Every portable tank with a capacity not less than 1 900 litres and every independent compartment of a
portable tank with a similar capacity, shall be provided with one or more pressure-relief devices of the
spring-loaded type and may in addition have a frangible disc or fusible element in parallel with the
spring-loaded devices except when prohibited by reference to 6.7.2.8.3 in the applicable portable tank
instruction in 4.2.5.2.6. The pressure-relief devices shall have sufficient capacity to prevent rupture of
the shell due to over pressurization or vacuum resulting from filling, discharging, or from heating of
the contents.

Pressure-relief devices shall be designed to prevent the entry of foreign matter, the leakage of liquid
and the development of any dangerous excess pressure.

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, portable tanks shall have a pressure-relief
device approved by the competent authority. Unless a portable tank in dedicated service is fitted with
an approved relief device constructed of materials compatible with the substance carried, the relief
device shall comprise a frangible disc preceding a spring-loaded pressure-relief device. When a
frangible disc is inserted in series with the required pressure-relief device, the space between the
frangible disc and the pressure-relief device shall be provided with a pressure gauge or suitable
tell-tale indicator for the detection of disc rupture, pinholing, or leakage which could cause a
malfunction of the pressure-relief system. The frangible disc shall rupture at a nominal pressure 10%
above the start to discharge pressure of the relief device.

Every portable tank with a capacity less than 1 900 litres shall be fitted with a pressure-relief device
which may be a frangible disc when this disc complies with the requirements of 6.7.2.11.1. When no
spring-loaded pressure-relief device is used, the frangible disc shall be set to rupture at a nominal
pressure equal to the test pressure. In addition, fusible elements conforming to 6.7.2.10.1 may also be
used.

When the shell is fitted for pressure discharge, the inlet line shall be provided with a suitable
pressure-relief device set to operate at a pressure not higher than the MAWP of the shell, and a
stop-valve shall be fitted as close to the shell as reasonably practicable.

Setting of pressure-relief devices

It shall be noted that the pressure-relief devices shall operate only in conditions of excessive rise in
temperature, since the shell shall not be subject to undue fluctuations of pressure during normal
conditions of carriage (see 6.7.2.12.2).

The required pressure-relief device shall be set to start-to-discharge at a nominal pressure of
five-sixths of the test pressure for shells having a test pressure of not more than 4.5 bar and 110% of
two-thirds of the test pressure for shells having a test pressure of more than 4.5 bar. After discharge
the device shall close at a pressure not more than 10% below the pressure at which the discharge
starts. The device shall remain closed at all lower pressures. This requirement does not prevent the use
of vacuum-relief or combination pressure-relief and vacuum-relief devices.

Fusible elements

Fusible elements shall operate at a temperature between 100 °C and 149 °C on condition that the
pressure in the shell at the fusing temperature will be not more than the test pressure. They shall be
placed at the top of the shell with their inlets in the vapour space and when used for transport safety
purposes, they shall not be shielded from external heat. Fusible elements shall not be used on portable
tanks with a test pressure which exceeds 2.65 bar unless specified by special provision TP36 in
Column (11) of Table A of Chapter 3.2. Fusible elements used on portable tanks intended for the
carriage of elevated temperature substances shall be designed to operate at a temperature higher than
the maximum temperature that will be experienced during carriage and shall be to the satisfaction of
the competent authority or its authorized body.

Frangible discs

Except as specified in 6.7.2.8.3, frangible discs shall be set to rupture at a nominal pressure equal to
the test pressure throughout the design temperature range. Particular attention shall be given to the
requirements of 6.7.2.5.1 and 6.7.2.8.3 if frangible discs are used.

Frangible discs shall be appropriate for the vacuum pressures which may be produced in the portable
tank.

Capacity of pressure-relief devices

The spring-loaded pressure-relief device required by 6.7.2.8.1 shall have a minimum cross sectional
flow area equivalent to an orifice of 31.75 mm diameter. Vacuum-relief devices, when used, shall
have a cross sectional flow area not less than 284 mm2.
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