Figure 7.7 Cargo refrigeration at sea
port with cargo temperatures below that of the shore tanks, thus minimising the amount of flash gas. Depending on the cargo and reliquefaction plant capacity, it can often take several days to cool the cargo by one or two degrees centigrade, but this may be sufficient. The need for this will often depend on the contractual terms in the charter party.
In this respect, poor weather conditions can sometimes present problems. Although most reliquefaction plants have a suction knock-out drum to remove liquid, there is a risk, in gale conditions, that entrained liquid can be carried over into the compressor. For this reason, it is preferable not to run compressors when the ship is rolling heavily, if there is risk of damage.
In calm weather conditions, if the condensate returns are passed through the top sprays, because of the small vapour space and poor circulation in the tank, it is possible that a cold layer can form on the liquid surface. This 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. To achieve proper cooling of the bulk liquid, the reliquefaction plant should be run on each tank separately and the condensate should be returned through a bottom connection to ensure proper circulation of the tank contents. After the cargo has been cooled, reliquefaction capacity can be reduced to a level sufficient to balance the heat flow through the tank insulation. Figure 7.7 shows the arrangement for cooling down cargoes on a loaded voyage.
If the reliquefaction plant is being run on more than one tank simultaneously, it is important to ensure that the condensate returns are carefully controlled in order to avoid the overfilling of any one tank.
Prevention of polymerisation
Where butadiene cargoes are being carried, the compressor discharge temperature must not exceed 60°C and the appropriate high discharge temperature switch must be selected. Similarly, in the case of vinyl chloride, compressor discharge temperatures should be limited to 90°C to prevent polymerisation (see also 2.6).
Condition inspections
Throughout the loaded voyage, regular checks should be made to ensure there are no defects in cargo equipment and no leaks in nitrogen or air supply lines. On LNG ships, it may be necessary to carry out visual cold-spot inspections of cargo tank surrounds even when the ships are fitted with temperature monitoring of the inner hull surfaces. Such inspections must comply with all relevant safety procedures for entry into enclosed spaces and due regard must be given to hazardous atmospheres in adjacent spaces.
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7.6.1 Operation of the reliquefaction plant
As already mentioned in 4.5, the reliquefaction plant is used during cargo loading to handle the vapours formed by evaporation and displacement. At this time, it is likely that the maximum compressor capacity will be required.
On the loaded voyage, and depending on cargo temperature, ambient temperature, and the design of tank insulation, the plant may be operated continuously or intermittently. If it is necessary to reduce the temperature of the cargo before reaching the discharge port, for example, to comply with the receiving terminal requirements or charter party stipulations, the plant will again be operating continuously.
Before starting the reliquefaction plant, it is necessary to ensure that oil levels in the compressors are correct and that the glycol/water cooling system is ready for operation (see 4.6.1). This will require a check to make sure the header tank is full and that the cooling fluid is circulating.
The lubricating oil in compressors must be compatible with the cargo being handled and must be changed if necessary. (When changing from butane/propane mixtures to other grades, it will be necessary to change the oil.) Before starting a cargo compressor, the condenser cooling system must be operating with sea water circulating or the R22 system running. Compressors should always be started and stopped in accordance with the manufacturer's instructions. Compressor discharge valves should be opened and suction valves opened slowly to minimise damage from liquid carry-over (see 4.6.3). The cooling water outlet temperature should be adjusted in accordance with the manufacturer's instructions. The following details should be checked regularly:
• Suction, inter-stage (see 4.5) and discharge pressures
• Lubricating oil pressures
• Gas temperatures on the suction and delivery side of compressor (high discharge temperature switches protect the compressor). Here, inspection of the appropriate Mollier diagram will assist in gaining maximum benefit from the compressor by ensuring that it operates along the appropriate line of constant entropy (see 2.19)
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• Current drawn by electric motor
• Oil leakage from shaft seal, and
• Cooling water temperature
Stopping the cargo compressor should always be carried out in accordance with the manufacturer's instructions. Generally, the first action is to stop the compressor. This is followed by closure of the suction and discharge valves. The glycol/water system (see 4.6.1) is left running to provide crankcase heating or, alternatively, the lubricating oil heater should be left switched on.
7.6.2 LNG boil-off as fuel
Although it is feasible to reliquefy LNG boil-off vapours, the equipment required is complex and expensive and, to date, full-scale equipment has not been installed on board ships. As methane vapours, at ambient temperature, are lighter than air (see 3.4.5/4.6.5), boil-off is used as fuel for the ship's main propulsion during sea passages. LNG is the only cargo which is permitted to be used as fuel in this manner. The equipment and safety devices used for this operation are described in 4.6.5.
Daily boil-off rates during the loaded voyage vary with changes in barometric pressure (unless absolute pressure control is adopted), ambient temperature and sea conditions. For this reason, a close watch must be kept on tank pressures and inter-barrier space pressures. On no account should cargo tank pressures be allowed to fall below atmospheric. Typical figures for LNG carrier boil-off rates are from 0.10 to 0.15 per cent of the cargo volume per day during the loaded voyage and 0.10 per cent per day for the ballast voyage. It should be noted that LNG often contains a small percentage of nitrogen, which will boil-off preferentially, thus reducing the calorific value of the boil-off gas at the beginning of the loaded voyage.
Normally, the compressors used on LNG ships have shaft seals pressurised with nitrogen. Thus, an adequate nitrogen supply must be available at all times when the compressor is running. Furthermore, as with LPG compressors, care must be taken to avoid liquid from being carried into the compressor via the vapour suction line.
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Receiving terminals often require cargo tank pressures on arrival to be below a certain value and this must be provided for by regular disposal of the boil-off through the voyage.
7.7 DISCHARGING
When a ship arrives at the discharge terminal, cargo tank pressures and temperatures should be in accordance with terminal requirements. This will help maximum discharge rates to be achieved.
Before the discharge operation begins, the pre-operational ship/shore procedures should be carried out along similar lines to the loading operation previously outlined, (see 6.3, 6.4 and 7.5).
The method of discharging the ship will depend on the type of ship, cargo specification and terminal storage. Three basic methods may be used:—
• Discharge by pressurising the vapour space
• Discharge with or without booster pumps
• Discharge via booster pump and cargo heater
These methods are discussed in 7.7.1, 7.7.2 and 7.7.3 below.
7.7.1 Discharge by pressurising the vapour space
Discharge by pressure using either a shore vapour supply or a vaporiser and compressor on board is only possible where Type 'C' tanks are fitted. It is an inefficient and slow method of discharge and is restricted to small ships of this type. Using this system, the pressure above the liquid is increased and the liquid is transferred to the terminal. An alternative method is to pressurise the cargo into a small deck tank from which it is pumped to the shore.
7.7.2 Discharge by pumps
Starting cargo pumps
A centrifugal pump should always be started against a closed, or partially open, valve in order to minimise the starting load. Thereafter, the discharge valve should be gradually opened until the pump load is within safe design parameters and liquid is being transferred ashore.
As the discharge proceeds, the liquid level in the cargo tanks should be monitored. Discharge and ballasting operations should be carefully controlled, bearing in mind ship stability and hull stress.
Removal of liquid from the cargo tank may cause changes in interbarrier space pressures and these should be monitored throughout the discharge.
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