Orifice Flange Size Question

I have been watching your post all day to see what responses you would get. None so far so I will offer the following:

• I could not find any site/Catalog showing any Orifice Flanges smaller than 1"
• I do not recommend using anything under 1", in fact I suggest you go up one or two sizes to get better readings on your Flow Control.
• I think you meant to write ASME B16.36 which covers Orifice Flanges (see below)
• You did not say what Flange Class you were dealing with but the minimum "T" dimension (Thickness of the Flange Rim) for a 1" or 2" is 1.50" for Class 300, 600, 1500 and 2500 (1" only). This extra Rim thickness is what allows for the Pressure Tap in the Rim.None of the Standard Weld Neck Flanges in 1" and 2" are that thick except the 2" Class 1500 & 2500.
  
• You did not share your Process Operating conditions (Pressure and Temperature) or the commodity so we cannot make any factual conclusions.
• As for "qualification/examination process before it is deemed safe/reliable to be used" I would suggest you check with your Insurance Company Representative.
  

The bottom line for me is I do not recommend that you use a B16.5 Flange and modify it.

ASME B16.36 Document Information:

Title
Orifice flanges

ASME International

Publication Date:
Nov 4, 2009

Scope:

This Standard covers flanges (similar to those covered in ASME B16.5) that have orifice pressure differential connections. Coverage is limited to the following:

(a) welding neck flanges Classes 300, 600, 900, 1500, and 2500. U.S. Customary units are presented in Mandatory Appendix I.

(b) slip-on and threaded Class 300.

(c) welding neck flanges Class 400 in U.S. Customary units in Mandatory Appendix II.