# Pipe Sizing & Flow Meters

Discussion in 'Plumbing Forum' started by kok328, Mar 27, 2014.

1. Mar 27, 2014

### kok328

#### Well-Known Member

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I have a situation where I can't seem to find a straight answer.
I've tried some engineering websites with no luck.

What I'm trying to determine is how much pressure loss with I experience by reducing a 3" pipe down to 2" with a 10hp, 120/gpm submersible well pump.

I have a flow meter installed in the 3" pipe but, it's minimum flow range is 3gpm. I need to find a way to detect flow as low as .5gpm.

It is being proposed that I reduce the size of my pipe from 3" to 2" to accommodate a lower gpm meter but, I'm concerned about pressure loss, thus my original query regarding pressure drop.

I also did some limited research on an ultra-sonic flow meters but, can't seem to find one that is correct for a 3" CPVC, Sched 80 pipe with well water inside it.

2. Mar 27, 2014

### JoeD

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That's a complicated question, because when no water is flowing there in pressure loss. It is equal. The more GPM you use the more potential there is pressure loss.

3. Mar 28, 2014

### Wuzzat?

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Try
http://neutrium.net/fluid_flow/pressure-loss-from-fittings-expansion-and-reduction-in-pipe-size/
and

If something here looks relevant, post back.

The trick seems to be to get or estimate all the coefficients for the correct formula - the rest is math on a calculator or a spreadsheet.

What info & numbers do you have besides what you posted? From the first link it looks like we need nine, total.
IIRC, determining whether the flow is laminar or turbulent is important and almost certainly you need the pump curve.

Worst case, have your library borrow this
http://www.constructionbook.com/mec...=16269335922&gclid=CPmgw76RtL0CFYN0Ogod3EYAYQ
from another library.

Chunks of it are online but you need exact quotes to pull up the right section.

Last edited: Mar 28, 2014
4. Mar 28, 2014

### Chris

#### Guest

As far as I know you don't loose any pressure by reducing pipe size. You will however loose volume. When I get to my office I will post a sheet from nfpa 24 that shows what to expect for gpm at different pressures from different pipe sizes.

Don't know if any of that will help but the info won't hurt.

Sent from my iPhone using Home Repair

5. Mar 28, 2014

### kok328

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Thanks guys. I'm not opposed to crunching numbers but, I don't have enough info to fill in the blanks.
I was looking for an easy answer but, ....
I didn't want to get into coefficient of drap using copper versus pvc versus etc....
How many bends, distance, etc .....

6. Mar 28, 2014

### Chris

#### Guest

Do you have access to put a pitot tube on it? If so that chart I was talking about will give you an easy way to see what GPM you are flowing.

7. Mar 28, 2014

### Wuzzat?

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Water is incompressible so you can't lose volume but the delivered GPM may drop because the pressure dropped. The water entering the system must leave the system.
This is kind of a chicken and egg problem.

8. Mar 28, 2014

### kok328

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Chris, anyway to make your charts bigger, I can't see them completely but, it looks like what I'm looking for.

9. Mar 28, 2014

### nealtw

#### Contractor retired

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Save the picture to your computer, there you will be able to rotate it and enlarge it and print it.

10. Mar 28, 2014

### Wuzzat?

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Last edited: Mar 28, 2014
11. Mar 28, 2014

### kok328

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Wuzzat, engineering tool box is where I was looking for info.
I plan on putting a VFD controller on it, maybe I can compensate with some bumped up pressure settings?

12. Mar 28, 2014

### Wuzzat?

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Yes, by making the pump speed variable you can adjust out a lot of problems. But, how'd you size the pump in the first place? Post a link to this bugger.

For your 10 hp pump, assuming 50% effficiency, (head in feet) x gpm = 20,000 so at 120 gpm you can have 170' of head. Does that sound right?

City water may give you 16 GPM @0 PSI with 50 PSI @0 GPM. These are the extremes of the pump curve you see at a house while looking back into the city water supply (=huge pumps and miles of pipe).

Last edited: Mar 28, 2014
13. Mar 28, 2014

### Chris

#### Guest

I can later from my computer, remember that the PSI the chart is referring to is Pitot pressure. For example, a couple weeks ago I was flushing a 4" line and my pitot tube was reading 40 PSI, now my lines static pressure was 150 psi. So this chart is pretty much useless with out a Pitot gauge. You would need to borrow or rent one to do this test. Although if you didn't need to know what yours has but just see what the loss would be, it will help.

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