Calculating Flow Rate From PSI Reading?

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flyboy320
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Calculating Flow Rate From PSI Reading?

Postby flyboy320 » Wed 26 Aug, 2009 20:02

We are installing a UV system for our pool, and the manufacturer recommends a max flow rate of around 50gpm. There is not enough room to install a flow meter, but there is a psi gauge at the inlet of the UV unit. Is there a way to estimate the flow rate using the psi gauge, and the inlet pipe diameter?

There is a by-pass installed at the UV system so some of the water is by-passed around the UV and onto the heater, so it's not enough to just get the flow rate of the pump, as that wouldn't accurately give the water flowing into the UV, so all I have to go on is the PSI gauge.


Me...
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Calculating Flow Rate From PSI Reading?

Postby Me... » Thu 27 Aug, 2009 09:59

UV wants to use ALL the water in your filtration system. It's not nearly as effective if you only use a portion of the circulated water.

That said, you can calculate roughly what the flow would be, but you need to know line sizes and possibly another gauge or 2. Just post some info and maybe someone can guesstimate a flow.

Think of a sand filter with both influent and effluent gauges. Influent measures pressure going into the filter before the sand while the effluent gauge measures backpressure from the pool and other equipment after the sand. Influent can be 30psi while effluent might be 15psi. The flow is identical passing both gauges. So you can see with enough info you can make a guess but a flowmeter is the ultimate. Keep in mind that a perfect placement might not even give you an accurate reading, but a flowmeter anywhere will at least give you an idea.
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mas985
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Calculating Flow Rate From PSI Reading?

Postby mas985 » Tue 08 Sep, 2009 10:57

I am new to this forum but have been posting to other pool forums for years. I know a little about hydraulics so I might be able to help you out. You can estimate the flow rate delivered by the pump by first determining the head loss of the plumbing. This can be done by measuring both the return side pressure (filter pressure works fine) and the suction side vacuum (need a vacuum guage at the pump basket). The head loss can then be determined from:

Return Head loss = 2.31 * Filter pressure + 3
Suction Head loss = 1.13 * Inches Mercury (vacuum gauge reading)
Total Head Loss = Return + Suction Head Loss

You can then use the pump head curve to look up the flow rate.

If you don't have a vacuum gauge and don't want to buy one, you can use one of these approximations depending on your set up.

Suction Dynamic Head = 75% of Return Dynamic Head for 1.5" suction and 1.5" return pipe
Suction Dynamic Head = 50% of Return Dynamic Head for 2" suction and 2" return pipe
Suction Dynamic Head = 30% of Return Dynamic Head for 2.5" suction and 2.5" return pipe
Suction Dynamic Head = 17% of Return Dynamic Head for 3" suction and 3" return pipe

Suction Dynamic Head = 22% of Return Dynamic Head for 2" suction and 1.5" return pipe
Suction Dynamic Head = 22% of Return Dynamic Head for 2.5" suction and 2" return pipe
Suction Dynamic Head = 14% of Return Dynamic Head for 3" suction and 2.5" return pipe

If you have multiple pipe runs, you can use the equivalent diameter.
Mark
Hydraulics 101 ; Pump and Pool Spreadsheets ; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
Jphilip
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My Pool: 25000 gallon no water features. Pentair intelliflo vs pump. Sand filter. 1.5" piping
Location: Mason oh

Calculating Flow Rate From PSI Reading?

Postby Jphilip » Thu 13 Jun, 2013 08:49

I am trying to figure out flow rates for my pentair intelliflo vs pump at various rpm.
I have taken vacume and psi readings at various rpm. I have used this to calculate the TDH at each rpm. I got pump curves from pentair and thought i could then see how my pump performs (flow rates) at various rpm. the problem is every rpm generates the same flow (within the inherent error of reading from a graph). There must be an error in my method. Stated differently, if I plotted rpm on the x axis and flow vertically, the graph is flat. Can anyone explain this result or the error I have made?

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