8.3.1 Where required by 15.1.4.2, an additional pressure relieving system to prevent the
tank from becoming liquid full at any time during relief under the fire exposure
conditions referred to in 8.5 should be fitted to each tank. This pressure relieving system
should consist of:
-
.1 one or more relief valves set at a pressure
corresponding to the gauge vapour pressure of the cargo at the reference
temperature defined in 15.1.4.2; and
-
.2 an override arrangement, whenever necessary, to
prevent its normal operation. This arrangement should include fusible elements
designed to melt at temperatures between 98°C and 104°C and to cause relief valves
specified in 8.3.1.1 to become operable. The fusible elements should be
located, in particular, in the vicinity of relief valves. The system should become
operable upon loss of system power if provided. The override arrangement should
not be dependent on any source of ship's power.
8.3.2 The total relieving capacity of the additional
pressure relieving system at the pressure mentioned in 8.3.1.1 should not be less than:
where:
|
Q′
|
= |
minimum
required rate of discharge of air at standard conditions of 273 K
and 1.013 bar. |
|
|
= |
|
with
|
ρr
|
= |
relative
density of liquid phase of product at relieving conditions (ρr =
1.0 for fresh water);
|
|
m
|
= |
-di/d
ρr = gradient of decrease of
liquid phase enthalpy against increase of liquid phase density (kJ/kg)
at relieving conditions. For set pressures not higher than 2.0 bar
the values in table 8.1 may be used.
For products not listed in the table and for higher set pressures,
the value of m should be calculated on the basis of the
thermo-dynamic data of the product itself;
|
|
i
|
= |
enthalpy
of liquid (kJ/kg); |
|
T′
|
= |
temperature
in kelvins (K) at relieving conditions, i.e. at the pressure at which
the additional pressure relieving system is set; |
F, A, L, D, Z and M are defined in 8.5.2.
8.3.3 Compliance with 8.3.1.1 requires changing of the setting of the relief valves
provided for in this section. This should be accomplished in accordance with the
provisions of 8.2.6 and 8.2.7.
8.3.4 Relief valves mentioned under 8.3.1.1 above may be the same as the pressure relief valves
mentioned in
8.2, provided the setting pressure and the relieving capacity are in
compliance with the requirements of this section.
8.3.5 The exhaust of such pressure
relief valves may be led to the venting system referred to in 8.2.9. If separate venting arrangements
are fitted these should be in accordance with the requirements of 8.2.9, 8.2.10, 8.2.11, 8.2.12, 8.2.13, 8.2.14 and 8.2.15.
Table 8.1 FACTOR (m)
| Product
|
m = -di/dρr
(kJ/kg)
|
| Ammonia, anhydrous
|
3,400
|
| Butadiene
|
1,800
|
| Butane
|
2,000
|
| Butylenes
|
1,900
|
| Ethane
|
2,100
|
| Ethylene
|
1,500
|
| Methane
|
2,300
|
| Methyl chloride
|
816
|
| Nitrogen
|
400
|
| Propane
|
2,000
|
| Propylene
|
1,600
|
| Propylene oxide
|
1,550
|
| Vinyl chloride
|
900
|
| The values in this table may be used for set pressures
not higher than 2.0 bar.
|