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!

 

[1503-44]

I have always used total volume +10. Mostly because large tanks were always individually diked and the ones that were grouped, were usually small tanks and the total volume wasn't a lot anyway.

 

[georgeverghese]

See if this helps : especially para 5.8.2 discussing AS1940 requirements

https://jyotisconsulting.com/wp-content/uploads/2020/10/Publication-Tank-Farm-Bund-Walls-Hydrocarbon-Processing-2006.pdf

 

[Muhammad@ZEL]

I have always used total volume +10. Mostly because large tanks were always individually diked and the ones that were grouped, were usually small tanks and the total volume wasn't a lot anyway.

Hello again,
Thanks a lot for your response earlier — really appreciate you sharing your practical experience. It helped clarify a lot.

I have a few follow-up questions related to dyke sizing when multiple products are stored in the same or separate dyke areas:

1. If different classes of flammable/combustible liquids (e.g., Class I, II, and III) are stored within the same dyke enclosure, should we size the dyke based on:
   • Only the Class I tanks (most hazardous) + 10% freeboard,
   • Or the total volume of all tanks regardless of class,
     
2. If tanks are separated class-wise into different dyke areas (e.g., all Class I tanks in one dyke, all Class II in another and all Class III in another), then for Class II and Class III, do we still consider a conservative approach like total volume of all tanks or just follow the code minimum (i.e., largest tank only)?

Would love to hear your thoughts or any practical guidance from your experience.
Thanks again!

 

[Muhammad@ZEL]

See if this helps : especially para 5.8.2 discussing AS1940 requirements

https://jyotisconsulting.com/wp-content/uploads/2020/10/Publication-Tank-Farm-Bund-Walls-Hydrocarbon-Processing-2006.pdf

Thanks a lot for sharing this!
I’ll go through it in detail — really appreciate your guidance.

 

[pierreick]

for your consideration.
Pierre

 

[Muhammad@ZEL]

for your consideration.
Pierre

Thanks a lot for sharing this!
I’ll go through it in detail

 

[LittleInch]

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!

NFPA 30, chapter 22 applies and has a lot of options wrt spacing of tanks and separation distance.

NFPA allows you to look online at any code for free, but not download. I suggest you have a look and register at www.nfpa.org

Section 22.11 applies to bunds.

There are many subclauses, but in essence the volume as a minimum is the largest tank minus the volume of all the other tanks up to the top of the bund (22.11.1.2). Nothing about types of liquid, but then there are sub sections which deal with intermediate bunds within bunds (subdivision). These are often sized for 10% of the tank contents so tha a mior spill and fire doesn't spread over the whole bunded area.

Most companies use these guidelines as a minimum and add additional volume. How much is quite variable as are the requirements of different countries.

I've not head of OGRA, but there are numerous other guidelines around.