I've got to install a drain pipe from a low spot to the street gutter - about 100 feet. Is there a formula to figure the maximum flowrate that a 4-in pipe will carry using the 1/4 per foot slope assuming the inlet is full?
GPM = 0.0408 x pipe ID ^2 x distance / water velocity
0.0408 * 16 sq.in. * 100 ft.
--------------------------- = ~520 gal/minute
re: DeWalt Plumbing Professional Reference
I've got to install a drain pipe from a low spot to the street gutter -
about 100 feet. Is there a formula to figure the maximum flowrate that a
4-in pipe will carry using the 1/4 per foot slope assuming the inlet is
500 gpm seems like a rather high flow rate for gravity flow in a 4"
plastic pipe with a drop of 1/4" per foot.
If my calcs are correct....500 gpm thru a 4" pipe yields about 12 ft/
sec not 2 ft/sec
at 2 ft/sec, completely full, the 4" pipe, the pipe will flow ~80
I haven't done the gravity flow calcs for a 4" pipe at 1/4" per ft but
I'm guessing its more like 50 gpm max (just a WAG).
Like I said, I haven't done the calcs, but my 3" drainline with ~1/4"
per foot drop easily handles the ~15 gpm output of a garden hose but I
doubt if it would handle 15x that flow.
After a lot of searching I found
page 15 gives the Manning formula for gravity flow......stumbling
through the calcs at midnight, I got
~70 gpm for a 4" PVC line with a 1/4" drop per foot
somebody please double check these numbers?
I have no idea how I came up with that, correct answer would be around 33
(New calculator, still have figured the blasted thing out. Somehow it took
16 and squared it, when 16 was already a square. Everything seems backward -
best excuse I can think of....)
The Dewalt formula is simply listed as gallons per minute through a pipe. It
doesn't give the type of pipe (roughness or corrosion) so I would tend to
believe that the .0408 takes the worst case into consideration which would
correlate to the low figure being generated.
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