I found an answer with charts, but I want to know where I am going wrong in the calculations. Note: I do not have the pump curve.

Problem. A pump feeds a heat exchanger with cooling water and drains to atmosphere in a wash water tank. Each day, a valve closes to wash the heat exchanger and kills the pump, which causes the wash tank to drain. The pump discharge is 45psi feeding 1500gpm to a 10in line.

I have 5ft of 2in HDPE pipe that I can use for a bypass to the wash tank when the heat exchanger valve shuts. The wash tank drains at 15gpm. Will the 2in line allow enough flow to match or exceed 15gpm?

Solution Attempts: Darcy-Weisbach I need velocity in the 2in, which I can't assume with A*V=constant because that would mean the flow rate in the 2in is equal to the pump discharge (1500gpm), which it certainly cannot be. Friction factor requires the mean velocity people to calculate. Velocity head requires flow rate. Every solution I try comes up with an unrealistic answer or I require something I am trying to find. Industry charts show the 2in line has a max water flow rate of 50-150gpm for this pressure. Can this be calculated without the pump curve in a way that I cannot see?

A thousand thanks if some light can be shed.