Figure 4.8 Vertical booster pump

Figure 4.9 Horizontal booster pump

Ice prevention at cargo pumps

The formation of ice or hydrates (see 2.7) may occur in ships carrying refrigerated or semi-pressurised LPG. Furthermore, hydrates may be transferred from the terminal during loading operations. Hydrates from the shore can be removed by cargo filters in the terminal loading lines.

Hydrate formations may enter cargo pumps, block lubricating passages, unbalance impellers and seize bearings. To prevent such damage it is common practice to inject a small quantity of freezing-point depressant into the cargo pump, especially submerged pumps, to facilitate de-icing. Because of the danger of methanol con­tamination to certain LPG cargoes, injection of this product should not normally be allowed without cargo receivers agreement.

When deepwell pumps are not in operation, it is recommended that manual rotation of the shafts be carried out during cool-down and loading to prevent freezing of the impellers.

4.3 CARGO HEATERS

When discharging refrigerated cargoes into pressurised shore storage, it is usually necessary to heat the cargo so as to avoid low-temperature embrittlement of the shore tanks and pipelines.

Cargo heaters are normally of the conventional horizontal shell and tube type ex­changer. Most often, they are mounted in the open air on the ship's deck. Sea water is commonly used as the heating medium and this passes inside the tubes with the cargo passing around the tubes.

The heaters are typically designed to raise the temperature of fully refrigerated propane from -45°C to -5°C; however, it should be noted that the cargo flow rate at

Figure 4.10 Cargo heater

which this temperature rise may be achieved can be significantly reduced in cold sea water areas. Under such circumstances only very slow discharge rates may be possible and when sea water temperatures fall below 5°C it becomes increasingly difficult to use sea water as a heating medium.

Figure 4.10 shows a typical heater arrangement; note the requirement for temperature controls and alarms to avoid freezing. This is a very real risk which always has to be guarded against.

4.4 CARGO VAPORISERS

A means of producing cargo vapour from liquid is often required on gas carriers. For example, vapour may be needed to gas-up cargo tanks or to maintain cargo tank pressure during discharge. This latter need will be more obvious in the absence of a vapour return line from shore. Accordingly, a vaporiser is usually installed on board for these purposes.

Cargo vaporisers may be either vertical or horizontal shell and tube heat exchangers. They are used with either steam or sea water as the heating source.

4.5 RELIQUEFACTION PLANTS AND BOIL-OFF CONTROL

With the exception of fully pressurised gas carriers, means must be provided to control cargo vapour pressure in cargo tanks during cargo loading and on passage. In the case of LPG and chemical gas carriers, a reliquefaction plant is fitted for this purpose. This equipment is designed to perform the following essential functions:

• To cool down the cargo tanks and associated pipelines before loading;

• To reliquefy the cargo vapour generated by flash evaporation, liquid displacement and boil-off during loading; and

• To maintain cargo temperature and pressure within prescribed limits while at sea by reliquefying the boil-off vapour.

There are two main types of reliquefaction plant and these are described in the following sections.

4.5.1 Indirect cycles

Indirect cycle is descriptive of a system where an external refrigeration plant is employed to condense the cargo vapour without it being compressed. This cycle is relatively uncommon as its use is limited to a small numbers of cargoes. It requires, for efficiency, a very cold refrigerant and large surfaces for heat exchange.

This type of reliquefaction plant is, however, required by the Gas Codes when carrying any of the following cargoes

• Chlorine

• Ethylene oxide

• Ethylene oxide — propylene oxide mix

• Propylene oxide

Figure 4.10(a) Examples of indirect cooling cycles

Reference to Table 2.5 shows that with respect to propylene oxide it is unlikely, but dependent on ambient conditions, that refrigeration will be required on voyage.

Two indirect cycle systems are shown diagramatically in Figure 4.10(a).

4.5.2 Direct cycles

Direct cycle is descriptive of a system where the boil-off is compressed, condensed and returned to the tank. This is the most common system, but may not be employed for certain gases (see IGC Code, Chapter 17 and 4.5.1 above).

There are three main types of direct cycle reliquefaction plant and these are described in the following sections.

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