• If applicable

Posted Tuesday, March 8th, 2022


When you see a single skin tank and a self-bunded tank, (also known as a double skin tank) side by side, you may not notice a difference; but really, they’re worlds apart.

The one big difference between the two is how they’re constructed; a self-bunded tank can be described as a ‘tank inside a tank’ making it ‘double skinned’. This is often a requirement when it comes to compliance, maintenance, and environmental purposes. It is very important to understand the difference between a single skin tank and a self-bunded tank before making the decision on what best suits your application.

What is a Single Skin Tank?

Essentially, a single skin tank is exactly as it sounds – the fuel held within one layer (a single skin) of plastic or steel. This straightforward construction means that the single-skin tanks are usually the cheapest option to purchase and in some rare cases may be easier to install and maintain. However, the compliance requirements for a single skin tank may require it to be stored in an external bund or spill pallet that has a capacity of 110% of the tanks’ content when full. This is an environmental safety measure, so if there is a rupture in the tank, the fuel would be captured in the bund, and the surrounding environment would not be contaminated.

If you are planning to build a ‘compound bund’ around the base of a single skin tank, it is still compliant however there are some associated disadvantages that you will need to consider. The bund can fill up with rainwater, leaves and debris, meaning that ongoing maintenance is required. If the fuel then gets into the water and contaminates it, it is classed as contaminated waste and will need to be removed by a waste management specialist so as it does not become a risk to the environment. If this happens often, it can be a costly exercise.

Click here to download more information and guidelines around installing a secondary containment bunding system.

What is a Self-Bunded Tank?

A self-bunded tank is also known as a ‘double-skinned’ tank or a tank within a tank, where the inner (primary) tank is fully encased by an outer (secondary) tank. The space between the inner tank and outer wall will capture any leaks, effectively removing the chances of environmental contamination or loss of product. To reach compliance, the secondary layer needs to be able to contain 110% of the total volume of the primary tank, should the primary tank fair or leak (in some cases 100% containment is suitable). This also means that no external bund is required.

Although it is still possible for a rupture to occur in the primary skin of a self-bunded tank, as it doesn’t have the exposure a single-skin tank has, this is rare. If the primary skin does rupture, the fuel will remain in the interstitial area (space between the primary & secondary tanks), keeping it from the open environment. An interstitial monitoring device is required to be used to monitor the interstitial space, should any liquid seep into this area, either by a tank overfill or tank rupture. In reverse, it also means if the outer tank was subject to impact that causes damage, the inner tank would most likely be safe.

Click here for more information around the ‘self-bunding’ requirements, sometimes known as ‘secondary containment’.


DC200 Double skinned

Here are 5 main benefits of a self-bunded tank:

  1. A self-bunded tank allows you to store a much greater column of fuel without the requirements of large bunded areas that take up space and are difficult to keep clean.
  2. They remove the need for external bunding and final installation utilizes a much smaller footprint and saves on yard space.
  3. Self-bunded tanks are very readily relocated & are transportable should the site requirements change, and can become operational within a few hours of delivery..
  4. Most self-bunded tank models come with secure and lockable pump bays that house all pumps and electrical componentry. This means the tank can be shipped to site almost ready to go, resulting in a faster, and much less expensive on-site set up.
  5. The strength of a self-bunded tank is much greater than a single skin wall tank due to the double wall construction. This is a benefit for structural and seismic strength or when moving the tank to site or to another site in the future.

In conclusion, while both the single skin & double skin tanks are compliant options, it always makes more sense to go with a double skin tank, meaning your operation is kept safe, clean efficient & legally sound.