Irrigation--Lost output pressure
I have a 20HP centrifugal pump capable of 400 gpm at 60 psi. The setup is a short length of 4" sealed intake with 5' head from a pond. The outlet is a 4" main that runs 700' with 3" feed extending another 300'. The end is a single water gun with 100gpm nozzle. All water line are mixed length of sectioned aluminum pipe.
According to rough estimation, even with losses including frictional say 10psi, I shall still be able to run the water gun non-stop. However, it seldom do so. Instead, it start off with full 60 psi. Overtime, it slowly drift downward until it tripped at shut off pressure of 40 psi over a period of 4-5 hours. I am at a lost of where else I loss the heads to. Is it possible total loses is greater than 20 psi? Appreciate any helps. Nick.
Outside United States
If your flow is truly 100 gpm, then there is about 2.6 psi loss in the 4-in section and 4.7 psi loss in the 3-in section (I am using aluminum pipe with couplings table). Don't know if you have a 5 ft. suction lift or if you actually have 5 ft of positive head on the pump, but even if suction that is only a couple psi lift loss plus miscellaneous. Fittings such as elbow add loss but all told I don't thing you would have more than 15 psi head loss at 100 gpm. I am assuming that you are not going uphill with the delivery either. If you pump can deliver 400 gpm at 60 psi, then it will deliver more than 60 psi at a lower flow rate (100 gpm). I do not know where you are measuring the pressure, but if you are measuring at the pump discharge, then the pressure should start off low as the pipes are filling and then build as the system pressurizes. Once pressurized it should maintain that pressure. I am assuming it is an electrical pump. I don't know why the pressure would drop by 20 psi over 4-5 hours. If you had bad seals on the pump, that should show up initially. Garry_Graybow@ncsu.edu provided this answer.