Pipe Undertolerance

Date 04-23-2013 19:33

Minimum wall thickness is the calculated wall thickness, including corrosion allowance, to withstand the internal or external pressure the vessel will be subjected to.
When the vessel will consist of a pipe and the minimum calculated wall thickness is less than the minimum pipe thickness required by the applicable specification, including 12,5% tolerance, then this latter shall be adopted.
Giovanni S. Crisi
Sao Paulo - Brazil

By strat

Date 04-24-2013 13:19

Thanks Giovanni,
I think i'm still some what confused, if i have a vessel with a minimum calculated wall of .057 with 0 corrosion allowance and the pipe I have is 12" xh .500 nominal wall thickness and I measure the wall at .403 which is more than 12.5% under ( 12.5% of .5 =.0625, .5-.0625=.4375) .4375 being 12.5% under I can still use the pipe becuase the calculated wall thickness is less than the minimum pipe thickness required by the applicable specification, including the 12.5% tolerance.
Again thanks
Strat

By ozniek

Date 04-24-2013 15:32

Hi Strat

It looks to me like your pipe does not meet the material specification requirements, but is suitable for your application. All you need to do is make sure that the designer specifies the material in terms of the minimum wall thickness, rather than a nominal pipe size. One has to be careful here, because some client specifications may specify a minimum thickness, irrespective of what the design calculations say, hence the possibility that the thicker shell is called for. Also, the designer may ask for a thicker shell so that compensation is not required for nozzles. - Bottom line is that the designer needs to specify the minimum required thickness, rather than a nominal pipe size, then all will be clear.

Hope that helps.

Regards
Niekie

By strat

Date 04-24-2013 16:41

Thanks Niekie,
That helps greatly, I have spoken with the AI and he says not to use for material specification requirements and reason being on the drawing the design data specifies 12" XH in which the material in question does not meet Section II Part A, BUT, if the designer would change the design data to a specified minimum wall (to be used) and the material meets his specified minimum wall that then we would be ok to use. (with the AI blessing)
Am I correct in making this statement?

Thanks to you both
Strat

By G.S.Crisi (****)

Date 04-24-2013 21:36

Yes.
Giovanni S. Crisi

By ozniek

Date 04-25-2013 00:26

Hi Strat

In short - YES.

Depending on how pedantic the AI wants to be, he may state that the material does not meet the specification requirement, therefore it is "non-conforming" and that it should not be used. This can be overcome by specifying the pipe by actual OD and thickness, rather than NPS and schedule. (This is what the designer needs to do on the drawing.) There is nothing that stops you then from ordering a "non-standard" size pipe diameter and wall thickness to A312. All the limitations that are causing you grief, regarding the tolerances on wall thickness, only kick in once you specify standard NPS and schedule designators.

Regards
Niekie

By strat

Date 04-25-2013 10:52

Thanks Niekie and Giovanni
Information is greatly appreciated
Strat

By MBSims

Date 04-28-2013 01:03 Edited 04-28-2013 01:06

Clearly you can use the 0.403" wall pipe. If you calculated a Code-required minimum wall thickness of 0.057" with no corrosion allowance and intend to use a pipe material that has a manufacturing tolerance of 12.5%, then the minimum required wall thickness with manufacturing tolerance would be 0.057 divided by (1 minus 0.125 or 0.875), which is 0.066". If there is a corrosion allowance, it would be added to the 0.057" then divide the result by 0.875. Always round up to the next significant digit on this type of calculation. Since 0.403" is greater than 0.066", the pipe is acceptable.

Since the drawing specifies Schedule XH, you would need the engineer to accept the out of tolerance material. This could be documented on a nonconformance report with engineer disposition or on an RFI.