The required capacity of the relief devices shall be calculated in accordance with a well-established
technical code recognized by the competent authority7.
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;
7 See for example CGA S-1.2-2003 "Pressure Relief Device Standards - Part 2 - Cargo and Portable Tanks for
(c) The reference temperature corresponding to the rated pressure for frangible discs;
(d) The rated flow capacity of the device in standard cubic meters 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 the requirements of 126.96.36.199 are always fulfilled. 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. Pipework to vent the vapour or liquid from the outlet of 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
refrigerated liquefied gases, the escaping vapour shall be directed away from the tank and in such a
manner that it cannot impinge upon the tank. 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 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.
A connection for a vacuum gauge shall be provided in the jacket of a vacuum-insulated portable tank.
Portable tank supports, frameworks, lifting and tie-down attachments
Portable tanks shall be designed and constructed with a support structure to provide a secure base
during carriage. The forces specified in 188.8.131.52.12 and the safety factor specified in 184.108.40.206.13 shall be
considered in this aspect of the design. Skids, frameworks, cradles or other similar structures are
The combined stresses caused by portable tank mountings (e.g. cradles, frameworks, etc.) and portable
tank lifting and tie-down attachments shall not cause excessive stress in any portion of the tank.
Permanent lifting and tie-down attachments shall be fitted to all portable tanks. Preferably they shall
be fitted to the portable tank supports but may be secured to reinforcing plates located on the tank at
the points of support.