Measured system GPM vs TDH not consistent with pump performance data

[SgtWilko]

Hello,

I just cleaned up my inground pool plumbing and added a flowmeter at the outlet of the filter. All plumbing is 1.5" with separate skimmer and bottom drain lines on the suction side. Heatpump and salt cell are plumbed in the usual serial manner on the pressure side.

Actual measured flow via a Blue-White F-300 flowmeter is about 44GPM. This results in a 6hr turnover rate which is certainly adequate for my ~16,000 Gallon pool but...

I measure about 68 ft of head using vacuum (4.5 in HG) and pressure (27.5 PSI) gauges. Pump performance data indicates flow should be more around 58 GPM at 68ft of head. Vacuum and pressure gauges are new. Filter is freshly backwashed. Does anyone more versed in hydraulics have any idea what might explain this 14GPM delta? The pressure gauge method seems widely used or at least written about. Is it just that imprecise or should I be closer to actual flow ?

My pump is 15 yrs old and I'm looking at moving to dual/variable speed to reduce energy consumption.

I have skimmed through the "Hydraulics 101 - Hydraulics, Pipes and Filters " sticky post and will take a closer look eventually. However, before investing too much time on the simulation front, I'd like to get a handle on the dicrepancy between my simple flow from TDH & pump performance data vs actual flowmeter result.

Any insight would be appreciated.

Regards,

Sgt Wilko

 

[mas985]

Did you make the pressure and suction measures on the pump drain plugs?

That type of flow meter is not all that accurate and a very sensitive to placement in the plumbing. I would go by the direct head measurements.

Full Scale Accuracy:

+/- 10% (5x pipe ID min. inlet pipe length / 2x pipe ID min outlet pipe length) +/- 5% (10x pipe ID min. inlet pipe length / 5x pipe ID min outlet pipe length)

Error increases with shorter distances to the nearest fitting.

Actual measured flow via a Blue-White F-300 flowmeter is about 44GPM. This results in a 6hr turnover rate which is certainly adequate for my ~16,000 Gallon pool but...

Hopefully you are not setting run time by turnover. That will lead you to running the pump far longer than you really need to.

 

[SgtWilko]

Thank you for taking the time to respond @mas985.

Did you make the pressure and suction measures on the pump drain plugs?

Vacuum at pump drain plug. Pressure at filter gauge.

That type of flow meter is not all that accurate and a very sensitive to placement in the plumbing. I would go by the direct head measurements.

Full Scale Accuracy:

+/- 10% (5x pipe ID min. inlet pipe length / 2x pipe ID min outlet pipe length) +/- 5% (10x pipe ID min. inlet pipe length / 5x pipe ID min outlet pipe length)

I understand from your response that you think flow meter accuracy Is the main issue and that I am probably closer to a 58 gpm flow, as expected from the pump performance data. Is that correct? That said, isn’t 25% a bit too big of an error to attribute to this single source ? Especially since I have the “new version” which B-W claims meets the 5% accuracy at 5x/2x ID inlet/outlet length for 1.5” pipe. I’m also actually at about 6x/4x ID Inlet/outlet And have taken great care to level the meter to within the allotted +/-2 degrees from vertical. I thought maybe the turbulence at the output of the filter could be an issue but I really dont have a clue.

What is the “Direct head measurement” method? Pipe lengths + equivalent lengths of all fittings? I don’t have filter and heat pump equivalent lengths in my equipment documentation. I can estimate these by bypassing each and noting the TDH change from the pressure gauges but there is a lot of interplay between variables for this to provide an accurate result.

Sgt Wilko

 

[mas985]

Direct head measurements are what you did. However, when you measure at the filter, you need to take into account the plumbing between the pump and filter plus the gauge height.

In the pump tools workbook there is a tab called "PSIVac2GPM" which will do this for you. Just based on your equipment and and a few guesses, the spreadsheet comes up with 42 GPM which is pretty close to the flow meter.

Your plumbing has extremely high head loss for the flow rate.

 

[SgtWilko]

Direct head measurements are what you did. However, when you measure at the filter, you need to take into account the plumbing between the pump and filter plus the gauge height.

In the pump tools workbook there is a tab called "PSIVac2GPM" which will do this for you. Just based on your equipment and and a few guesses, the spreadsheet comes up with 42 GPM which is pretty close to the flow meter.

Your plumbing has extremely high head loss for the flow rate.

Thanks for the clarification. I will take a look at the pump tools workbook's "PSIVac2GPM" tab. I'll also move the pressure gauge to the pump to compare. The filter pressure port is about 36" off the slab. Plumbing btwn the pump outlet and filter MPV is roughly 5ft of 1.5" pipe with a 90° elbow (pics attached).

About the high TDH, I know... My pool guy went ahead with the plumbing redo while I was away, despite my telling him to wait untill I was back so I could provide further imput. His focus was apparently on effort and cost. He reused as much of the existing as he could (1.5" pipe), all 90° elbows (and lots of them), not to mention lots of Tees, and did not provide a 12"-15" straight run from the skimmer/drain suction line couplings to the pump inlet. I counted a mix of 18 (!) elbows and Tees for the heatpump/salt cell circuit to the main pool pipe runs... When I expressed my disatisfaction, he said doing it how I wanted would have "been a lot of work and extra cost for nothing" since this was "good enough".

Hence my stab at instrumenting to assess the "damage".

Believe it or not, my pool guy is not usually that lazy. He might have gotten a little too informal with me over time. Since he has provided great service over the past 15yrs, I'll let that one slide. Looking at the bright side, it gives me a chance to tinker and learn for the $200 he charged me for that mess .

Sgt Wilko