Fusible elements

Fusible elements shall operate at a temperature between 100 °C and 149 °C on condition that the
pressure in the shell at the fusing temperature will be not more than the test pressure. They shall be
placed at the top of the shell with their inlets in the vapour space and when used for transport safety
purposes, they shall not be shielded from external heat. Fusible elements shall not be used on portable
tanks with a test pressure which exceeds 2.65 bar unless specified by special provision TP36 in
Column (11) of Table A of Chapter 3.2. Fusible elements used on portable tanks intended for the
carriage of elevated temperature substances shall be designed to operate at a temperature higher than
the maximum temperature that will be experienced during carriage and shall be to the satisfaction of
the competent authority or its authorized body.

Frangible discs

Except as specified in 6.7.2.8.3, frangible discs shall be set to rupture at a nominal pressure equal to
the test pressure throughout the design temperature range. Particular attention shall be given to the
requirements of 6.7.2.5.1 and 6.7.2.8.3 if frangible discs are used.

Frangible discs shall be appropriate for the vacuum pressures which may be produced in the portable
tank.

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.
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