IMDG Code has three arrangements for bottom openings in portable tanks

1. Two serially mounted means of closure
2. Three serially mounted means of closure
3. Not Allowed

When bottom openings are prohibited, there must be no openings below the liquid level of the shell when it is filled to its maximum permissible filling limit. If an existing opening is closed, it must be done by internally and externally welding one plate to the shell.

Bottom openings are not permitted for packing group I and certain packing group II substances which are highly corrosive to steel or aluminium. These are indicated through tank instruction in column 13 of dangerous goods list in chapter 3.2 of IMDG Code.

Below table indicates bottom opening provisions for tank types             T1 to T22

Two serially mounted means of closure

1. 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

• A liquid-tight closure at the end of the discharge pipe, which may be a bolted blank flange or a screw cap.

Three serially mounted means of closure

1. 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: (For a lined shell this may be replaced by an additional external stop-valve)
2. the control devices for the operation of the valve are designed so as to prevent any unintended opening through impact or other inadvertent act;
3. the valve may be operable from above or below;
4. if possible, the setting of the valve (open or closed) shall be capable of being verified from the ground;
5. except for portable tanks having a capacity of not more than 1000 L, it shall be possible to close the valve from an accessible position of the portable tank that is remote from the valve itself; and
6. the valve shall continue to be effective in the event of damage to the external device for controlling the operation of the valve;
7. an external stop-valve fitted as close to the shell as reasonably practicable; and
8. a liquid-tight closure at the end of the discharge pipe, which may be a bolted blank flange or a screw cap.