GTM-8.7 Gas Tanker Manual
GTM Carriage of Cargoes
 Form No.: GTM-8.7
Revision: 01
Date: 15 Oct 2024
Issued by: DPA
Approved by: MD

7.1 INTRODUCTION

7.1.1 Pressure and Temperature

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.

During normal operations at sea and to prevent cargo tank over-pressurisation, the vessel’s tank pressure is not to exceed 80% of the pressure at which the safety valve lifts. Should the tank pressure increase to above this pressure then you are to use all possible methods to reduce the pressure to below the 90% level. Should the vessel be pressured at any time to exceed the 90% pressure you are to alert the office immediately.

For Refrigerated LPG Carriers, normal integral, membrane or rectangular independent tanks are not permitted for pressures above about 0.7 bar.

For Pressurised LPG Carriers a maximum temperature of 45ºC is normally assumed, which corresponds to a pressure of about 15 bar for propane and 17 bar for ammonia. 17 bar is the common design pressure for this type of cargo tank although a lesser design can be used for ships with restricted ambient requirements. For Refrigerated LPG Carriers, the carriage temperature is the atmospheric pressure boiling point of the cargo. If the cargo is refrigerated to below ambient temperature, but is not fully refrigerated, it can still exert a significant vapour pressure.

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.

7.1.2 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.

7.1.3 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.

7.2 RELIQUEFACTION

The reliquefaction plant is designed to maintain a constant tank pressure at a maximum ambient air and sea water temperatures.

The method of operation will depend on the design of the plant installed.

Refer to the Builders Cargo Manual for the correct operation of the plant.

The plant may be capable of being operated in a number of ways. Selection depends upon the grade of cargo, suction and discharge temperatures of the LPG compressors, and suction and discharge pressure differential across the LPG compressors. In general, reliquefaction without desuperheating between the first and stage compression is possible only if the 2nd stage suction temperature is less than +150°C.

For Butadiene the temperature of the compressor discharge must be less than +60°C. With VCM this must not be allowed to exceed +90°C. This is to prevent polymerisation.

With ammonia it is usual to run the compressors with desuperheating between the first and second stages.

When reliquefying Propylene and Propane using two stage compression with intercooling, control of the 1st stage discharge temperature can be affected by changing the level of the liquid in the intercooler. Increasing the level reduces the temperature.

An accumulation of non-condensable gases in the condenser will reduce the refrigerating capacity of the plant. These should be separated out in the purge condenser and blown to atmosphere. If, for any reason, the non-condensable gases comprise mainly hydrocarbons (methane, ethane, ethylene) or cargo gas, and it is not possible to vent via the masts, they can be injected back to the cargo tanks via the stripping/condensate lines.

Purge control should always be done slowly, carefully and under supervision. Pressure fluctuations can lead to high temperatures at the compressor causing automatic shut down of the plant.

To prevent this, the following precautions are recommended:

During the voyage, periodically turn the cargo pump shaft. If movement is difficult, the motor is to be rotated for some time to clear. If the pump is blocked, methanol injection can be tried, although this is an expensive and not altogether reliable method of clearing a blockage. There is no guarantee that injecting an anti-freeze liquid onto a block of frozen material will cause it to unfreeze any more than the immediate contact surface.

7.2.1 Reliquefaction with Intercooling

Intercooling is used in conjunction with two stage compression. The first stage discharge is de-superheated in the intercooler and returned to the 2nd stage. The discharge from the 2nd stage is condensed in the condenser, and thence to the intercooler for further cooling. The condensate is returned to the cargo tank.

Non-condensable gases are separated out in the purge condenser and transferred by the cross over line to the collector and mast, or via the stripping or bottom distribution lines back into the cargo tank.

If two or more tanks are being reliquefied simultaneously via one cargo system, the distribution of condensate between the tanks is to be controlled manually.

If, during intercooling, there is insufficient condensate in the intercooler drum, additional liquid can be transferred from the cargo tank using the stripping/condensate line and a cargo pump. This is more likely to occur during the early stages of intercooling, or if the 2nd stage compression temperature is too high.

The condenser pressure is to be maintained at approximately 1 bar above the saturation pressure of condensate at the condensate temperature.

7.2.2 Reliquefaction without Intercooling

Gas is drawn into the 1st stage of the compressor from the tank, via the surge drum (if fitted), compressed and discharged through the intercooler, but without cooling. The gas is liquefied in the condenser, expanded to tank pressure and returned via the spray or stripping/condensate line.

When several tanks are being cooled simultaneously by one reliquefaction plant, the operation should start with the tank having the highest pressure. Pressure is to be equalised before the tanks are interconnected. Also, when several tanks are being cooled simultaneously, a careful watch must be kept on the liquid return to ensure equal filling.

7.3 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°C. With VCM this should not be allowed to exceed +90°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.

7.4 ETHYLENE

7.4.1 General

LPG Ships have in many cases ethylene capability. The gas is carried fully refrigerated at its atmospheric pressure boiling point of -104°C. 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°C. Therefore some other cooling medium other than sea water must be used.

7.4.2 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.

7.4.3 R22 System Operation