ERECTING TANKS
In tank construction there
are several ways to erect
a tank. The most
traditional method is to start
with the tank bottom and,
after that, place the first
course and any next course
on top of the previous one.
This method has proven
itself in the past and is still
being used on many sites.
There are a few
disadvantages with this
construction method, of
which 'working at height' and
'instability during
construction' are the major
ones. Working at height
requires extensive
scaffolding and involves the
difficulties of working with
rather heavy equipment at
this high level.
Instability can be caused by
wind during construction,
because at certain stages of
the construction process the
wind girders/gangway or roof
structure, which normally act
as stiffeners on a completed
tank, are not yet in place.
Strong winds can result in
extreme damage to the tank
shell if the correct (and
expensive) precautions are
not taken.
Another potential difficulty
is that with small settlements
it can be difficult to place the
final courses.
The use of jacking systems
during construction can avoid
the problems mentioned
above. As each course is
installed at ground level both
inside and outside welding
works can be executed with
scaffolding only reaching
about two meters high. The
automatic welding machines
that are frequently used in
tank construction are easily
installed at this low level. In
terms of safety, speed and
quality this is a big
advantage.
The jacking itself is a setby-
step procedure where the
top shell courses are lifted in
consecutive steps of 100 mm
each. When the shell is at the
level of the shell course
height (2000-3000 mm), the
new plates are put in place.
As the lifting process is a
matter of 2-3 hours
(depending on the tank-size)
the new plates are installed
and temporarily connected
the same day. This to avoid
the possibility that changing
weather conditions will affect
the lifted tank.
The easy use of fully
automatic machine-welding at
low level is one of the biggest
advantages of this method,
as a welding quality of almost
100 % can be achieved. This
reduces the time lost for
repairs. Also, automatic
welding reduces large
temperature differences in
the tank shell and therefore
reduces the 'buckling' effect.
If the roof is already in
place, the roof acts as a
stiffener to secure the
roundness of the tank shell.
Alternatively, with floating
roof tanks, the wind girder
which is placed immediately
on the first (top) course
guarantees the roundness of
the shell.
