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Discharge length from open ended pipe

Discharge length from open ended pipe

Discharge length from open ended pipe


First time post here! I'm looking for some guidance on how to calculate how far water will discharge from the open end of a horizontal pipe that is gravity draining storm water.

For instance, a 6" diameter pipe takijg water from the roof of a structure, routed vertically down through that same structure, but then taken horizontally for a short distance and discharged to a bioretention area.

The task is to find out how far the water will "shoot" from the pipe when flowing 100 gpm in a 6" pipe by gravity only.

Thanks in advance!

RE: Discharge length from open ended pipe

Answering your own question - smart!!!

RE: Discharge length from open ended pipe

Not quite as easy as that....

Your issue here is that the water has some velocity and energy when coming down and along the pipe which the water in the tank doesn't have.

The other key issue / problem is that unless the inflow into your horizontal section exceeds outflow rate, your pipe won't be completely full and will glug and surge or maybe just flow out slowly.
Hence you can't find out what the height h is compared to a tank water level.

100gpm in a full pipe is only 0.4m/sec. Not very fast so the distance will be very small.

Storm water by definition is a sudden event and not really suitable for steady state applications.

Water flow into a vertical drain pipe is a well known issue and again it takes a lot of water flow to get the pipe full. a lot more than 100gpm in a 6" pipe.

If its a problem just add an elbow to the end and point it down.... Or add a large pipe ( 12" ) say 1m from the end to slow it down.

You could just do a calc for various full pipe or 50% full pipe velocities and see what you get once you can figure out a way of estimating your max inflow into the pipe using simple velocity and vertical acceleration formuale.

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

RE: Discharge length from open ended pipe

Also check out this post http://www.eng-tips.com/viewthread.cfm?qid=409750

for vertical flow in pipes like your drain pipe.

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

RE: Discharge length from open ended pipe

why not to use the blow-down system as per boiler so to have the water discharge quite close to the end of the pipe ? that is a larger pipe as suggested by LittleInch but with a blind end and full of bores (total area of bores bigger than the pipe section)

RE: Discharge length from open ended pipe

You need to think about the results of your analysis (estimates, really.) Do you "want" to know how far the water might be spewing out horizontally the maximum distance possible? Or do you want to estimate how far it might be spewing out so you can minimize that distance or protect some some other object below the downspout or use that horizontal distance for some other purpose (like trying to get it further away from a basement wall)?

RE: Discharge length from open ended pipe

I'd point LI's elbow "up", to make a fountain, rather than down to make a mudhole. But that's just me. Just make sure your retention pond overflow is below road grade, and you are +1 versus our local county roads department.

RE: Discharge length from open ended pipe

This is an interesting problem in differential calculus

RE: Discharge length from open ended pipe

The equations that you have posted are for full pipe flow and will tend to overstate the distance. The downcomers will generally be partly full and the horizontal sections will tend to be full flow.

Here is an another calculator:


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