Dangerous Goods by Road
Every shell shall be provided with not less than two independent spring-loaded pressure-relief devices.
The pressure-relief devices shall open automatically at a pressure not less than the MAWP and be
fully open at a pressure equal to 110% of the MAWP. These devices shall, after discharge, close at a
pressure not lower than 10% below the pressure at which discharge starts and shall remain closed at
all lower pressures. The pressure-relief devices shall be of the type that will resist dynamic forces
including surge.
Shells for non-flammable refrigerated liquefied gases and hydrogen may in addition have frangible
discs in parallel with the spring-loaded devices as specified in 6.7.4.7.2 and 6.7.4.7.3.
Pressure-relief devices shall be designed to prevent the entry of foreign matter, the leakage of gas and
the development of any dangerous excess pressure.
Pressure-relief devices shall be approved by the competent authority or its authorized body.
Capacity and setting of pressure-relief devices
In the case of the loss of vacuum in a vacuum-insulated tank or of loss of 20% of the insulation of a
tank insulated with solid materials, the combined capacity of all pressure-relief devices installed shall
be sufficient so that the pressure (including accumulation) inside the shell does not exceed 120% of
the MAWP.
For non-flammable refrigerated liquefied gases (except oxygen) and hydrogen, this capacity may be
achieved by the use of frangible discs in parallel with the required safety-relief devices. Frangible
discs shall rupture at nominal pressure equal to the test pressure of the shell.
Under the circumstances described in 6.7.4.7.1 and 6.7.4.7.2 together with complete fire engulfment
the combined capacity of all pressure-relief devices installed shall be sufficient to limit the pressure in
the shell to the test pressure.
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
Compressed Gases".
(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
in mm².
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 6.7.4.7 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.