Gravity fed water calculations

[packaging001]

Hey all
I am trying to work out the minimum angle and pipe diametre I can use for feeding water down an incline.
The problem I have is that I wish to recirculate water from a open weir that feeds to a cooling tower using a gravity fed pipe at atmospheric pressure. Currently this process uses a pump to recirc the water but is causes a safety hazard if the pump fails.
It produces a flow from the weir at atmaspheric pressure of up to 20litres/sec and needs to flow approx. 20 metres in length to the inlet of the cooling tower.
I can only have approx. 1200mm of fall over this distance.
What I need to know is what formular I need to work this problem out and how do I work out the min. pipe diametre needed.
Any information on the above will be very much appreciated as I have come from a packaging machinery building background and this one is a curly one for me.

 

[byrdj]

here is a web site with Mannings Eq for gravity flow in round pipe.

http://www.cpp-pipe.com/techpapr.htm#references.

Hope this helps.

 

[byrdj]

using the above chart; If I did the conversersions correctly, it looks like a 4" smooth plastic, 6% slope will be just shy of 20l/s. going to 6" should give a little over 50l/s

 

[packaging001]

Thanks heaps for the reply Byrdi.
Im currently operating out of Sydney Australia and Im wondering if yourself or anyone else out there is able to be contracted to work out the above formular. There has been a little more detail added to the equation and its probably best I assign someone who has the experience in this industry.

 

[sailoday28]

byrdj (Mechanical)
With the incline, will the pipe flow full?

 

[byrdj]

Packaging001, I am not experanced or qualified to make the design discssion. (i found that site when i was installing drains around my house.)

sailoday28, the manning equation gives flow, for a given hydrailic radius R. Normally R is assumed to be "full pipe" for maximun flow value.

 

[sailoday28]

byrdj and packaging001
My question is relating to the system design. With the physical piping configuration and the given parameters will the pipe flow full? If not, the Manning equation will not be appropriate. For example, if there is a void or air space above or alongside the water, then the hydraulic radius will have to be modified.

 

[sailoday28]

I believe you may get some additional expertise/thoughts by posting the original question on "piping and fluids mechanics.." section of this website.