For sections of piping which can be closed at both ends and where liquid product can be trapped, a
method of automatic pressure-relief shall be provided to prevent excess pressure build-up within the
piping.

Each connection on a closed cryogenic receptacle shall be clearly marked to indicate its function (e.g.
vapour or liquid phase).

Pressure-relief devices

Every closed cryogenic receptacle shall be provided with at least one pressure-relief device. The
pressure-relief device shall be of the type that will resist dynamic forces including surge.

Closed cryogenic receptacles may, in addition, have a frangible disc in parallel with the spring loaded
device(s) in order to meet the requirements of 6.2.1.3.6.5.

Connections to pressure-relief devices shall be of sufficient size to enable the required discharge to
pass unrestricted to the pressure-relief device.

All pressure-relief device inlets shall under maximum filling conditions be situated in the vapour
space of the closed cryogenic receptacle and the devices shall be so arranged as to ensure that the
escaping vapour is discharged unrestrictedly.

Capacity and setting of pressure-relief devices

NOTE: In relation to pressure-relief devices of closed cryogenic receptacles, maximum allowable
working pressure (MAWP) means the maximum effective gauge pressure permissible at the top of a
loaded closed cryogenic receptacle in its operating position including the highest effective pressure
during filling and discharge.

The pressure-relief device shall open automatically at a pressure not less than the MAWP and be fully
open at a pressure equal to 110% of the MAWP. It 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.

Frangible discs shall be set to rupture at a nominal pressure which is the lower of either the test
pressure or 150% of the MAWP.

In the case of the loss of vacuum in a vacuum-insulated closed cryogenic receptacle the combined
capacity of all pressure-relief devices installed shall be sufficient so that the pressure (including
accumulation) inside the closed cryogenic receptacle does not exceed 120% of the MAWP.

The required capacity of the pressure-relief devices shall be calculated in accordance with an
established technical code recognized by the competent authority1.

Approval of pressure receptacles

The conformity of pressure receptacles shall be assessed at time of manufacture as required by the
competent authority. Pressure receptacles shall be inspected, tested and approved by an inspection
body. The technical documentation shall include full specifications on design and construction, and
full documentation on the manufacturing and testing.

Quality assurance systems shall conform to the requirements of the competent authority.
 
1 See for example CGA Publications S-1.2-2003 "Pressure Relief Device Standards-Part 2-Cargo and Portable
Tanks for Compressed Gases" and S-1.1-2003 "Pressure Relief Device Standards-Part 1-Cylinders for Compressed
Gases".
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