Inside WN Diameter tolerances between B16.5 and B16.36

[BFZHA]

Hi,
I want to know why for orifice flanges the tolerances for internal diameter shall as per ASME B16.5 (+/- 1mm) except for welding neck flange except which is specified in ASME B16.36 equal to +/- 0.5% of nominal diameter.

Please advise because I didn't see any issue to consider ASME B16.5 internal diameter tolerance especially for small diameters.

Thanks and Best Regards.

 

[BFZHA]

Hi,
Since I started this post, it seems correct to me to present the conclusion I obtained after a brief research, however I liked and like to have the point of view of colleagues to better understanding of this issue.

First,I recall for welding neck orifice flange some provisions shall be taken for their welding to the pipe :

- Welding neck orifice flanges should be the same bore as the pipe to which they are attached and must be aligned accurately.
- Welds attaching welding neck orifice flanges to pipe should be ground smooth on the inside.

This two provisions are taken in order to avoid any flow disturbance in the neighborhood of the orifice flange and orifice plate which is in fact used for flowrate measure. And as we know any singularity isn't welcome upstream the orifice plate because it affect accuracy of flow measured.

This is, I guess, why in ASME B16.36 the tolerance for indise diameter of orifice welding neck flange is more stringent than in ASME B16.5.

Best Regards.

 

[nickelkid]

I think you have things covered with your understanding/approach. On small bore pipe I would think the weld more impact than on larger bores. I do not have access to my notes from the last time I had to design some metering runs, but I recall using ISO and GPA standards for additional guidance.