ASME B31.3, clause 302.2.2 interpretation

[Santha]

Clause 302.2.2 in ASME B31.3 states that "...piping component having same allowable stress as the straight pipe shall be rated using not more than 87.5% of nominal thickness of seamless pipe..." - I would like to know the exact interpretation of this. Does this mean the thickness of pipe fittings like elbow, tees, reducers should be more than the mating pipe thickness by 12.5%.
Also there is a mismatch of mill tolerance (M.T) between pipe and fittings when we use pipes welded from rolled plates (like ASTM A 671 , A672 or A 358) as in these cases the M.T for pipe is 0.01" while the corresponding fittings (say A 234 or A 403) have M.T of 12.5%. How is this problem overcome.

 

[prex]

As you correctly reported, that clause tells that, to define the pressure rating of a fitting, you use the straight seamless pipe with the same rating or class, you subtract the 12.5% M.T. plus any C.A. or thread depth, and the fitting is assumed to take the same pressure.
Now it's true that not all the seamless pipes have a M.T. of -12.5% (from 20" and over should be 10% if I recall correctly), but as the Code has the last word...
Another question is: what is the thickness of a fitting having a specified schedule? I would expect that the minimum thickness at any point is higher than the minimum thickness of the pipe with the same schedule: so a reducer at its larger end could have the same thickness as the mating pipe. It is up to the fabricator to define a starting higher thickness in order to have a sufficient margin against thinning during the forming process.

prex

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[bvi]

just to add to what prex wrote,

If you were to calculate the pressure rating of a fitting (and in practice, this is not done), you calculate the pressure rating of matching straight seamless pipe, considering a 12.5% mill tolerance (thus the 87.5%). In practice, however, you calculate the pressure rating of the pipe and select a matching fitting.

Regarding pipe rolled from plate, it is not the plate tolerance that governs, it is the pipe tolerance. The pipe manufacturer could order thinner plate than nominal thickness, and as long as the weight tolerance and thickness tolerance of the piping specification are complied with, it is OK.