Application of Mill Tolerance in Pipe Wall Thickness Calculation from ASME B31.3

[SeanL12]

Hi All,

First time post, but I have been reading this site for a while.

I have a quick question about application of the Mill Tolerance used in ASME B31.3.

Looking at the formula for the wall pressure thickness calculation it is:
T = (P*OD)/(2(S*E*W+P*Y))
Then to find the minimum required thickness (t_m):
t_m = T + c

When taking into account mill tolerance (m_t) the minimum required thickness should not be greater than the nominal thickness Nominal Thickness (Tb) times the mill tolerance, where the nominal thickness, Tb, is the selected schedule pipe wall thickness from ASME B36.10M:
t_m <= Tb * (1 - m_t)

Now to my question... I know that the required thickness (t_m) must be less than or equal to Nominal Thickness (Tb) times the mill tolerance (m_t), but can the Mill Tolerance be transposed to the minimum required thickness side of the equation?
eg: t_m / (1 - m_t) <= Tb

The reason i ask is that with the ASME calculation it requires iteration by trying to guess if the pipe schedule wall thickness (from ASME B36.10M) will be thicker than the minimum required thickness taking mill tolerance into account. Where as by transposing the mill tolerance to the minimum required thickness side of the equation allows you to instantly look for a pipe schedule with a wall thickness above that value thus saving the iteration steps.

In both cases the mill tolerance is being applied so my belief is that it is being taken into account but i wanted to check to see if my assumption of being able to transposing the mill tolerance is correct.

If not, could you please detail why (for my understanding)?

Kind Regards,
Sean Lewis
Associate Piping Engineer (Oil & Gas)

 

[StevenHPerry]

So in summary, is your question:
"Can x*y = z be rewritten as x = z/y?"
?

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[JakeR82]

I deal with VIII and B31.1 a lot.

The way you can look at it is this.

Calculate the required thickness for the pressure boundary of the pipe.
T = (P*OD)/(2(S*E*W+P*Y)) + CA where CA is corrosion allowance

Add in the mill tolerance (12.5%). This can be done to either side.
Tr = T/0.875
Tr = Required thickness with Mill Tolerance
T = Pressure Retaining thickness Required
Tn = Nominal Thickness of pipe (thickness of pipe from pipe schedule chart)

Check if Tr is less than Tn. If true, then pipe thickness is acceptable.

Conversly you could also do Tn*0.875 = Tred
Tred = Nominal pipe thickness with mill tolerance added in
Check if this reduced thickness is greater than T (The pressure retaining thickness).

Note that these calculations do not account for any external loadings. In regards to that, you would need to figure out if the stresses in the material are greater than the allowable stress of the material. (UG22 in VIII).