The elements of MEGCs used for the carriage of UN No. 1013 carbon dioxide and UN No. 1070
nitrous oxide shall be divided into groups of not more than 3 000 litres each isolated by a valve. Each
group shall be fitted with one or more pressure relief devices. If so required by the competent
authority of the country of use, MEGCs for other gases shall be fitted with pressure relief devices as
specified by that competent authority.
When pressure relief devices are fitted, every element or group of elements of an MEGC that can be
isolated shall then be fitted with one or more pressure relief devices. Pressure relief devices shall be of
a type that will resist dynamic forces including liquid surge and shall be designed to prevent the entry
of foreign matter, the leakage of gas and the development of any dangerous excess pressure.
MEGCs used for the carriage of certain non-refrigerated gases identified in portable tank instruction
T50 in 126.96.36.199.6 may have a pressure-relief device as required by the competent authority of the
country of use. Unless an MEGC in dedicated service is fitted with an approved pressure relief device
constructed of materials compatible with the gas carried, such a device shall comprise a frangible disc
preceding a spring-loaded device. The space between the frangible disc and the spring-loaded device
may be equipped with a pressure gauge or a suitable telltale indicator. This arrangement permits the
detection of disc rupture, pinholing or leakage which could cause a malfunction of the pressure relief
device. The frangible disc shall rupture at a nominal pressure 10% above the start-to-discharge
pressure of the spring-loaded device.
In the case of multi-purpose MEGCs used for the carriage of low-pressure liquefied gases, the
pressure-relief devices shall open at a pressure as specified in 188.8.131.52.1 for the gas having the highest
maximum allowable working pressure of the gases allowed to be carried in the MEGC.
Capacity of pressure relief devices
The combined delivery capacity of the pressure relief devices when fitted shall be sufficient that, in
the event of total fire engulfment of the MEGC, the pressure (including accumulation) inside the
elements does not exceed 120% of the set pressure of the pressure relief device. The formula provided
in CGA S-1.2-2003 "Pressure Relief Device Standards - Part 2 - Cargo and Portable Tanks for
Compressed Gases" shall be used to determine the minimum total flow capacity for the system of
pressure relief devices. CGA S-1.1-2003 "Pressure Relief Device Standards - Part 1 - Cylinders for
Compressed Gases" may be used to determine the relief capacity of individual elements. Springloaded
pressure relief devices may be used to achieve the full relief capacity prescribed in the case of
low pressure liquefied gases. In the case of multi-purpose MEGCs, 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 the MEGC.
To determine the total required capacity of the pressure relief devices installed on the elements for the
carriage of liquefied gases, the thermodynamic properties of the gas shall be considered (see, for
example, CGA S-1.2-2003 "Pressure Relief Device Standards - Part 2 - Cargo and Portable Tanks for
Compressed Gases" for low pressure liquefied gases and CGA S-1.1-2003 "Pressure Relief Device
Standards - Part 1 - Cylinders for Compressed Gases" for high pressure liquefied gases).
Marking of pressure-relief devices
Pressure relief devices shall be clearly and permanently marked with the following:
(a) The manufacturer's name and relevant catalogue number;
(b) The set pressure and/or the set temperature;
(c) The date of the last test.
(d) The cross sectional flow areas of the spring loaded pressure-relief devices and frangible discs
The rated flow capacity marked on spring loaded pressure relief devices for low pressure liquefied
gases 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 pressure relief device. No stop-valve shall be installed between the element
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 184.108.40.206. There shall be no obstruction in an opening leading to or leaving from a vent
or pressure-relief device which might restrict or cut-off the flow from the element to that device. The
opening through all piping and fittings shall have at least the same flow area as the inlet of the
pressure relief device to which it is connected. The nominal size of the discharge piping shall be at
least as large as that of the pressure relief device outlet. Vents from the pressure-relief devices, when
used, shall deliver the relieved vapour or liquid to the atmosphere in conditions of minimum backpressure
on the relieving device.
Siting of pressure-relief devices
Each pressure relief device shall, under maximum filling conditions, be in communication with the
vapour space of the elements for the carriage of liquefied gases. The devices, when fitted, shall be so
arranged as to ensure that the escaping vapour is discharged upwards and unrestrictedly as to prevent
any impingement of escaping gas or liquid upon the MEGC, its elements or personnel. For flammable,
pyrophoric and oxidizing gases, the escaping gas shall be directed away from the element in such a
manner that it cannot impinge upon the other elements. Heat resistant protective devices which deflect
the flow of gas are permissible provided the required pressure relief device capacity is not reduced.