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Thread: How to calculate my total dynamic head loss

  1. #1

    How to calculate my total dynamic head loss

    So my suction gauge reads 9 in Hg, my filter gauge reads 17 PSI and my flowmeter reads 80 GPM using my 1.5HP Jandy Stealth pump for my system circulation. According to this:
    TDH = Hsuction + Hdischarge
    = (1.13x9) + (2.31x17)
    = 49.44 ft head

    So then I pull apart my discharge line and rebuilt it, making it more efficient by eliminating my heater, backwash valve and removed most of my 90's and used 2 45's when I really needed a 90 elbow. I hooked up the same pump and presto! My pump IMMEDIATELY pulls more water, just by site alone. My new flowrate is higher @ 105 GPM and my filter pressure drops to 13 PSI. But my suction head reads higher @ 13 in Hg (i.e. MORE suction). I guess this can be expected since my flow has not remained constant but increased. So my DYNAMIC head pressure rises with increased flow rate. According to my new setup:
    TDH = Hsuction + Hdischarge
    = (1.13x13) + (2.31x13)
    = 44.72 ft head

    Did I do this right? Because when I go to the Jandy pump curves online (link is http://www.jandy-downloads.com/pdfs/SHP_curve.pdf) , 90 GPM correlates to 68 ft head and 105 GPM correlates to 62 ft head. So why don't these curves show the feet of head that I calculated? Am I using these pump curves incorrectly?
    20,000 in-ground pebble-coat pool with 60sf Jandy DE filter, Pentair Variable-Flow pump, Goldline Aquarite SWG, Dolphin Premiere robot, passive solar heating.

  2. #2
    Administrator JasonLion's Avatar
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    Re: How to calculate my total dynamic head loss

    Is it possible that you have a 1.5 HP Max Rated pump? That would fit your test results better than the full rated pump does.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  3. #3

    Re: How to calculate my total dynamic head loss

    Quote Originally Posted by JasonLion
    Is it possible that you have a 1.5 HP Max Rated pump? That would fit your test results better than the full rated pump does.
    Actually, the EXACT pump I have is the Jandy Stealth SHPF 1.5 HP, which is the full-rated pump and NOT the max-rated pump. I know this for a fact since it has a label on the pump. So what's going on here? Do I seem to be doing my calculations correctly? Do my values make sense?

    I guess if I really want to compare apples to apples, then I need to keep one parameter constant and vary the other, right? In other words, I should drop my flow to the SAME rate as BEFORE I did the plumbing so as to measure the NEW head pressure with the NEW plumbing. In that case, I don't see any reliable means of doing this unless I have a variable flow (or variable speed) pump for this type of test. Or should my test be that my TDH should equal the SAME value (BEFORE and AFTER my plumbing has changed) and the flows would vary along the designated pump curves to give a new flow rate? If so, that didn't seem to work with these charts. So what could I be doing wrong?
    20,000 in-ground pebble-coat pool with 60sf Jandy DE filter, Pentair Variable-Flow pump, Goldline Aquarite SWG, Dolphin Premiere robot, passive solar heating.

  4. #4
    Administrator JasonLion's Avatar
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    Re: How to calculate my total dynamic head loss

    You appear to be doing everything correctly. The only thing that occurs to me is that all of the measurements are subject to fairly large errors. Filter pressure gauges are not all that precise and neither are vacuum meters. You are off by noticeably more than I would expect from a gauge error. Still, it isn't out of the question that that is what is happening.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  5. #5
    Senior Member mas985's Avatar
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    Re: How to calculate my total dynamic head loss

    It sounds as though it may be a mislabeled pump. What does motor label say about service factor and HP?

    The measurements shouldn't be that far off for the kind of error you are seeing. Also, what is the size of your plumbing on the return and suction side? This can help determine where the error might be.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study
    18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  6. #6

    Re: How to calculate my total dynamic head loss

    Quote Originally Posted by mas985
    It sounds as though it may be a mislabeled pump. What does motor label say about service factor and HP?

    The measurements shouldn't be that far off for the kind of error you are seeing. Also, what is the size of your plumbing on the return and suction side? This can help determine where the error might be.
    Well, according to the label the horsepower says 1.5 HP but the service factor says nothing or "----" on the label which I guess means it is really 1.0 since it is a full-rated pump.

    Is there another test I could do validate my pump/motor is truly what they say it is?

    I am using 2" suction and 2" return lines with the standard 2" Jandy pump unions that come with the pump.

    I guess I should call Jandy to verify that their pump is what they say it is. The pump is over 2 years old, but I've never evaluated it to this extent when I bought it. I don't think a warranty is even valid at this point, nor do I think they would replace it based on my tests alone.
    20,000 in-ground pebble-coat pool with 60sf Jandy DE filter, Pentair Variable-Flow pump, Goldline Aquarite SWG, Dolphin Premiere robot, passive solar heating.

  7. #7
    Senior Member mas985's Avatar
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    Re: How to calculate my total dynamic head loss

    If there is no service factor then it is an up rated pump not a full rate pump. Full rated pumps have service factor between 1.5 & 1.9.

    So the pump you have is likely to be the 1.5 HP SHPM even though is says otherwise. The 1.5 HP SHPM fits the data that you have much better so I suspect, the pump was mislabled somehow.

    Another check is the full load amps if it is on the label. A 1.5 HP up rated pump (SHPM) will use about 7 amps @ 230 V where the full rated pump (SHPF) will use a little over 10 amps @ 230v.

    Also to be exact, you need to add 3 feet of head for the filter guage height since it will read slightly lower than the pump pressure.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study
    18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  8. #8

    Re: How to calculate my total dynamic head loss

    Quote Originally Posted by mas985
    If there is no service factor then it is an up rated pump not a full rate pump. Full rated pumps have service factor between 1.5 & 1.9.

    So the pump you have is likely to be the 1.5 HP SHPM even though is says otherwise. The 1.5 HP SHPM fits the data that you have much better so I suspect, the pump was mislabled somehow.

    Another check is the full load amps if it is on the label. A 1.5 HP up rated pump (SHPM) will use about 7 amps @ 230 V where the full rated pump (SHPF) will use a little over 10 amps @ 230v.

    Also to be exact, you need to add 3 feet of head for the filter guage height since it will read slightly lower than the pump pressure.
    Okay, I will speak to Jandy to see exactly what pump I have. I will see why there is no service factor listed as well.

    I measured the amperage of this pump and it pulls around 9.3 amps at 230 volts and it's labeled as 9.6-8.8 max load amps, which also made me think that this pump was labeled correctly. But I always find it hard to determine exactly what sort of draw to expect from these pumps since they label them with their "maximum" draw amperage. My understanding is the maximum draw is the draw at start-up, since the pump tends to level off at a lower "running" amperage, once the pump has overcome inertia/primed itself. Correct?
    20,000 in-ground pebble-coat pool with 60sf Jandy DE filter, Pentair Variable-Flow pump, Goldline Aquarite SWG, Dolphin Premiere robot, passive solar heating.

  9. #9
    Senior Member mas985's Avatar
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    Re: How to calculate my total dynamic head loss

    Quote Originally Posted by mgianzero
    I measured the amperage of this pump and it pulls around 9.3 amps at 230 volts and it's labeled as 9.6-8.8 max load amps, which also made me think that this pump was labeled correctly. But I always find it hard to determine exactly what sort of draw to expect from these pumps since they label them with their "maximum" draw amperage. My understanding is the maximum draw is the draw at start-up, since the pump tends to level off at a lower "running" amperage, once the pump has overcome inertia/primed itself. Correct?
    Unfortunately, there doesn't seem to be much of a standard for the pump labels. Sometimes they will use the service factor amps or max load amps. These are the motor ratings and may not directly reflect the operating point of the pump but they usually represent running amps at maximum load and not startup amps (startup is much higher). However, the ratings are usually pretty close to what the motor will draw when operated with the wet end since pumps are designed for nearly full load.

    The amps do suggest that it may indeed be a 1 HP full rated motor but the missing service factor is troubling. If it is a full rated motor, the impeller could still be for an uprated motor and this will determine the head curve. An impeller can always be downsized for a given motor but it should never be sized greater than the motor's capability. BTW, does the motor (not the pump) have a model number and manufacture (eg. A.O. Smith)?

    Also, what is the history of this pump (e.g. recently new, recently repaired, inherited, etc)?

    Jandy will probably just reflect what the pump model would suggest so a phone call may not be all that productive but it doesn't hurt to ask.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study
    18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  10. #10

    Re: How to calculate my total dynamic head loss

    So, I finally heard back from Jandy and they tell me my 2.5 hp 2-speed pump is up-rated with a service factor of 1.1 and the 1.5 hp SHPF pump is full rated and has a service factor of 1.47.

    So how to I put these numbers into my calculations?

    I also discovered something else ... I put a pressure side gauge (same make/brand as the suction side) and it reads much higher than my expected 3 feet of head (pressure gauge on top of filter is roughly 3 feet. I figured you would simply add 3 to whatever PSI I read on the filter. But instead, the reading on my pressure gauge at the pump reads 20 (instead of 14). Why is this?

    So here are my new head calculations for my 2 speed pump:
    TDH = HS + HD
    = (1.13 * 14) + (2.31 * 20)
    = 15.82 + 46.20
    = 62.02 ft head

    Considering inaccuracies in the pressure/vacuum gauges, does this now seem to agree with the pump curve I provided for the SHPM 2.5 hp pump below or am I still way off?
    Attached Files Attached Files
    20,000 in-ground pebble-coat pool with 60sf Jandy DE filter, Pentair Variable-Flow pump, Goldline Aquarite SWG, Dolphin Premiere robot, passive solar heating.

  11. #11
    Administrator JasonLion's Avatar
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    Re: How to calculate my total dynamic head loss

    You are much much closer to matching the curves. I doubt you will ever match them exactly, there are too many possible sources of error.

    The extra pressure at the pump might indicate a partial blockage of the pipe, or it might be a more complex (higher TDH) main valve than expected, or it could just be errors in the pressure measurements.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    TFP Admin. Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  12. #12
    Senior Member mas985's Avatar
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    Re: How to calculate my total dynamic head loss

    Quote Originally Posted by mgianzero
    So, I finally heard back from Jandy and they tell me my 2.5 hp 2-speed pump is up-rated with a service factor of 1.1 and the 1.5 hp SHPF pump is full rated and has a service factor of 1.47.

    So how to I put these numbers into my calculations?

    I also discovered something else ... I put a pressure side gauge (same make/brand as the suction side) and it reads much higher than my expected 3 feet of head (pressure gauge on top of filter is roughly 3 feet. I figured you would simply add 3 to whatever PSI I read on the filter. But instead, the reading on my pressure gauge at the pump reads 20 (instead of 14). Why is this?

    Dynamic head loss between the pump and filter is likely to increase the pressure difference beyond what would be expected with just the filter height. So it depends on the plumbing you have between the pump and filter but normally that is not much of a difference so I suspect one of the gauges may not be reading correctly so you may want to swap gauges and see if they read the same.

    Also, note that what I was suggesting was to add 3' to head loss not pressure. Pressure due to 3' of elevation is 3/2.21 = 1.3 PSI.


    So here are my new head calculations for my 2 speed pump:
    TDH = HS + HD
    = (1.13 * 14) + (2.31 * 20)
    = 15.82 + 46.20
    = 62.02 ft head

    Considering inaccuracies in the pressure/vacuum gauges, does this now seem to agree with the pump curve I provided for the SHPM 2.5 hp pump below or am I still way off?
    The 2.0 HP SHPM seems to fit the data better but again one of the gauges could be reading incorrectly. The 2.0 HP SHPM has the same head curve as the 1.5 HP SHPF so this would seem likely.

    One other thought is that the flow meter does not have enough straight pipe to read correctly. They usually require quite a bit of straight pipe to read correctly.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study
    18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, 450 sq-ft EPDM Solar Panel, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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