Dangerous Goods by Road
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
in mm².
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 6.7.5.5. 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.
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 MEGC overturning.
Gauging devices
When an MEGC is intended to be filled by mass, it shall be equipped with one or more gauging
devices. Level-gauges made of glass or other fragile material shall not be used.
MEGC supports, frameworks, lifting and tie-down attachments
MEGCs shall be designed and constructed with a support structure to provide a secure base during
carriage. The forces specified in 6.7.5.2.8 and the safety factor specified in 6.7.5.2.10 shall be
considered in this aspect of the design. Skids, frameworks, cradles or other similar structures are
acceptable.
The combined stresses caused by element mountings (e.g. cradles, frameworks, etc.) and MEGC
lifting and tie-down attachments shall not cause excessive stress in any element. Permanent lifting and
tie-down attachments shall be fitted to all MEGCs. In no case shall mountings or attachments be
welded onto the elements.