All floating roof tanks provide safe, efficient storage of volatile products with minimum vapor loss to the environment. They're particularly useful in the petrochemical industry, for products with high vapor pressure such as naphtha, gasoline, and crude oil.
Reducing filling and breathing losses benefits the bottom line, and helps to meet environmental regulations for the emission of volatile organic compounds (VOC) and air pollutants.
Floating Roof Tank Overview
Floating roof tanks are suitable for storage up to a vapor pressure of 11.1 psi. The roof comprises a steel, aluminum, or fiberglass deck that floats on the contents. A full-contact roof lies directly on top of the contents, whereas a skin and pontoon roof floats above it.
Guide on Floating Roof Tanks
A floating roof is a passive system. As the level of liquid product in the tank varies due to filling, emptying, contraction, and expansion, its buoyant roof rises or falls automatically to adjust to these changes. The floating roof maintains contact with the sidewall of the tank as it slides up or down.
The presence of a floating roof covering the contents can reduce evaporation loss by over 90%. It also prevents the collection of harmful gasses and vapors that can burn or explode when mixed with air. Furthermore, floating roof tanks drastically reduce shell corrosion by eliminating vapors.
Types of Floating Roof Tanks
1. Internal Floating Roof Tanks (IFRs)
These all-weather tanks have an exterior domed or conical roof that shields the interior floating roof from loading by sand, rain, or snow. Additionally, they separate the contents from the environment, reducing the dangers of air pollution and combustion. The gas phase layer in the ullage (vapor space) has a heat insulation effect, further reducing evaporation.
2. External Floating Roof Tanks (EFRs)
They consist of an open-topped cylindrical steel shell with a single or double deck that floats on the surface of the stored liquid. There is no ullage. A rim seal system between the tank shell and roof reduces evaporation and prevents the ingress of contaminants.
Two Basic Components of a Floating Roof Tank
Regardless of their design, all floating roof tanks have two basic components:- The roof itself: This is the pan-like or pontoon deck that floats on the liquid.
- A peripheral seal: A flexible curtain-like seal called the shoe closes off the rim where the floating roof meets the sidewall.
Advantages of Floating Roof Tanks
A floating roof tank is the best, safest option for storing highly volatile petrochemical products because it:
Provides flexible use of storage capacity by adjusting to the volume of the contents.
Offers the most economical and efficient solution for reducing vapor emissions and product loss.
Reduces harmful VOC and pollutants entering the atmosphere to comply with environmental regulations.
Provides fire and lightning strike protection by minimizing or eliminating the ullage, which significantly reduces the risk of implosion or explosion.
Protects the stored product from adverse weather such as wind, rain, and snow, and prevents the entry of sand, dust, or other contaminants.
Disadvantages of a Floating Roof Tank
It’s important to note that a floating roof tank is only intended for "stabilized" products that can’t form large quantities of vapor underneath the floating roof.Some possible drawbacks differ for IFR and EFR tanks:
- IFR tanks are suitable for liquids only. They can’t be used for liquid-solid mixtures. Also, they are more expensive to manufacture.
- EFR tanks can be used for liquid-solid mixtures. However, they require adequate drainage systems for accumulated rainwater or snow which could sink the roof.
Safety Requirements for a Floating Roof Tank
The greatest risks to a floating roof tank are rim fires or incidents that result in the roof sinking or tilting. Safety is paramount when storing volatile petrochemical products:- The floating roof may not move past the specified low landing or vertical clearance positions. Conventional floating roof tanks have a low landing position of approximately 3’ (<1m) and require 5’ (1.5m) or more vertical clearance.
- All primary and secondary seals, adjustable roof supports, mixers, drains, swing lines, rolling ladders, etc. must be designed to operate without compromising the integrity of the floating roof.
- Rolling ladders, roof drains, and fire foam systems must be designed and installed so that the floating roof remains balanced.
- Measures should be taken to prevent the buildup of static electricity from friction around the rim.
- The pontoons must be liquid and vapor tight, and not distort if the roof deck is loaded with rain or snow.
Conclusion
Internal and external floating roof tanks both offer effective solutions for the control of vapor emissions. T BAILEY, INC. can advise you on the best storage option for volatile petrochemicals including gasoline, light diesel oil, crude oil, aviation oil, etc., and noxious chemicals such as benzene, toluene, and xylene.
We provide IFR and EFR tank design, manufacturing, assembly, and after-sales support, according to the highest industry standards. Contact us today.
