Ethylene and gas/chemical carriers construction, capacity and temperature control
Ethylene carriers are the most sophisticated of the semi-pressurized tankers and have the ability to carry not only most other liquefied gas cargoes but can also transport ethylene fully-refrigerated at its atmospheric pressure boiling point of -104°C. Designed to carry most liquefied gas cargoes except LNG.
Of this ethylene carrier fleet, about one dozen form a special sub-group of ships able to handle a wide range of liquid chemicals and liquefied gases simultaneously. The ships can load or discharge at virtually all pressurized and refrigerated terminals, making them the most versatile gas carriers in terms of cargo-handling ability.
Many ethylene carriers can also carry LPG cargoes, which increases their flexibility.They feature cylindrical or bi-lobe, insulated, stainless or low temperature nickel steel cargo tanks able to carry cargoes at minimum temperatures of -104ºC and at tank pressures of up to about 6 bar. The ships can load and discharge at virtually all pressurised and refrigerated terminals, making them the most versatile LPG carriers in terms of cargo handling ability. Powerful reliquefaction units are installed.
Cargo capacitiy depends on the trade for which the vessel was constructed and range from 1,500 - 15,000 cubic metres.
Fig: Ethylene carrier underway
Thermal insulation and a high capacity reliquefaction plant is fitted on this type of vessel.
Ethylene Carriers have Containment systems can either be Type C, Type B or Type A prismatic free-standing.
If Type C pressure vessel tanks are used, then no secondary barrier is required If Type B is used a partial secondary barrier is required If Type A is used a full secondary barrier is required. Because of the cargo temperature of -104°C, the hull cannot be used as a secondary barrier in this case and a separate secondary barrier must be fitted.
For reasons of economy, Type C tanks have predominated in this trade.
Ballast is carried in a full double bottom and wing tank ballast system.
Fig: Ethylene carrier cargo tank
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