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Thread: Calculating Pump Requirements for a Horse and Buggy IG

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    Calculating Pump Requirements for a Horse and Buggy IG

    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.
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    JasonLion's Avatar
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    Re: Calculating Pump Requirements for a Horse and Buggy IG

    Figuring out the dynamic head of the entire plumbing system from scratch is extremely difficult. Many of the fittings don't have published equivalent pipe length numbers. It looks like you have missed a few steps somewhere, dynamic head loss can't be as low as you think it is.

    If the equipment is above the water level in the pool, it is unlikely you will be able to get an above ground pump to prime. A few of them are self priming, but the great majority won't.

    If you are concerned about noise, a two speed pump is a great idea. A two speed on low is way quieter than any single speed pump.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Calculating Pump Requirements for a Horse and Buggy IG

    Quote Originally Posted by JasonLion
    Figuring out the dynamic head of the entire plumbing system from scratch is extremely difficult. Many of the fittings don't have published equivalent pipe length numbers.
    Well, other than skimmer and eyeball the only fittings in my pool are a bunch of 90 degree elbows. Everything is direct plumbed, no valves or fittings of any kind. I realize that different elbows have somewhat different equivalent lengths but I can't see one elbow being more than three times the equivalent length of another.

    In my original calculation I used an equivalent length of 4 feet per fitting. If I raise that estimate to 12 feet per fitting that only adds around another 7.65 feet of dynamic head loss to the total.

    Quote Originally Posted by JasonLion
    It looks like you have missed a few steps somewhere, dynamic head loss can't be as low as you think it is.
    That's the feeling one gets but without being able to point to an obvious missing step I'm inclined to think that my dynamic head loss is roughly in the ballpark of my calculated 16.84 feet H2O. Maybe it's somewhere between 16 and 32 feet of head. That would be assuming my calculation was off by at most a factor of two.

    I'm hoping Mark aka mas985 will see this thread and set me straight one way or the other.

    Quote Originally Posted by JasonLion
    If the equipment is above the water level in the pool, it is unlikely you will be able to get an above ground pump to prime. A few of them are self priming, but the great majority won't.
    The Pentair Dynamo Two Speed that I'm looking at does happen to be self priming.
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