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Figure 5.6 LPG loading terminal — vapour return using a shore based blower



A typical shore-based blower for removing LPG vapours from ships' tanks and re­turning them to shore is shown in Figure 5.5. A pipeline drawing showing a typical terminal arrangement, with an in-line blower, is shown in Figure 5.6. Liquid is pre­vented from entering the blower by a knock-out drum. The knock-out drum is provided with a high-level switch. In order to protect against low pressures being created in the ship's tank, a low suction pressure control is provided for in blower design. This opens a bypass valve between the discharge and the suction sides of the machine.

Certain loading terminals, although providing vapour return facilities, restrict their use due to the risk of receiving contaminated vapour ashore. At such terminals, vapour returns are usually fitted in case of unacceptable pressures developing in the ship's tanks. Similarly, some loading terminals require that a vapour return line be connected for reasons of safety. Receiving terminals may also require vapour return facilities as


an integral part of the cargo handling system. In such circumstances the ship will be so advised prior to cargo transfer.

5.1.4 Insulating flanges

Discussion in 2.22 centres on the need to prevent electrical flow through a hard arm or hose. Such currents can be generated by electrolytic differences between ship and shore. It describes the recommended practice of inserting an insulating flange in the lower end of the outer hard arm to achieve this aim.

Cargo hoses are normally insulated by a similar flange but this is usually fitted on­shore close to the shore presentation flange. Its position in the pipework should ensure that no supports to the jetty deck exist between the insulation flange and the point of hose connection.

The external surfaces of insulating flanges should be kept clean and unpainted and its insulating properties should be regularly tested by a 500 volt insulation resistance tester (such as a megger) and recorded in terminal maintenance documents.

5.2 SHORE STORAGE

In the same way that gas cargoes are transported by sea through control of their pressure and temperature, liquefied gases are stored on shore in either a pressurised, semi-pressurised or refrigerated condition. The most common methods utilised for storing liquefied gases can be itemised as follows:

1. As a liquid at ambient temperature under pressure in:

• Spherical tanks above-ground

• Mounded horizontal cylindrical tanks

• Underground storage caverns

2. As a semi-pressurised liquid at a temperature above the product's atmospheric boiling point

3. As a fully refrigerated liquid at atmospheric pressure and at low temperature equal to the cargo's boiling point in:

• Single-wall tanks (LPG)

• Double-wall tanks (LNG, LPG, chemical gases)

• Double-containment tanks (LNG, LPG)

• In-ground tanks (LNG)

It should be noted that methods 1 and 2 above do not apply to LNG storage.

5.2.1 Pressurised storage at ambient temperature

Pressurised storage of liquefied gases is undertaken at refineries, chemical plants, import and export terminals, filling/bulk distribution centres, retail sales outlets and large industrial premises. Individually, pressure tanks are small compared to fully


refrigerated gas storage tanks. For example, the largest spheres have a capacity of about 5,000 m3, but many distribution terminals have large numbers of cylindrical tanks or spheres to provide total capacities equivalent to the average-sized refri­gerated terminal. In Spain, for example, there are several gas storage terminals in ports with total capacities in excess of 50,000 m3 in 30 or more pressure tanks.

Pressure vessels should be designed, fabricated, inspected and tested in accordance with a recognised pressure vessel code, such as British Standard BS 5500 (Speci­fication for Unfired Fusion Welded Pressure Vessels) and the American Standard ASME Boiler and Pressure Vessel Code; Section VIII. The layout of a storage in­stallation should be such that if product leaked, and if that leakage ignited, the effect on other parts of the complex or on people or property located outside the terminal is minimised.

The storage of quantities of liquefied gases is recognised by regulators as a haz­ardous industrial activity and has been subject to stricter controls in recent years. For example, the recent amendments to the European Union's Seveso Directive, covering major hazard sites, calls for greater distances, not only between gas storage vessels within the plant but also between the industrial installation itself and adjacent public facilities, than has previously been the case.


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