filling or discharge; or
(b) The maximum effective gauge pressure to which the shell is designed, which shall be:
(i) for a non-refrigerated liquefied gas listed in the portable tank instruction T50
in 4.2.5.2.6, the MAWP (in bar) given in T50 portable tank instruction for that gas;
(ii) for other non-refrigerated liquefied gases, not less than the sum of:
- the absolute vapour pressure (in bar) of the non-refrigerated liquefied gas at the
design reference temperature minus 1 bar; and
- the partial pressure (in bar) of air or other gases in the ullage space
being determined by the design reference temperature and the liquid phase
expansion due to an increase of the mean bulk temperature of tr -tf (tf = filling
temperature, usually 15 °C, tr = maximum mean bulk temperature, 50 °C);
(iii) for chemicals under pressure, the MAWP (in bar) given in T50 portable tank instruction
for the liquefied gas portion of the propellants listed in T50 in 4.2.5.2.6;
Design pressure means the pressure to be used in calculations required by a recognized pressure
vessel code. The design pressure shall be not less than the highest of the following pressures:
(a) The maximum effective gauge pressure allowed in the shell during filling or discharge; or
(b) The sum of:
 
(i) the maximum effective gauge pressure to which the shell is designed as defined in (b) of
the MAWP definition (see above); and
(ii) a head pressure determined on the basis of the static forces specified in 6.7.3.2.9, but not
less than 0.35 bar;
Test pressure means the maximum gauge pressure at the top of the shell during the pressure test;
Leakproofness test means a test using gas subjecting the shell and its service equipment to an effective
internal pressure of not less than 25% of the MAWP;
Maximum permissible gross mass (MPGM) means the sum of the tare mass of the portable tank and
the heaviest load authorized for carriage;
Reference steel means a steel with a tensile strength of 370 N/mm2 and an elongation at fracture
of 27%;
Mild steel means a steel with a guaranteed minimum tensile strength of 360 N/mm2 to 440 N/mm2 and
a guaranteed minimum elongation at fracture conforming to 6.7.3.3.3.3;
Design temperature range for the shell shall be -40 °C to 50 °C for non-refrigerated liquefied gases
carried under ambient conditions. More severe design temperatures shall be considered for portable
tanks subjected to severe climatic conditions;
Design reference temperature means the temperature at which the vapour pressure of the contents is
determined for the purpose of calculating the MAWP. The design reference temperature shall be less
than the critical temperature of the non-refrigerated liquefied gas or liquefied gas propellants of
chemicals under pressure intended to be carried to ensure that the gas at all times is liquefied. This
value for each portable tank type is as follows:
(a) Shell with a diameter of 1.5 metres or less: 65 °C;
(b) Shell with a diameter of more than 1.5 metres:
(i) without insulation or sun shield: 60 °C;
(ii) with sun shield (see 6.7.3.2.12): 55 °C; and
(iii) with insulation (see 6.7.3.2.12) : 50 °C;
Filling density means the average mass of non-refrigerated liquefied gas per litre of shell capacity
(kg/l). The filling density is given in portable tank instruction T50 in 4.2.5.2.6.

General design and construction requirements

Shells shall be designed and constructed in accordance with the requirements of a pressure vessel code
recognized by the competent authority. Shells shall be made of steel suitable for forming. The
materials shall in principle conform to national or international material standards. For welded shells,
only a material whose weldability has been fully demonstrated shall be used. Welds shall be skilfully
made and afford complete safety. When the manufacturing process or the materials make it necessary,
the shells shall be suitability heat-treated to guarantee adequate toughness in the weld and in the heat
affected zones. In choosing the material the design temperature range shall be taken into account with
respect to risk of brittle fracture, to stress corrosion cracking and to resistance to impact. When fine
grain steel is used, the guaranteed value of the yield strength shall be not more than 460 N/mm2 and
the guaranteed value of the upper limit of the tensile strength shall be not more than 725 N/mm2
according to the material specification. Portable tank materials shall be suitable for the external
environment in which they may be carried.

Portable tank shells, fittings and pipework shall be constructed of materials which are:
(a) Substantially immune to attack by the non-refrigerated liquefied gas(es) intended to be
carried; or
(b) Properly passivated or neutralized by chemical reaction.

Gaskets shall be made of materials compatible with the non-refrigerated liquefied gas(es) intended to
be carried.

Contact between dissimilar metals which could result in damage by galvanic action shall be avoided.

The materials of the portable tank, including any devices, gaskets, and accessories, shall not adversely
affect the non-refrigerated liquefied gas(es) intended for carriage in the portable tank.

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.

Shells shall be designed to withstand an external pressure of at least 0.4 bar (gauge pressure) above
the internal pressure without permanent deformation. When the shell is to be subjected to a significant
vacuum before filling or during discharge it shall be designed to withstand an external pressure of at
least 0.9 bar (gauge pressure) above the internal pressure and shall be proven at that pressure.

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 6.7.3.2.9, the safety factor to be observed shall be as follows:
(a) For steels having a clearly defined yield point, a safety factor of 1.5 in relation to the
guaranteed yield strength; or
(b) For steels 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
and 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 steel in question, the value of yield strength or proof strength used shall be approved by the
competent authority.

thermal insulation, the thermal insulation systems shall satisfy the following requirements:
(a) It shall consist of a shield covering not less than the upper third but not more than the upper
half of the surface of the shell and separated from the shell by an air space about 40 mm across;
(b) It shall consist of a complete cladding of adequate thickness of insulating materials protected
so as to prevent the ingress of moisture and damage under normal conditions of carriage and so
as to provide a thermal conductance of not more than 0.67 (W.m-2.K-1);
(c) When the protective covering is so closed as to be gas-tight, a device shall be provided to
prevent any dangerous pressure from developing in the insulating layer in the event of
inadequate gas tightness of the shell or of its items of equipment; and
 
1 For calculation purposes g = 9.81 m/s2.
 
(d) The thermal insulation shall not inhibit access to the fittings and discharge devices.
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