To my understanding, an orifice can restrict the flow, therefore reduce the flow rate. I want to know what should the diameter of the orifice be compared to the pipe, to result the desired flow rate.
The Chemical Engineering issue of August 17, 1987, under the heading of Plant Notebook brings an article (with example) titled Quick sizing of restrictive orifices by Herman E. Waisvisz.
You didn't tell us if your flow is gas or liquid. That makes a big difference.
If it is a gas, then the upstream and downstream pressures define whether the flow rate is "choked" or "non-choked".
Read this Wikipedia article to learn how to determine if the flow is choked or non-choked, and how to calculate the choked flow rate for a given diameter of the hole in the orifice plate:
If the flow is non-choked, then read this Wikipedia article to learn how to calculate the non-choked flow rate for a given diameter of the hole in the orifice plate:
Seems that the problem is more complex than i thought, though i've reached to a methodology:
1. Look up pump’s head at existing flow rate (from the pump’s Head-Flow rate chart)
2. From the same chart, find pressure drop (which will be caused by the orifice) for the desired, reduced flow rate (from the requested flow, look up bigger head and calculate the difference from the first value)
3. Assume an orifice diameter
4. Check if differential at selected orifice agrees with the experimental from calculator (i.e.
On #4, be sure you calculate and use the "permanent pressure loss" of the orifice. A lot of references give flow coefficients for the differential pressure from 1 pipe diameter upstream to 1/2 pipe diameter downstream of the orifice, which is useful when sizing an orifice meter, but it's not the end result you need when sizing a restriction orifice. My Perry's 6th Ed. speaks to this on page 5-16.