As the name suggests, floating roofs float on top of a product, such as crude oil, in a storage tank. As the level of the product fluctuates, the floating roof moves up and down with it, maintaining a sliding contact with the tank wall. The objectives are to prevent evaporation loss or minimize evaporation loss and maintain regulatory compliance for emissions.
Floating roofs are more expensive to install than conventional fixed roofs, as they are generally used in conjunction with fixed roofs. However, they are much better at keeping flammable vapors from releasing into the atmosphere and are therefore much more environmentally friendly. They are also more cost advantageous over time by preventing product loss from evaporation.
There are two basic components to a floating roof tank — the roof itself and a peripheral seal. The seal is analogous to a tube that goes inside a bicycle tire. It runs against the side of the tank so that, as the product level moves up and down, constant contact with the tank wall is maintained so that the product can’t escape past the seal.
The most typical designs for steel floating roofs are either pan or pontoon. Originally, there was just a single steel deck with a rim around the edge. If you got a puncture in the deck or there was much water infiltration, the water would sink the roof. Engineers came up with a pontoon-style model with individual compartments.
The idea is that if there is a leak, it is contained within the compartments or the center. The roof doesn’t sink, and it gives tank owners time to fix the problem. Some aluminum floating roofs look like pontoon boats with tubes and pipes that are sealed at the end, while others have the buoyant elements as internal components. Both kinds of aluminum roofs can’t be used with certain crude oils because interaction with the oil corrodes the aluminum.
A mix of materials can also be used to construct floating roof tanks. Sometimes stainless pontoons are paired with carbon steel roof decks. There’s also a full contact floating roof, which is like a honeycomb panel that’s on the oil. The honeycomb panel is equipped with hundreds of thousands of little compartments, making it less likely to sink in the event of failure of individual compartments.
There are a variety of different products stored across the world in floating roof tanks. Many are highly flammable products with low flashpoints. Floating roof tanks are nearly ignition proof. The rim of a tank is most likely to catch fire. To help prevent ignition and therefore fires, it’s best to regularly check rim seals for deficiencies and to keep rim seals well maintained. If the rim seals are in good shape, there’s less chance that vapor will escape. After all, products like crude oil aren’t what actually burns, it’s the vapors that are flammable.
Lightning strikes continue to be the most likely cause of storage tank fires. The U.S. has relatively few lightning strikes compared to some regions globally. For example, Singapore has one of the highest lightning strikes per kilometer per year in the world and has three facilities totaling 1.295 million barrels per day refining capacity. Ideally, people would build on locations that are not problematic but then again, one of the most popular tourist destinations in the world is located on an active volcanic island.
An internal floating roof’s purpose is to create a seal between the tank and the atmosphere. Sealing a tank leaves no gap through which to distribute foam. Foam can be pumped in through a manhole. However, if there’s a fire in the tank, it’s not safe to enter the tank to open the manhole. Leaving a manhole continuously open at all times defeats its purpose.
Alternatively, foam can be distributed in the area over the sealed rim of the floating roof. Spray foam systems have a ring both internal and external on floating roofs to spray the foam exactly in place. There would be concerns about someone spraying water instead of foam if regulatory changes were made.
“Water in a tank fire is death,” said Matt Fegenbush, structural/design engineer for Pittsburg Tank & Tower Group.
Oil is lighter than water so any water that finds itself into the tank will sink to the bottom until it gets hot enough to boil. Once it does boil, an emulsion is created, and it becomes almost volcano-like. People have been known to get third-degree burns from 200 meters away, Fegenbush said.
If a fire breaks out, the tank’s sides should be sprayed so that the steel shell doesn’t fail. While this is being done, the contents inside the tank should be pumped out as safely and quickly as possible. As soon as the product level is dropped to the inlet level, it is possible to draw product that’s hot enough to burn, so it is advisable to stop pumping and allow the rest of it to burn off.
In March, a fire broke out at a tank farm storing petrochemical liquids and gasses in the Houston suburb of Deer Park. Firefighters initially responded to two tanks burning, but over the next several days more tanks caught fire. According to news reports, more than 170,000 gallons of products were lost, dozens of animals were killed and toxic gasses released into the atmosphere. The Houston Chronicle reported that officials said the fire was likely caused by a containment wall breach, but the official cause had not been determined. Investigations involving local and federal authorities were launched.
Following the fire, questions have arisen on how a floating roof’s design could be altered to enable foam to more easily enter the tank, thus making it easier to douse a fire.
“There’s an inherent risk in tank fires,” said Fegenbush.
Addressing issues with floating roof tanks would require the input and collaboration of the NFPA, API and other tank experts. It also comes down to process availability. There are only so many contractors who design, fabricate and manufacture floating roofs. There’s also only so many experts who can install and maintain lightning protection on storage tanks.
Seals can be vented. However, steel technology is not such that 50-square inches of coverage can be sacrificed and meet emissions standards. Why aren’t seals designed to allow one-way fluids to pass through? That’s not something that can physically happen, said Fegenbush. The foam can be applied onto the seal, but the pressure on that foam from the product on the bottom is not going to allow it to go down as there is no air gap. Internal floating roofs are designed to eliminate air gaps because that’s where vapors form and vapor is what is combustible and most dangerous.
Regular inspections and maintenance are the keys to ensuring functional tanks. Tanks should be inspected when they are constructed and periodically once they are in service. General visual inspections should be performed once every six months at least. A complete inspection should be done every other year. If the tank is struck by lightning or damaged in any way, it should be immediately inspected.
Floating roof tanks should be electrically grounded to the main tank shell (Bouquegneau, 2007). Careful consideration should be given to the design and placement of the seals and shunts to reduce the risk of ignition. Multiple shunt connections should be placed between the tank shell and floating roof at intervals of no less than 1.5-meters.
Erin Schmitt is a technical writer and director with the Pittsburg Tank & Tower Group.
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