Head loss in partially-full pipe? 1

[KernOily]

Ok here is a dumb gear head (mechanical engineer) question for you sewer rats .

I am trying to estimate head loss in a partially full pipe. Good old Round Pipe, NOT a trapezoidal or rectangular channel or wood flume or etc. I have looked in all my references (Crane, Cameron, Marks, Lindeberg, Streeter & Wylie, etc.) but none _definitively_ say whether the Darcy equation, modified for hydraulic radius, is correctly applied to circular cross-sections, i.e. a pipe.

I assume the procedure is to calculate the velocity using Manning, and then use that in the Darcy equation with the correct hydraulic radius for my flowrate and pipe size - yes?

By the way, this is for design work for a new combined oily water/storm water/fire water sewer in a process plant. I am dropping 60' in elevation from my source (tank diked area) to my collection point (below-grade sump tank, 1200' away) so I am working on line sizes, head loss, estimating the potential for hammer events, etc. Thanks for your help!


Thanks!
Pete

 

[dicksewerrat]

This really sounds more like a gravity sewer. No head loss. Use Manning's to get your Q.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[KernOily]

Thanks Dick. Yes this is a gravity sewer.

But... I don't understand why I don't need to consider friction loss. It seems to me that the motive force supplied by sloping the pipe is offset by friction loss. Why do I not need to consider that? Isn't the theoretical Q predicted by Manning reduced by the amount of head lost due to friction, as would be the case for a full pipe? Thanks!

Thanks!
Pete

 

[bioengr82]

Isn't friction loss only a consideration when you have to size a pump & motor? Why do you care? Just point it down hill @ appropriate slope.

Also, why are worried about water hammer?

 

[dicksewerrat]

Friction losses are taken into account by the 'n' value of the pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[LHA]

bio:
Friction decreases the capacity of the pipe. Insufficient capacity could overtop the diked with oily water.

74:
Just run Manning's for full flow, as dick said. It isn't going to change much for partial flow. At 5% you should be fine, and velocity shouldn't get excessive. But with 60' of head, plus however deep your diked volume is, you might get a reduced diameter using Hazen-Williams or Darcys. If the oil content is high, use Darcy's to accurately account for density. Finally, again due to the potentially huge difference between headwater and tailwater, run a culvert check, for type of control, with conservative inlet loss and minimum tailwater. You don't want to design too close and flood that dike.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[stanier]

Download flowpro2 freeware for gravity line simulation