I have a sustained pressure pump that runs at 80 PSI. I'm trying to work out what the GPM rate is through a 3/4 inch pipe.
Pipe flow is tricky. Does it discharge to open air at the other end (so 80 psi is the actual pressure drop down the length of pipe). You also need the length of pipe, inside surface roughness, and whether there is any kind of nozzle or other constriction at the end.
John is absolutely right with all of those factors coming into play. another MAJOR, MAJOR factor is your lift. Are you trying to pump through a level pipe or is your discharge higher than your pump? Is the pump already at the fluid level or is it having to draw it up to itself. Also, what are you pumping? Gasoline will flow at a different rate than water through the same system. Keep in mind that you could close off the discharge end, your flow rate would be 0, and your pressure would actually go up. If you're doing something like a pond, there are usually tables available for different pump models that will give you an approximate flow rate for that pump and various lift heights.