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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
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
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.
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.
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
- Start R22 system before cargo plant.
Ensure liquid separator is not flooded.
- Ensure R22 compressor discharge valve is open.
- The compressor is started and the suction valve opened slowly to prevent cut out due over
- Check that compressor oil level and crankcase pressures are within manufacturer's limits.
- Check R22 liquid levels for correct operation during reliquefaction.
- On stopping R22 compressor ensure oil separator return to the crankcase is closed.
More info pages
- Various type LPG tanker - Design characteristics and usability
- Preparations for loading compatible cargo onboard LPG tanker
- Preparation for changing different grade cargo or drydocking -LPG tanker guideline
- Cargo tank inerting prior to gassing up - LPG tanker procedure
- LPG cargo tank purging & safety guideline
- LPG cargo tank cooling safety procedure
- LPG cargo loading special guideline
- Tackling fire onboard LNG & LPG ships
- Detail guideline for Ballast operation at sea by LPG carrier
- Handling cargo related documents for LPG carrier
- Cargo sampling procedure for liquefied gas cargo
- Cargo measurement and calculation guideline for LPG carriers
- Handling Propylene oxide, Ethylene oxide mixtures
- Special characteristics of Vinyl Chloride Monomer & Butadiene
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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