Capacity of pressure-relief devices
The spring-loaded pressure-relief device required by 6.7.2.8.1 shall have a minimum cross sectional
flow area equivalent to an orifice of 31.75 mm diameter. Vacuum-relief devices, when used, shall
have a cross sectional flow area not less than 284 mm2.
The combined delivery capacity of the pressure relief system (taking into account the reduction of the
flow when the portable tank is fitted with frangible-discs preceding spring-loaded pressure-relief
devices or when the spring-loaded pressure-relief devices are provided with a device to prevent the
passage of the flame), in condition of complete fire engulfment of the portable tank shall be sufficient
to limit the pressure in the shell to 20% above the start-to-discharge pressure of the pressure limiting
device. Emergency pressure-relief devices may be used to achieve the full relief capacity prescribed.
These devices may be fusible, spring loaded or frangible disc components, or a combination of springloaded
and frangible disc devices. The total required capacity of the relief devices may be determined
using the formula in 6.7.2.12.2.1 or the table in 6.7.2.12.2.3.
To determine the total required capacity of the relief devices, which shall be regarded as being the
sum of the individual capacities of all the contributing devices, the following formula shall be used:
As an alternative to the formula above, shells designed for the carriage of liquids may have their relief
devices sized in accordance with the table in 6.7.2.12.2.3. This table assumes an insulation value of
F = 1 and shall be adjusted accordingly when the shell is insulated. Other values used in determining
this table are:
M = 86,7 T = 394 K
L = 334,94 kJ/kg C = 0,607
Z = 1
Minimum required rate of discharge, Q, in cubic metres per air per second at 1 bar and 0 °C (273 K)
A
Exposed area (square metres)
|
Q
(cubic metres of air per second)
|
A
Exposed area (square metres)
|
Q
(cubic metres of air per second)
|
2
|
0.230
|
37.5
|
2.539
|
3
|
0.320
|
40
|
2.677
|
4
|
0.405
|
42.5
|
2.814
|
5
|
0.487
|
45
|
2.949
|
6
|
0.565
|
47.5
|
3.082
|
7
|
0.641
|
50
|
3.215
|
8
|
0.715
|
52.5
|
3.346
|
9
|
0.788
|
55
|
3.476
|
A
Exposed area (square metres)
|
Q
(cubic metres of air per second)
|
A
Exposed area (square metres)
|
Q
(cubic metres of air per second)
|
10
|
0.859
|
57.5
|
3.605
|
12
|
0.998
|
60
|
3.733
|
14
|
1.132
|
62.5
|
3.860
|
16
|
1.263
|
65
|
3.987
|
18
|
1.391
|
67.5
|
4.112
|
20
|
1.517
|
70
|
4.236
|
22.5
|
1.670
|
75
|
4.483
|
25
|
1.821
|
80
|
4.726
|
27.5
|
1.969
|
85
|
4.967
|
30
|
2.115
|
90
|
5.206
|
32.5
|
2.258
|
95
|
5.442
|
35
|
2.400
|
100
|
5.676
|
Insulation systems, used for the purpose of reducing venting capacity, shall be approved by the
competent authority or its authorized body. In all cases, insulation systems approved for this purpose
shall:
(a) Remain effective at all temperatures up to 649 °C; and
(b) Be jacketed with a material having a melting point of 700 °C or greater.
Marking of pressure-relief devices
Every pressure-relief device shall be clearly and permanently marked with the following particulars:
(a) The pressure (in bar or kPa) or temperature (in °C) at which it is set to discharge;
(b) The allowable tolerance at the discharge pressure for spring-loaded devices;
(c) The reference temperature corresponding to the rated pressure for frangible discs;
(d) The allowable temperature tolerance for fusible elements; and
(e) The rated flow capacity of the spring-loaded pressure relief devices, frangible discs or fusible
elements in standard cubic metres of air per second (m3/s);
(f) The cross sectional flow areas of the spring loaded pressure-relief devices, frangible discs and
fusible elements in mm2.
When practicable, the following information shall also be shown:
(g) The manufacturer’s name and relevant catalogue number of the device.
The rated flow capacity marked on the spring-loaded 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 where 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 in use. 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. Vents or pipes from the pressure-relief device outlets, when used, shall deliver
the relieved vapour or liquid to the atmosphere in conditions of minimum back-pressure on the
relieving devices.
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 the escaping vapour is discharged unrestrictedly. For
flammable substances, the escaping vapour shall be directed away from the shell in such a manner that
it cannot impinge upon the shell. 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 pressure-relief devices by unauthorized persons
and to protect the devices from damage caused by the portable tank overturning.
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