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The Ship - Equipment and Instrumentation

This chapter covers gas carrier cargo handling equipment and related instrumenta­tion. It reviews pipeline and valve design issues and considers cargo pumps and ancillary equipment. The plant associated with cargo reliquefaction is also described along with some of the special operational and maintenance issues. The design of inert gas generation equipment is also covered.


4.1.1 Cargo pipelines

Gas carriers are normally fitted with liquid and vapour manifolds situated amidships. These are connected to liquid and vapour headers — or pipelines — (see Figure 7.2) with branches leading into each cargo tank. The liquid loading line is led through the tank dome to the bottom of each cargo tank; the vapour connection is taken from the top of each cargo tank. On semi-pressurised and fully refrigerated LPG ships a vapour connection is taken from the vapour header to the cargo compressor room where reliquefaction of the boil-off takes place. After reliquefaction the cargo is piped, via a condensate return line, to each cargo tank. In the case of LNG ships the boil-off vapours are usually fed to the ship's boilers, via a compressor and heater, for use as main propulsion fuel.

As stated in Chapter Three, cargo pipelines are not allowed beneath deck level on gas carriers; therefore, all pipe connections to tanks must be taken through the cargo tank domes which penetrate the main deck. Vapour relief valves are also fitted on the tank domes; these are piped, via a vent header, to the vent riser. The vent risers are fitted at a safe height and safe distances from accommodation spaces and other such gas-safe zones as specified in the Gas Codes.

Provision must be made in the design and fitting of cargo pipelines to allow for thermal expansion and contraction. This is best achieved by the fitting of expansion loops or, by using the natural geometry of the pipework, as appropriate. In a few specific cases, expansion bellows may be fitted and, where this is planned, corrosion resistant materials should be used and Section of the IGC Code should be considered. Where expansion bellows are fitted in vapour lines, it should be ensured that their pressure rating at least meets the liquid pipeline design criteria. Furthermore, expansion bellows are often subject to a considerable amount of wear and tear while a ship is in service — in particular, sea-water corrosion must be carefully avoided otherwise pin hole leaks are liable to develop.

It is also important not to alter or adjust adjacent pipeline supports once the ship has entered service since they form an integral part of the expansion arrangements.

Furthermore it should also be noted that parts of pipeline systems are fitted with strong anchor points to resist lateral or vertical displacement from surge pressures. Similarly, when replacing parts such as bolts and restraining rods, care must be taken to ensure that the new parts are of the correct material for the service.

Removable spool pieces are taken in or out of pipelines to interconnect sections of line for special operational reasons such as using the inert gas plant or ensuring se­gregation of incompatible cargoes. These spool pieces should not be left in position after use but should be removed and pipelines blanked to ensure positive segregation.

4.1.2 Cargo valves and strainers

Isolating valves for cargo tanks must be provided in accordance with Gas Codes. Where cargo tanks have a MARVS greater than 0.7 barg (Type 'C' tanks), the principal liquid and vapour connections on the tank dome (except relief valve connections) should be fitted with a double valve arrangement. This should comprise one manually operated globe valve and a remotely operated isolation valve fitted in series. For Types 'A' and 'B' cargo tanks (with the MARVS less than 0.7 barg) the Gas Codes allow single shut-off valves for liquid and vapour connections. These valves can be remotely actuated but must also be capable of local manual operation.

Remotely operated emergency shut-down valves are provided at the liquid and vapour manifolds for all gas carriers.

Figure 4.1 shows the piping system on a cargo tank dome including the valving arrangement. This particular drawing is typical for a semi-pressurised ship.

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