On completion of a dry dock period or delivery of a new Vessel, the Interbarrier Spaces (IBS) and Insulation Spaces (IS) may require re-inerting to remove any traces of oxygen, or to reduce the moisture content of the atmosphere inside the insulation spaces. This can be carried out by using the vessel’s nitrogen generation system.
Prior to any inerting of these spaces, great care must taken to ensure all sampling and control systems are fully operational, pressure relief valves are fitted and in full working order and personnel are fully conversant with the operation. If possible, the cargo tanks should be maintained at a pressure of at least 3kPa(g) during the inerting and drying of the insulation spaces to reduce the risk of over-pressurising the spaces.
In the extreme case where the oxygen content in the IS is above 10%, shore supplied vacuum pumps would be required, as the vessel is not fitted with these, to pull a 20kPa absolute vacuum in both the IBS and IS spaces simultaneously and back-fill with nitrogen.
The nitrogen provides a dry and inert medium for the following purposes:
The nitrogen is produced by two nitrogen generators located in the engine room 2nd deck and is stored in a pressurised buffer tank, ready to be supplied to the nitrogen headers through make-up regulating valves. From the headers, branches are led to the IBS and IS spaces of each tank. Any excess pressure is vented through exhaust valves to the nitrogen vent mast from the IBS and to the deck from the IS.
The system is automatically maintained by alternate exhaust and make-up as the atmospheric pressure and the temperature rises and falls between a pressure of between 0.5kPa and 1.0kPa above atmospheric. Both the IBS and IS spaces on each tank are provided with pressure relief valves which open at 3kPa(g) for the IBS and at 3.5kPa(g) for the IS. A manual bypass with a valve is provided for local venting and sweeping of a space if required.
The nitrogen generators are maintained in automatic mode, during normal operations, one generator should be able to maintain the pressure in the buffer tank. When a high nitrogen demand is detected, the second generator will start automatically.
Insulation Space Pressure Control
Nitrogen is supplied from the buffer tank and pressurisation headers through a pressure reduction unit and supply regulating valve which reduces the header pressure to 50kPa(g). Branch lines lead from the pressurisation header to automatic supply valves for the Interbarrier Space (IBS) and Insulation Spaces (IS) at each tank liquid dome. Nitrogen is released from the IBS and IS by automatic exhaust regulating control valves at each cargo tank gas dome to the respective IBS and IS vent mast.
The IBS and IS on each tank are provided with two pressure relief valves, one on the liquid dome and one on the gas dome, they will open when a pressure of 3kPa for the IBS and 3.5kPa for the IS above atmospheric. A manual bypass is provided for local venting and purging of the space if required. In addition, a manual flow gauge is fitted to a bypass line on the inlet side for the IBS and IS so that manual flow control can be performed in the event of failure of a automatic make-up valve.
Interbarrier and Insulation Spaces
The supply and exhaust control valves for both spaces on each cargo tank are operated under split range control by the output of the reverse-acting pressure controller for that space. Therefore, when the pressure in the space falls below the set value, the supply valve opens and the exhaust valve remains shut. When the pressure in the space rises above the set value, the exhaust valve opens and the supply valve remains shut.
The nitrogen header control valve reacts to the demand from the system to maintain the IBS/IS header at the pressure set by the operator. High, low and differential pressure alarms are fitted to the pressure control systems for each interbarrier and insulation space.
Pressure Control Logic for Interbarrier and Insulation Spaces Set Pressures
