1. Introduction
The tanks with floating roof are most
widespread method for decrease the loss when volatile oil products are stored
in them. They can be used
when the tanks are open on top or they can be constructed inside in the
tanks with
fixed roofs. They can be used also when the tanks are closed and it is
not
admissible any product leak in the environment.
Floating roof
Floating roofs are strongly dependent on the good
geometry of the
shell, from
vertical position of the antirotation device
and the roof itself must not have a slope.
The cases of discrepancy from the project condition of the tanks
elements in the tanks lead to the key on the
floating
roof and to danger from fires.
Big part from tanks with floating roofs that
are in exploitation in Bulgaria are constructed in
the end of 70 years and in the beginning of the 80
years. One
of the most important problems is to find out whether these facilities
are good
for further use. In Bulgaria serious research concerning the above mentioned tanks
and
their behavior during the years and their status now have not been
made.
The main advantages of the tanks with floating
roofs are as follow:
1. Minimizing of the loss of the stored product due to its
evaporations. The factors influencing the product evaporation are:
a) temperature of the liquid;
b) the presence and the dimension of the vapor
space above the liquid;
c) the possibility for ventilating of the vapor
space;
d) the presence of the free surface of the
product.
2. As a result of the decreased evaporating of the
product the corrosion activity in the upper part shell courses is
decreased too.
3. The safety from fire is increased which is a result of the decreased
evaporation of the volatile stuff hence the lower risk from compound
between the air and evaporating product which can be easily detonated.
4. Comparing with the tanks with fixed roofs there are smaller spaces
inside the tanks with floating roofs which must be used for foam
extinguish and it allows fitting fire extinguished installations with
smaller power and their pipes to have smaller diameters.
5. The floating roof almost does not transmit the vertical longitudinal efforts to the shell. The risk from
loss of stability is decreased.
The floating roofs are implemented for the
exploitation for the first time in 1923 by “Chicago Bridge
& Iron” Company and their construction is developed in the time.
The
correct
choice of the type of floating roof can decrease the risks during the
exploitation. The factors which influence the tank
construction and kind of the floating roof are as follow:
- characteristic of the stored product;
- climate conditions and conditions on the
site;
- volume of the stored product;
- the requirements toward of the tank as a
facility as a whole.
The floating roofs may be separated in the different groups
depending on
the different criteria:
According to the presence of the fixed roof in the tank:
- external
floating roofs.
The floating roofs projected and used for open
on the top tanks.
They are made from steel and are welded. They
must bear loadings from:
a) weather conditions (rain, snow, wind);
- 250 mm rain water for the period of 24 hours.
With tank owner agreement the real values for
geographical area can be accepted for calculating the rain loading;
- the normal snow loading for Bulgaria is
st =
2,0 kN/m2. The coefficient for snow
overloading is
accepted to be equal to γf
=1,5. In this case the snow
loading may be valid condition depend on area where the tank is
situated. Snow load can be irregularly distributed on the roof .
b) live loading on the roof;
- 2,2
kN movable concentrated
loading of two persons upon 0,1 m2, who can stay on each one
point on
the roof, which is floating or is in the lower position upon its
supports;
c) loading
from rolling ladder and supporting truss;
d) loading from additional manholes, nozzles, roof drain, and seal;
e) loading of the product upon the roof;
f) single
deck roof – membrane and two neighboring compartments are punctured.
double deck roof - two neighboring compartments
are punctured.
During this check, independently from the kind
of the roof, there are not water and mobile load.
- internal floating roof
They appear because of the increased
requirements of environment safety and to decrease the loss of oil
product. These
floating roofs are fit inside under the fixed roof itself. They can be
fit even
in the tanks with supported cone roof.
They can be made from steel, aluminum, or
synthetic materials. They must bear the loading from:
a) 2,2
kN live concentrated load from
two persons upon the 0,1m2, who can stay upon any point on
the roof,
when it is floating or is in its lower position supported by its
supports;
b) if there are not other conditions, all project
calculation must be done when the product density is 0,7t/m3;
According to the kind of the construction,
the
floating roofs must be separated into the following groups:
- single
deck, without pontoons, with and supporting truss upon
the membrane
This is one from the first kind of the floating
roofs. The original design was developed by "Chicago Bridge & Iron" company. It was installed on the tank which diameter was up to 45 m. Due
to its
high boards the floating roof can continues to float even when there
are water
or snow upon it.
Advantages:
a) simplicity for execution, erecting and
maintenance;
Disadvantages:
a) this kind of roof has not positive buoyancy and
if its membrane be punctured it will sink;
b) the wind can push the accumulated water upon
the roof toward one board and it will cause the bending and probably
sinking.
Single deck floating
roof without pontoons,
with supporting trusses
- single
deck, with outsider ring of pontoons on the periphery and a membrane in
the
middle.
Area of application
a) in the regions with small wind pressure where
it is not possible to tear the membrane from the product;
b) in the regions with small sun shining where the
lower temperature of the membrane helps the decrease of evaporation of
the
product;
c) more often when tankПръстеновидно поставени ребра са използвани и при 7-те резервоара с обем V = 20 000 m3 в ПБ "Поликраище", България, виж снимката по-долу:Пръстеновидно поставени ребра са използвани и при 7-те резервоара с обем V = 20 000 m3 в ПБ "Поликраище", България, виж снимката по-долуПръстеновидно поставени ребра са използвани и при 7-те резервоара с обем V = 20 000 m3 в ПБ "Поликраище", България, виж снимката по-долу:Пръстеновидно поставени ребра са използвани и при 7-те резервоара с обем V = 20 000 m3 в ПБ "Поликраище", България, виж снимката по-долу::s have diameter D ≤ 50 m.
Advantages:
a) they have simple construction which can be prefabricated
and mounted easily;
b) they have little expenses for metal to unit of
the covered surface;
c) they do not need additional special equipment for
mounting.
Disadvantages:
a) increased possibility of membrane deformation;
b) there are peaks and dents on the central part
which are obstacle on the way of the water
to the roof drain;
c) difficulty to executing the membrane with slope toward roof
drain without waves.
The partial solution of the problem with the deviation of membrane
itself and its project is to put the stiffening
girders.
They may be put in radial or circle direction.
When the stiffening girders are put in radial direction
it is more convenient to put them above the membrane. All erecting
joints will
be executed in bottom position.
When the stiffening rings have circle shape it
is more convenient to put them under the membrane. They are not an
obstacle for
movement of the water to the roof drain. But now all erecting joints
must be made on the ceiling which is inconvenient.
Another solution for improving the correct
geometry of the membrane is to put the heavy objects (more often sand)
on the center
of the roof. The additional tension of the central part is assured.
Both solutions require more steel
and take higher manpower consumption.
Single deck roof with peripheral ring of
pontoons
- single
deck floating roof with peripheral ring of pontoons and one central pontoon
The design decision with central pontoon has
two advantages:
a) the weight put upon in the center helps
keeping of the membrane in its projected form;
b) the weight element is executed as a pontoon.
This additional pontoon allows decreasing the dimensions of the
peripheral ring.
Area of application:
a)
in the regions where
the wind pressure is
increased, where it is possible to tear the membrane from the product
and it
must be avoid;
b) when the central part is executed by roll of
sheets and it is required
to stretch the membrane;
c) more often in the tanks with diameter D > 50
m.
Advantages:
a) improved geometrical form of the central
part;
b) increased buoyancy of
the roof in the case of punctured membrane;
c) simple construction for execution, erecting
and maintaining;
d) relatively small metal expenses for unit
covered surface.
Disadvantages:
a) complicated detail for roof drain.
Single deck floating roof with peripheral and
central pontoons
- double deck
floating roof, with two membranes
covering all surface of the roof and structure between them.
Area of application:
a) in regions with increased wind pressure, where
it is possible to tear the membrane from the product and it must be
prevented;
b) in regions with increased sun shining when it is
necessary to decrease temperature of the membrane which contacts with
the
product;
c) more often when the tank has diameter D > 50 m.
Advantages:
a) the construction has enough stiffness and
the geometrical form is well supported;
b) increased buoyancy of
the roof comparing with the single deck roof;
c) decreased deformation when the snow load is irregular, when there is
blast of wind;
d) when the blast of wind are strong it is
possible the roof come off from the
liquid.
Disadvantages:
a) increased metal and labor consumption;
b) many welding joints must be done in the closed
space;
c) many erection devices are necessary.
Double
deck floating roof
- single deck floating roof with ring of
pontoons
in the periphery and many pontoons upon the membrane
This kind of roof combines the strengths of the
single deck and double deck floating roofs.
Area of application:
a) in region with increased wind pressure, where
it is possible to tear the membrane from the product and it must be
prevented;
b) when the central part is executed by rolled
sheets and it is required
to stretch the membrane;
c) more often in the tanks which diameter D > 50 m.
Advantages:
a) they are simple enough as a construction which
can be done easily;
b) they do not need the additional special erection
devices;
c) increased buoyancy of
the roof comparing with the single deck roof;
d) decreased deformation when the snow load is
uneven and when there is blast of wind;
e) when the blast of wind are strong it is
possible the roof come off from the
liquid.
Disadvantages:
a) increased consumption of metal for one unit
surface;
b) increased length of the welding
joints.
Single deck roof with pontoons on
the periphery and many pontoons in the middle
2. Criteria for determining the
type of the floating roof
The purpose of the tank, kind and the characteristics
of the stored product, the conditions of the site must be taken into
account
when the type of floating roof is being chosen. The correct
determination of
its kind and the facilities connected with it can assures easer
maintenance and
quick return of the investments.
a) stability of the floating roof:
It is one of the most important conditions for
the correct work. Moving loads which are not balanced upon the roof can
cause inclination
of the roof and floating with slope. There
is the higher risk for
the roof to sink there. It is necessary to assure the work of the roof
in the
projected position by means of correct constructing;
b) product
- aggressiveness
of the stored product. It is necessary to take care of chemical
characteristic of
the liquid and the possibility to damage the roof equipment which is
not metal.
Some chemical products can destroy the seals of the roof in several
weeks of
exploitation;
- density of
the product. According to the API Std. 650 volume weight of the product
which are
used when the calculations for the buoyancy are made is ρf = 0,7 t/m3. It is
not likely to store product with smaller density but it is possible to
happen.
The information for the minimal values of the ρf
allows the designer to determine the sinking of the
roof;
- steam
pressure of the product. It is one of the most important characteristic
of the
roof when the kind of the roof is being determined. According to the
NFPA the
use of the floating roofs in the tanks for storage of oil and oil
products is
recommended when the steam pressure of the liquid bigger or equal to
1,5 psia
(78 mm Hg), but not bigger than 11,1 psia (570 mm Hg). If the steam
pressure
exceeds 11-12 psia the heating during the day will causes the
considerable
increase of the steams under the floating roof. The inflation of the
central
part as balloon is possible. The cooling of the product during the
night will
decrease the pressure and will allow to the roof to restore its
previous form
with slope toward the center. It is possible when there is the
combination of
the considerably inflated membrane by insider pressure, which slope is
from the
center toward the periphery and the water upon the roof, the correct
drain of
the water to be difficult.
When the
steam pressure of the liquid is high, approaching 11,1 psia or higher
then it
is better to use double deck floating roof. Its rigid construction
keeps
it in the projected form, even when there are the steam under it and it
assure
uninterrupted drain. On the other side the double deck roofs protect
the roof
from excessive heating.
c) technological process
It is necessary to measure the speed of
filling/emptying. When we do not consider this speed and it do not
correspond to
the erected technological facilities it is possible damage and even
destroying
of the roof.
In order to fix the floating roof according to
its designed position (for avoiding its rotation), the antirotation
devices
must be put inside in the tank, parallel to the shell.
In the past the number of antirotation devices
usually was two per tank. Now during the repairing works one of the
antirotation
devices can be removed and only one antirotation device can left. It
must be
done in order to neutralize of the inclinations of the antirotation
devices
from their designed position during the erections and during the
exploitation.
When the antirotation devices are not vertical it is possible to key of
the floating roof with all consequences connected with it. It is
necessary that
the antirotation device supports all horizontal efforts which were
supported
before by two devices.
3.
Constructive requirements
a) approximately
200 mm free space must have
between external floating roofs and the shell, where the seal will be
mounted.
The modern seals assure good jam and normal work when the distance
between
shell and roof is from 100 to 300 mm;
b) minimal
thickness of the steels in the pontoons
in the membrane is 5mm (tr ≥ 5
mm);
c) the maximal distance between the roof supports
of the floating roof must not be bigger than 1000 x tr
(thickness of
the membrane);
d) distance from roof supports to welding
joint in
the membrane is not smaller than 300 mm;
e) when there are the girders stiffening the
membrane, roof supports must be put in the area in this stiffening.
f) the length
of the supports is deferent depending on its status:
- during the
exploitation – minimal height must grant the tank filling without the
liquid
jet to touch the vertical board. It is unacceptable that the floating
roof in
its lowest position to lie upon the technological facilities;
- during the
repairing works on the tank - the minimal distance depends on
considerations for
easy doing of the repairing works. It is assumed to be 1800 mm in the
lowest
point of the roof.
g) the floating roof , stepped through its
supporting constructions on the bottom or when floats on the liquid
must have
the slope toward the roof drain not less
than 1:100. For insider roofs this slope have not be considered;
h) upon the floating roofs must be
constructed foam dam which height must
not be less than 300 mm at 1 m from shell of the tank;
i) for fixing
the roof according to the design conditions the antirotation devices
must be
considered. They do not allow the rotation of the roof. They must bear
the
horizontal forces, caused by the rolling ladder, by the uneven load
from snow,
by wind and by rotation of the stored product when it is mixed or the
tank fills;
j) the drain
pipe must have the diameter not smaller
than 3” for the tank
which have the diameter D ≤ 36 m. The
drain pipe must have the diameter not
smaller
than 4” for the tanks which
have the diameter D > 36 m;
k) the pontoons situated on the periphery must be
separated by waterproofed barriers and every pontoon must have manhole
for
inspection;
l) in the
central part of the roof must have one manhole for inspection, through
which
people can go to the roof when the roof is in its lower position and
the tank
is empty.
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