AS 1940 - Crest Locus Interpretation 3

[ChalMel13]

Hi guys,

My first post here from Down Under.

So, I've started working in petroleum industry and reading a lot about secondary containment/bunding. One of the things confusing me at the moment is about the crest locus requirement (cl. 5.8.3 (h)) saying:
"The location of a bund relative to the closest tank shall be such that the top inside perimeter of the bund is not inside the crest locus limit specified in Figure 5.2, except where it can be demonstrated that a reduced distance would be appropriate due to viscosity or other considerations."

Although, it is straight forward to check compliance or calculate the distance to comply with Crest Locus I'm not sure about the reasoning behind this requirement. So far, people I speak with are referring to the Jetting Failure (see below):

So, the economic solution for that risk is shielding the tank shell as shown below:

However, from my research the main risk associated with the bund wall height (hence crest locus) is mainly the overtopping after a catastrophic failure of the tank (like below).

So, going with my assumption, if the bund wall is within the crest locus of the tank will require higher bund wall or a board on the wall.

Do you guys have a clear understanding of the Crest Locus requirement?

Cheers.

 

[LittleInch]

Well first welcome, but what code are you quoting from?

I might not have the very latest API 650 (2018), but I can't find any reference to crest or that section.

I think your original supposition is correct, even though your picture is clearly not a containment bund, but a spill bund.

Catastrophic failure of the type your lower picture shows is very rare and difficult for most people to quantify

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChalMel13]

Thank you!

I'm referring to the AS 1940-2017 (but even very old versions include that Crest Locus requirement) and specifically cl. 5.8.3 (h). Haven't checked the API 650 before to be honest but I'll have a look.

BTW, what is the difference between the containment and spill bund?

I understand that the specific type is rare but I just added a picture as an explanation about the overtopping. I actually found it on an article I came across (

https://www.sciencedirect.com/science/article/pii/S0950423017305624#fig5).

I am just a bit confused as there is no explanation on the Australian Standard on the reasoning behind this requirement.

 

[LittleInch]

Sorry, I saw the reference in the title.

Not many people will have this specific code so you may need to snapshot that page or section and post it.

A full containment bund holds normally 110% of the tank contents, but usually allow you to include the tank area within the bund.

A spill bund to me is as shown in your photo, i..e it just caches drips, small spills etc and directs the oily water into an oily water drain system.

I would have thought the reasoning was quite clear - whilst you have quite a lot of flexibility as to the location of the bund wall and its height, you shouldn't make it so close to the tank that in the event of a hole in the tank the liquid has enough velocity to make it over the bund wall and hence you have liquid outside of the bund which can pollute or catch fire.

The articles listed look very interesting and you might add something extra to the bund wall to prevent overtopping, but in most cases, I think people generally think the risk is so low that a bit of over topping isn't going to cause a big enough issue to do something extra. If the tank has burst like that then it's a very bad situation. A bit of liquid slopping over the top isn't normally going to make a very bad situation that much worse.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChalMel13]

The reason I’m getting confused is because the standard indicates an angle of 26.5 deg

Whereas the Torricelli law indicates an angle of 45 degrees

Which I’d assume that the standard is based on.

I think tho your thought process is correct and the design is based on the likelihood of the event and not just on the theoretical knowledge.

 

[LittleInch]

Looking this up ( a new one on me) shows that the theoretical law is as you show. However it is frictionless which is not reality.

I think looking at this if you do the actual physical experiment, you don't get 45 degrees but something more like the rather precise 26.5 degrees once you take exit losses and other frictional effects into account.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChalMel13]

Great conversation!

LittleInch, thank you for your time, sharing your knowledge and experience in the field!

It’s great to have reachable people with your knowledge/experience.
First time using a forum for professional questions but I’ll definitely stay in touch here!

 

[LittleInch]

No problem. There isn't a lot of knowledge about Australian design codes on ET, but so long as it involves general principles you should be good.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BJI]

The AS1940 requirement is for a 'pin hole' leak, not catastrophic failure of the tank. Where this could not be achieved splash screens have been used, either inside the bund wall or sitting on top of the wall. An alternative would be self bunded tanks, where there is a double wall with an interstitial space and leakage detection. You may also see spill boxes or bunds on some portable double wall tanks to provide containment at connections outside the double wall tank.

 

[ChalMel13]

BJI, thanks for your clarification.

The AS1940 requirement is for a 'pin hole' leak, not catastrophic failure of the tank.

May I ask how did you find this out? I couldn't find that information and that's why I was confused.

Thanks for your response!

 

[BJI]

Previous experience working in the petroleum/petrochemical industry applying AS1940 mostly. But the height requirement with viscosity adjustment is only really applicable to leakage considerations. This clause was in AS1940 before my time in engineering so can't cast any insight on the origins unfortunately.