Ask what the difference is between a self bunded tank and a double walled tank and you will usually be told they are the same thing. For diesel storage that answer is close enough. For chemical storage it is not, and specifying on it can leave you with a tank that does not satisfy your site’s containment requirement.
One term describes construction, the other makes a compliance claim
Double walled describes how a tank is built: an inner vessel inside an outer shell, with an interstitial space between them. It is a construction fact and nothing more. Some double walled tanks exist so that gap can be monitored for leaks, not so the outer shell can hold a full spill.
Self bunded is a claim about compliance. It says the outer shell has been designed, sized and documented to contain the full contents of the inner vessel, so the tank satisfies your secondary containment obligation without an external bund, dike or compound around it. AS 1940 deals with this format directly in Section 5.9, above-ground tanks with integral secondary containment, and that is the clause worth naming when you ask a supplier what their tank is built to.
Every self bunded tank is double walled. Not every double walled tank is self bunded. The question worth asking a supplier is not how many walls the tank has, it is what the outer shell is rated to hold and what documentation proves it.
Where the 110% figure comes from, and why your spec may say more
If your specification calls for 110% of the largest tank, it is right, and it is the most common number written into Australian site specifications for good reason. For flammable and combustible liquids it is the figure AS 1940 itself requires, and several environmental regulators land in the same place or go further.
The standards set the floor for the class of goods you are storing. State environmental regulators and site licences then apply their own figures on top, which is where 120% and 133% come from.
| Source | What it applies to | Containment capacity |
|---|---|---|
| AS 1940:2017, Section 5.8 | Flammable and combustible liquids, tank compounds | The greater of 110% of the largest tank in the compound, or 25% of the aggregate capacity of all tanks in it. Increased by the output of any fire suppression system serving the compound over 20 minutes |
| AS 1940:2017, clause 4.4.3 | Packaged storage, drums and small containers | 100% of the largest package plus 25% of storage capacity up to 10,000 L, then 10% between 10,000 and 100,000 L, then 5% above that |
| AS 1940:2017, Section 5.9 | Above-ground tanks with integral secondary containment | The clause that deals with the self bunded format, where the tank carries its own containment instead of sitting in a compound |
| AS 3780:2023 | Corrosive substances, in packages, IBCs and bulk | Package stores: the greater of 25% of the aggregate volume kept or the capacity of the largest container, capped at 5,000 L where only packages are stored. Bulk containers and fixed tanks are dealt with separately |
| AS/NZS 3833:2024 | Mixed classes of dangerous goods, in packages and IBCs up to 1,600 L | Segregation, separation and spill containment where classes share a store, written to give the same level of safety as applying each single class standard. It does not reach bulk tanks |
| EPA Victoria, Publication 1698 | Bulk tank storage | 100% of the largest container as the minimum, plus a suggested 10% of the second largest tank and a suggested 10% freeboard for rainwater and firewater. Illustrated in the guide as an effective capacity of 110% |
| ACT petroleum storage guideline, June 2019 | Single wall tanks | At least 110% of the tank volume, stated as a flat requirement, alongside roofing, bollard and spill recovery requirements |
| EPA South Australia, EPA 080/16 | Tank storage | 120% of the largest tank, rising to 133% for flammable liquids |
| Fire system allowance | Bunds served by automatic sprinklers | AS 1940 and EPA Tasmania add the system output over 20 minutes. EPA South Australia takes the greater of 20 minutes of sprinkler output or 133% of the largest tank |
| Queensland | Any site | WorkSafe Queensland’s guide points to AS 1940 without adding a figure of its own, so the operative numbers are the standard plus your development approval and licence conditions |
| Outdoor installations | Uncovered bunds | Add rainfall allowance and freeboard, or provide a controlled drainage and testing regime |
So 110% is not an arbitrary rule of thumb. For flammable and combustible liquids it is the figure in AS 1940 itself, and EPA Victoria arrives at the same effective number by a different route. Where it goes wrong is when it is treated as the universal answer. Four things change it. Once several tanks share a compound, the 25% aggregate test can govern instead. Water based fire suppression adds the system output over 20 minutes, or forces 133% in South Australia. Some regulators simply require more, with 120% and 133% both in current Australian guidance. And corrosives, mixed classes and packaged goods are each governed by a different standard with a different rule.
Two further consequences are worth noting. An outdoor self bunded tank is only weatherproof secondary containment if the interstitial space is genuinely sealed, which is one of the real advantages of the format over an open concrete bund that fills with rainwater and eats into the capacity you paid for. And if you store more than one class of dangerous goods in packages or IBCs in the same area, AS/NZS 3833 and the segregation rules can decide the layout before capacity does.
Almost every self bunded tank on the Australian market is a diesel tank
Search the term in Australia and you will find steel self bunded fuel tanks, mine site refuellers and transportable diesel cubes. That is where the product category grew up, and it is a good fit: diesel is chemically undemanding, the duty is well understood, and the tanks are built to AS 1940 and sold as a compliance package.
Chemical duty is a different problem. Sodium hypochlorite gasses off and attacks metals. Sulfuric acid changes behaviour with concentration. Caustic soda needs heat traced and insulated design in cold conditions. Alum and ferric are corrosive to most metals and to concrete. For these, the containment question is secondary to the material question: choose the wrong polymer or the wrong grade and the primary vessel is the thing that fails.
That is why chemical storage in Australia is generally polymer, and why a tank intended for chemical service should be designed to a tank design standard rather than assembled to a fuel tank pattern. We design welded thermoplastic tanks to DVS 2205, the German standard that sets wall thickness from contents, specific gravity, temperature, service life and weld factor. It is the difference between a tank that is thick enough and a tank that has been calculated.
Rotomoulded or fabricated, and what to settle before you specify
There are two ways to get an integrally bunded plastic tank. Rotomoulded twin wall tanks are moulded as a tank within a tank, usually in polyethylene to AS/NZS 4766, and bought off a shelf in fixed sizes. They are quick, cost effective and well suited to small dosing duties where a standard capacity fits and the chemistry is mild. The limits are the mould: you take the sizes, shapes, wall thicknesses and fitting positions that exist, temperature capability is modest, and a damaged unit is generally replaced rather than repaired.
Fabricated tanks are cut and welded from certified sheet to DVS 2205, so the tank and its bund are built to the duty instead of the duty being fitted to the tank. Capacity, footprint, height limits, nozzle positions, instrumentation, venting and restraint details become design decisions rather than catalogue options, material is selected for the chemical (HDPE for general chemical and water treatment duty, polypropylene where service temperatures run higher, PVDF for the aggressive end), and because the joints are welded the tank or bund can be modified, extended or repaired in place years later. We compare the two technologies in detail in rotomoulded vs fabricated plastic tanks.
The same choice applies to the containment itself. Where a plant room, an existing tank farm or an awkward footprint makes an integrally bunded unit impractical, a fabricated chemical bund built to the actual floor area is usually the faster and cheaper answer, and it can be retrofitted around equipment that is already installed.
Decide the containment strategy before the tank, because it changes the tank. Confirm which standard governs your goods, then confirm what your state regulator and your site licence require on top of it, since that is where the 120% and 133% figures come from. Establish whether the installation is indoors or outdoors, because rainfall and fire water can both drive the number. Check the footprint you actually have, since an integrally bunded tank needs less floor area than a tank plus a bund, and that alone often decides it.
Then define the duty properly: contents, concentration, specific gravity, operating and maximum temperatures, fill and discharge arrangement, venting, and design life. Those inputs decide polymer, grade and wall thickness, and no amount of secondary containment compensates for getting them wrong.
If you want that checked before it goes into a specification, send us the duty. We have fabricated chemical storage in Brisbane since 1971, every enquiry is reviewed by an engineer, and material selection advice costs nothing.
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