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The storage of crude oil and volatile petroleum products in tanks fitted with floating roofs confers significant advantages over storage in fixed roof tanks where the entire surface of the stored material is exposed to a vapour space. Where a free liquid surface exists, molecules will escape from it to the vapour phase at a rate depending on a number of physical characteristics in an attempt to raise the vapour content of the atmosphere above the liquid to its saturation limit.

Major factors influencing vapour loss from a fixed roof tank containing a free liquid surface are:-

· Temperature
· Vapour pressure
· Wind
· Vapour space volume
· Tank size and proportions
· Time
· Frequency of filling and emptying
· Surface condition of the shell plates

Since the mechanism of vapour loss is a feature of the free liquid surface, then the placing of an impermeable barrier between the liquid surface and the vapour phase will prevent liquid molecules from escaping to the vapour phase. This is achieved by the floating roof and a variety of floating roof types have long been in service in the oil industry.

Storage tanks, being generally large and constructed in the field from fairly thin steel plate are subject to deviations from the ideal cylindrical form. The extent to which these deviations are permissible in a new tank has been defined in the various Codes of Practice to which these structures are built e.g. BS 2654 and API 650. During the lifetime of a tank, further changes in shape may take place due to a variety of circumstances e.g. foundation settlement and seismic disturbance. Because of these departures from the perfect tank shape, it is not possible to build the floating roof such that it fits snugly against the tank shell, like a piston. For the practical reasons stated, it is necessary to build the roof such that its maximum diameter is less than the inside diameter of the tank shell. The diametrical amount by which the floating roof is smaller than the tank shell is generally 400mm i.e. with the roof centrally situated, an annular gap exists between roof and shell of 200mm (8 ins nominal).

The gap between floating roof and shell requires to be closed, as effectively as possible, with some system which will permit the free vertical movement of the roof and yet have sufficient flexibility to cope with local, or general, changes in the annular gap. Various mechanisms have been designed to meet this need and many have given excellent service over long periods and these items are known as PRIMARY SEALS. A few of the main types of primary seal are shown in figs. 1, 2, 3, and 9.


Fig. 2

Fig. 3

Fig. 9

It should be noted that while the nominal rim gap is normally 200mm (measured radially) it may be deliberately increased in response to particular conditions. For example, where exceedingly poor foundation conditions exist and tank settlement is likely or where the tank diameter is around 100m then rim spaces of 300mm may be adopted. Seal designs to accommodate these special conditions can be arranged.

Riveted tanks, due to the shell plate overlaps and rivet heads present problems requiring special consideration in seal design and fitting. Always be sure to advise on whether the tank shell is riveted or welded when enquiring for seals.