Types of Atmospheric Storage Tanks: Fixed Roof vs Floating Roof

Designing Atmospheric Storage Tanks for Volatile Liquids

[Vapor Control Systems]: The selection between fixed and floating roof configurations determines how a storage facility mitigates product evaporation, controls hazardous emissions, and complies with environmental regulations.

To understand why we choose one design over another, we must look at the physics of vapor spaces. In a standard fixed roof tank, a vapor space (or ullage) exists between the liquid surface and the roof. As the tank is filled, vapor is pushed out. As temperature changes, the tank “breathes,” expelling vapor during the day and drawing in air at night. This breathing cycle is a major source of product loss and environmental pollution.

Fixed Roof Tanks: Structural Variations

Fixed roof tanks are the simplest and least expensive to build. They generally fall into two categories:

• Cone Roof Tanks (CRT): These feature a conical roof with a slight slope (typically 1-in-16). They can be supported by internal columns and rafters or be self-supporting for smaller diameters.
• Dome Roof Tanks (DRT): These feature a spherical dome roof. They are usually self-supporting, relying on the shell for structural integrity, which leaves the tank interior completely free of columns.

Floating Roof Tanks: Eliminating the Ullage

Floating roof tanks solve the vapor loss problem by placing a deck directly on the liquid surface. The deck rises and falls with the liquid level, eliminating the vapor space.

• External Floating Roof Tanks (EFRT): These have no fixed roof. The floating deck is exposed to the elements. They require robust primary and secondary rim seals, as well as a sophisticated roof drainage system to handle rainwater.
• Internal Floating Roof Tanks (IFRT): These combine both designs. A floating deck sits on the liquid inside a fixed roof tank. This protects the floating deck from wind, rain, and snow, making it highly reliable but slightly more complex to vent.

Vapor Loss Calculations

To quantify the emissions from these tanks, we use the methodologies outlined in API MPMS Chapter 19. The total loss (L_T) is calculated as:

Where:
L_S = Standing storage loss (lb/year)
L_W = Working loss (lb/year)

For a fixed roof tank, standing loss is driven by daily temperature cycles. For a floating roof tank, standing loss is almost entirely replaced by rim seal losses, deck fitting losses, and deck seam losses, which are significantly lower in magnitude.

Inspecting Atmospheric Storage Tanks During Commissioning

[Pre-Commissioning Inspection]: A systematic field verification protocol ensures structural integrity, seal alignment, and venting capacity prior to introducing hydrocarbons into the containment system.

Before any tank is handed over to operations, a rigorous inspection must be performed. In my experience, skipping these checks often leads to premature seal wear, roof binding, or environmental non-compliance.

Field Case Study: Real-World Application

Direct Recommendation: When dealing with high-ambient-temperature regions and volatile products, do not rely solely on fixed roofs with vapor recovery. Retrofitting to an IFRT is almost always the most robust and economically viable long-term solution.