I'm looking for standards/specification numbers that cover the calculations for, and manufacturing of, 'restriction orifices'(and not 'flow orifice plates').
Are they covered by the same standards/specifications for flow orifice plates e.g., ISO 5167, AGA Report No.3, API 14.3.2 or something else ?
I have searched through the above and they seem to be more 'flow orifice plate' specific and I cannot find any mention of restriction orifices.

Why not design per flow orifice? After all, an RO will require the same strength considerations as a flow orifice when it does see flow - it's just that you are not trying to accurately measure the flow presumably.

See following for some manufactured ROs - http://www.thermocouple.co.uk/Downloads/FM-OP_ROPA.pdf

There is a Steam Trap() Handbook that provides info on orifice plates regarding orifice size and capacity curves versus differential pressure. Capacities are given for both steam and condensate. I think the author is McKelvey.

Data was developed to show relative loss if orifices vice traps are used for ridding condensate from lines. Regarding construction there is a spec MS18301 to use that type of plate in line betwen two flanges.

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