Floating Roof Tanks - an overview (2024)

Internal floating roof tanks (IFRTs) are also used to store fluids with a moderate vapor pressure under normal atmospheric conditions. Internal floating roofs adjust position with the liquid level in the tank, just like the EFRT, but the IFRT has an additional fixed roof above the floating roof to prevent wind and rain from affecting the internal floating roof, see Figure16.

Atmospheric storage tank—this type of tank operates from atmospheric pressure to 0.5psi/0.034bar.

Cone Roof Tank—this type of tank is a low-pressure storage tank with a fixed, cone-shaped roof

Closed floating roof tank—this has an internal floating roof but eliminates natural ventilation of the tank vapor space. Instead, the CFRT is equipped with a pressure-vacuum (PV) vent and may even include a gas blanketing system such as that used with fixed roof tanks, these tanks are designed as in Appendix C of the API Standard 650

Fixed roof—this is a low-pressure tank with a roof welded to the shell, regardless of roof design or support methods.

Double wall storage tank—is a tank with an inner wall to contain a liquid (as used in LNG storage tanks), it has an annulus space filled with insulation and an outer wall.

Bullet—this type of tank is a long cylindrical high-pressure storage vessel that is shaped like a bullet.

Horton sphere—this is a spherical vessel used to store liquids and gases at high pressure.

Intermediate storage (holding) tank—used for temporary storage of liquid until it reaches a specified state, after which it is pumped downstream for process.

Fires in floating-roof tanks are usually in the area between the tanks steel shell and the rim of the floating roof. A fire detection system should be installed on floating roof tanks. The fire detection system consists of a foam system designed to deliver a flame-smothering foam mixture that is injected into the space between the tank shell and the foam dam. The foam extinguishes the fire. The foam dam, consisting of a short vertical plate, is welded to the top pontoon plate at a short distance from the seal. The height of the foam dam is higher than the upper tip of the seal; thus allowing the whole seal area to be flooded with the foam and extinguishes the fire effectively. Fig. 17.39 shows a typical foam dam installed on a floating roof tank.

Figs. 17.40A and 17.40B show “multiple chamber” method and a rim seal with an automatic foam fire suppression system installed on a floating roof tank in which the foam is discharged, by the foam chambers or foam pourer that are mounted at an equally spaced distance around the tank perimeter. The system is designed in accordance with NFPA-11, Standard for Low-Medium High-expansion Foam.

Figs. 17.41A and 17.41B are schematics showing typical arrangements of the fire protection for a floating roof tank. In this system, when a fire is detected, a propriety foam compound is injected into the firewater system which is routed into a foam generating point. The foam generator draws air into the mixture which causes the foam to expand. The foam is injected into the tank via a pourer which injects the foam onto the surface of the roof where the foam flows down the roof and collects in the rim space. Fig. 17.42 shows a tank fire where foam is being released which suppresses the fire.

A fire at a floating roof tank was caused by switching operations (CCPS, 2007b).

An 80,000 barrel … floating roof storage tank exploded and burned while being filled with diesel oil …. The tank contained approximately 7,000 barrels of diesel oil at the time of the incident and had previously contained gasoline ….

Initially, the fire was blamed on a lightning strike, but a thorough incident investigation … found the causes included an improper procedure for switching the content of the tank from gasoline to diesel oil, and an unsafe filling procedure.

In general the following guidelines should be followed:

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The truck should be bonded to the loading piping before opening any compartment.

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The loading arm should be extended down to the bottom of the tank so as to avoid splashing.

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Liquid velocities should be kept below 1m/s until the bottom of the fill spout is covered with liquid and there is no splashing. The velocity can be increased after the fill spout is covered.

(a)

Most fires on floating-roof tanks are small rim fires caused by vapor leaking through the seals. The source of ignition is often atmospheric electricity. It can be eliminated as a source of ignition by fitting shunts—strips of metal—about every meter or so around the rim to ground the roof to the tank walls.

Many rim fires have been extinguished by a worker using a handheld fire extinguisher. However, in 1979, a rim fire had just been extinguished when a pontoon compartment exploded, killing a firefighter. It is believed that there was a hole in the pontoon and some of the liquid in the tank leaked into it.

Workers should not go onto floating-roof tanks to extinguish rim fires [5]. If fixed firefighting equipment is not provided, foam should be supplied from a monitor.

(b)

The roof of a floating-roof tank had to be replaced. The tank was emptied, purged with nitrogen, and steamed for six days. Each of the float chambers was steamed for four hours. Rust and sludge were removed from the tank. Demolition of the roof was then started.

Fourteen days later, a small fire occurred. About a gallon of gasoline came out of one of the hollow legs that support the roof when it is off-float and was ignited by a spark. The fire was put out with dry powder. It is believed that the bottom of the hollow leg was blocked with sludge and that, as cutting took place near the leg, the leg moved and disturbed the sludge (Figure 5-15).

Figure 5-15. Oil trapped in the leg of a floating-roof tank caught fire during demolition.

Before welding or burning is permitted on floating-roof tanks, the legs should be flushed with water from the top. On some tanks, the bottoms of the legs are sealed. Holes should be drilled in them so they can be flushed through.

(c)

Sometimes a floating roof is inside a fixed-roof tank. In many cases, this reduces the concentration of vapor in the vapor space below the explosive limit. But in other cases, it can increase the hazard, because vapor that was previously too rich to explode is brought into the explosive range.

A serious fire that started in a tank filled with an internal floating roof is described in reference 6.

As a result of a late change in design, the level at which a floating roof came off-float had been raised, but this was not marked on the drawings that were given to the operators. As a result, without intending to, they took the roof off-float. The pressure/vacuum valve (conservation vent) opened, allowing air to be sucked into the space beneath the floating roof.

When the tank was refilled with warm crude oil at 37°C (100°F), vapor was pushed out into the space above the floating roof and then out into the atmosphere through vents on the fixed-roof tank (Figure 5-16). This vapor was ignited at a boiler house some distance away.

Figure 5-16. Tank with internal floating roof.

The fire flashed back to the storage tank, and the vapor burned as it came out of the vents. Pumping was therefore stopped. Vapor no longer came out of the vents, air got in, and a mild explosion occurred inside the fixed-roof tank. This forced the floating roof down like a piston, and some of the crude oil came up through the seal past the side of the floating roof and out of the vents on the fixed-roof tank. This oil caught fire, causing a number of pipeline joints to fail, and this caused further oil leakages. One small tank burst; fortunately, it had a weak seam roof. More than 50 fire appliances and 200 firefighters attended, and the fire was under control in a few hours.

The water level outside the dike rose because the dike drain valve had been left open, and the dike wall was damaged by the firefighting operations. The firefighters pumped some of the water into another dike, but it ran out because the drain valve on this dike had also been left open.

An overhead power cable was damaged by the fire and fell down, giving someone an electric shock. The refinery staff members therefore isolated the power to all the cables in the area. Unfortunately, they did not tell the firefighters what they were going to do. Some electricity-driven pumps that were pumping away some of the excess water stopped, and the water level rose even further. Despite a foam cover, oil floating on top of the water was ignited by a fire engine that was parked in the water. The fire spread rapidly for 150 m. Eight firefighters were killed and two were seriously injured. A naphtha tank ruptured, causing a further spread of the fire, and it took 15 hours to bring it under control.

The main lessons from this incident are as follows:

1.

Keep plant modifications under control and keep drawings up to date (see Chapter 2).

2.

Do not take floating-roof tanks off-float except when they are being emptied for repair.

3.

Keep dike drain valves locked shut. Check regularly to make sure they are shut.

4.

Plan now how to get rid of firefighting water. If the drains will not take it, it will have to be pumped away.

5.

During a fire, keep in close touch with the firefighters and tell them what you propose to do.

(d)

Roof cracks led to an extensive fire on a large (94,000-m³) tank containing crude oil. The cracking was due to fatigue, the result of movement of the roof in high winds, and a repair program was in hand. A few days before the fire, oil was seen seeping from several cracks, up to 11 in. long, on the single-skin section of the floating roof, but the tank was kept in use, and no attempt was made to remove the oil. The oil was ignited, it is believed, by hot particles of carbon dislodged from a flarestack 108m (350ft) away and 76m (170ft) high, the same height as the tank. The fire caused the leaks to increase, and the tank was severely damaged. Six firefighters were injured when a release of oil into the dike caused the fire to escalate. The fire lasted 36 hours, 25,000 tons of oil were burned, and neighboring tanks, 60 m away, were damaged. The insulation on one of these tanks caught fire, and the tank was sucked in, but the precise mechanism was not clear [9,10].

Determine the evaporation loss for an internal floating roof tank given the following:

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Tank diameter 200 ft

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Liquid-mounted primary seal only and an average seal fit

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Product true vapor pressure of 10 psia

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Welded deck with self-supporting fixed roof

1.

Use Figure1 for the welded deck and self-supporting fixed roof.

2.

From Table 1 select the seal axis. The seal axis for the example problem is F.

3.

Locate the point of intersection F1 between the seal axis F and the tank diameter contour for the 200-ft diameter tank.

4.

From the point F1 traverse horizontally to intersect the reference axis R at R1.

5.

Locate the true vapor pressure point P1 corresponding to 10 psia on the pressure axis P.

6.

Connect the point R1 on the reference axis R and the point P1 on the pressure axis P and extend in to intersect the evaporation loss axis L at L1.

A dense-plume model was used to simulate the atmospheric dispersion of acrylonitrile, which is heavier than air. Reference Table 19 (dense gas, 60-min release, rural conditions, atmospheric stability D, wind speed 3m/s) of the RMP guidance was utilized to determine the end-point distances. In accordance with the definition of damage state DS1, no release occurs although there is some seismic loading. For damage states DS2–DS5, RAPID-N predicts the release of acrylonitrile and evaporating pool formation, which results in an end-point distance of 2.4km with occurrence probabilities ranging between 1.5×10⁻¹ and 2.2×10⁻³. The released amount of substance shows a pronounced increase for DS4 and DS5, but the end-point distance is not affected. Similar to the analysis for tank T-1, the dike’s holding capacity is sufficient to keep the substance confined within the dike, this time even for the worst-case scenario, and hence evaporation is limited. This demonstrates the importance of measures to contain the spill. The results of the analysis for tank FR-1 are summarized in Table10.8 while the impact areas are shown in Fig.10.6.