×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bernoulli and Required Head

Bernoulli and Required Head

Bernoulli and Required Head

(OP)
I need a little push with this calculation. Even though my initial reaction is that this is a simple calculation, I think the lack of sleep is catching up to me (crazy deadline for a different project)and not letting me visualize the approach.

So I have a 54 inch pipe (193 lf) which outfalls into a sump situation. Water builds up in the pipe and is allowed to bubble up over a ridge (top of ridge elevation = 7.6 ft) to eventually end up in open space (wetlands). The invert of the pipe at the outfall (sump) is 2.62. The 1st manhole upstream is at invert 3.94 and the rim is at 9.5, at which point pipe changes in size to a 42 inch line. There are 243 lf  of this 42 inch pipe before it reaches the 2nd manhole upstream at invert elevation of 4.96 and rim elevation of 10.6.

Assuming the 54 inch pipe is completely submerged, what is the required head you would need to push that water so that it will continue to bubble up over the ridge?

I started with Bernoulli's equation, from the point of interest of unknown elevation to the ridge at 7.6.

pressue head at either end is 0 since I'm assuming both ends open to the atmosphere (assuming water surface will not exceed the rim elevation creating pressurized flow)

velocity in both the 54 inch and 42 inch pipes is determined as 7.2 fps and 12 fps respectively based on assumption that pipes are running full and the Q = 115 cfs. Velocity head at 54 inch determined to be 2.24 and the veolocity head at the 42 inch is 0.80.

I'm ignoring friction losses for now.

Therefore am I oversimplifying this and determiing the head required to push the water over the ridge to be 0.80-2.24 or 1.44 ft? Just doesn't sound right to me. Any input on what I'm missing is greatly appreciated.   
 

RE: Bernoulli and Required Head

You can't ignore friction losses. If there are no friction losses then you have a level hydraulic gradient (similar to a lake). Thus, water will crest the ridge when the upstream water level is equal to the elevation of your ridge plus a smidge.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Bernoulli and Required Head

Have you got a picture of this?  I can't follow the ups and downs of your description.

David

RE: Bernoulli and Required Head

CPENG78
Grab a copy of FHWA HEC22 and run through (see link below) the hydraulic gradeline computation.  The proceedure I would follow is:
1. Weir analysis on bubble up section to establish flow and head at outfall.
2. Exit loss (1XVh-54"Dia)
3. Friction loss to next upstream node (MH) (Q/K)^2xLpipe
4. Node loss (Manhole loss)(See chapter 7)
5. Friction loss to next upstream node
6. Node loss

Continue this until you reach your inlet or point of interest. You can either compute the HGL upstream or the entire headloss through the system.

http://www.fhwa.dot.gov/engineering/hydraulics/index.cfm

RE: Bernoulli and Required Head

(OP)
Everyone - Thank you for your input.

Zdas04 - attached is a sketch (not to scale) but hopefully clears up what I was trying to describe.

I have donwloaded the HEC22 and certainly the losses are a bit more than I expected. Thank you fel3 and qbam.

Any additional input will certainly be appreciated.

RE: Bernoulli and Required Head

Do what gbam suggests.

The sort of thing you're describing is pretty common in central/southern Florida designs.

 

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources