Capacity of relief devices
The combined delivery capacity of the relief devices shall be sufficient that, in the event of total fire
engulfment, the pressure (including accumulation) inside the shell does not exceed 120% of the
MAWP. Spring-loaded relief devices shall be used to achieve the full relief capacity prescribed. In the
case of multi-purpose tanks, the combined delivery capacity of the pressure-relief devices shall be
taken for the gas which requires the highest delivery capacity of the gases allowed to be carried in
To determine the total required capacity of the relief devices, which shall be regarded as being the
sum of the individual capacities of the several devices, the following formula5 shall be used:
Insulation systems, used for the purpose of reducing the venting capacity, shall be approved by the
competent authority or its authorized body. In all cases, insulation systems approved for this purpose
(a) Remain effective at all temperatures up to 649 °C; and
(b) Be jacketed with a material having a melting point of 700 °C or greater.
Marking of pressure-relief devices
Every pressure-relief device shall be plainly and permanently marked with the following particulars:
(a) The pressure (in bar or kPa) at which it is set to discharge;
(b) The allowable tolerance at the discharge pressure for spring-loaded devices;
(c) The reference temperature corresponding to the rated pressure for frangible discs;
(d) The rated flow capacity of the device in standard cubic metres of air per second (m3/s); and
(e) The cross sectional flow areas of the spring loaded pressure-relief devices and frangible discs
When practicable, the following information shall also be shown:
(f) The manufacturer's name and relevant catalogue number of the device.
The rated flow capacity marked on the pressure-relief devices shall be determined according to
ISO 4126-1:2004 and ISO 4126-7:2004.
Connections to pressure-relief devices
Connections to pressure-relief devices shall be of sufficient size to enable the required discharge to
pass unrestricted to the safety device. No stop-valve shall be installed between the shell and the
pressure-relief devices except when duplicate devices are provided for maintenance or other reasons
and the stop-valves serving the devices actually in use are locked open or the stop-valves are
interlocked so that at least one of the duplicate devices is always operable and capable of meeting the
requirements of 126.96.36.199. There shall be no obstruction in an opening leading to a vent or
pressure-relief device which might restrict or cut-off the flow from the shell to that device. Vents from
the pressure-relief devices, when used, shall deliver the relieved vapour or liquid to the atmosphere in
conditions of minimum back-pressure on the relieving device.
Siting of pressure-relief devices
Each pressure-relief device inlet shall be situated on top of the shell in a position as near the
longitudinal and transverse centre of the shell as reasonably practicable. All pressure relief device
inlets shall under maximum filling conditions be situated in the vapour space of the shell and the
devices shall be so arranged as to ensure that the escaping vapour is discharged unrestrictedly. For
flammable non-refrigerated liquefied gases, the escaping vapour shall be directed away from the shell
in such a manner that it cannot impinge upon the shell. Protective devices which deflect the flow of
vapour are permissible provided the required relief-device capacity is not reduced.
Arrangements shall be made to prevent access to the pressure-relief devices by unauthorized persons
and to protect the devices from damage caused by the portable tank overturning.
Unless a portable tank is intended to be filled by weight it shall be equipped with one or more gauging
devices. Glass level-gauges and gauges made of other fragile material, which are in direct
communication with the contents of the shell shall not be used.
Portable tank supports, frameworks, lifting and tie-down attachments