Liquefied Gas Carrier

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Carriage of LPG cargo during sea passage - LPG tanker safety procedures

Carriage of LPG : During the loaded passage, the cargo is warmed by heat input from sea water and atmosphere, causing the temperature and saturation pressure (cargo tank) to rise. It is therefore necessary to maintain strict control of the cargo temperature and pressure at all times during the loaded passage. On vessels other than pressurised LPG carriers this is achieved by reliquefying the boil-off and returning it to the tanks.




Also, there are frequent occasions when it is necessary to reduce the temperature of the cargo on passage so that the ship can arrive at the delivery terminal with her cargo temperature below that of the shore tanks, thus minimising the amount of "flash gas" discharged during the discharge operation. This is achieved by use of the reliquefaction plant on board. It can often take several days to cool by 0.5°C, but this may be sufficient.

Heavy weather can sometimes present problems as there is always a risk that slugs of liquid can be carried over into the compressor. For this reason, it is preferable not to run the compressors during heavy weather

Where weather conditions are calm, it is possible that, because of the small vapour space in the tank and the absence of liquid circulation in the tank, a cold layer of liquid can form at the surface when the condensate returns from the reliquefaction plant through the top sprays. This, in turn, enables the compressors to reduce the vapour pressure after only a few hours running, when in fact the bulk of the liquid has not been cooled at all.

In order to avoid this, full reliquefaction plant capacity should be run on each tank separately and the condensate returned from the cargo condenser should be returned through the bottom connection to ensure circulation of the tank contents. After the cargo has been cooled, the reliquefaction plant capacity can be reduced to a level sufficient to balance the heat flow through the tank insulation. If the reliquefaction plant is being run on more than one tank simultaneously, it is important to ensure that the condensate returns are carefully monitored and controlled to avoid overfilling.

NOTE: It is very important to regularly check and equalise tank levels. Throughout the loaded passage, regular checks must be made to ensure that there are no defects in the cargo equipment and no leaks in the nitrogen or air supply lines.

Read more on LPG reliquefying plant detail guideline


Internal cargo tank inspection

Cargo tanks are to be inspected internally during every dry docking and in circumstances where a particular tank is gas-freed to effect repairs. Particular attention is to be paid to examine any debris which may be left inside tanks after the repair period. The debris will cause damage to filters, pumps and valves on ship and on shore installation. Any water accumulated should also be removed to avoid any formation of hydrates.


Inspection of cargo tank insulation ( Hold space inspection)

Hold spaces are to be inspected when the vessel has loaded its first cargo after delivery and after dry docking. This is required to find the cold spots and evaluate the condition of tank insulation. Location and number of cold spots within hold spaces are to be recorded for monitoring purposes. It will be necessary to have more frequent inspection if any problem is encountered. The frequency will also depend on the duration of the voyages. In any case, every hold space shall be inspected at internals not exceeding 3 months.


VCM and Butadiene

VCM and Butadiene have a tendency to polymerise during transportation and require the use of an inhibitor.

During cooling the reliquefaction selector switch is always be set to "VCM/Butadiene". The compressor suction pressure is not to exceed 1.5 bar with these gases. If the tank pressure before starting the compressors is greater than 1.5 bar, set the suction pressure controller on the compressor and after stabilisation of the suction pressure, manual operation can be resumed.

Condensates produced by reliquefaction do not contain any inhibitor, and as polymerisation is accelerated by high temperatures, this is most likely to occur downstream of the compressor discharges, in the condenser, and in the lines to the cargo tank.

To counter this tendency, inhibited liquid cargo is to be circulated from the tank to the condenser and back to the tank using a deep well cargo running at reduced capacity.

The condenser level should not exceed 50% full, thus maintaining the ability of the incoming gas to blow inhibited liquid back to the tank. Care must be taken to ensure that the level does not rise above 50% otherwise the condenser and/or intercooler may be flooded. The level can be controlled manually or automatically.

For the reliquefaction of Butadiene or VCM intercooling is not normally required. For Butadiene the temperature of the compressor discharge should be less than +60 deg.C. With VCM this should not be allowed to exceed +90 deg C. If above these limits intercooling may be required. This is to prevent the possibility of polymerisation. Normally, it is not necessary to use the purge condenser with these cargoes.


Ethylene

LPG Ships have in many cases ethylene capability. The gas is carried fully refrigerated at its atmospheric pressure boiling point of -104C. When IMO Type 'C' pressure vessel tanks are fitted, no secondary barrier is required. High value thermal insulation and high capacity reliquefaction plant is fitted to vessels with this capability.

The maximum practical pressure of which ethylene can be compressed is about 15 bar and at this pressure the condensate temperature would be -40 degree C. Therefore some other cooling medium other than sea water must be used.


Cascade Cycle

This system uses a refrigerant to condense cargo vapour. The compression of cargo vapour is similar to the direct cycle however the cargo condenser is cooled using the refrigerant in place of sea water. The cargo in condensing evaporates the liquid refrigerant and the vapours are then taken through a conventional closed refrigeration cycle condensed by sea water - hence the term cascade.


R22 System Operation







Related Information:

  1. Various type LPG tanker - Design characteristics and usability


  2. Preparations for loading compatible cargo onboard LPG tanker


  3. Preparation for changing different grade cargo or drydocking -LPG tanker guideline


  4. Cargo tank inerting prior to gassing up - LPG tanker procedure


  5. LPG cargo tank purging & safety guideline


  6. LPG cargo tank cooling safety procedure


  7. LPG cargo loading special guideline


  8. Tackling fire onboard LNG & LPG ships


  9. Detail guideline for Ballast operation at sea by LPG carrier


  10. Handling cargo related documents for LPG carrier


  11. Cargo sampling procedure for liquefied gas cargo


  12. Cargo measurement and calculation guideline for LPG carriers


  13. Handling Propylene oxide, Ethylene oxide mixtures


  14. Special characteristics of Vinyl Chloride Monomer & Butadiene




More info pages

Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for segregation of Liquefied Gas Cargoes

Procedure for Stripping Liquefied Gas Cargoes

Procedure for Changing Liquefied Gas Cargoes

Displacing Atmosphere with Inert Gas (Inerting)

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer

Procedure for commissioning the cargo system

Cargo emergency shutdown requirement for liquefied gas carrier

Gas analyzing equipment





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