Figure 5.8 Rock cavern LPG storage
The principle of mined rock cavern storage is that the caverns are set at such a depth below ground level that the static head of the water-table is greater than the pressure of the stored product. There is, therefore, a pressure gradient towards the inside of the cavern and leakage of product to the rock strata is thereby avoided.
Ground water inflow is collected in a water pit on the cavern floor and then pumped to the surface. The LPG is discharged from the cavern by means of submerged pumps. Figure 5.8 shows a mined cavern suitable for LPG.
In the case of a cavern leached from salt deposits the product is stored above a brine solution. On filling, LPG is pumped into the upper part of the cavern and displaces the brine. After passing through a degassing process, the brine is stored in a pit. The LPG is normally retained in the cavern by brine pressure. For delivery to ship or pipeline, LPG may be discharged either by brine displacement or by means of submerged pumps. Figure 5.9 shows the basic layout of a salt cavern.
Figure 5.9 Salt cavern LPG storage
5.2.2 Storage in semi-pressurised spheres
Spheres for the semi-pressurised storage of liquefied gases can be fabricated on site. Each may provide up to 5,000 tonnes of storage capacity. Normally, this type of storage requires vapour pressure control by means of a reliquefaction plant (see Figure 5.10).
Figure 5.10 Semi-pressurised storage in spheres
5.2.3 Refrigerated storage at atmospheric pressure
Refrigerated storage is considered economical when storing liquefied gas in quantities of more than 5,000 tonnes.
Because cargo leaking from a refrigerated storage tank is already close to atmospheric pressure, it would boil rather than flash-off. Therefore, any liquid release which takes place requires heat inflow for evaporation. The source of heat is the ground on which the liquid has fallen. The larger the surface area the greater the heat inflow and hence the greater the vapour generation rate. The use of a walled bund area around a refrigerated tank, therefore, reduces the potential vapour cloud and prevents the spread of a spilled liquid.
The tanks commonly used to store gases in a fully refrigerated state are of the following types:
Single containment — single-wall tanks (for LPG)
Traditionally, refrigerated LPG has been stored in single-wall tanks. One such is shown in Figure 5.11. There are large numbers of this tank type throughout the world. Specific details of construction vary from tank to tank but the main features are those of a low-temperature steel shell surrounded by insulation. The insulation is held in place by an outer cladding which should be waterproof.
For some varieties of this tank type, additional insulation can be provided above the liquid level in the form of a suspended ceiling. Within the tank base, heating coils, or sometimes an air gap, are fitted to avoid freezing the ground and damaging the foundations by frost heave.
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