Will 6-inch gravity flow handle 100 gpm water? 3

[aRoseChemE]

I have searched many threads and found many useful tips leading me in the right direction (Manning equation and Hazen-Williams) but not one that directly answers my question or offers a good reference.
I am designing a system with two bent screens (separating solid material from water). The total water flow rate from both screens is estimated to be 100 gpm. This water must gravity flow 31 feet horizontally and drop down only 2.5 feet to tie-in to an existing pipe connection. My question is what size pipe do I need to handle this flow? I figure 6-inch pipe will work, but would 4-inch pipe be sufficient? Is there a table somewhere that shows flowrate versus slope for a certain diameter pipe (one chart for 3-inch pipe, another chart for 4-inch pipe, etc.)? I will be using 304 stainless steel pipe, although I don't feel that matters much (PVC also has a smooth inside surface). I'm really just looking for a chart or table comparing slope, pipe diameter, and maximum gravity flow rate (of water).
My concern is having the pipe too full and blocking the natural vent at the top of the pipe in the bent screens. I believe this may cause a serious hinderance to flow, much like placing your finger over the top of a straw - when you pull the straw out of a filled glass, the liquid stays in the straw. One potential solution I have to that problem is to install a 2-inch U-shaped vent in the top of the gravity-flow pipe before it ties into the existing connection. But I think a 6-inch pipe will run empty enough, even with much more than 100 gpm of flow, that this small vent will not be necessary to purge the air - the air will purge at the top of the pipe in the bent screens. FYI, I work in a starch-gluten processing plant, so anyone in wet corn milling or a similar industry should be very familiar with this equipment. Any help would be appreciated, especially pointers to a book or website with the aforementioned tables.

Thanks,
aRoseChemE

 

[hacksaw]

a lot of handbooks (Crane,Cameron,Marks,etc.) have that kind of data.

you have to decide what flow velocity works for you, given your experience with the drains. a 6" pipe will pass a lot more than 100 gpm under the conditions you describe.


for example, under liquid full conditions, a 6" pipe at 3 fps velocity produces a 1 ft head loss for every 100' of pipe (effective length of fittings included in pipe length) for 260 gpm.

at 450 gpm ( 5 fps) the loss factor is 2.7 ft head/100 ft pipe. Drains are not usually designed for greater than 5 fps.


you also need to know what kind of elbows are being used (long/short radius, etc) and need to include the entrance and exit losses.

 

[25362]

On the Hydraulic Institute's Pipe Friction Manual you'll find the friction losses for water running in steel pipes (not drains). It gives for an ID=6.055 in. and a rugosity ratio [ε]/D=0.000293, for 100 gpm water, 1.11 fps velocity, a friction loss of 0.0843 ft/100 ft.

For a pipe with ID=4.026 in, rugosity ratio=0.000447, 100 gpm (a velocity of 2.52 fps), the same manual gives a friction drop of 0.624 ft/100 ft.

It appears that an available head of 2.5 ft over, say, 50 ft equivalent length (fittings and other losses included) is more than enough to move -without air bubbles or pockets- the fluid down by gravity in both cases.

A 3 in. pipe would probably be small or on the limit with a [Δ]P_f=2.39 ft/100 ft of pipe.

 

[friartuck]

You need the chezy equation

its something like v=C*SQRT(mi)

where C = constant (55)

m = hydraulic mean depth
i=incline i.e. 1 in 100 fall or 0.01 etc

and v=velocity m/sec

hope that helps



Friar Tuck of Sherwood

 

[TD2K]

If I just do it as a standard piping calculation, I would use your 31 foot length with an entrance and exit loss (K = 1.5) and find that flow that gives me a dP of 2.5 feet of water, your elevation difference. That gives me a flow of about 275 gpm for a 4" line.

Sorry, don't have a reference for tables as you asked for.

 

[quadtracker]

http://www.piping-toolbox.com

is a great site with lots of info on piping including reference tables!

 

[katmar]

A more comprehensive set of tables than those at piping-toolbox can be downloaded as a pdf file from

http://www.cispi.org/handbook/chapter8.pdf

These tables are based on cast iron with n=0.012, but would be safe to use with Stainless Steel. The Manning formula is also given in this article if you want to do exact calcs, but the tables are so comprehensive you probably won't need to.

 

[PEDARRIN2]

The International Plumbing Codes have tables relating gravity flow, diameters, and flow rates.

 

[aRoseChemE]

Thank you all for your quick and concise responses. I have decided to go with a 4-inch pipe, based on quadtracker and katmar's references (GREAT online references). The PDF file (Table 2 on pages 9 and 10 of the PDF) that katmar directed me to shows that a 4" pipe with a 0.0806 ft/ft slope will handle 125 gpm running only half-full; at 100% full, max flow is just over 250 gpm. (Quadtracker's reference, The Piping Toolbox, has very similar values but doesn't tell you what equations were used to create the tables. Has excellent articles, though.)

This was my first post on eng-tips.com, and I want to thank you all again for making it a successful experience.