Figure 5.11 Typical single-wall tank — LPG storage



Tank filling is through a top entry spray ring; tank discharge is taken from the bottom via submerged pumps.

Any leakage of liquid from such tanks is contained in a bunded area around the tank. Tanks of this type will normally be designed to an appropriate code which requires a bund to contain a spillage equal to the tank contents in the unlikely event of tank failure.

Such tanks are classified as a single containment storage system.

Single containment — double-wall tanks (for LPG and LNG)

Single containment tanks are most usually provided with an outer shell surrounding the primary tank. They are constructed so that only the primary containment is re­quired to meet the low-temperature requirements for product storage.

The outer shell is primarily for the retention and protection of loosely filled insulation and to contain nitrogen purge-gas pressure. The outer shell is not designed to contain refrigerated liquid in the event of leakage from the primary container.

Figure 5.12 shows a typical double-wall tank for LNG. This example shows two com­plete tanks with an annular gap of about 0.5 metres in between. The annular space may be filled with granular insulation such as perlite (see Table 3.1) and a nitrogen breather system is provided to accommodate volume changes in the inter-space, re­sulting from atmospheric pressure changes and inner tank expansion or contraction. A suspended roof, inside the outer tank dome, fits inside the inner tank. Here, the insulating space is filled with cargo vapour flowing freely from the main vapour space.


Figure 5.12 LNG tank—concrete bund

Figure 5.13 LNG tank — double-wall


Such a tank is also classified as a single containment storage system. More recent versions of this design are provided with a full-height concrete bund wall. This is constructed close to the tank, usually within one or two metres from the outer shell.

Figure 5.13 shows another typical double-wall tank. In this case an elevated founda­tion, to eliminate frost heave problems, is used. Furthermore, there is a complete dome cover on the inner tank to enable inner tank leakage to be detected in the nitrogen gas in the insulation space.

The bund wall contains any liquid leakage. Also, its position minimises boil-off rate from any leakage by preventing liquid from spreading over a large area of warm ground. Another advantage is that any leaked vapour within the bunded area is released at a high level which assists with dispersion of the gas cloud.

Double containment storage tank (for LPG and LNG)

Double containment storage tanks are a development of the single-wall tank. They provide increased safety margins against tank leakage by introducing an extra inner tank.

Figure 5.14 illustrates this design. Here the liquid is contained in an inner shell which is surrounded by an outer shell, also of low-temperature steel.

The outer shell acts in the same way as a bund and contains any liquid leakage from the inner shell — but in this case it also avoids vapour release to atmosphere.


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