Water pressure at end of a outfall?

[coatsandrew]

I'm trying to determine the water pressure at the exiting end of a 66" storm water pipe. Max flow is 10 ft/s. Suggestions?

Thanks

 

[XR250]

0 psig if it discharges to the atmosphere

 

[coatsandrew]

I'm not following how water moving at 10 ft/s won't have a pressure as is exits the pipe. I'm designing a support structure for debris screen that is approximately 10 ft away from the end of pipe, and I'm trying to determine the pressure/force the exiting water will exert on a clogged debris screen.

Thanks

 

[LittleInch]

It has no "pressure", but it has energy in the form of kinetic energy or perhaps think of it as momentum.
A 66" pipe at 3m/sec sounds like a big force to me if it stops suddenly
SO it becomes a different calculation.

search force water jet screen and take your pick.

I would be more worried about the screen, especially if it's doing its job and actually screens something...

A screen at an outlet like that sounds like a pretty bad idea to me.

Do you have a sketch or a drawing to explain a bit more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SnTMan]

Static pressure vs dynamic pressure vs total pressure. Fairly elementary fluid mechanics.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[coatsandrew]

Below is a link to the debris screen that we'll be placing at least 10 feet away from the open end of the pipe outfall. The screen is 4'-0" tall, and floats on the water surface by use of floats along the top portion of the screen. It is anchored to precast piles at each end of the screen. As I indicated, I'm looking for the surface pressure (psf) the moving water will have on the screen if it becomes clogged with debris.

http://www.silt-barriers.com/floatingdebrisbarriers.html

 

[btrueblood]

Why not ask the manufacturer?

 

[LittleInch]

Well if it clogged a lot then it's like a flat plate.

Many examples such as this

http://www.roymech.co.uk/Related/Fluids/Fluids_Jets.html

I still would like to see how your end of the pie and screen is actually built as the website seems to be other things.

Also still seems like a bad move to me. Can you give a bit more explanation.

If you're really talking about a pipe end full of water at 3m/sec, don't be surprised if the force is really quite high.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[thaidavid40]

F = 1/2(mv²)

Thaidavid

 

[LittleInch]

I think you'll find that's kinetic energy equation, not force.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[coatsandrew]

There'll be a 12" square precast prestressed concrete pile embedded 20 ft or so into the soil, and sticking out 13 feet or so above grade at each end of the 50 feet long floating debris screen. The debris screen will attach to a sliding anchor point mounted to the face of the pile, which will allow the debris screen to float with the changing tides.

 

[SwinnyGG]

The screen is 50 feet long (implying that your reservoir is 50 feet wide) but the pipe discharge is only 10 feet upstream?

In order to calculate force on the screen, you need more information. What is the mean flow velocity in the reservoir at the exact position where the screen is located?

 

[SlideRuleEra]

The worse case loading is straight forward. Assume the clogged screen is a wall and velocity change is 10 fps (from 10 fps to 0 fps). Then:

Force = Mass per Second x Velocity Change

Assume the pipe is flowing full, calculate the mass flow (Slugs per second) and multiply by 10 fps. The answer is the maximum force for those conditions. The correct answer will be less force.

See this link: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[coatsandrew]

SlideRuleEra - so for a pipe with a 4 ft diameter, and a water flow rate of 10 ft/s, the force is 78,414 lbs?

 

[SlideRuleEra]

No, I get 2440 lb. Divide your answer by the value of "g" (32.2 ft/sec²). That is necessary to get mass (slugs) in conventional US units.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[coatsandrew]

Great, thank you for all of the help.

 

[dhengr]

Coatsandrew:
But, rather than the 2440lbs. acting on a small area or length of the screen, immediately in front of the outfall pipe, isn’t the real design load 6 or 8" of water head behind the screen and over the full 50' length when the screen might start getting clogged up with floating junk? Why not ask the manufacture of that screen system what various load conditions should be considered? They’ve got far more experience with their screen system than any of us do. Why wouldn’t the loading on the screen system be based on some drag coef. and the flow of a given volume per second of water through the screen and some clogging debris being held back? Given floating debris, what are the wave action forces on the screen. If a 12" log might be transported through that 66" storm water pipe, I imagine it would be quite a concentrated projectile impinging on the screen, too close to the outfall. The screen should probably not be to close to the outfall. Maybe you should provide some sort of energy dissipating system at the outfall.

 

[LittleInch]

I can't understand where the water goes if it doesn't go through the screen?

Over the top?

Round the side?

Through the big hole in the middle when the screen fails? The linked material seems to be valid for a pretty gentle tidal flow to avoid getting debris in or containing debris from something like a floating log store. As the website says "You will want to periodically remove floating debris that has build up to prevent the structure from receiving too much pressure."

No where near enough information other than vague guesses all round.

It seems like a very strange idea to me all round....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SlideRuleEra]

I don't know the OP's application, but he does mention "changing tides". I know of an installation where the layout he describes is reasonable - the inlet/outlet of a tidal impoundment. My sketch is shown below. The pipe is totally underwater most of the time. For this application, especially if it is in an urban environment (say a coastal city park) the debris screen would likely collect only light debris. The screen is somewhat self-cleaning with reversing tidal flow. Normally, flow trough the pipe is fairly slow. For the real installation I know of, flow is very fast during tidal extremes (king tides). The floating screens the OP linked to are only 2', 3' or 4' high - water can go under the screen.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[LittleInch]

SRE - that looks quite reasonable but a 66" storwater pipe at 10ft/sec sounds like a lot more water flow in one direction to me...

Anyway I got about 2 tonnes force on direct flat plate, so maybe not a huge force for "a 12" square precast prestressed concrete pile embedded 20 ft or so into the soil, and sticking out 13 feet or so above grade "

Sounds quite strong to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.