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