Using Pipe as a Shell while Considering UG-16(d)

[Rmclach]

I am currently designing a Category H fitting for use inside of Canada to CSA B51 and ASME Sec VIII, Division 1. The fitting consists of 1/2" Sch 10S pipe, SA-312 TP316L, that is both bent and coiled. The pressure is 300 psi and temperature is 350°F. The required thickness is less than the minimum thickness for pressure retaining components per UG-16(b). Therefore, the minimum required thickness for the application is 1/16".

My question is, can you define the minimum wall thickness of the pipe to be below the 87.5% undertolerance in the calculations, i.e. 1/16" per UG-16(b)? One of my reasons for wanting to do this is to account for potential thinning due to the additional forming/fabrication that is to be done to the pipe.

I have read UG-16(d) and have interpreted it two ways.

1) You cannot use pipe below it's specified undertolerance thickness
2) You must ensure that the pipe used is above the minimum thickness required after considering undertolerance, i.e. 1/16" * 1.125 = 0.0703"

I believe that the second interpretation makes more sense when comparing to rolled plate.

The pipe material will be received to SA-312 material specification and in the unformed state, will have to be equal to or greater than the undertoleranced thickness.

However, after additional processing I would like to define the minimum thickness as below the 87.5% thickness, aka 1/16".

I look forward to other peoples views on this scenario.

 

[SnTMan]

Rmclach, you may use any product form at not below 1/16" plus any corrosion allowance. Note also the language states "after forming". You can state something like "thickness after forming shall be 1/16" minimum". Ths meets UG-16, assuming CA is zero.

There ere exceptions given as well, does your application meet any of these?

Regards,

Mike

 

[Rmclach]

Corrosion allowance is 0.

The argument made to me was that once your thickness is below the pipe undertolerance, the pipe is no longer acceptable to the material specification. Is this valid?

Thank you,

Ryan

 

[SnTMan]

Well, yeah, if it were supplied below undertolerance before forming. Forming is a secondary operation.

Same with heads, rolling, or any forming operation. The supplied material must meet all its specification requirements, including thickness undertolerance, before forming. Subsequent operations may reduce the as-supplied thickness. Which reduced thickness is then subject to other Code rules, such as UG-16.

I think you are OK, as you have described it.

Regards,

Mike