Transport tank design under BPVC Section VIII -- using design margin = 4.0 or 3.5 ?

[Sean999]

Hello:

The current US hazardous materials reg's require that transport tanks for many commodities (such as non-pressure liquid tanks for gasoline built to MC-306/DOT-406, and high-pressure tanks for propane built to MC-331) use the 1998 Edition of ASME BPVC Section VIII, which calls for a design margin on ultimate tensile strength of 4.0 .

In BPVC editions from 2001 on, Section VIII allows a lower design margin of 3.5, but transport tank designs for hazardous material service are not supposed to be designed to these later BPV codes.

On the other hand, ASME says that each new BPVC becomes "mandatory" 6 months after it's issued.

Is there a conflict here? What do transport tank designers actually do, in practice? Are you still digging up the old 1998 BPVC for current work, and also complying with ASME's requirement to use the newest code? If so, is that difficult to do?

I'm trying to understand how designers deal with this apparent conflict, and any thoughts or observations would be appreciated.

Thanks,
Sean Streiff

 

[jtseng123]

The government regulation has not been updated to the latest code and it may take years for them to catch up with the latest practice. And no vendor is willing to design pressure vessels based on old, out of date 1998 code to get a code stamp.

You will need to write a petition to DOT or Authority to get approval by using the latest code. What their major concern is the "safety". In the old code it has higher safety margin as well as higher hydrotest pressure of 1.5 multiplier instead of 1.3 in the latest. The authority does not realize why it was changed from 1.5 to 1.3. It is simply due to reduction of safety margin in the new code so the multiplier has to be reduced, otherwise, the vessel may be exceeding yield stress during hydrotest. We have one project got approval from Authority to use the latest code but with additional requirements: hydrotest by using 1.5 multiplier, higher schedule for small bore nozzle, and fully NDE for all nozzle welds.

The major issue is the hydortest pressure. If the MAWP is based on calculation, the stress from the higher hydrotest pressure may exceed the yield and if you reduce the test pressure, it will not meet the Authority requirement of using 1.5 multiplier. Simply increasing the thickness may not solve this problem because it is just like a dog chasing its tail. So we decide to make MAWP = design pressure, such that it can be tested at higher pressure (by increasing thickness as required) and stress can be managed to under 5% or 10% of the yield.

Good luck.

 

[Sean999]

Thanks for your thoughts, jtseng123.

I think that vendors who build *transport* tanks for hazardous materials have it a bit different: they *have* to get an ASME code stamp in order to comply with the hazardous materials regulations. This includes some very common tank trailers and tank trucks, so it's a big deal.

I'm still hoping to hear from engineers at some of these transport tank builders about how they cope with this issue.

-Sean

 

[jte]

Indeed. I'm not in that business but would think that the appropriate Code would not be Section VIII at all, but Section XII.

https://www.asme.org/products/codes-standards/bpvcxii-2015-bpvc-section-xiirules-construction

I once accidentally sat in on one of the code committee meetings for XII, then excused myself when I realized it wasn't Section VIII. The guys laughed and asked me to stay and said that XII was basically "Section VIII Light".

 

[Sean999]

I hadn't heard the "VIII Light" comment about XII before, but I guess that sounds about right, from what I can tell.

You're right that XII is more *technically* appropriate for transport tanks -- that's what it was created for, with input from the DOT. But the hazardous materials reg's haven't yet been updated to allow the use of Section XII. The DOT is actually moving in that direction, but they do have lots of rules (*not* created by them: Congress does that job) to follow in order to complete that process.

And I'm trying to figure out what competent, responsible, knowledgeable design engineers are doing in this situation.

Thanks again,
-Sean