You have not provided enough information for anyone to even venture a guess as to what the flowrate is.
What are the lengths of pipe? What is the pipe material? How old are the pipes? What is the configuration? Where was the differential pressure measured? etc, etc.
Do you have a 14 inch orifice plate inside an 18 inch pipe?
or is this a reducer going from 18 inch to 14 inch?
in both cases we need more information.
Hydrae