Pontoon roofs, also known as single decks, consist of several closed chambers or pontoons separated by radial bulkheads, which increases the floating stability of the roof. The ratio of each pontoon area to the total deck area will vary depending on the overall size of the tank and the liquid being stored. True to its name, the roof has only one deck covering the liquid, which slopes down toward the center to allow rainwater to flow into a drainage system. Although this type of tank provides the least protection against evaporation, it's also the cheapest and easiest to install.
Double Deck Roof
The double deck roof uses two separate, stacked steel plate decks over the entire tank surface area. This way, the surface of the liquid is never in contact with the underside of the roof deck that's being exposed to the sun, thereby maximizing protection against evaporation. The top deck slopes downward for the rainwater drainage system while the bottom deck slopes upward to allow water vapor inside the tank to accumulate. The space between the decks is called the flexible bay, and boasts several compartments separated by radial bulkheads. This bay can be inflated or deflated based on filling operations or to reduce accumulated rainwater on the top deck.
Covered Floating Roof Tank
The covered floating roof tank, also known as an internal floating roof system, has a permanently-affixed roof at the top over the floating roof. This type of scenario affords protection for the floating roof from outdoor elements such as rain, frost and snow, thereby minimizing maintenance and repair costs over the long term. In addition, covered floating roof tanks minimize, and in some cases eliminate, the potentially harmful gaseous zone that accumulates over the stored liquid and ultimately leads to corrosion since the space between the floating roof and fixed roof is always well-ventilated.
Common Characteristics Between All Types
While each tank is different, certain features remain consistent across all three. First, they all use flexible rubber seals on the floating roof's edges that form a vapor-minimizing barrier with the seal ring on the inside of the tank shell. Second, the underside of the roof is always fitted with support legs so it doesn't scrape the tank bottom when the tank is empty. Third, roof access ladders are mounted to the side of the tank and across the top of the floating roof, which allow for sampling, gauging and maintenance. Fourth, crucial rainwater drainage systems are always installed on the floating roofs of pontoon and double deck tanks or on the fixed roofs of covered tanks.