Bottom openings

Certain substances shall not be carried in portable tanks with bottom openings. When the applicable
portable tank instruction identified in Column (10) of Table A of Chapter 3.2 and described
in 4.2.5.2.6 indicates that bottom openings are prohibited there shall be no openings below the liquid
level of the shell when it is filled to its maximum permissible filling limit. When an existing opening
is closed it shall be accomplished by internally and externally welding one plate to the shell.

Bottom discharge outlets for portable tanks carrying certain solid, crystallizable or highly viscous
substances shall be equipped with not less than two serially fitted and mutually independent shut-off
devices. The design of the equipment shall be to the satisfaction of the competent authority or its
authorized body and shall include:
(a) An external stop-valve, fitted as close to the shell as reasonably practicable, and so designed as
to prevent any unintended opening through impact or other inadvertent act; and
(b) A liquid tight closure at the end of the discharge pipe, which may be a bolted blank flange or a
screw cap.

Every bottom discharge outlet, except as provided in 6.7.2.6.2, shall be equipped with three serially
fitted and mutually independent shut-off devices. The design of the equipment shall be to the
satisfaction of the competent authority or its authorized body and include:
(a) A self-closing internal stop-valve, that is a stop-valve within the shell or within a welded flange
or its companion flange, such that:
(i) The control devices for the operation of the valve are designed so as to prevent any
unintended opening through impact or other inadvertent act;
(ii) The valve may be operable from above or below;
(iii) If possible, the setting of the valve (open or closed) shall be capable of being verified
from the ground;
(iv) Except for portable tanks having a capacity of not more than 1 000 litres, it shall be
possible to close the valve from an accessible position of the portable tank that is remote
from the valve itself; and
(v) The valve shall continue to be effective in the event of damage to the external device for
controlling the operation of the valve;
(b) An external stop-valve fitted as close to the shell as reasonably practicable; and
(c) A liquid tight closure at the end of the discharge pipe, which may be a bolted blank flange or a
screw cap.

For a lined shell, the internal stop-valve required by 6.7.2.6.3 (a) may be replaced by an additional
external stop-valve. The manufacturer shall satisfy the requirements of the competent authority or its
authorized body.

Safety-relief devices

All portable tanks shall be fitted with at least one pressure-relief device. All relief devices shall be
designed, constructed and marked to the satisfaction of the competent authority or its authorized body.

Pressure-relief devices

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).
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