If two phase flow is possible, there are concerns of light phase hold-up with the hole down and heavy phase hold-up if the hole is up. The relative amounts of the phases come into play.
Thanks for the reply Latexman, but these are concerns specific to eccentric orifice plates installed in horizontal lines.
Since hold-up from orifice position in the vertical line is not a concern, the correct installed position is unclear. I was thinking that positioning the plate off-center with respect to the taps could cause some measurement error. Such as the taps on opposite sides of the pipe but with the eccentric orifice closer to one tap.
After noodling on this, whether eccentric or concentric, vertical downflow has little 2 phase downflow concerns, but vertical upflow may cause accumulation of the heavy phase.
Vertical upflow is likely to cause concentration of the heavy phase even if the orifices are not there. The heavies fall back and the vapors ascent easily. The orifices would actually help, concentrating the flow at each orifice into a jet and atomizing the heavy phase
You do not specify fluid, one phase or two-phase, temperature, pressure, batch-pumping/varying flow or regular flow under constant parameters, and choke or measuring purpose of orifice plates.
Specific to your question I suspect the orifice plates are for measurement purposes (delta p over plates).
In any case I suspect under circumstances (if plates are too near each other) you could have more turbulence on both plates if excentric than if 'lined up' centric or excentric.
If this is the case you might have a tendency of varying and more unaccurate mesurement if measuring purpose, varying with the degree of turbulence. If choking perhaps (extra) varying degree of choking with flow or perhaps turbulence or cavitation problems and extra wear.
Certain is, however, that the flow picture/flowlines will vary with the distance (long distance less or no influence) and orientation of the orifice plates and flow.
To get a more accurate answer on influnce the only way is to test out 1:1 on actual site against a 'neutral' calibration. (If this is at all possible in practice???)
Eccentric orifices may have 2 useful purposes, in my mind.
In horizontal pipes, they would allow drainage if offset toward the invert.
In any pipe, if 2 or more eccentric orifices are in series and offset, then the effective restriction effect is increased, and solid particle erosion may be decreased on the downstream piping. As I recall, this configuration was used in the 1960's on supercritical power boilers in their startup systems, downstream of a block valve ( drain)..
Perhaps the AGA has some quantitative data on accuracy of ecentric orifice plates, but my own company's documentation uses the same plot for orifice coefficients and merely notes that the best case accuracy (2 std deviations) when applied to eccentric orifi is 2% vs 1.2% for a concentric orifice. I read this as when all the errors are considered, if you typically think that an orifice plate is good to +/-6% of scale, then a eccentric orifice would be +/-10%.
I don't think there are any general concerns to using an eccentric orifice in the vertical, but I can't think of any advantages either. Can you clue us as to why an eccentric is being used? Usually eccentric and segmental plates are used to address damming of seperated phases and/or solids in horizontal runs. If you are measuring in the vertical, then the flow direction is probably the most important detail- i.e. liquid upflow.
