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.1
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.
