The pump on our little 12,000 gallon IG died and I'm trying to choose an appropriate replacement. Since the pump is totaled I can't take pressure/vacuum measurements to calculate dynamic head. I'm stuck trying to ballpark it from calculations. I'm hoping some of the more knowledgeable folks will look over my work and let me know if I'm on the right track.
The pool is a 15x30 oval vinyl IG with a 3 foot shallow and 6 foot deep end. It's around 11,500-12,000 gallons. We're in the northeast and the pool was installed in the 70's so it's an antiquated setup. There's a single skimmer and a single return. There are no goodies like spa, chlorinater, heater, etc. It's "no frills" plumbing too. The skimmer is plumbed directly to the pump inlet. The pump is connected directly to the DE filter. The output of the filter is plumbed directly to the "eyeball" in the pool. There are no fancy check values, bypass valves, or manifolds. The pump sits on a pad a few feet from the edge the pool. The center of the pump impeller is between 1 and 2 feet above the water level of the pool. All pipe is 1.5 inch black pvc. The filter is a Sta-Rite "System 2 Modular DE Filter" Model PLD50. Max flow rate for the filter is 60 gpm.
I'd like to hit the sweet spot of 3/4 max flow rate of the filter so my target for the system is 45 gpm.
First is head loss to pipe and fittings:
11 feet of pipe from skimmer to pump + 19 feet of pipe from filter to outlet eyeball = 30 feet of actual pipe
8 right angle elbows * 4 feet equivalent length per fitting = 32 feet
20 feet + 32 feet = 62 feet (equivalent) of 1.5 inch pvc
Equivalent length of fittings was found here: http://www.engineeringtoolbox.com/pvc-p ... d_801.html
Head loss in 1.5 inch nominal pvc is about 11.95 feet of head loss per 100 feet of pipe.
62 feet of 1.5 inch pvc * 11.95 feet of head loss per 100 feet of pipe = 7.4 feet of head loss to pipe and fittings
Friction head loss numbers from here: http://www.engineeringtoolbox.com/press ... d_404.html
Now head loss from the filter:
According to the filter manual's pressure drop curve, 45 gpm yields a 4 psi pressure drop.
4 psi * 2.3106 = 9.24 feet H2O
PSI to feet of water conversion from: http://www.engineeringtoolbox.com/press ... d_406.html
Now add up head loss of all components:
0.20 feet H2O - skimmer
7.40 feet H2O - pipe and fittings @ 45gpm
9.24 feet H2O - filter loss @ 45gpm
total dynamic head loss: 16.84 feet H2O
I realize that I've neglected head loss for the outlet "eyeball" and the additional pressure drop of a dirty filter. Does anyone have numbers for those?
Even accounting for additions due to eyeball and dirty filter I'm still surprised. 16.84 feet is less than numbers I've seen for above ground installations. Am I missing something? Did I make any glaring errors?
Assuming no major errors, would it be safe to say that I could put an above ground pump on this pool just as well as an IG pump provided that it's self priming?
The previous pump was a 1 HP (Service Factor 1.0) AquaTools AT2310110. It was an above ground pump and it lasted nearly ten years. There were never any issues with priming. Priming was pretty much instant. Remember, the pump is only 1-2 feet above water level and the inlet pipe is only 11 feet long.
(For the record I was not involved in choosing the 1hp above ground pump that I'm replacing! )
All the local pool guys tell me the sky will fall and the Earth will spin backwards if I put an above ground pump on this pool. They want to sell me 1.5-2.0 BHP monsters.
If it's fine to use an above ground pump I'm thinking that a Pentair Dynamo Two Speed would be a good choice. Run it on low for filtering; use full speed for sweeping.
Here's the curve for the Dynamo Pumps:
Even if my dynamic head numbers are off by a factor of two the Dynamo 3/4 HP will still be above my target of 45 gpm on full speed.
Does Pentair provide curves for low speed? I haven't been able to find any. How are we supposed to calculate turnover time on low?
The main motivation for a two speed is noise. It's nice to save energy but our electric is dirt cheap, our pool is small, and it's only open 3-4 months a year. We would never save enough to recoup the cost of a variable speed or even a high efficiency $500 pump. That said, noise is a big deal. A pump that's inaudible at 10-12 feet would be awesome.