Location of Pressure Taps for Orifice Plates 1

[qwerpoiu]

From some documents, I have read recommendations to use radius taps (pressure taps located at 1D u/s and 1/2D d/s of an orifice plate) with regular flanges in lieu of using orifice flanges for pipe size NPS 12 and larger. The same documents recommend using orifice flanges for pipe size NPS 2 to 10.

I would like to understand the basis behind this recommendation. As orifice flange spec (ASME B16.39) specifies size up to NPS 24, what would be the advantage for using radius taps over orifice flanges? Is it because the 1/2D tap location d/s of the orifice plate is closer to the vena contracta location? (from literature search, vena contracta is about 0.3D to 0.8D d/s of the orifice plate).

But I thought different equations are used to size the orifice plates depending on the location of the pressure taps; shouldn't this have been accounted for in the equations? Also, why does it apply only to piping NPS 12 and up?

Or is it because AGA 3 generally specifies flange taps but not radius taps? If the measurement is not for custody transfer, would I still need to comply with AGA 3?

I am not experience with orifice plates, and am wondering if I can get some directions, thanks.

 

[DLite30]

AGA has different calculations for different tap types and pressure monitoring locations. So in order for the AGA calculations to be their more accurate, it is wise to install the taps per the specs laid out in the standard.

Regarding the difference between pipe and flange taps, flange taps are probably going to be more consisent and thus more accurate, than a welder installing pipe taps.

If it's not custody transfer, I would go with tapped flanges.

 

[Cockroach]

ALWAYS 1.0 inch from the front face of the orifice plate and ALWAYS 1.0 inches off the back face of the same orifice plate.

This is the upstream and downstream tap location. It is specifically spelled out in AGA Report No. 3 or API 14.3.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada