API-650 Tank Hot Tap Size Limit 2

[quantexhung15]

Hi All,

For performing hot-tapping on API-650 storage tanks, API-653 Table 9.1 only list nozzle size up to NPS18. Does it limit the hot-tap nozzle size up to NPS18? For certain cases if larger size hot-tap is needed, which code/standard shall be followed to validate the design?

Thank you!

 

[IFRs]

API 653 does not list sizes over 18" in table 9.1 and to my knowledge there is no current agenda item considering increasing the size limit. You could send in a technical inquiry and see what response you get. Use this link to do so:

http://rfi.api.org/Index.aspx

I know of no other competent code for hot-tapping API 650 tanks. Could you use multiple 18" hot-taps to get what you need?

 

[quantexhung15]

Thank you, IFRs. Yes multiple 18" hot-taps is an option. This is a specific request from client for us to design the 24" hot-tap. I will use the link provided to send out a technical inquiry.

 

[IFRs]

I'd like to follow the inquiry - please post your actual question text and their response. I'd be happy to review your actual question if you post it here before hitting the Submit button.

 

[Duwe6]

A thought: consider adding some plug-welds to the repad that will be necessary to allow this big nozz to be cut. These will keep the shell and repad acting as one solid sheet. I am a little concerned that the shell and the repad may not stay snug up against each other during the entire Hot-Tap operation.

 

[quantexhung15]

Thanks for your sharing above. Finally hear from API on the RFI submitted. Posted below for your reference. No helpful answer though...

RFI: In API-653 the table 9.1 only lists hot-tap up to NPS18. Is this for a specific reason? For hot-tap over NPS18 which code/standard shall be followed to validate the design?

API Standard Dept. Answer:

Thank you for your recent inquiry. Due to limited committee resources, API staff cannot respond to questions seeking the rationale or intent for requirements in its standards. These requirements are based upon consideration of technical data and the judgment and skill of experienced engineering and technical personnel representing both users and manufacturers who serve on the standards-writing committees. Also, the American Petroleum Institute (API) is not able to provide an answer to your inquiry because API does not provide consulting on specific engineering problems or on the general understanding of its standards. Based on the information you submitted, we recommend you seek the assistance of a consultant or engineering company familiar with the requirements of the standard.

 

[IFRs]

Not too surprising an answer, I guess. Your question was not asking for clarity of the code or pointing out an inconsistency or perceived error. The reason for stopping at 18" may be lost in the fog of history and it is impossible to tell if there was a technical reason to stop at 18". Looks like you are either using multiple 18" hot-taps or striking out on your own, use good engineering judgment, emulate the API details, commit to removing them as soon as the tank is empty, etc.

 

[Duwe6]

When the Code is silent, you may proceed. Make sure that the 24-inch hole is fully reinforced, and go ahead and do it.

 

[IFRs]

Duwe6 brings up an interesting concept.
There are two viewpoints on this:
1) you can do anything the code does not specifically prohibit
2) you can only do the things that the code describes
Your company must choose which philosophy to follow.

 

[Duwe6]

Personally, due to the way API and ASME are written, I tend to go with #1 for all API items - not forbidden = allowed. Reasoning is that API tends to expect more from the Inspector and Engineer, and leaves a LOT of wiggle-room in the Code. The 'Fine Points' are not covered nearly as well as in ASME.

For ASME, if it is not forbidden I proceed carefully, with a LOT of research. ASME is VERY well documented, and there is very little chance that somebody else has not already run into the same problem, and generated a Code Case. And if it is still silent, I proceed.

 

[IFRs]

I'm not trying to be argumentative here, or question your wisdom or advice.
When asked if your work is "per the code" do you say "yes" if you have done something that the code is silent on, like a 24" hot-tap? My view is that this would not be "per the code" because the code stops explicitly at 18". Would you PE seal a 24" hot-tap and note it is per API 653?

 

[Duwe6]

Ah, a 'fine point'. Having started out as a Nuke Inspector, am fully certified to count the total number of angels dancing on the head of that pin.

653 does not forbid Hot Taps over 18", they just do not have it listed as pre-approved. Thus I would handle it like an Alteration - the document(s) directing the work would require the approval of a tank engineer and an API 653 Inspector. Would probably use the wording "in the manner of API-653", assuming that what is calculated looks like the preapproved 18" nozzle.

 

[IFRs]

While not the OP's question, I find this conversation useful, and so continue it.

API says that their publications are to facilitate the broad availability of proven, sound engineering and operating practices without obviating the need for applying sound engineering judgment while not inhibiting anyone from using any other practices.

The text says "the connection size and shell thickness limitations are shown in Table 9.1.". This would appear to expressly limit the size of a hot-tap to 18 inches. A 24 inch hot tap would be outside the code. Would it be in violation of the code? Perhaps that depends on the reader's inclination ( engineer, lawyer, inspector, owner, insurance agent, etc ), interpretation and appetite for risk. I also consider the present and possible future circumstances of an inquiry ( initial design, after the fact inspection, incident investigation, deposition, etc ).

I am always cautious when scaling things up, as you mentioned earlier there is a concern of the repad and shell interaction. There may be other issues to consider...if this opening is near the corner weld, if this were a cyclically heated tank, if this nozzle were to experience piping stresses, etc.

I would also be cautious because as we all know, temporary modifications often become permanent and used for unforeseen applications beyond the original intent.

 

[Duwe6]

". . .temporary modifications often become permanent and used for unforeseen applications beyond the original intent."

Absolutely. Thus the comment about taking care with the repad calc and attachment weld details. When you are above [or below] the 'cookbook' API-650/653 sizing details, proceed very carefully and conservatively. ASME PCC-2 has some excellent methods of Post-Construction additions.

 

[quantexhung15]

Thank you all for contributing experiences and engineering inputs in the conversation above. I truly appreciate it.