1 The International Convention for the Safety of Life at Sea, 1974, as amended,
includes requirements for fire safety measures for tankers in regulations II-2/59 and 62.
These regulations contain arrangements for venting, inerting, purging, gas-freeing and
2 The Sub-Committee on Fire Protection has considered problems associated with the
design of cargo tank venting and gas-freeing arrangements and the Maritime Safety
Committee, at its fifty-third session (8 to 17 September 1986), approved MSC/Circ.450 on
main factors that should be considered in the design of the arrangements referred to in
paragraph 1 above. That circular was revised by the Committee, at its fifty-fifth session
(11 to 20 April 1988), and issued as MSC/Circ.450/Rev.1.
3 The Committee, at its sixty-sixth session (28 May to 6 June 1996), taking into
account the development of MSC/Circ.677 (Revised standards for the design, testing and
locating of devices to prevent the passage of flame into cargo tanks in tankers), revised
the aforementioned main factors as contained in the annex.
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.
Regulation II-2/220.127.116.11 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
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
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
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
.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);
.4 the densities of the cargo vapour air mixtures for the range of cargoes to be
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.