Storage in above-ground spheres or cylinders
When considering pressure storage in above-ground tanks, the storage terminal layout requirements effectively mean that pressure storage vessels, pump bays and loading and discharge facilities must be at a sufficient distance from sources of ignition. Furthermore, they must be accessible for fire-fighting and be laid out in such a way that spillage from one tank or work area does not flow under any other tank or to any other important work area. In addition, each pressure vessel must be provided with at least one pressure relief system connected directly to the vapour space, a level gauge, pressure and temperature indicators and a maximum level device such as a high level alarm.
Figure 5.7 Fully pressurised storage in horizontal cylindrical tanks
If liquid should leak from a pressurised tank, vapour evolves due to rapid flashing. Therefore, maximum ventilation is essential to ensure dispersion; this is facilitated by laying out the ground under a tank in such a way that any leak drains to a catchment area where spill vaporisation can be more closely controlled. Retaining walls around a catchment area should not be more than 0.6 metres high and should not totally surround the area to allow for air circulation.
Figure 5.7 shows a typical horizontal, cylindrical pressure vessel tank and associated pipework.
Each gas storage vessel and each bulk loading or discharge facility must be provided with a fire protection system which will ensure the tank's structural integrity under fire conditions. Fire protection may be achieved by the use of spacing, location, insulation or similar systems and by the use of cooling water. All above-ground pressure storage should be provided with fixed spray systems to ensure that all tank surfaces and product pipelines in the immediate vicinity exposed to thermal radiation are protected by a film of cooling water.
Storage in mounded horizontal cylinders
In recent years increasing concern about the safety of areas surrounding gas storage depots in case of a major containment failure has prompted greater interest in mounded horizontal cylindrical storage tanks. Some countries have even gone so far as to specify the use of mounded tanks in certain circumstances. In Germany, for example, planning approval is conditional on burying all pressurised gas storage tanks over 1,000 m3 in capacity under an earth cover while existing larger capacity above-ground pressure vessels will have to be replaced by mounded pressure tanks in time. In the early 1990s France announced that it was to prohibit the construction of LPG spheres greater than 500 m3 in capacity.
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The first mounded gas storage tanks were installed in Germany in 1959 and today such units can not only be found throughout Europe but also on most continents. The largest mounded cylinders are 4,000 m3 in size but several countries utilise small mounded, semi-underground and fully buried tanks in the size range 10 to 100 m3 to support marketing operations such as automotive LPG retailing and networked LPG supply systems.
The earth covering mounded tanks protects them from fire engulfment and radiation from close-proximity fires so that extensive water deluge systems are unnecessary and the BLEVE form of catastrophic tank failure is impossible. Furthermore, the amount of space required between mounded tanks is much less than that specified for above-ground tanks so that less land area is required. In practice, mounded vessels are usually installed in batches of up to six units with the spacing between individual tanks as little as 0.8 to 2.0 metres. Other groups of up to six tanks are then situated a safe distance away.
The question of appropriate landscaping can also be an important factor in terminal layout and design. For example, at a recent LPG storage development in Europe, immediately adjacent to an area designated as one of outstanding natural beauty, the company's choice of mounded cylindrical tanks was heavily influenced by the need to ensure that no part of the facility could be seen above the tree-line.
Another advantage of mounded tanks relates to the sun impingement factor. Because direct sunlight does not fall upon such units, the design pressure of a tank for the storage of LPG can be set at about 12 barg, in contrast to a figure of 16 to 18 barg for an above-ground tank. Thus, the shell thickness of the mounded vessel does not have to be so great. The use of internal ring stiffeners along the length of the tank also helps avoid the need for uneconomically thick shells.
Most mounded tanks are placed on beds of sand, a type of foundation which provides uniform support over the length of the vessel. The number of connections and nozzles fitted to mounded tanks is kept to a minimum; experience has shown that the optimum arrangement is to have a top outlet only, in conjunction with a submersible pump.
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One of the alleged drawbacks of earth-covered pressure tanks is the difficulty in providing adequate protection against corrosion although the service records of these units have shown that this is not a problem. After the finished tank is shot blasted, it is then primed prior to being wrapped with a heavy anti-corrosion coating. Polyester glass-flake coatings have proved to be suitable in this respect. A compatible cathodic protection system, usually of the impressed current type, completes the corrosion protection package.
Underground storage
Pressurised LPG may also be stored in underground caverns either leached from salt deposits or mined from rock formations.
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