PVC Maximum Flow Rate?

[catnip]

Brad, thanks for your response on the limits in your area. Presuming even flow through each of the three suction lines (not necessarily so), I would never exceed 6 fps there. If I can go to 10 fps on the return, that gives me more pump choices.

Mark, I don't have a spa, so, I've been comparing curves for high-efficiency 1/2 hp and 3/4 hp full rated pumps (with service factors of around 1.9 and 1.67 respectively). The Sta-Rite Max-E-Glas, Sta-Rite Max-E-Pro and the Pentair Challenger seem like good choices with "steep" pump curves that could still move water as the filter gets clogged. The clean filter pressure is about 18.5 lbs. which translates to 43 feet of pressure head, I believe. I have not measured the suction head but guess it's around 5 - 10 feet. Does that make sense? Unfortunately, I'm away from my pool for several weeks, so can not give you the details on the current pump.

[mas985]

Just a word of caution. Some pump manufactures will simply change out the impeller of a 3/4 HP pump to 1/2 HP impeller but use the same 3/4 HP motor. The result is an under loaded motor which fairly inefficient. You can this is the case when the maximum current draw is the same for a 1/2 HP and 3/4 HP pump.

It sounds as though you have about 50 feet of head with your current pump. However, the head loss will change with a new pump depending on if the new pump has a higher or lower brakiing HP (BHP). A fairly good approximation for the head loss change is:

Head Loss New Pump = Head Loss Old Pump * (BHP(New)/BHP(Old))^2/3

[catnip]

Mark, thanks for the caution about the impeller vs. HP possibility. I checked that on the Sta-Rite Max-E-Pro line and there were both different impellers and different motors with different current draws for their 1/2 and 3/4 HP models.

I also appreciate the dynamic head loss vs. HP equation. I presumed there had to be some kind of change with HP since the term is "dynamic" after all!