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.

 

[1503-44]

Companies tend to group tanks by similar contents. Only makes sense. But there is no hard rule that I ever heard of.

 

[Muhammad@ZEL]

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.

Thank you so much for your detailed response — it really helped clarify many aspects for me. Your explanation of NFPA 30, especially the part about Section 22.11 and its application to bund sizing, was very helpful.

From what I’ve understood so far — based on this discussion, insights from senior engineers, and some additional reading — the "largest tank volume" approach appears to be the minimum requirement under NFPA, and is generally the one followed in most designs.

That said, in some real-world scenarios — especially for Class I liquids — certain companies still prefer to adopt that conservative approach, using 110% of the total volume of all tanks within a dyke. This is often done to align with local authority or client-specific requirements.

Also, just to clarify — OGRA refers to the Oil & Gas Regulatory Authority here in Pakistan. They also issue local design requirements for petroleum storage facilities, which often follow NFPA as a base but may include some additional constraints.

Really appreciate your support and guidance!

 

[Muhammad@ZEL]

Thanks for the input — that makes sense

Companies tend to group tanks by similar contents. Only makes sense. But there is no hard rule that I ever heard of.

 

[LittleInch]

Thank you so much for your detailed response — it really helped clarify many aspects for me. Your explanation of NFPA 30, especially the part about Section 22.11 and its application to bund sizing, was very helpful.

From what I’ve understood so far — based on this discussion, insights from senior engineers, and some additional reading — the "largest tank volume" approach appears to be the minimum requirement under NFPA, and is generally the one followed in most designs.

That said, in some real-world scenarios — especially for Class I liquids — certain companies still prefer to adopt that conservative approach, using 110% of the total volume of all tanks within a dyke. This is often done to align with local authority or client-specific requirements.

Also, just to clarify — OGRA refers to the Oil & Gas Regulatory Authority here in Pakistan. They also issue local design requirements for petroleum storage facilities, which often follow NFPA as a base but may include some additional constraints.

Really appreciate your support and guidance!

That's a decent summary I think.

NFPA is the minimum and is generally exceeded to allow for sloshing and waves if indeed it ever happened.

The consequence of bund failure or overtopping if further tanks collapse in the event of a fire from the biggest one ( think rivers of fire!) often cause people to add a relatively small amount of height to their bund to allow for the volume of all tanks.

What OGRA requires is the start point / minimum as that is legislation / authority approval.

If you want further reading try this. As you can see there are many different guidelines and each large operating company tends to have their own as well. Some insurance companies also create guidelines, but I can't lay my hands on it at the moment.

Plant Layout

www.hse.gov.uk

 

[georgeverghese]

While you may be blinkered on this task of determination of containment dyke volume, it should be obvious this is the final containment barrier in the event of a fire and tank rupture. There are many other prior safety / containment measures that must be implemented and maintained in order to match up with the selected dyke volume. Some of these are an independent SIL2 overflow trip loop, normal operating level indication and alarm integrity, adequate firefighting measures, intertank spacing, tank to dyke wall separation distance, operating procedures etc.

Older storage tanks are notorious for contamination of the water table due to tank bottom plate corrosion. See if the Plant Owner has walked the talk on this also.

 

[Muhammad@ZEL]

That's a decent summary I think.

NFPA is the minimum and is generally exceeded to allow for sloshing and waves if indeed it ever happened.

The consequence of bund failure or overtopping if further tanks collapse in the event of a fire from the biggest one ( think rivers of fire!) often cause people to add a relatively small amount of height to their bund to allow for the volume of all tanks.

What OGRA requires is the start point / minimum as that is legislation / authority approval.

If you want further reading try this. As you can see there are many different guidelines and each large operating company tends to have their own as well. Some insurance companies also create guidelines, but I can't lay my hands on it at the moment.

Thanks a lot — that explanation really helped tie everything together. The points about sloshing, bund overtopping, and fire spread risks make a lot of sense.
Appreciate the time and insight!

 

[Muhammad@ZEL]

Thank you for highlighting these important points — really appreciate it.

At the moment, I'm specifically working on dyke sizing for a project, so my focus has been on reviewing the applicable guidelines and volume requirements. But you're absolutely right — dyke containment is just one layer, and a complete safety system includes much more (like level monitoring, SIL systems, spacing, and operating procedures).

hat said, even when we do consider these broader safety layers, we still need to ensure that the minimum dyke sizing as per code is met

Your input definitely helped expand my perspective on this. Thanks again!

While you may be blinkered on this task of determination of containment dyke volume, it should be obvious this is the final containment barrier in the event of a fire and tank rupture. There are many other prior safety / containment measures that must be implemented and maintained in order to match up with the selected dyke volume. Some of these are an independent SIL2 overflow trip loop, normal operating level indication and alarm integrity, adequate firefighting measures, intertank spacing, tank to dyke wall separation distance, operating procedures etc.

Older storage tanks are notorious for contamination of the water table due to tank bottom plate corrosion. See if the Plant Owner has walked the talk on this also.