Subject: Dyke Wall Capacity Criteria as per NFPA/OGRA – Class-wise Tank Scenarios in General Plot Plan

[Muhammad@ZEL]

Hello all,
I’m working as a junior design engineer at a consultancy firm, and currently involved in preparing the General Plot Plan (GPP) for a petroleum facility, including dyke wall sizing for vertical storage tanks. We follow NFPA and OGRA standards for design compliance.

Based on my understanding from NFPA guidelines:

• For Class I flammable liquids: dyke capacity = largest tank volume + 10% freeboard
• For Class II & Class III liquids: dyke capacity = volume of the largest tank only

However, some senior engineers recommend a more conservative approach:

• For Class I: dyke should contain the total volume of all tanks + 10%
• For Class II: sum of all tank volumes
• For Class III: only the largest tank volume

I would like guidance on the following:

1. What do NFPA or OGRA officially require in terms of dyke sizing class-wise? If possible, please share a clause reference or source document.
2. What is the industry best practice in such cases? Do most companies stick to the minimum code requirement, or follow the conservative “all tanks + 10%” method for Class I products?

I appreciate any insights or official references you can share.
Thank you in advance!