Why use floating roof? SafetyReduces chance of fire as vapor
because the vapor space is normally well below the explosive limit
EffectivenessNo filling & evaporation losses. Controls HCs
emissions EconomyMost cost effective method of controlling vapor.
The vapor saved is the money saved!!!
Evaporative loss control It depends upon:1. Liquid temperature2.
Space above liquid3. Ventilation of liquid4. Available liquid
surface area
External Floating Roof Appurtenances
Roof support legs When the tank is empty, the floating roof
needs to be supported at some distance above the tank floor.
Necessary so that the roof does not foul any heating coils, drain
lines, etc. Also access will be required via the shell manholes for
the maintenance personnel. The legs consist of two concentric
tubes. The outer, shorter tube forms a housing which is welded into
the roof. The inner tube, which forms the support and is secured to
the housing with a steel pin which passes through both tubes.
Guide pole Its purpose is to prevent the floating roof from
rotating in the tank. The lower end is connected to the tank floor
and at the top to the gaugers platform, which is an extension to
the tank top access stair. Radial movement of the roof is not
restrained here as this is provided by the roof seal system which
tends to centralize the roof in the tank. The guide pole is very
often used to house level-indicating equipment. To ensure that the
product level in the pole is the same as the level in the tank,
slots are cut in the pole to allow the liquid levels to
equalize.Roof sealsThe gap between the inside of the tank shell and
the outer rim of the floating roof is normally about 200 mm. To
prevent the escape of vapor from this gap and to minimize the
amount of rain entering the product here, a sealing system is
required. Characteristics of sealing systemSealing system has to be
flexible enough to Allow for any irregularities in the construction
of the roof and shell when the roof is travelling up and down. For
any radial or lateral movement of the roof due to wind or other
action.TypesMechanical sealsThe seal consists of a ring of thin
galvanized or stainless steel plates each about 4 meters long and
1.2 meters deep bolted together with sealing strips and countersunk
bolts. This ring of sealing plates is kept in close contact with
the shell by a series of weighted or spring-loaded pantograph
mechanisms mounted on the outer rim of the pontoons. The lower edge
of the plates is immersed in the product and the upper edge is
roughly level with the top rim of the pontoons. The gap between the
plates and the pontoons is sealed by a flexible-shaped fabric which
is connected to the top of the ring of plates and to the pontoon
rim by clamp bars and bolt. Vapor can escape however where
irregularities in the shape of the shell allow gaps between the
plates and the shell. To alleviate this problem the seal ring can
be made to accommodate such changes in shape by the introduction of
flexure points in the seal plates. These flexure points are formed
by vertical shallow V-shaped creases in the plates at about 560 mm
centers and the open top of these creases is capped to prevent
vapour emission. The creases, as well as allowing the seal ring to
conform to the shape of the shell, also act as stiffeners where the
thrust from the pantograph mechanisms is transmitted to the seal
ring. To ensure the dispersal of any static or lightning, a series
of thin flexible stainless steel shunts are connected between the
bolt rings of the roof and the seal ring,
Liquid-filled fabric sealThe liquid-filled fabric seal consists
of a petroleum and abrasion resistant synthetic rubber type tube
filled with 200 to 250 mm depth of sealing liquid. This tube is
positioned in the rim space and is supported at its lower end by a
bottom ring on a hanger system. The sealing liquid ensures close
contact of the tube on the tankshell and the outer rim of the
floating roof. The liquid may be fuel oil or the same liquid as
that stored in the tank. In non-freezing climates water may be used
as the sealing liquid. The sealing liquid makes the tube take up
whatever rim space is available around the circumference and
automatically compensates for discontinuities in the shell or roof
rim profile.. This flexible ring has a fixed circumference and
therefore automatically aligns to any discontinuities in the major
or minor axes of the tank and roof.
Resilient foam-filled sealThis type of seal, is similar to the
liquid-filled seal except that the tube is filled with pre-formed
blocks of resilient urethane foam, rather than a liquid and
there-fore does not require a bottom hanger support system
Advantages of this type of seal are that when it is mounted just
above the liquid level in the rim gap, any small tears or abrasions
in the tube will not cause a serious collapse of the seal. Also,
when replacement is finally necessary this may be done entirely
from above the roof.Compression plate type sealsSecondary sealsIt
was found that even properly maintained primary seals, permitted
vapour losses from the rim gap due to the swirling, scouring action
of the wind within the tank. To counter this, independently mounted
spring action compression plate secondary seals, formed from thin
galvanized steel or stainless steel sheet, were mounted above the
primary seal thus excluding the wind from the rim gap. The spring
action, due to the induced compression in the plates ensures a
close seal between the abrasion resistant polymer seal tip and the
shell.Primary sealsThe success of compression plate secondary seals
led manufacturers to develop this type of design as a primary seal
also the main difference being that the primary seal deflects
downwards such that the tip of the seal is usually just above the
level of the stored liquid.
Seals incorporating foam damsIt is a short vertical steel wall
that ensures that as the top-injected fire fighting foam spills
down the inside face of the shell; it is concentrated within the
rim space and does not spill out over the surface of the roof Rim
ventsTo vent off excessive vapors, a vent tube may be fitted
between the outer rim and the upper deck of the pontoon where
either a pressure relief valve or a free vent is fitted.Drain
plugsUsed when tank is out of service to drain rainwater etc.Fire
fightingFires in floating roof tanks are usually limited to the
area between the shell and the rim of the floating roof i.e. the
rim space. Several sets of foam generating and injection equipment
areProvided, equi-spaced around the tank periphery which ejects
foam incase of fireRim fire detection1. A small bore Plastic tube
which runs around the whole circumference of the rim area. This
tube is connected into a more substantial piping system in both
flexible and hard piping, which is connected into a fire fighting
alarm or initiation control unit on the gaugers platform. The rim
tubing is subjected to an internal pressure and in the event ofa
fire, the tubing melts releasing the pressure thus triggering an
alarm and/or actuating the fire fighting system.
2. Another method is to have a series of tensioned wires with
fusible links managed around the rim space. Again, in the event of
a fire a fused link would cause the alarm to be raised.
Roof drainsThe rainfall which accumulates on the surface of the
floating roof is drained to one or more sumps set into the low
points of the top roof membrane. The sump is drained through a
closed pipe work or hose system which operates within the tank.
Articulated piping systemThis type of drain uses a solid steel
piping system with a series of articulated knuckle joints; it is of
rugged construction but can suffer from seizure of the articulated
joints due to the slow movement of the roof or lengthy periods of
inactivity due to the roof being stationary
Armoured flexible hoseThis type of system eliminates the need
for articulated joint, but it has been known for the hose to snag
on internal tank fittings or for it to be trapped under a roof
support leg as the roof grounds on the tank floor.
Syphon drainsThis system automatically drains water from the
roof membrane and discharges it directly into the product where it
gravitates to the bottom of the tank, to be collected in the floor
sumpBleeder vents lts purpose is to vent the area below the landed
roof in its stationary position, allowing air to enter the space
under the roof as the product is evacuated from the tank, thus
avoiding a vacuum in the space and then to allow the air under the
roof to escape when the tank is being refilled, avoiding a pressure
under the roof.The valve is a simple device consisting of a short
vertical trunking which forms a valve seating and this is welded to
a corresponding aperture in the deck. Through the centre, and
supported off of this trunking, passes a vertical guide tube which
houses a push rod on to which is attached a disc which forms the
valve lid. The length of the push rod is such that as the tank is
emptied, the rod contacts the floor plating before the roof support
legs land and the valve opens. Freely venting the space beneath the
deck. Similarly, on refilling the tank the valve closes after alI
the air beneath the roof has been expelled and the roof floats.
The gaugers platformThe gaugers platform is a small access area
of about 4 square metres; the platform overhangs the shell to allow
the guide pole to pass through it so that access can be gained to
the guide pole which usually houses the product level indicating
equipment .Also the platform is used as an attachment for the
rolling ladder which gives access to the floating roof.
Rolling ladderThe rolling ladder is the means of access on to
the floating roof from the gaugers platform. The upper end of the
ladder is attached to the gaugers platform by hinged brackets. The
lower end is provided with an axle with a wheel at each side of the
ladder .The wheels run on a steel track mounted on a runway
structure supported off the roof so that, as the roof moves up and
down, the hinged ladder can take up a varying angle as
required.Deck manholesOne or more of these square or circular
manholes are provided in the deck of the roof to allow access to
the underside of the roof from the top, when maintenance work is
required whilst the tank is out of servicePontoon manholesEach
pontoon of a floating roof is a separate buoyancy compartment and
must be periodically checked to ensure that it is dry and free from
leaks. Hence each compartment has its own inspection
manholeSample/dip hatchThe sample/dip hatch is fitted either to a
nozzle which projects through one of the pontoons or it is fitted
to the top of the gauge pole. It is illustrated in Figure 6.31 and
may be used as follows: To measure the depth of product in the tank
using a dip tape. This may be done as a check on the correct
functioning of the automatic level gauge. To take a sample of the
tank contents To take the temperature of the tank
contentsElectrical continuityIn the event of a lightning strike on
the tank, or a build-up of static electricity within the tank due
to product movements, there needs to be a secure electrical bond
between the roof and the tank to make certain that any electrical
charge is conducted directly to earth, thus ensuring that a spark
cannot be created between the roof and the tank which could cause a
fire