Water Flow Rate in Residential Water System

[Bkg73321]

Hello,

Was hoping to find a calculator online that i could plug this data into simply but can't find. Tried to teach myself the calculations i found but didn't stick. Could someone please calculate this and explain how you calculated it if you have time?

Trying to calculate the GPM Flow Rate at the open air outlet for the following hypothetical residential water system.

Entire system is PEX 1/2 ID tubing.
Inflow at entry ground level 95PSI @ 10.75GPM.
Immediately goes up vertical 8FT.
Then horizontal 8FT.
Then down vertical 6FT into a water filter.
The water filter restricts the flow rate to 7GPM on its outlet.
Upon exiting the filter the tubing then goes up vertical 6FT.
Then horizontal 10FT.
Then down vertical 4FT.
Then horizontal 2FT.
Then up vertical 4FT.
Then horizontal 40FT.
Then down vertical 4FT.
At this point the water flows freely from the end of the tubing at atmospheric pressure.

For simplicity no elbows or tees or couplers.

Can you please tell me what the flow rate in GPM will be at the outlet?

Thank you so much!

Sincerely,
Brandon

 

[LittleInch]

7 gpm.

Inflow cannot be more than anywhere else if the are no offtakes.

To check just add up the vertical distances. Subtract from inlet head. Add up total length. Check flow rate possible with reduced head. If >7GPM Your filter will restrict as you say it restricts flow to 7 gpm.

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[Bkg73321]

Thank you littleinch.
So drop in psi does not mean drop in flow rate? The psi will be dropping with every foot of horizontal and vertical pipe. By the time it gets to the outlet i would guess it is somewhere around 80psi. Wouldn't the accumulated drop in psi after the filter drop the flow rate by the time it reached the outlet?

 

[chicopee]

What you should have mentioned for a more accurate answer are: 1)is the static pressure of water at the inflowmeter under no flow condition and 2) the pressure drop thru the filter when 7 GPM is flowing. With those two values then we can plot on a hydraulic graph the supply curve and the friction loss curve for an approximation to you query.

 

[Bkg73321]

Hi Chicopee, Thank you. Yes the 95 psi at inflow meter is in static condition. I don't know the pressure drop after the filter just the restricted gpm.

 

[cvg]

water is basically incompressible. inflow equals outflow. has absolutely nothing to do with the pressure.

 

[LittleInch]

Bkg,

Apologies if this is a little simplistic, but it seems there are some basic issues of which you are not aware.

1) Conservation of mass - basically this states that over a period of time (especially for gas systems) the mass of material entering a system will be equal to the mass exiting that system. For a basically incompressible system like water, this means that the flowrate in all parts of a single system (i.e. without any offtakes, inlets or branches) will be the same at all times.

Hence unless there is an offtakes somewhere you haven't told us your description
Inflow at entry ground level 95PSI @ 10.75GPM.
The water filter restricts the flow rate to 7GPM on its outlet.

simply cannot happen otherwise mass is disappearing somewhere and that is against the laws of the universe...

"So drop in psi does not mean drop in flow rate?" No -see above

The psi will be dropping with every foot of horizontal and vertical pipe. By the time it gets to the outlet i would guess it is somewhere around 80psi." Yes, but this is caused by friction in the pipe. Your system is complicated by the number of vertical jumps, but if you allowed for them then the drop in head/pressure would be a straight line when plotted pressure versus distance.
You stated the outlet was " at the open air outlet for..." This is normally taken as virtually 0 psig at the outlet.

Wouldn't the accumulated drop in psi after the filter drop the flow rate by the time it reached the outlet?" - No see above

I'll be honest here and say I haven't checked whether 7gpm is actually accurate, but that was quite a lot of pressure for a short distance so if indeed your filter outlet is actually controlled to 7gpm (how??) then that is the most you will get, but it might be less - you need to work out total length of tubing, add 10% for all the bends, calculate the end elevation compared to the start elevation then find a pressure loss chart for your tubing.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

No your original post states 110 psi with a flow of 10.75 gpm. Under no flow I suspect that the static pressure could be 105psi.As far a the filter data that I mentioned, the manufacturer would have that info and not the retailer.
Supposing a static pressure of 110psi under no flow and 95 psi at 10.75 gpm at the inflow meter, a pressure drop thru the filter of 10 psi at 7gpm, 18 psi pressure drop at 7gpm thru the 1/2" tubing and 90 deg. elbows whose friction loss was assumed to be half that of steel tubing, the supply curve and friction loss curve intersect at 12.8gpm and 88psi.

 

[Bkg73321]

OMG trying to get that information from the company. Not something they are either capable or willing to share. Got a little contentious. Still waiting for the info. I am afraid you are mistaken chicopee. My original post states 95psi which i clarified later to be static. I do not know the pressure during flow. Thank you all for your help and explanations.

 

[chicopee]

OK from your post "Inflow at entry ground level 95PSI @ 10.75GPM" or is that an error?

 

[Bkg73321]

Chicopee, yes that is what i originally stated. A few posts later i clarified to you,"Hi Chicopee, Thank you. Yes the 95 psi at inflow meter is in static condition." Sorry If I am misunderstanding but i don't know where you are getting the 110 & 105 psi from.

Inflow at entry ground level is:
95psi in static no flow condition.
77psi @ 10.75gpm in flowing condition.

Thank you. B

 

[LittleInch]

So if water is flowing in at 10.75 gpm then if they are no other places for it to flow out then it will be flowing out of the open end at 10.75 gpm regardless of what the filter thinks. I ask again, what device on the filter restricts the flow rate to 7 GPM. Just because the filter might be rated for 7 gpm doesn't mean that is all the flow it will allow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cvg]

1/2 inch pex has a headloss of about 100 feet per 100 foot of piping at about 6 gpm. and your filter is rated for 7 gpm maximum. Including pressure drop through the filter, which is unknown, I would seriously doubt you can get more than 6 gpm in your system.

https://www.uponorpro.com/~/media/E.../PressureLossTables_H191_1210.aspx?sc_lang=en

 

[Bkg73321]

Inflow at entry ground level is:
95psi in static no flow condition.
77psi @ 10.75gpm in flowing condition.
10psi drop from filter.
7GPM flow rate after filter.

Only thing that restricts flow at filter cis filter itself. It actually 6 filters in series.

Thank you everyone.

 

[LittleInch]

"77psi @ 10.75gpm in flowing condition"

Therefore you appear to have answered your own question....

BUT, If you allow for bends you have approx. 100 ft of 1/2" tubing. This gives you 100ft of head loss for 6 GPM ( then end of the chart) You have a resultant 4 ft of head gain - 104 ft = 45 psi

Your filters take 10psi - 55 psig

You claim 77 psig @ 10.75 GPM

By proportioning and allowing 10 psi across your filters I get pretty close to 7GPM ( pressure drop rises as a square of the flow rate) Note that this is not "controlled " other than by the resistance of the piping system.

So where the missing 3.75 GPM is going (10.75 - 7) I don't know and you're not telling us...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bkg73321]

Thank you everyone for the analysis. The numbers i provided are the best i got. Manufacturer gave me the 10psi drop number and the 7gpm number. I measured the Inflow psi and gpm myself. Not sure why the numbers aren't adding up. I will be actually installing the system in the next week. I am installing pressure gauges after each stage so I will be able to get accurate data on the whole system. I will update this thread with the final accrual results.

 

[IRstuff]

How are you actually measuring the "flow?" If the pressure at the filters was actually 50 psig, that could be consistent with a pipe diameter change, so if you were only measuring velocity and assuming constant the cross-sectional area, that could possibly account for the discrepancy.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[LittleInch]

Aaah, so this thing isn't built yet??

SO I assume you've measured 10.75 gpm at 77 psig by opening a connection valve and somehow measuring volume over a period of time??

From the calc done before I think that more by luck than any judgement, your flow will actually be between 7 to 8 GPM as your inlet pressure will be a little bit higher as the flow reduces.

In the absence of any sort of control valve or regulating element the system will rapidly come to a hydraulic balance where the incoming pressure will balance out with the system friction drop and minor elevation climb at a fixed flow.

The filters will affect the flow as they start clean prob 2-3 psi differential and end up dirty (10 psig). The 7 GPM is just the recommended maximum flow.

Because of the changes in elevation, your pressure gauges will need to be corrected to a single elevation otherwise you will find numbers jumping around all over the place.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bkg73321]

Correct, not built yet. This is what I meant by hypothetical. I measured flow with a 5gal bucket and a stopwatch.

 

[LittleInch]

Great.
Problem solved
Your flow will be between 7 and 8 GPM.

What you get out of a connection before you attach the connection is good data, but the flow will be what it will be given the physical parameters given.
E.g. If you added another 100 feet of pipe without changing inlet pressure, you wouldn't expect the same flow would you?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.