Threaded pipe min wall thickness

[ARenko]

API 570 defines required thickness as the lesser of pressure design thickness or structural design thickness. Pressure design thickness is defined as "minimum thickness without corrosion allowance based on design code calculations for pressure."

Let's say pressure design governs our threaded pipe and the code is B31.3. Pressure design thickness is 't' in 304.1.1. 'c' is the sum of all allowances - corrosion, erosion, and mechanical allowances. Say I calculate t=1.5, CA=.06, and thread allowance =.06 (i.e. c=.12). Am I correct that the required thickness would be 1.44"? In other words we don't take out the mechanical allowance in figuring the required thickness as defined in API 570?

 

[BigInch]

No.
The pressure design thickness would be 1.5
The required design thickness using threaded pipe is 1.56
The required B31.4 design thickness including thread and the required corrosion allowance is 1.62

Not having read it, but not that it makes a difference, I can't imagine anybody caring what API 570 may, or may not, say in regard to the required thickness for a B31.3 design code. B31.3 decides the minimum thickness that it needs to be, given a typical installation. The piping engineer decides if it should be more. Nobody else can say it should be less.

 

[ARenko]

My brain is fried today - I wasn't clear and subtracted when I should've added. I am considering when to take the pipe out of service (the "required thickness" in API 570). I should've added 1.5 to .06 for 1.56 instead of 1.44. So, the pipe pressure design thickness is 1.5 and where I might normally take it out of service, but due to threads I'd have to take it out of service at 1.56. Correct?

 

[ARenko]

Ugh, and I meant .15, not 1.5, but the idea is the same.

 

[BigInch]

Take it out of service as soon as the corrosion allowance is gone.

 

[ARenko]

I guess my question is really how to present the min wall thickness info. For welded pipe it's corrosion allowance is gone when it hits the pressure design thickness of .15, so we can simply say take it out at .15 wt. Do we keep that same number for threaded pipe and rely on the inspector to know to accomodate for thread depth? Can the inspector measure wall thickness in the threaded area while the joint is made up, or does he measure wall thickness in the main body and then subtract for thread depth? Or does he just measure in the main body, in which case we should consider acoomodating for the thread depth in our spec for min wall thickness.

 

[dcasto]

I suggest you get a copy of ASME B1.20 Threaded pipe to understand the strength of threaded pipe. It suprises a lot of people.

 

[ARenko]

I'll try to get my hands on a copy. I've probably been way overthinking this - I appreciate the responses. dcasto, are you suggesting I may not need to worry if the wt within the threaded area (due to thread cut) drops below pressure design thickness? Would that be within the intent of the code? I assume we add thread allowance so that we stay above pressure design thickness in all sections of the pipe.

 

[Gilmiril]

Does ASME B1.20 exist? Perhaps ASME B1.20.1 was intended, but that only deals with the actual form of the pipe thread and not the strength of the pipe on which it's cut.