Tank bund capacity is a common topic of discussion during process safety reviews, COMAH assessments, and environmental protection studies. One question that frequently arises is whether the liquid that remains inside tanks that have not failed can be included when calculating the required bund capacity.
The short answer is no.
When determining tank bund capacity, no credit should be taken for liquid that would remain inside other tanks within the bund. The design approach is intentionally conservative to ensure adequate containment during a worst-case loss of containment event.
How is tank bund capacity calculated?
Industry guidance sets minimum requirements for tank bund capacity.
In general, the bund should be capable of containing:
- 110% of the volume of the largest tank within the bund, or
- 25% of the total capacity of all tanks within the bund
The greater of these two values should be used when determining the minimum bund capacity.
This approach helps ensure that the bund can effectively contain a significant release and minimise the risk of environmental contamination.
Can you take credit for liquid remaining in other tanks?
No.
Although it may appear reasonable to assume that other tanks within the bund would continue to retain their contents, this volume cannot be credited when calculating tank bund capacity.
Guidance in HSG176 states that the space occupied by other tanks should be considered when estimating bund capacity. In practice, this means that the physical displacement of the tanks reduces the available containment volume within the bund.
However, the liquid contained within those tanks cannot be counted as part of the available containment capacity.
This distinction is important because it ensures that bund sizing remains conservative and provides protection under worst-case conditions.
Why is this approach used?
Tank bund capacity calculations are based on the principle of ensuring adequate secondary containment during a major release.
If designers assumed that neighbouring tanks would always remain intact, the available containment volume could be overestimated. As a result, the bund might not perform as intended during an incident.
By excluding any credit for liquid retained in other tanks, the design remains robust and conservative.
This approach supports:
- Environmental protection
- Pollution prevention
- Regulatory compliance
- Major accident hazard management
Are there any exceptions?
In some circumstances, smaller bund capacities may be acceptable.
For example, liquid may be directed to a separate impounding basin or evaporation area. Diversion walls may also be used to route liquid away from the primary bund area.
Where these arrangements are proposed, they should form part of the overall containment strategy and be supported by appropriate engineering justification.
Any alternative approach should demonstrate that the containment system can safely manage the credible release scenarios for the site.
Why is tank bund capacity important?
A correctly sized bund forms a critical layer of protection against environmental damage and loss of containment incidents.
Insufficient tank bund capacity can increase the likelihood of:
- Environmental pollution
- Regulatory non-compliance
- Increased clean-up costs
- Damage to surrounding assets
- Escalation of major incidents
For this reason, bund design should always be considered alongside wider process safety, environmental, and COMAH requirements.
Need support with bund design?
Understanding tank bund capacity requirements is an important part of designing safe and compliant storage facilities.
If you have questions about bund sizing, secondary containment, COMAH compliance, or pollution prevention measures, our team can help assess whether your arrangements meet current guidance and industry good practice.
For further guidance, useful references include HSG176, the Health and Safety Executive guidance on storage of hazardous substances, and environmental containment guidance published by the Environment Agency.