Help on calculating flow rate

[mscolvin]

I'm trying to calculate flow rate so that I can set the pump RPM to turn over the pool once per day (and ultimately lower my electrical costs). Here are some data points:
Pump: Intelliflo VS-3050
Pool volume: 8500
Current RPM: 2050
Current pressure measured at filter gauge: 14

So, using the equations as best I understand them:
TDH@2050 RPM = 14*2.31+50% = 48.5 (let’s call it 50 for ease of calcs)

To use the Pentair charts, convert TDH@2050 to TDH@3110 using Mark’s equation:
TDH@3110 = 50 * (3110/2050)^2 = 115
But, when I read the Pentair Charts, TDH of 115 is above the maximum operating curve. I'm not sure where to go from here - where did I go wrong? Can the smart people of this forum offer some guidance?

 

[Bama Rambler]

For starters, don't get hung up on turnovers. It's been shown that most pools don't need nearly a complete turnover to stay clean.

I's suggest that you run your pump starting at 1000 rpm and see if that's enough to get adequate skimmer action and then adjust the run time to where the pool stays clean.

I'm sure that Mark (mas985) can tell you pretty close to what your flow rate is, but there's no real reason to need to know.

 

[mas985]

There are a few things that can through the calculation off. 14 PSI is pretty high for 2050 RPM so I suspect that the gauge may not be reading correctly. Does the gauge go to zero when the pump shuts off? Also, what is the gauge elevation relative to water level (e.g. 3' below water level)?

 

[mscolvin]

Mark - The gauge does go to 0 when the pumps shuts off. The gauge sites on top of a Pentair Clean & Clear cartridge filter, so it's probably a little over 3 ft above the water level of the pool.

Dave - Thanks, good point, but I would like to know for my own edification. By "adequate skimmer action", I assume you mean that debris gets sucked into the skimmer?

Editing this post: just noticed that the cleaner was on, so turned it off, and pressure dropped to 13.

 

[mas985]

At 2050 RPM, you should not be getting anywhere close to 14 or even 13 PSI so I still suspect the pressure gauge as being incorrect. The pump would be dead headed at that head loss and RPM. If you tap the gauge while the pump is running, does the needle drop down to a lower PSI? Do you have another gauge you could try?

 

[jblizzle]

As Dave said, no real reason to need to know, but Mark can give you a decent estimate.

Read this regarding determining pump run time: http://www.troublefreepool.com/content/152-determine-pump-run-time

 

[mscolvin]

I don't have another gauge, but I thumped it, and it dropped down to 12. I should note that I have an air leak - waiting on a strainer basket o ring to come in as the most likely fix for that. I shut the pump off, the skimmer lids popped off, and then I reprimed. The gauge now reads 10.

 

[mas985]

How about purging the air from the filter, does the pressure change? You may not get an accurate reading until you fix the air leak.

What is the pressure at full speed? That may be a better way to calibrate for flow rate. The air might purge from the filter easier too.

 

[mscolvin]

Yes, I purged the air from the filter - that's when it dropped from 12 to 10.

I can try measuring the pressure at 3450 - and cross-correlate. But, as you suggest, I think I'll wait until I get the air leak fixed before pursuing this further.