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Maximum flow and slope in sewer pipe?


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#1 Dleg

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Posted 08 September 2010 - 12:41 AM

This is where my degree in Mechanical Engineering lets me down in the field of environmental engineering: I am very familiar with the minimum velocity requirements in the design of gravity sewers, but today was the first time I heard about a maximum slope or velocity. None of my engineering references mention a maximum, but an older copy of the Metcalf & Eddy sewage collection and pumping book mentions a maximum velocity of 8-10 feet per second. Any velocity above that, it says, will cause the liquids to bypass the solids, and the solids to adhere to the invert of the pipe, to the point where they might not be dislodged by subsequent flows, and eventually clog the pipe.

I ask because it was raised by an older, more experienced engineer, and seconded by another engineer that I work with, but then we couldn't find anything on other than the old M&E text. The copy of the UPC we have (an old one - 1988) doesn't mention a maximum slope, and neither do any of our other design references.

Is this a valid design concern? Or is this a relic that has somehow been disproven in the past 20-30 years?

Thanks.

Edit: We found several instances where protections against high flow velocities were mentioned, including in the 10-state standards, but with the exception of the old M&E text, the concerns were related to pipe erosion and displacement, and could be overcome with material selection and bedding design. The 10-state standards, in fact, appear to explicitly condone higher slopes and velocities, provided these concerns are addressed in the design.

#2 Chucktown PE

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Posted 08 September 2010 - 12:50 AM

QUOTE (Dleg @ Sep 7 2010, 08:41 PM) <{POST_SNAPBACK}>
This is where my degree in Mechanical Engineering lets me down in the field of environmental engineering: I am very familiar with the minimum velocity requirements in the design of gravity sewers, but today was the first time I heard about a maximum slope or velocity. None of my engineering references mention a maximum, but an older copy of the Metcalf & Eddy sewage collection and pumping book mentions a maximum velocity of 8-10 feet per second. Any velocity above that, it says, will cause the liquids to bypass the solids, and the solids to adhere to the invert of the pipe, to the point where they might not be dislodged by subsequent flows, and eventually clog the pipe.

I ask because it was raised by an older, more experienced engineer, and seconded by another engineer that I work with, but then we couldn't find anything on other than the old M&E text. The copy of the UPC we have (an old one - 1988) doesn't mention a maximum slope, and neither do any of our other design references.

Is this a valid design concern? Or was it somehow disproven in the past 20-30 years?

Thanks.



Dleg, it's definitely a valid concern, but I've never heard it being a concern due to clogging a pipe. It's a concern because in turbulent flow you'll get hydrogen sulfide coming out of solution. If the sewer isn't full, you'll get a little monster called Thiobacillus bacteria which will oxidize the hydrogen sulfide, produce sulfuric acid as a byproduct, and corrode the crap (no pun intended) out of your sewer.

I've never heard of solids not staying suspended at those kinds of velocities. 2.5 ft/s is enough to resuspend any solids that have settled out.

#3 Dleg

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Posted 08 September 2010 - 01:18 AM

Thanks, Chucktown. It's come to my attention primarily in the realm of on-site septic system design, in the maximum slope of the building sewer to the septic tank, usually a 4-inch ABS or PVC pipe, and then the 4-inch pipe from the septic tank to the drainfield. I did some more Googling and found it in a number of local plumbing codes around the US (with reference to the solids clogging problem, not H2S), but I wonder why it's not in the UPC, and since it's apparently not, I wonder if it's just some holdover, superstition kind of thing.

#4 VTEnviro

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Posted 08 September 2010 - 02:55 AM

I know generally the MINIMUM is 2 fps with around a 0.6% slope, but I haven't run into max conditions often.

For some reason, 7 fps is sticking out in my mind as a rough upper limit before you run into issues of scouring the pipe itself and causing damage.

As for max slopes, if you have a manhole with an external drop, it's essentially an infinite slope. I don't remember ever seeing anything in the regs.

I've never heard of liquid bypass. Wouldn't fast moving water cause less adhesion of the solids to the pipe?

#5 jregieng

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Posted 08 September 2010 - 05:54 PM

There's a joke in here, right?

On a serious note, Vesilind (1979) provided a correlation based on the Hazen-Williams Equation for a modified coefficient "C" based on sludge solids content > 1% for the difference in the fluid behavior (e.g. non-newtonian). The "economical range" of flow velocities was considered to be between 5 fps and 8 fps, so Vesilind put together a nomograph of flow velocity vs. friction headloss with curves representing % solids based on field-measured data (reproduced below).



Sucks!? multiplespotting.gif

JR

#6 Dleg

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Posted 09 September 2010 - 03:44 AM

No, we're not talking sludge. We're talking turds sticking to the pipe, to be direct about it. Building sewer stuff, but the UPC (1988) doesn't mention it, and I would think that would be the go-to for building sewers. Anyone have an up-to date UPC? This would be under both "drainage systems" and "building sewers". But, according to these "old timers", the same principle applies to larger gravity sewer lines as well.

#7 IlPadrino

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Posted 09 September 2010 - 11:23 AM

According to UFC 3-240-04A:

CODE
(2) Velocity. Sewers will be designed to provide a minimum velocity of 0.60 meters per
second (2.0 feet per second) at the average daily flow, or average hourly flowrate, and a
minimum velocity of 0.75 to 1.05 m/s (2.5 to 3.5 fps) at the peak diurnal flowrate, as determined
in paragraph 3-1. When velocities drop below 0.30 m/s (1.0 fps) during periods of low flow,
organic solids suspended in the wastewater can be expected to settle out in the sewer.
Sufficient velocity (0.75 to 1.05 m/s (2.5 to 3.5 fps)) must be developed regularly, once or twice
daily as a minimum, to resuspend and flush out solids which may have been deposited during
low flows. A velocity of 0.75 m/s (2.5 fps) minimum is required to keep grit and sand suspended.
However, new sewers which are properly designed and constructed should contain only minor
quantities of grit or sand. Maximum velocity is set at 3.00 m/s (10.0 fps) in the event that grit
becomes a problem.


I think I have an electronic version of the IPC somewhere... if I can find it, I'll give you the relevant sections.
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#8 Capt Worley PE

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Posted 09 September 2010 - 01:38 PM

So will my low flow toilet generate enough flow so my dukes don't back up, m'kay? I don't want any chocolate dragons hanging out in my plumbing.

Seriously, I always wondered if fifty year old sewer lines work well with low flow toilets. Seems the gpm/fudge monkey ratio wouldn't be high enough.

#9 Dleg

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Posted 10 September 2010 - 12:20 AM

Thanks, IlPadrino. Yet another source that fails to cite soilds separation as a reason for a maximum velocity - only pipe erosion, just like my references. In other words, probably not much of an issue for 4-inch building sewers from ordinary residences.

Still, there must be some reason the older engineers I know both know this, and why it was mentioned in the old M&E. At any rate, the maximum velocities recommended for both purposes are the same - 10 fps.

#10 jregieng

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Posted 10 September 2010 - 02:09 AM

The Standard Handbook of Environmental Engineering, 2nd Edition, 2004 Chapter Six, Page 6.44, Hydraulic Design for Sanitary Sewers states,

QUOTE
Slope. The slope of the gravity sewer should be sufficient to provide a minimum velocity of 1 ft/s (0.3 m/s) during average to low-flow conditions. Typical design practice is to provide a minimum velocity of 2 ft/s (0.6 m/s) when the pipe is flowing full. Maximum velocities should not exceed 10 ft/s (3 m/s) due to the development of hydraulic problems in the sewer.


Just sayin' ....

JR
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#11 Dleg

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Posted 10 September 2010 - 03:30 AM

Yeah, pipe displacement would be considered a hydraulic problem. But pipe erosion? And more to the point, poop getting permanently stuck??




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