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Shawner (Civil/Environmental) (OP)
15 Sep 11 17:47
How would I go about determining the maximum amount of flow through a pipe of certain diameter within our distribution system?

The info I would have at hand would be pressure, diameter, pipe type, etc.   
bimr (Civil/Environmental)
15 Sep 11 22:17
Are you suggesting that you would like to model your system?
Shawner (Civil/Environmental) (OP)
15 Sep 11 23:45
Not necessarily no.  We have a 6" C900 line feeding an orchard/vineyard off of a 24".  They are allowed a certain amount of water due to water licenses, but there is no meter on the line.  Just wondering what the maximum flow that 6" would be capable of?

Would a modeling system like EPAnet be a program to start with?
cvg (Civil/Environmental)
16 Sep 11 9:52
get a flow test on the nearest hydrant, on the 6 inch line if possible. then you could use epanet to do an analysis
Shawner (Civil/Environmental) (OP)
16 Sep 11 10:03
I'm afraid there is no hydrant nearby.  The line comes off our system right before our chlorine contact chamber (it's irrigation water only) at the start of the system.  

Is there a theoretical maximum for a 6" line with a certain amount of pressure?
zdas04 (Mechanical)
16 Sep 11 10:23
There is an empirical equation that describes the minimum flow rate (in gpm) required for a line to run full of water.  That equation is:


Since it is an empirical equation it only makes sense if the ID is in inches.

Other than that, if you know the pressure in your 24" header, the length of the 6" and can estimate the backpressure caused by their end devices you can use D'Arcy-Weisbach.  One version of that is:


for FPS units:
"C" is 2*g
"fm" is the Moody friction factor (if you're not familiar with that term, then you need to get help)
"dP" is in lbf/ft^2
"ID" is in ft
"A" is in ft^2
"ρ" is in lbm/ft^3
"L" is in ft
q works out to ft^3/sec

bimr (Civil/Environmental)
16 Sep 11 11:05
You need to know the length of the 6-Inch pipe, the pressure in the 24-Inch pipe, the elevation change, and the headloss in the sprinkler head if any to calculate the flow.

However, as a rough estimate, the maximum velocity through the 6-Inch pipe is expected to be about 10 feet per second which gives you about 900 gallons per minute.

You can use the calculator in the link. The headloss from a 6-Inch pipe, 1000 feet long, 10 feet per second velocity is about 20 psi.
cvg (Civil/Environmental)
16 Sep 11 16:29
you also need to know what type of pipe, condition of the pipe and also any fittings or valves in the line
Shawner (Civil/Environmental) (OP)
16 Sep 11 23:40
Thanks for all the replies!  

bimr, how did you know that the flow can be expected to be around 10 ft/s?  I'm familiar with the Hazen-Williams equation/nomograph but wasn't sure what velocity I should plug in   
bimr (Civil/Environmental)
17 Sep 11 10:41
Headloss is related to the square of the fluid velocity. At a velocity of 8-10 ft/sec, the headloss starts to be significant. The ability to transfer the water any significant distances is limited by the headloss.

City water mains are designed for 3-5 ft/sec.   
Pumpsonly (Mechanical)
18 Sep 11 3:19
You are having an existing system.The recommended flow velocity in pipe line sizing is of no relevance for your case.

You are suppose to do the reverse calculation.

You have a X pressure in the 24 inch head.
You know the length of the 6 inch pipe, the material and the other properties of the water.
If at the end of the 6 inch pipe is open ended and not too many bend and turns,no major fitting that causes significant pressure /hea losses,just use the formula provided by Zdas04 and get the estimate flow rate.
Of cause you have to add or deduct the equivalent of the static height difference between the header and exit point of the 6 inch pipe in the value of the dp ( pressure in the header).

ImroZ (Civil/Environmental)
18 Sep 11 16:21

In case of having enough data and your system is not too complicated (I mean there arent closed circles in that) maybe a simple Bernoulli equation can be a really useful help !
rconner (Civil/Environmental)
18 Sep 11 16:59
The questions you are asking are those many new engineers associated with piping or distribution ask, "frequently asked questions" if you will. It is a good general idea to first check what is available with a few keywords using the "search" feature of this and all forums before posting. While not a guarantee that your real root questions will be answered by the voluminous past information you retrieve (as you know better than any of us exactly what you are dealing with), the information you find may at least help you refine your inquiry. I believe there have been many past threads discussing these general subjects (with a lot of information available).
Beyond this I will only caution that "C900" does not really nail down much the specific features of all pvc pipe that you may be dealing with. This specification has meant different things at different times, with regard to dimensions, thicknesses and/or pressure ratings etc. Again, searches will help in understanding this history. You will find also with web keyword search that in general many authorities are most unomfortable with high fluid flow velocities (as have been mentioned in this thread) and pvc piping in general, I suspect based on lessons, experience or other authoritative references.         

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