Maximum loading/discharge rate
The venting system should be designed to take into consideration
the maximum permissible loading/discharge rate for each cargo tank
and in the case of a combined venting system, for each group of tanks.
These loading and discharge rates should also be used for the design
of the inert gas system, regulation II-2/62.3.
1.
Gas evolution
Regulation II-2/59.1.9.5 requires at least
25% to be added to the maximum loading rate to account for the increased
volume due to gas evolution from the cargo. A higher gas evolution
factor may be considered for highly volatile cargoes.
Pressure
loss across devices
Data relating to pressure loss
across devices to prevent the passage of flame, approved in accordance
with MSC/Circ.677 and referred to in regulation II-2/59. 1.5, is to be considered
in the design of the venting system. Fouling of devices should be
taken into account.
Pressure loss in the venting
system
Pressure loss calculations of systems including
pipes, valves, bends, fittings, etc., should be made to ensure that
the pressure inside the cargo tanks does not exceed the pressure these
tanks are designed to withstand taking into consideration .2 and .3
above. In the case where a combined venting system is used in association
with loading of cargo tanks simultaneously, the combined effect of
vapour pressure generated in the tanks and venting system should be
considered.
Pressure at which the vents open
The initial opening pressure of the vent valves should be considered
in selecting the appropriate valves for the venting system.
Prevention of hammering
In the case of high velocity
vents, the possibility of inadvertent detrimental hammering leading
to damage and/or failure should be considered, with a view to eliminating
it.
Density of the gaseous mixture
The
maximum density of the gaseous mixtures likely to be encountered in
the cargo tanks having regard to the types of cargo intended to be
carried and their temperature is to be considered.
Design
to prevent liquid overfill
Where overflow control
systems are fitted, consideration is to be given to the dynamic conditions
during loading.
Location of vent outlets
Horizontal and vertical distances of the vent outlets are to
be in accordance with regulation II-2/59.
Types of venting systems
Due
regard is to be given to cargo segregation when considering a venting
system or inert gas system common to more than one tank. Where the
inert gas main is designed for venting of cargo tanks, additional
means for venting of these tanks are to be in accordance with regulation II-2/62. 11.3.
Vent
draining arrangements
The draining arrangements
for venting systems are to be designed in accordance with regulation II2/59.1.4.
Gas-freeing
In designing a gas-freeing system in conformity
with paragraphs 2.2.2 and 2.2.3 of regulation
II-2/59 in order to achieve the required exit velocities, the
following should be considered:
-
.1 the flow characteristics of the fans to be
used;
-
.2 the pressure losses created by the design of
a particular tank's inlets and outlets;
-
.3 the pressure achievable in the fan driving
medium (e.g. water or compressed air); and
-
.4 the densities of the cargo vapour air mixtures
for the range of cargoes to be carried.
Others
Repairs and renewal of the venting
system should conform to the original design parameters. Factors in
the above paragraphs are to be taken into consideration when modifications
are carried out to the venting system.
The master is to
be provided with a manual containing information relating to the maximum
loading and unloading rates for each tank or group of tanks established
during the design of the venting system, as per paragraph 1 of this
circular.
Data referred to in paragraph
4.3 of MSC/Circ.677 should be taken into consideration when
renewing devices referred to in the above circular.