Primary Means of Venting : Vessels utilising a common gas / vapour system as the ―primary means of venting‖ which is isolated from a cargo tank by a valve, or other means, which is shut due to the normal operation of the vessel (such as in the case of a vessel carrying parcel cargo with non compatible vapours) are not in compliance with the requirements of SOLAS Reg. II-2/4.5.3 unless they have a second independent means of venting which can not be isolated from the cargo tank.
Secondary Means of Venting : (Reg. II-2/220.127.116.11.2) Where the arrangements are combined with other cargo tanks, either stop valves or other acceptable means shall be provided to isolate each cargo tank. Where stop valves are fitted, they shall be provided with locking arrangements which shall be under the control of the responsible ship‘s officer. There shall be a clear visual indication of the operational status of the valves or other acceptable means. Where tanks have been isolated, it shall be ensured that relevant isolating valves are opened before cargo loading or ballasting or discharging of those tanks is commenced. Any isolation must continue to permit the flow caused by thermal variations in a cargo tank in accordance with regulation 18.104.22.168.