Figure 7.13 Pipeline diagram of a cargo booster pump and heater




pump and heater, it is necessary to first establish sea water flow through the heater. Thereafter, the booster pump and heater may be slowly cooled down (prior to full operation) by very slow throughput of liquid from the cargo pump discharge. Once cooled down, the discharge valve can be opened until the desired outlet temperature is reached. It is important to ensure that the cargo pumps maintain adequate flow to the booster pump at all times. Figure 7.13 shows the usual layout.

Heating cargo during discharge always entails a risk of freezing the circulating water in the heater. In addition to checking the cargo outlet temperature and the booster pump suction during operation, attention should also be paid to the sea water inlet and outlet temperatures and pressures. The sea water outlet temperature must not be allowed to fall below the manufacturer's recommended limit. A low temperature switch should stop cargo flow through the heater in case of low sea water discharge temperature.

As will be noted, this method of cargo heating depends on a suitable sea water temperature. In cold sea water areas, the efficiency of the system can be seriously affected and slow discharge rates can result and if sea water temperatures are below 5°C the risk of freezing becomes much greater. To cover such possibilities, sometimes thermal oil heaters are fitted to ships.

7.7.4 Draining tanks and pipelines

It has already been noted in 4.2 and illustrated in Figure 4.3 that in order to avoid cavitation of a centrifugal pump, the pressure of the liquid at the pump suction needs to exceed the saturated vapour pressure (SVP) by an amount termed the minimum Net Positive Suction Head (NPSH). The required minimum NPSH, expressed as an equivalent head of liquid above the pump suction, may vary from one metre (at maximum pump capacity) to 200 millimetres (at reduced flow). If the vapour space pressure can be increased above the SVP by the supply of extra vapour from the shipboard vaporiser, the onset of cavitation, as the liquid level approaches the bottom of the tank, can be delayed. Such augmentation of vapour space pressure is usual practice on fully pressurised and semi-pressurised ships and may also be carefully applied to fully refrigerated cargoes, particularly where maximum cargo out-turn is required in preparation for gas freeing. Whether this extra vapour pressurisation is used or not, there will be a liquid level at which the pump becomes erratic. Gradual reduction of the flow rate at this point, by careful throttling of the discharge valve, reduces the NPSH requirement and permits continued discharge to a lower level. It should be remembered, however, that a pump discharge valve should not be used for flow control if the pump is operating with a booster pump since the booster pump might cavitate, resulting in damage (see 7.7.2).

On completion of discharge, liquid cargo must be drained from all deck lines and cargo hoses or hard arms. Such draining can be done from ship to shore using a cargo compressor. Alternatively, it may be carried out from shore to ship, normally by blowing the liquid into the ship's tanks using nitrogen injected at the base or apex of the hard arm. Only after depressurising all deck lines and purging with nitrogen should the ship/shore connection be broken (see Reference 2.40).

7.8 THE BALLAST VOYAGE

It is frequent practice in some refrigerated trades to retain a small quantity of cargo on board after discharge and the amount retained is known as the heel. This product is


used to maintain the tanks at reduced temperature during the ballast voyage but this procedure only applies when the same grade of cargo is to be loaded at the next loading terminal.

In general, the quantity retained on board as a heel depends on:—

• Commercial agreements

• The type of gas carrier

• The duration of the ballast voyage

• The next loading terminal's requirements, and

• The next cargo grade

In the case of a large LNG carrier, as much as 2,000 to 3,000 cubic metres of liquid may be retained in the tanks on departure from the discharge port; the actual volume, depending on the size and type of cargo containment, the length of the voyage and fuel policy. These ships are normally fitted with spray cool-down pumps in each cargo tank to provide liquid to spray lines fitted in the upper part of each tank. This system is used from time to time on the ballast voyage to minimise tank thermal gradients. The frequency of this operation will depend on ship size and type and the duration of the ballast voyage.

With LPG cargoes, the small amount of liquid remaining after discharge should be sufficient to provide the necessary cooling effect during the ballast voyage. This is carried out by intermittent use of the reliquefaction plant, returning the condensate to the tanks to ensure arrival at the loading port with tanks and product suitably cooled.

If the ship is proceeding to a loading terminal to load an incompatible product, none of the previous cargo should be retained on board but if small amounts exist they may be stored in the deck-mounted pressure vessels. This avoids contamination of the following cargo and allows the maximum quantity of the new cargo to be loaded (see 7.9).

7.9 CHANGING CARGO (AND PREPARATION FOR DRYDOCK)

Of all the operations undertaken by a gas carrier, the preparation for a change of cargo is the most time consuming. If the next cargo is not compatible with the previous cargo, it is often necessary for the tanks to be gas-freed to allow a visual inspection — see Table 2.3(b). This is commonly the case when loading chemical gases such as vinyl chloride, ethylene or butadiene.

When a ship receives voyage orders, a careful check must be made on the com­patibility of the next cargo. (It is also necessary to check compatibilities and the ship's natural ability to segregate, if more than one cargo grade is to be carried. On such occasions, special attention must be given to the ship's reliquefaction system.) There may also be a need, when changing cargoes, to replace the lubricating oil in compressors for certain cargoes — this is discussed in 7.6.1 and 4.6.1.

Tables 2.3(a) and 2.3(b) provide a guide to the compatibility of gases. The tables also cover cargo compatibility with respect to the construction materials commonly used in cargo handling systems. For a more detailed exposition of these points, reference should be made to the IGC Code and Reference 2.1.

In order to obtain a gas-free condition, the full process is as shown below. However, depending on the grade switch, it may not be necessary to include all these steps:


• First, make the tank liquid free

• Then, warm the tank with hot cargo vapours (if necessary)

• Next, inert the tank, and

• Finally, ventilate with air

These procedures are preliminary to tank entry for inspection or when gas freeing the ship for drydock.

7.9.1 Removal of remaining liquid

Depending upon cargo tank design, residual liquid can be removed by pressurisation, normal stripping or, in the case of fully refrigerated ships with Type 'A' tanks, by using the puddle heating coils fitted for this purpose. (An older method of warming Type 'A tanks with hot vapours from the compressor — but without puddle heating — is now generally out of favour due to the extended time taken, although on some ships, and particularly those in LNG trades, there may be no other choice).

The first operation to be carried out is the removal of all cargo liquid remaining in the tanks or in any other part of the cargo system. Due to enhanced evaporation in a non-saturated atmosphere, residual liquid can become super-cooled to a temperature which could result in brittle fracture of the tank. Furthermore, any liquid retention will frustrate the future inerting operation.

As an aid to liquid removal, many general purpose LPG ships are provided with special pressure vessels mounted on deck. These tanks can be used for the recovery of liquid and vapour from the cargo tanks. The contents of the deck tanks may also be used, at some future time, to provide vapour for gassing-up purposes when changing grades.

When all cargo tank liquid has been removed, the tanks can be inerted either with inert gas from the ship's supply or from the shore, as required by the next cargo. Alternatively, gassing-up using vapour from the next cargo may be carried out — but this is increasingly unusual (see 7.2.3 and 7.3 for more detail of the procedure).


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