Recently had a job with a wet-pipe fire sprinkler system (smaller project) that has a 6-inch double check backflow. The area static and residual pressures are pretty consistent (had a flow test of ~40 psi static and ~38 psi residual at 1,300 gpm).
We ran a forward-flow test through the backflow preventer, needing a minimum flow of about 800 gpm, which includes the hose allowances for the project (no standpipes or interior hose connections).
The most flow we could get was about 450 gpm, even with three 2-1/2" hose outlets completely wide open. The static pressure following the test at the riser was about 40 psi. We used three 100 ft, 2-1/2" hoses with a 1-3/4" smooth nozzle and diffusers at the end of each hose, measuring about 3 psi pitot on each and full flow (which equates to roughly 450 gpm).
Do you have any tips or advice in why we were not able to get enough flow through the backflow, or where to start to troubleshoot?
Thanks in advance!
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