Internal Floating Roof Tank VS External Floating Roof Tank, Which Is Better?

What is a floating roof tank?

Difference between internal floating roof tank and external floating roof tank

         Internal floating roof tank                    external floaitng rof tank                             fixed roof tank

The transmitter and the receiver of the two ways of storing liquid are same, specific differences between external floating roof tank and fixed internal floating roof tank as follows:

• Both can reduce evaporation loss greatly;
• There is a floating roof covered upon the floating tank liquid surface, liquid stored in the storage tank is separated with the air, to reduce air pollution and the danger of fire and explosion, ensure the liquid quantity easily. It is especially suitable for storing advanced gasoline, jet fuel, and toxic and easily contaminated liquid chemical;
• Fixed roof tank, including internal floating roof tank, can prevent the stored liquid from being pollute by wind and sand storms, rain and snow, and dust. They can guarantee the stored liquid quality in all weather conditions, are famous as "all – weather storage tank";
• In the same sealing condition, compared with external floating roof tank, internal floating roof tank can further reduce the volatilization loss, as a result of that, compared with double deck floating roof on external floating roof tank, shaded by fixed roof and the gas phase layer between fixed roof and internal floating plate have a more significant effect on heat insulation;
• Unlike floating plate opened upon external floating roof tank, there can be no rain and snow load on the internal floating roof in internal floating roof tank. With less load, simple and light structure, internal floating plate can save accessories like central drain and ladder, constructed  and maintained easily, also can prevent the sealing material from aging by the intense light;
• Compared with external floating roof tank, internal floating roof tank is a promising tank with more advantages, also its applications are wider and wider. So far, perfect-designed internal floating roof tank is the best and the least investment method to control volatilization loss from the fixed roof tank. Internal floating roof tank can be used to store liquid chemical product, including volatile oil like gasoline and jet fuel, aldehyde, alcohol like methanol and ethylalcohol, ketone like acetone, benzene like benzene, toluene, xylene, and styrene.

Internal floating roof tank

Characteristic of internal floating roof tank

Characteristic of internal floating roof structure

           Internal floating roof              Internal floating roof tank structure             Internal floating roof tank seal

An internal floating roof or internal floating plate is a head cover floating with oil and liquid up or down on the oil surface. Adopting the internal floating roof cover in the liquid surface is recognized as the most ideal economic and practical method to reduce the evaporation loss of oil products.

Internal floating roof tank advantages

• Obvious environmental economic benefits：Under specific climate conditions, according to the historical data shows that the economic value of the volatilization of oil and gas reduced by internal floating roof tank within 3-6 months is equal to the internal floating roof installation and procurement costs, which can recover the cost of internal floating roof in half a year. It can be suitable for all kinds of butt and lap pot, it does not pollute oil product or chemical, but can reduce the volatilization loss of medium, prevent the air pollution, it is an ideal internal floating roof and floating plate with low cost, short construction period, corrosion resistance, space-saving and long service life, etc.
• High safety：The internal floating plate floats on the liquid surface, which makes the liquid phase have no evaporation space, reducing the evaporation loss of 85% ~ 90%. At the same time, through the floating plate, it can cut off air and liquid, reduce air pollution and the occurrence of fire.
• Long economic life: As there is no gas space upon the liquid surface in internal floating roof tank, it can reduce the oscillation of the liquid in the process of conveying the liquid in the tank, prevent the inside wall of the tank from impact or the internal pressure by the fluid, reduce corrosion of tank top and tank wall, and prolong the service life of the storage tank.

• Working pressure: atmospheric storage tank;
• Working temperature: -30℃～150℃;
• Liquid medium: various oil products, including naphtha, gasoline, diesel, kerosene, crude oil, fuel oil, heavy oil, benzene, xylene, methanol, ethanol, etc;
• Tank volume: ≤10,000 m³.

External floating roof tank

Concept of external floating roof tank

Characteristic of external floating roof tank

• Compared with fixed roof tank without a floating roof, there is no evaporation space in external floating roof tank, eliminating the so-called “big breathing loss” in principle.
• Evaporation can not only resulted in the loss of product quantity, but also affects product quality, and pollute the environment simultaneously. It is prone to explode when space combustible medium mixed with air reaches a certain limit, so it is a necessary measure to fix a external floating roof on the open top cylindrical steel tank.
• There is a 100 mm – 500 mm gap between tank shell and external floating roof to prevent the tank from jamming during the running process. External floating roof tank also uses edge sealing system to reduce edge evaporation. 
• The structural form of external floating roof is similar to internal floating roof, both of them have a steel tray form, floating cabin form, single deck floating roof or double deck floating roof, even external floating roof  with aluminum alloy sandwich structure. The supporting leg of floating roof are retractable activity, usually suspended in the liquid.
• External floating roof is the most suitable for large crude oil storage tank, form 20,000m³, 30,000m³, 50,000m³  to 100,000m³, etc.

                 External floating roof tank seal                                                 External floating roof

Safety requirements of floating roof tank

• During operation, the floating plate is not allowed to run over the high level, also should not be located in the low level, to prevent the occurrence of a chuck or a floating plate sinking accident.
• The transport flow of the floating roof tank shall be adapted to the allowable lifting speed of the floating plate, Generally, the lifting speed should not exceed 3.5 m/h.
• When the floating plate is lower than 1.8m, the flow speed of the oil pipe fixed on the tank should be limited within 1m/s, to ensure the stability of the floating plate, and to prevent the occurrence of floating plate sinking accident.
• Artificial measurement and sampling are not allowed in 12~18h after the floating plate floated, to Prevention the fire and explosion caused by static electricity accumulation.
• When adjusting the height of the floating roof support, valve stem of floating roof automatic vent valve must be adjusted together with all floating roof support, omission is not allowed.
• For floating roof tank, due to the low temperature, the outlet of the drainage tube may freeze, thermal insulation should be taken at the exit, and the water in the drain pipe should be cleaned before cooling.
• When the drain pipe is kept close during the normal operation of the oil tank, the outlet valve should be opened before the  water on the top of the floating roof amount to 75mm precipitation; when the drain pipe is fully open during the normal operation of the oil tank, workers should always check to prevent oil leakage.