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Mounded Lpg Vessels Are Here to Stay

Don Barber, principal and senior partner at Enmat International, summarises his presentation to a joint meeting of the Energy Institute and the Welding Institute on his experiences in the design, construction and commissioning of large mounded lpg storage projects.

For any lpg storage project, all the systems available should be evaluated and approached with caution. Safety and environmental issues are important elements in making a decision, and project costs must not dominate against the safety case.

Capital cost is important, but it should not influence the chosen design or compromise safety and operational requirements. Problems have arisen from using unskilled parties during the design phase – the project team should always be ‘competent’ for the job in hand.

Mounded tanks are semi-conventional pressure vessels, covered by a layer of soil or sand – when should mounded storage be used?

Legislation, codes of practice and practical requirements will determine the final choice. This will not automatically lead to mounded storage systems, but often it will be the preferred choice.

Two scenarios dominate the decision.

The Safety Case

The safety record of the lp gas industry is generally good, but it has been blighted by some serious incidents. The worst was the 1984 Mexico City disaster, when lpg tanks at a storage complex ruptured under fire engulfment conditions. There was considerable loss of life.

By mounding lpg tanks with an adequate layer of sand and earth, the possibility of fire engulfment and a BLEVE is removed.

Mounding allows safety distances around the facility to be considerably reduced, compared to an equivalent unprotected design. This does not imply that urban development can then be permitted up to the boundary fence of the storage complex. Every effort must be made to ensure local administrative bodies do not permit such activity.

The Environmental Case

The environmental case is important. Originally justified by reducing the area of ground required by above ground systems, it has developed into an aesthetic argument. Mounded lpg gas tanks can be landscaped to blend with the environment.

Further environmental concern results from the large volume of water required for the fire systems of conventional storage. This imposes a demand on valuable water sources, and problems associated with the drainage and disposal of the water.

The extra cost associated with a mounded system is offset by the lower cost of the land required, and the virtual exclusion of fire fighting systems. Mounded systems have also been installed where the available space, or environmental pressure, precluded traditional storage methods.

A geotechnical study is essential, based on test borings and the predicted load bearing capability of the ground. This includes predicting tank settlement and any stabilisation required. This work, carried out by a geotechnical specialist, provides an input to the lpg tank design.

Sites with high load bearing capability require only a prepared sand bed. Poor sites may require stabilisation before mounded lpg tanks can be placed. Stabilisation includes preloading the site or improvement by vibro-replacement or vibro-compaction. A poor site may have to be abandoned, or the tanks placed on a concrete foundation.

Drainage is important, and mounded tanks have the advantage over underground lp gas tanks, which require protection against possible floatation.

The difference between mounded tanks and those for conventional use is the outside superimposed load. To avoid an uneconomically thick shell, a thinner pressure vessel with flanged internal stiffening rings is constructed.

Experience indicates an economic maximum diameter of 8m and a length of 80m, giving a capacity of around 3,500m3. Most installations employ vessels around half of this capacity. Long tanks of small diameter should be avoided.

Relief valves are not designed for fire engulfment conditions. Smaller relief valves are used, typically sized at 30 per cent of the requirement for an equivalent above ground tank. In constructing the mound, an important consideration is the preparation of the sand bed. Sand close to the tank should be carefully graded and built up progressively in layers. The thickness under and around the tank should be not less than 1m. The mound surface should be protected against erosion by means appropriate to the location.

To ensure access for future inspection, individual tanks must be adequately spaced. This can vary between 0.8 and 2m, according to the tank diameter.

Mounded tanks require protection against corrosion. Different systems are used, including reinforced bituminous wrapping and high strength epoxy coatings. This requirement cannot be underestimated, and specialist advice should be sought, particularly if a long tank life is planned.

Coatings are only as good as the preparation of the tank surface, and the quality of application. Strict quality control is essential. Cathodic protection systems are required, provided for large tanks by impressed current. The cathodic protection design is part of the geotechnical site investigation.

Because the tanks have an anticorrosion coating and are buried within the mound, external examination of the tank surfaces is not possible. For a correctly designed and installed system, it is also not necessary. The regular inspection and examination of lpg tanks is site specific, and should be included in the operating manual.

Above ground lpg gas tanks have an internal inspection at not longer than 10 year intervals. For well-engineered mounded tanks, the same inspection frequency may be employed.

Conclusion

Mounded systems are here to stay and they will be used in more and more situations in the coming years.

This article is extracted from a presentation made by Don Barber to a joint meeting of the Energy Institute and the Welding Institute in London on March 9 2009.

More detailed information, including the experience of two recent mounded storage projects, can be obtained by contacting Don Barber at Enmat International on 01494 783142 donb@enmat.co.uk