API-650 - Leak Testing Thin Reinforcement Pads 1

[NorthWest250]

Hello there,

How is everyone pneumatically leak testing thin nozzle reinforcement pads that are only 3/16" or 1/4" thick?, given that a 1/4" NPT connection has roughly a 3/8" thread engagement to be leak tight.

Each time we connect a nipple to the repad we lose pressure out of the nipple to weep hole threaded connection, because the nipple bottoms out on the tank shell before making an air tight seal.

Thank you in advance.

 

[JStephen]

Weld a #101 flange over it.
See page 6 of this pdf, numbered as page 5 in the print:

https://www.phoenixforge.com/catalogs/phoenix catalog.pdf

 

[IFRs]

I see a lot of threaded couplings welded to thin repads

 

[r6155]

1) Check if a reinforcing pad is required. Try to avoid them.
2) I agree with IFRs.
3) Remember that the inspector must be inside the tank to verify the leak during air test.

Regards

 

[MJCronin]

Why is the nozzle pad so thin ?... Why isn't it at least the same thickness as the underlying plate ?

Is this simply a design mistake ?

Has you company fabricated and installed MANY OTHER tanks with these thin-walled pads ?

MJCronin
Sr. Process Engineer

 

[JStephen]

API tanks can and do have 3/16" and 1/4" shells.

 

[IFRs]

The reinforcing pad is sometimes only as thick as it needs to be to supply proper reinforcing. Depending on the tank design, nozzle elevation and neck thickness (sometimes exceeding API minimum) the repad can be thinner than the tank shell, in some cases (for instance using Sch 160 neck) eliminated altogether. There are some cases where there is a reinforcing plate on the tank inside and the one on the outside is thinner than the tank shell, or eliminated.

 

[MJCronin]

Sorry,...

IMHO, while I understand what is "theoretically possible" regarding the reinforcement rules of API-650, I believe that usage of ultra thin sheet (3/16") is simply bad policy ... I do not understand why API-650 permits this for repads

There is nearly zero downside in making all repads at least the same thickness as the underlying shell plate (there is no significant cost savings !)

Furthermore, making nozzle necks thicker and saving all of those welding costs should always be the first choice (as noted above)

I would like to hear the thoughts of others on this issue ...

MJCronin
Sr. Process Engineer

 

[JStephen]

API permits this for repads because it permits it for shell plate.
Repads can be eliminated in many cases by individually checking nozzle reinforcement calculations, but that is not always advantageous or allowed by customers. Similar issues come up in vessel reinforcing.

 

[NorthWest250]

Thank you everyone for the suggestions. I'm familiar with couplings, but I'd never heard of a #101 flange. I'm glad to know there are no code restrictions on adding one or the other to a weep hole if required.

These are small tanks (10ft / Ø12ft) which are designed to code minimum thicknesses, where Appendix E, F, J, M & S apply.

The custom fabrication shop I work for relies on a subcontractor to design & detail API-650 tanks, typically all under Appendix J to allow for complete construction and shipment in one piece. We only fabricate a few tanks per year (among many other things) and are familiar with API-650 construction details but are not as knowledgeable of the intricacies of the design, and sometimes question details proposed by the designers. I am keen to learn what others do to maximize efficiency and practicality in storage tank construction.

From a shop fabrication point of view, I agree with MJCronin in that it would me much faster and cheaper to weld a nozzle with a thicker wall, in lieu of attaching a reinforcement pad (cut, drill, tap, form, fit, weld, test). But as JStephen stated and as I understand it, would then require individually checking nozzle reinforcement calculations resulting in increased engineering cost... As a fabricator, I would prefer the former lol.

 

[r6155]

1) Fire your designer.
2) This is not API 650. See API 12F

Regards

 

[JStephen]

This would fall under API 650 Annex J, shop fab tanks. Note that API 12F only allows specific standardized tank diameters, which do not fit many projects.

 

[r6155]

I already know it.
But API 12F is cheaper than API 650 for 12 ft diameters. Did the designer not consider API 12F?.

Regards

 

[NorthWest250]

I don't think API-12F would have been feasible for this project given the clients requests (specific tank dimensions, large nozzles and manways, plethora of shell & roof nozzles, high temp service, etc.)

If a client asks us for a tank of "X x X" dimensions, complete with "X" nozzles in "X" locations, designed to API-650, then that's what we typically provide them. There are many shops who supply API-12F tanks at a lower cost than a custom tank, and I have to assume that if that's what would have worked for the client then that's what they would have procured.

 

[r6155]

Please, next time send more information: diameter, height, ..etc
More details- better answer (by LitleInch)

Regards

 

[r6155]

@ NorthWest250
ANNEX M (not Appendix M as you said) is for elevated temperature operation.
So this tank requires thermal insulation.
So how do you think leaks will be detected (during service) through the tell tale hole in the pad?

Regards