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Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

(OP)
Help me to settle a debate I have.  B31.3 states for strait pipe:

"...the internal pressure design thickness for strait pipe shall not be less than that calculated in accordance with eq. (3a) OR eq. (3b)."

I am picking up on the word OR here, meaning that you can use either formula to calculate the required thickness.  One formula is based on OD and the other on ID.

I am in a situation where I have a very over designed tube.  Formulas 3a and 3b give different requirements because the tube has a small ID, and a very thick, overdesigned wall.  In this design scenario it seems that only the ID formula is needed because calculating the required thickness based on the OD would give a required thickness based on an OD diameter that is essentially just providing extra material.

What are your thoughts?  Should one or both equations always be applied?

RE: Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

How thick? Are you still within t<D/6?

Take a look at K304.1.2 for thick walled pipe, where the two equations should give you more consistent results.

Beware that I'm still pretty new to the piping codes.

RE: Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

(OP)
Yes, the calculated required thickness satisfies t<D/6.  It is not a very high pressure application, so not much actual thickness is required.

RE: Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

The required thickness is < D/6, but the actual thickness is above the limit, right?

You should be able to use either formula. The fact that they give different results wouldn't matter, since your actual thickness should be greater than either one of them. Using the OD one might make it easier to calculate a corrosion allowance revision. With corrosion allowance adjusted to take up most of your wall, the two formulas should give consistent results.

RE: Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

The equations render the same result. Refer to Interpretation: 21-13.  

RE: Use B31.3 Equation 304.1.2, 3a OR 3b for strait pipe

beh188 (Mechanical)

ASME B31.3 2010
read and review changes 304.1. General (b) nomenclature is used in the equations for pressure design of straight pipe:

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