ORIFICE

or⋅i⋅fice [awr-uh-fis, or-] –noun
an opening or aperture, as of a tube or pipe; a mouthlike opening or hole; mouth; vent.

Orifice Plate –
From Wikipedia, the free encyclopedia

An orifice plate is a device used to measure the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.
An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called vena contracta point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation.

Orfice Plates are normally the responsibility of and purchased by the Instrument/Control Systems Engineering group. The Orifice plates are shipped to the job site and installed by the field piping installation contractor.
www.emersonprocess.com/daniel/Pr ... 0563KB.pdf


Orifice Flanges –
Orifice Flanges are normally purchased by the Piping Engineering and Design group. They are normally shipped to the pipe fabrication shop.
www.thermocouple.co.uk/Downloads/FM-CR_FLGA.pdf
and
www.imperialco.net/flange/flange.html



Orifice Run –
Orifice Runs are normally horizontal. However there are sometimes when the runs can be in the vertical. Vertical runs are very rare and require the complete approval of the Process Engineer and the Instrument Engineer.
Orifice Runs require very specific up-stream and down-stream lengths.
The up-stream should be 20 diameters of straight run with no obstructions (Thermal Wells, etc). This distance may be reduced with the approval of the Instrument Engineer for various reasons. These reasons may include the configuration of In-plane elbows and the distance between the Ells.

The down-stream run should be a minimum of 5 diameters of straight run before any obstruction or elbow.

An FO (including all the items above) will also have the Flow transmitter, one or more Flow indicators (Dials) and a Flow Control Valve. This is all part of a complete Loop which is alive and self adjusts the flow.

A Restriction Orifice (RO) is just a plate (or other device) with a fixed hole which cuts down the flow through the pipe.

Per your profile you are a Piping Designer.
The sizing of the orifice is not the responsibility of the piping designer.
I know that all piping designers are curious about a lot of things and that is fine if it is just curiosity.
What I am afraid of is that you may be trying to take over the responsibility of something you are not trained for and not authorized to do.
What if you try to do it and do it wrong.
Please consider finding an Instrument/Controls Systems Engineer. Ask him or her to explain how it is done but let them do it for the project.
You will learn but will also stay out of trouble.