How Much Secondary Containment Capacity Is Required: Dike, Tank and Freeboard Sizing
Created on 09.09
How Much Secondary Containment Capacity Is Required: Dike, Tank and Freeboard Sizing
Secondary containment capacity is sized from the largest single container the area can release: for a fixed-roof tank the containment volume must equal the tank shell capacity plus a freeboard allowance, and for a floating-roof tank it must equal the volume at the highest rim level the roof can reach. Many specs then add a rainfall or snowmelt increment and a deflection margin. The practical sizing sequence is list the containers, take the worst-case release volume, add freeboard and rain, check the footprint against available land, and only then fix the diameter and shell height. Getting the freeboard right matters more than the dike wall height, because a tank filled to the exact top level has no margin for thermal expansion or foam.
The question arrives at the worst moment: a site permit is pending, the environmental consultant needs the containment volume on the drawings, and the structural engineer wants a dike height before the concreting date. Everyone quoted at you a hundred percent rule, or a hundred and ten, or a number from a different state or country, and none of them agree. The honest answer is that containment volume is calculated, not copied: it starts from the maximum credible liquid release and then adds whatever allowances your authority or your own risk assessment requires. This article sets out the calculation in the order we use it on industrial projects, and finishes with the geometry that turns a required volume into a diameter and a shell height. How much secondary containment capacity is required depends on the maximum credible release plus the rain and freeboard allowances the authority applies, not on a percentage copied from another site.
What counts as the worst-case release volume
The governing volume is the largest single container that can be released into the area, not the total of all containers. A battery of four 500 m³ tanks does not require 2,000 m³ of containment; it requires the volume of the largest one, because a single failure is the credible scenario and a multi-tank release is a design accident rather than a design basis. For a fixed-roof tank that means the full shell capacity up to the overflow or the highest liquid level the inlet can deliver. For a tank with an external floating roof, the correct figure is the volume at the maximum roof seat level, which is lower than the shell capacity and is the number that is commonly over-specified by mistake.
Two adjustments sit on top of that base volume. The first is freeboard: an allowance for thermal expansion and contraction, for foam or scum accumulation, and for the fact that no operator wants the level touching the roof plate. Two to five percent of the contained volume, with a minimum of 300 millimetres, is a defensible specification on most industrial sites. The second is a precipitation increment where the authority requires rainfall or snowmelt to be drained or stored; this is site-specific and should be confirmed before the concrete is poured, not after.
From required volume to diameter and height
Once the required containment volume is fixed, the same geometry problem appears for every tank on site: volume equals the circular area times the usable liquid depth. A bolted glass-fused-to-steel tank is well suited to this exercise because the panel system allows the height to be chosen almost independently of the diameter, and bolted tanks are supplied for single units up to 60,000 m³. Panel widths near 1.2 m mean the shell can be built in a large diameter with a modest number of course levels, and the tank can be raised later by adding panels if the process demand grows.
For a quick check: a tank of diameter D and liquid height H holds roughly 0.785 × D² × H cubic metres. If the required volume is 3,000 m³ and the site gives a 15 m diameter footprint, the liquid depth works out near 17 m, which tells you immediately that either the diameter has to grow or the dike wall has to do the work. Large diameter, moderate height, is usually the better civil answer on a compact plot because the ring beam and anchor pattern are cheaper than a tall slender shell. Counter-intuitively, the tall slim tank is more expensive to assemble offshore or on a soft foundation, and it is harder to keep round during the erection sequence.
Materials, linings and the corrosion interaction
Containment volume is worthless if the inner surface that holds the fluid fails first. Many secondary containment duties carry aggressive service: refinery slops, chemical spillage, landfill leachate, acidic process drains. A carbon steel shell without an internal lining is a maintenance liability in those duties because corrosion thins the plate at the liquid line and under the sediment layer. Glass-fused-to-steel panels carry an enamel layer fused to steel at 820–930 °C with a layer thickness of 0.25–0.45 mm, rated above 3,450 N/cm², spark tested at 1500 V DC after assembly, and effectively inert across pH 1–14. That combination removes the lining from the critical path and moves the inspection focus to gaskets, bolts and the external coating.
Bolting and sealing are where bolted tanks earn their maintenance budget. 8.8-grade bolts in a corrosion-inhibited system, EPDM gasketed panel joints, and a defined re-torque interval after the first hydrotest keep the shell tight. A dress of the joint, a recoat of the exterior, and a periodic spark retest define a realistic inspection interval. It is also worth stating plainly at the design meeting that a GFS tank is a normal-pressure or low-pressure container, not a pressure vessel; if the process requires pressure, the roof and the code path change completely.
Technical Specification
Parameter | Typical Value / Range | Note |
Worst-case release volume | full shell capacity, or roof seat level for floating roof | single largest container governs |
Freeboard allowance | 2–5 percent of volume, minimum 300 mm | thermal expansion, foam, sediment |
Rainfall / snowmelt increment | per local authority or site risk assessment | confirm before civil works |
Panel width | about 1.2 m | modular bolted panel system |
Bolt grade | 8.8-grade bolts | corrosion-inhibited bolted connection |
Enamel fusion temperature | 820–930 °C | single thermal fusion to the steel |
Enamel layer thickness | 0.25–0.45 mm | 1500 V DC spark test after assembly |
Enamel layer compressive strength | above 3,450 N/cm² | rated for the enamel layer |
Surface roughness | Ra < 0.8 µm | easy to decontaminate after a spill |
pH resistance | 1–14 | inert across the full service band |
Design life | at least 30 years | under the specified chemical service |
How we document the calculation
A defensible submittal shows the sequence rather than just the result. It lists every container in the area with its shell capacity, identifies the governing container, states the freeboard and rain allowances applied with the reason, shows the final required volume, and then shows the actual delivered volume of the proposed tank or dike with a stated margin. Owners and regulators accept this format because the margin is explicit. Where a project has no ledger project of its class, we still describe what is delivered in comparable glass-fused-to-steel duties for municipal and industrial service, with capacity range verified by project specification, rather than implying a specific past site.
The final geometry check is always a site one: road access for the largest shipped panel, crane pick radius, soil bearing under the ring beam, and the separation distance to a property line or a drainage outfall. A tank that is technically correct and cannot be erected on the plot is the most common late-stage change order on containment projects.
Project Case
Project | Location | Product | Capacity | Scope |
Large-diameter water tank | Namibia | GFS tank | 44,900 m³ | supply and supervision |
Firewater tanks, two units | China, Sichuan | GFS tanks | 8,930 m³ | φ19.87 × 14.4 m each, supply |
Chemical wastewater tank, large diameter | China, Zhejiang | GFS tank | 11,613 m³ | φ24.45 × 19.8 m, supply |
Potable water tank | Indonesia | GFS tank | 21,099 m³ | φ42.04 × 15.2 m, supply |
Center Enamel Engineering Capability
Center Enamel (Shijiazhuang Zhengzhong Technology Co., Ltd) has designed and fabricated bolted storage tanks since 2008. As the first glass-fused-to-steel (GFS) tank manufacturer in China, the company holds close to 200 enamel-related patents, produces roughly 300,000 enamel-coated steel plates a year, has completed more than 30,000 installed projects and supplies its tanks to over 100 countries. The new 150,000 m² production base was added to raise output capacity, and single tanks are supplied up to 60,000 m³. Manufacturing runs under ISO 9001 and ISO 45001, with product certification including NSF/ANSI 61, WRAS, FDA, LFGB, CE (EN 1090), ISO 28765, FM, BSCI and EUROCODE, and design referenced to AWWA D103-09, AWWA C550 and NFPA where the application requires it.
That capability applies directly to containment service: modular panel sizing that lets the height be set after the volume is calculated, an inert enamel surface for aggressive spill duty, and documentation packages that show the freeboard and rain allowances explicitly.
Frequently Asked Questions
Q1: Is 100 percent of the largest tank always required?
A1: For a fixed-roof tank the common basis is the full shell capacity of the largest container plus freeboard. Where a regulator or a site standard asks for more, the extra is usually expressed as a percentage of that base volume rather than as a percentage of the total tank battery.
Q2: How do you size containment for an external floating roof tank?
A2: Use the volume at the maximum roof seat level rather than the shell capacity. The roof cannot release liquid above its seat, so the governing volume is lower, and specifying the shell capacity simply over-builds the dike.
Q3: Does the dike have to be impermeable as well as tall enough?
A3: Yes in most regulatory frameworks. The floor and the inside face of the wall need a lining or a compacted impervious layer, and the penetration from a drain or a culvert must be sealed or fitted with a closure that can be closed on an alarm.
Q4: Can the tank itself be the containment for a smaller container?
A4: Often yes, where a single bolted tank serves as both the process vessel and the spill basin. The key condition is that the tank interior can hold the second duty's release without overflowing and without a chemically incompatible mix.
Q5: Why add rain allowance at all if the dike drains?
A5: Because a draining dike cannot drain during the release itself. If the discharge is a one-off spill the drainage may be acceptable; if the containment is a permanent process basin the authority will usually require the increment to be stored, not drained.
Q6: What freeboard do you recommend on a bolted GFS tank?
A6: Two to five percent of the contained volume with a 300 millimetre minimum is a practical spec. It covers thermal expansion, foam and sediment, and it keeps the liquid line below the roof joint so the gasket never sees immersion.
Q7: Can the tank be expanded later if the volume requirement grows?
A7: Usually yes. Bolted panel tanks can take additional panels to raise the shell height and additional bays to widen the diameter, which is why many owners specify the foundation for the largest credible footprint at the start.
Secondary containment capacity is a calculation with a clear order: identify the governing container, take its worst-case release volume, add freeboard and any required rainfall increment, check the footprint, and only then fix the diameter and the shell height. The most frequent errors are specifying the whole tank battery instead of the single largest container, using shell capacity where a floating roof seat level governs, and leaving out freeboard entirely. A tank dimensioned without a stated margin will be full earlier than anyone planned.
Send us the container list, the governing fluid, the available plot and the applicable standard, and we will return the containment volume calculation with the freeboard and rain allowances shown explicitly, plus the diameter and shell height that satisfy it.
Talk to an Engineer
Share your tank list, the fluid and its volume, the site boundary you can use, and the standard or permit condition you are designing against. Our engineers will run the containment volume calculation, propose the diameter and shell height with the freeboard shown, and send the technical file package for your review. No obligation, no sales follow-up beyond the engineering answer.
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