PVC Maximum Flow Rate?

[catnip]

I'm about to replace my pump and want to get the best match to my piping size. The suction side is 3 1.5" PVC pipes joined just before the pump inlet. The return side is 1 (one) 1.5" PVC pipe, so I believe I am constrained by the single 1.5" return line.

Research across the web yielded several flow recommendations for PVC pipe of 2" of less based primarily on water/pipe abrasion effects:
5 fps or 32 gpm (1.5") - Conservative flow limit
7 fps or 44 gpm (1.5") - Maximum flow limit
10 fps or 63 gpm (1.5") - Absolute maximum flow limit

My question: based on practical experience in pools what is the flow limit I should shoot for to balance performance with pipe life/reliability (don't want to have to break up the deck to replace a leaking pipe!!).

BTW, the pool size is about 18,000 gallons, so the I can get a full turnover in about 7 hours with 7 fpm/44 gpm. And the sand filter is rated at 60 gpm.

[Waterworks]

The single return line is your bottle neck. I like to use 6 fps for suction side and 10 fps for discharge side. These are the commercial limits in my area.

Brad
Waterworks Pools

[mas985]

Unless you have spa, just go with the smallest pump you can. A 3/4 HP pump should be under 63 GPM with 1 1/2" pipe given the high head loss.

What is your current pump braking HP (service factor * HP) and filter PSI?

[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!