TYPES OF CARGO PIPELINE SYSTEMS
Tankers have a varied pipeline system which is basically based on three main types:
The Free Flow System
This system is mainly used on large crude carriers which carry only one grade of cargo which is required to be discharged quickly. Large gate valves built into the bulkheads of the tanks allow the oil to flow from the side tanks to the centre tanks, from where the stern trim of the vessel causes the oil to flow to the aftermost tank, where the suctions of the main cargo pumps are situated. The large bulkhead sluice valves permit a good drainage of the cargo. Any residues are discharged by the stripping pumps.
The Direct Line System
This system permits 2 or 3 grades to be carried and is suitable for VLCCs as it facilitates quick loading and discharging. The arrangement is simple with lines leading directly to the tanks. Due to the straight lengths of pipeline, there is better suction and less loss of pressure due to friction. Fewer bends and valves mean less erosion and leaks, reducing the maintenance required. The time spent in line washing is also reduced; however thorough washing of the line is not possible unless the washings are flushed into the tank and discharged from there.
Due to there being fewer valves, leaks are difficult to control and many grades cannot be carried as the required line and valve segregation is not provided.
The Ring Main System
This system is ideal for product carriers where several different products are to be carried simultaneously in the different tanks. Any pump can be connected to discharge any tank, making it very versatile. Cargo may have to be pumped in a roundabout route but the required two valve segregation between products can always be met. Line washing can be carried out thoroughly without flushing into the tanks but it takes a longer time due to the number of bends, joints and valves. Due to this the pumping rate also is
affected and leaks due to erosion become common as the ship becomes older.
Ref : Cargo work book by Capt. Errol Fernandes
