Maximum flow through pipe question

[pats1003]

Is there a calculation available to determine the maximum flow through 3/4" copper (type k) tubing? For example, if 60' of 3/4" copper tubing is connected to an 6" watermain, with a flow of 900 USGPM in the watermain and a static pressure of 60 psi, what is the maximum flow rate through the copper tubing?

 

[CountOlaf]

I'd say you're closing in on 25 gpm (quite a decent velocity however).

 

[CountOlaf]

Ooops...that was for 100' of pipe. For 60' it would be more like 31 gpm. Now of course, this was based on a straight pipe, no other fittings and losing the full 60 psi in that 60' of pipe run (i.e., going from 60 psi down to 0 psi at outlet).

 

[pats1003]

Thanks CountOlaf. But how did you calculate this? Your assumptions are correct, straight pipe with no fittings.

 

[CountOlaf]

Well, I first looked in my tabulated hydraulic handbooks to save me from doing a hand-calc or looking for an on-line calculator...but no matter which of the three methods, it's all based on either the Darcy formula or Hazen-Williams equations.

Anyway, your numbers are off most of the tabulated sources. If for instance, you were selecting a new pipe size that you wanted 31 gpm through and had more typical pressure drops and velocities, you'd be selecting something more along the size of a 1-1/2" pipe. A 3/4" line is more typicaly used for single digit flow rates.

But since your question was more theoretical (like you could actually stand to lose the full 60 psi in 60 feet), I had to go to some on-line calulators and keep iterating until the pressure drop was 60 psi.

The following is a link to the first one that came up on google.

http://www.pressure-drop.com/Online-Calculator/index.html

 

[pats1003]

Thanks again CountOlaf. Let me pose this question. The 6" dia. watermain has an operating pressure of 60 psi with a flow of 900 USGPM. The 60' long 3/4" copper pipe is connected to the watermain. How do I determine my flow at the outlet of the copper pipe? I know how determine the head loss through the pipe but this is based on a defined flow. Obviously pushing 900 USGPM is not possible through the 3/4" pipe.

 

[cvg]

use the Hazen-Williams formula to calculate the flow rate which gives a friction loss equal to 60 psi.

http://en.wikipedia.org/wiki/Hazen-Williams_equation

 

[coloeng]

Is the flow through the 3/4-inch the only flow in the 6-inch? If it is, you need to determine what the residual pressure is for the 6-inch at different flows so that you can develop a flow/pressure curve. It will look very similar to a pump curve. You then use an iterative process to estimate the flow in the 3/4-inch, determine the headloss in the 3/4-inch for that flow and then see if that headloss matches the pressure in the 6-inch for that flow. You continue to narrow down your value until the pressure in the 6-inch matches the presure loss in the 3/4-inch.

 

[bimr]

You can calculate this with Cranes Technical Paper No. 410.

You have to figure entrance loss, exit loss, head loss, and pipe loss. I calculated 28 gpm.

 

[cvg]

technically, coleng is correct, however for the range of flow rates in the 3/4 inch line (up to 31 gpm maximum), there will be little appreciable pressure drop in the 6 inch main. I would tend to ignore it.

 

[Artisi]

pats1003, What pressure do you want / need at the discharge of the 3/4" line, seems everyone is only considering using all of the the 60psi in the main with zero pressure at the 3/4"discharge.

 

[jonr12]

3/4" K copper has 14.41 PSI loss in 100' at 10 GPM. AT 10 GPM you would still have 51 PSI at the other end of the 60'. 10 GPM has a velocity of 7.35 fps, which I think is about all you would want.