18gpm through 1" poly is about 3gpm above the recommended velocity rating of the pipe. Every well installer I have ever dealt with shamelessly undersizes the supply line coming from the pump stating that the minimal amount of pipe that they are using will not make a difference. A bottleneck is a bottleneck where I learned how to design. A system flowing 18gpm should have utilized an 1 1/4" mainline/manifolds with 1 1/4" trunk line after the valve splitting the zone in the middle.
Last edited by Rcn971; 09-03-2013 at 10:50 PM.