Correct Pressure Tap Location for Diaphragm-Sealed FT

> pressure taps (both upstream and downstream) are located on the sides of the flanges (180° horizontal)

I don't understand where that tap is.

If looking down the center hole in the pipe, is the flange at a tangent at 3 o'clock (vertical flange face)?

or at what clock position is the flange at a tangent to the pipe wall?

Regardless, I think your initial assessment is correct, the purpose for specifying the tap locations for impulse lines is to prevent the impulse line from filling with condensate/liquid in gas service.

An impulse line with accumulated condensate/liquid will create a head pressure error on that line.But diaphragm seals will keep any process condensate/liquid out of the capillary tubing.

Any accumulation of process condensate/liquid in the line at the diaphragm seal will create some head pressure error in that impulse line, but presumably any such liquid is temporary and will move along in the fluid flow; after all, the assumption of DP flow measurement is a single phase fluid, not a 2 phase liquid/gas fluid, right?

If the flanges are tangent at 6 o'clock and condensate/liquid could pool in flange pocket, then you'd be justified pursuing a change in the orientation of the flanges.

API 551, covers diaphragms seals in 6.3 but is barely a page and half long. It mentions keeping capillary lengths the same and steps to avoid temperature gradients along the capillaries. I'm not aware of other standards, but that's not definitive statement on whether such are out there or not.