The application is a 6" line at 75 PSI which we insert an in-line valve with a throat diameter of 3" at the output of the water meter. The question posed is, will the 3" throat be able to provide the same (or close to) GPM rate. As for pressure drop, it would be variable, not knowing how many taps would be open at any given time. Can you make a calculation based on an open ended 6" line 10' long after the 3" throat is installed, how many GPM's would flow out the end of the 10' open-ended line. Wouldn't the GPM be the same as the 6" line without the 3" throat, just flowing faster?
I'm sorry I don't have the talent (or money) to figure this out, and I appreciate your help.