Flow into a downspout 2

[jay156]

I am really frustrated trying to find an actual equation on the internet for flow into a circular downspout so I can make a real calculation of its capacity. Can anyone direct me to this information?

When I look up downspout size formula/calculator on google I get mickey mouse tables from better homes and gardens giving me "engineering for dummies" info that says "a 6 inch downspout can drain xxxx SF of roof area". Yeah, HOW did you get that number? Even looking up vertical pipe flow gives me equations that let me calculate how much water is shooting UP and OUT of a vertical pipe, and that's the opposite of what I want.

I appreciate any help you guys can give me.

 

[SuperSalad]

Something like this?

Hazen-Williams Equation

Andrew H.

http://www.mototribology.com

 

[jay156]

No, I need to calculate the flow rate. I don't know what the pressure drop is or I would be able to use that.

The pressure at the outlet would be 0, right... The pressure at the top would be the depth of water above the opening times its weight? Would that work?

 

[cvg]

flow is generally controlled by the amount that can get into the pipe. the pipe capacity should be much greater so that it can handle air and debris also. so you could use a weir or orifice equation for the inlet capacity. and suggest a large factor of safety.

so for example, inlet has 6 inches of weir length; weir coefficient is 2.8 and depth of flow is 3 inches

flow equals 79 gpm

 

[SuperSalad]

The link I posted describes how to calculate the pressure drop from gravitational flow. You can rearrange the equation to solve for the flowrate with the other values known.

Andrew H.

http://www.mototribology.com

 

[jay156]

I found this.

https://www.tandfonline.com/doi/full/10.1080/00221680903568626#

I think this is probably what I want, though it's for orifices drilled into the vertical side of a tank, but I don't think it makes a difference if the orifice opens to the bottom, does it?

 

[RVAmeche]

It does make a difference but for what you're doing I doubt it matters. Why do you care about the flow rate through a downspout anyway?

 

[LittleInch]

Try this post and follow links to other posts ams open the attachment.

If you go down about halfway to a long post by the OP there are formula there.

https://www.eng-tips.com/viewthread.cfm?qid=409750

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

 

[cvg]

I would not get all wrapped up in two phase flow through the downspout. the flow rate of the gutter/downspout system is controlled by the inlet capacity. Orifice or weir flow equation. probably weir since the depth of flow is shallow compared to the diameter of the orifice. if it overflows the gutter before it can get through the inlet into the downspout, than the system fails. It would be unusual for a downspout to back up the flow into the gutter. If that happens, than it is plugged.

 

[LittleInch]

Althougyou need to be a little careful do a search for"glory hole calculation" or "bell mouth" or "morning glory"

But make sure you add "calculation"!!

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

 

[LittleInch]

This looks like it has a bunch of formula in it

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=2107

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

 

[PEDARRIN2]

Overcomplicating things.

If it is a downspout, it is likely a plumbing code issue.

You have to know where you are located so you can determine the typical rain fall rate to use.

In International Plumbing Code controlled areas, it gives you maximum gallons per vertical leader size. To determine the flow rate you will have, you use Q=AIR where I is the intensity inches per hour and R is the parameter related to the permeability of the roof. Typically, I use 0.95.

For a 3" rainfall, and the 0.95 factor, GPM/SF - 2.94E-02

 

[cvg]

that last link is a SCS design for an outlet structure on a dam. and yes, the inlet capacity does rely on weir flow (at normal operating depths) and orifice flow at higher depths.

https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=18611.wba

 

[jay156]

I did use the rational method to calculate the maximum flow from the roof. Then I needed to size the downspouts to accommodate that flow. There were no gutters actually, they are just roof drains that will be taking away the water.

I'm pretty sure orifice flow is what I needed. I just didn't know what it was called to search for the equation.

Thanks everyone.

 

[cvg]

orifice flow only applies when the hydraulic depth is high enough to completely submerge the inlet opening. when the water depth is shallow, than weir flow applies. You may over-estimate your capacity if you use orifice equation with very shallow water, as is typical on most roofs.

 

[georgeverghese]

For gravity flow in vertical down pipes, use the Froude equation to keep air entrainment in the flow stream to a minimum.

N[sub]Fr[/sub]= V[sub]L[/sub]/[(ρ[sub]L[/sub]-ρ[sub]G[/sub]).D/ρ[sub]L[/sub]]^0.5, with V in m/sec, rho in kg/m3, D = downpipe dia in metres.

Keep N[sub]Fr[/sub] at less than 0.31.

For water in air, this simplifies to V[sub]L[/sub] < = 0.31 x (D^0.5)

 

[PEDARRIN2]

Since it is a roof drain, you use the appropriate table in the plumbing code in your state. The flows or area listed is based on using the Rational Method.

 

[katmar]

I absolutely agree with cvg that the flow into the line will be controlling, and that the practical static head you would have at the inlet means that it would be preferable to use a weir formula rather than an orifice one.

If you want to understand a bit of the background and get some formulas then the best article I know of is PD Hills, "Designing piping for gravity flow", Chemical Engineering, Sept 5, 1983, pgs 111-114. If you have access to a university library they should have it, or you will find various pdf copies floating around the internet. Another article, which leans heavily on (I don't know if it would be fair to say plagiarizes) the Hills article, is "Overflows and Gravity Drainage Systems" by GBH Enterprises. I can't find it on their own web site anymore, but there are also multiple copies floating around.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"