Important Considerations for Aboveground Storage Tank Design

Tank owners and operators face a heavy burden when searching for aboveground storage solutions. Even when focusing on just one element of aboveground storage tank—floating roof design—countless decisions must be made.

As an operator, your priorities are pulled in multiple directions. You must consider safety, regulatory compliance, operating costs and capital expenditures. Material and design choices must meet your present operational goals but also look to your future needs.

New materials and innovative floating roof designs are exciting but can muddy the waters. With that in mind, it would be tempting to ignore recent innovations and stick with more time-tested options like the carbon steel roof.  However, more traditional options may ultimately cost more to build and maintain over time.

As you can see, choosing a floating roof is not an easy decision or a decision you should take lightly. The choices you make today and throughout your project will impact operations, cost, and safety for the lifecycle of your asset.

As you set out to narrow down your floating roof options, it’s important to walk through the five broad elements of your project: 

• Design Loading
• Stored Product
• Operational Plans
• Emissions Profile
• Budget

Through careful analysis of each of these elements, your best option should emerge. Taking this systematic approach eliminates the bias of familiarity and other external factors that might sway you one direction over another. These elements could also be formally evaluated through a FEED (Front End Engineering and Design) study. This process is incredibly beneficial to nearly every project, but especially ones where a large capital investment is required. 

Let’s examine the five elements of floating roof design and how each will impact your application.

Design Loading: How Will Mother Nature Impact Your Tank Roof

Design loading considers the environmental factors that will impact your roof. Forces from wind, seismic effects, snow and rain accumulation need to be accounted for.  A tank storing the same product will have different loading considerations based on regional weather patterns at its location.

Steel may offer the most strength but can still have pitfalls. Snow, for example, can create differential loading on an external steel floating roof making it vulnerable to capsizing. Designed appropriately, steel can withstand nearly any load condition with unmatched durability.

Aluminum is a lightweight alternative. It may not have the brute strength of steel, but can be used internally or paired with a geodesic dome to compensate. Used this way, medium to heavy-duty aluminum roofs are more than capable of handling several loading scenarios and more cost-effective than carbon steel. 

What’s Inside Matters: Choosing the Right Material for Your Internal Floating Roof

Both the properties of the product to be stored and the Reid Vapor Pressure (RVP) of a product must be considered.

Aluminum and composite roof materials are often not compatible with all products. Alcohol-based products such as ethanol and methanol can react with aluminum causing corrosion. Steel, while more expensive, offers the flexibility of multiple product storage options. Different coatings can be applied based on the stored product to prevent corrosion.     

Present and long-term plans for the storage facility should be included in this planning process.  Perhaps the current plan calls for one type of product while future expansion plans may require a different product to be stored. Of course, it is hard to predict the future, but it could provide tremendous cost savings to retrofit an existing storage facility when required instead of building an entirely new one.

The Reid Vapor Pressure (RVP) will determine how strong the roof needs to be to prevent the roof from bubbling and from pressure building up beneath the deck. A change in the stored product will also change the RVP. Roof design specifications should also account for this differential. 

How Operations Impact Durability and Load Requirements for Aboveground Storage

Operational plans are a key consideration in your floating roof and seal choices. Both cycle frequency and inlet/outlet rates of the tank will help determine the best material options.

Aggressive cycle frequency puts considerable strain on a floating roof, especially the rim. When making material choices for your internal floating roof, the forces of these loads must be taken into account as well as their long-term durability. When combined with an appropriate seal, aluminum and composite materials are often up to the task. Their lightweight strength is an asset in this application.

Filling and emptying rates cannot be overlooked in this process either. The forces exerted on the floating roof can vary greatly depending on the inflow/ outflow rates. This is especially important for lightweight internal floating roofs. Aluminum, for example, has lower yield strength which could result in decreased durability over time.

Reducing Environmental Concerns Through Effective Tank Roof Design

The regulations imposed by the Environmental Protection Agency (EPA) and other governmental agencies cannot be ignored. Manufacturers will often suggest that a full contact roof is the blanket solution for emission concerns.  However, the type, quantity of sources and level of emissions are really what must be evaluated and not just full contact roof vs. floating roof.

Steel roofs can be welded to provide a vapor-tight seam, but the need for support legs can reduce emissions effectiveness. Bringing in an independent expert is often a prudent decision to address these complex concerns. 

How The Top of Your Storage Tank Storage Roofs Impact Your Bottom Line

Finally, budget considerations are arguably the most important element of planning. Examining the budget is a twofold process. You must first consider the initial capital investment and how quickly that expenditure will provide a return on the investment.  You must also analyze the ongoing operational and maintenance costs. These costs can be significant and a larger preliminary investment may prevent expensive roof repairs or replacement throughout the lifecycle of your AST. Conversely, a bigger spend may not be justified based on your current and future needs.

Storage Tank Roof Design is Worth Your Consideration

In conclusion, whether as part of a FEED study or through careful examination, design, and material specifications, your AST tank design deserves a thorough investigation. Each element must be evaluated for their strengths and limitations. Conducting a full analysis of the cost of ownership will result in better operations, regulatory compliance, and use of financial resources. 

AST Solutions from Matrix PDM

Whether you are looking for custom tank design, pre-engineered storage, or modifications to your storage facility, Matrix PDM can provide a solution. Since 1984, we’ve built our reputation around aboveground storage tank design and construction. We are an industry leader in the engineering, procurement, fabrication, construction, maintenance, modification, repair and relocation of ASTs and specialty structures and related terminals.

Contact us today to see what storage solution is right for your operational needs.