Manning Calculation in pipes with partial full pipe

[Cristian Cox]

Hello , good morning:

I am looking for some help and I would be very thankful for a hand.

I have several tanks connected to a drain system.
I have used Manning equation to calculate flow inside this drain, assuming the pipe most of the time will behave as an open channel, let say 50%

Q = A* (Kn/n)*(Rh exp 2/3)* (S exp 1/2)

Assuming Sf (friction slope) = S (slope of pipe) because the type of flow allow it

I can get

Hf (head loss pipe) = S * L (length of pipe)

No problem with above and I have obtained the values

But since I want to model, and see how the "Q" varies in manning equation Q = A* (Kn/n)*(Rh exp 2/3)* (S exp 1/2, when I start adding "Minor losses" generated , for example, "elbows"
I was thinking in obtain the minor losses from Hfm= k* (v2/2g) which is fine, and then

get a final: Total head loss THL = Hf + Hfm

and with this final Total head loss THL, incert it in the equation below instead of Hfp and in that way I obtain a new S
but the new "S" will be larger and if I incert that S in Manning the flow Q will be larger (of course since you have more slope) and that should "NOT" happen since I am placing losses in the system "minor losses" so Q should be less.


Hfp (head loss pipe) = S * L (length of pipe)

Can I play with the sign (+ or -) to make the S smaller??
or maybe the Q that I am obtaining is the Q before the system has generated the losses?


I dont really know, I am lost.


The last question and short:


Can I use Hazzen Williams for this application? assuming that the pipes are full but not pressurized, just flowing as open channel.

My main concerns are of all these questions are:


To be 100% that I getting the right amount of water at the end of the pipe, flowing as open channel at some % full or full but not pressurized + but checking also the "minor losses"

If I can do this with Hazzen william for me it is fine, since I have used it before and the major concern is to get short becuase I did not calculate minor losses

thanks very much and please give me a hand.

 

[LittleInch]

I think the issue is that you have changed S, the slope of the pipe.

If instead you fix the slope of the pipe, then you will need to adjust Q downwards to accommodate the increased friction head until your new THL equals the head loss you actually have.

For a part full gravity system, your head loss can't exceed the actual static head without increasing the percent full of the pipe or increasing the pipe slope.

There was a discussion here a few weeks ago on which correlation to use, but if you have open channel flow, Mannings is usually the go to set of equations.

You just need to be careful about your assumptions or substituting head for slope etc without thinking about the impact.
If some items are fixed ( i.e. slope, but others can change then do that.



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

 

[1503-44]

If your head loss is more than elevation change, that's always a sign that you need to iterate and try a reduced flow rate. Alternatively, you could increase the slope of the pipe, or make some combination of both.

Your fitting losses K factors are probably based on full flow.

You can use most flow equations for pipe, if you can find a way to compensate for hydraulic radius. Flow equations for pipe assume water contact everywhere on the inside circumference. The typical formulation for Manning open channel flow leaves that variable for the user to calculate, which is why it is customarily used to solve channel flow problems. You can use other pipe flow equations, if you go back to their derivation and do not reduce and eliminate all the terms in the hydraulic radius that assume it is based on water contact with the full inside circumference of the pipe.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[Cristian Cox]

Little inch, thanks for your answer.

I am digesting what you have said and it looks you are right in the way
that I can not plug just the new value THL in manning. I should start lowing Q to match the new THL

thanks very much

I will let you know how it goes


regards

Cristian

 

[Cristian Cox]

Thanks 1503-44 too , I will check this too


regards

 

[1503-44]

LI was correct. To have more flow, you must increase diameter, or slope. You could try 60%full, if you only need a little more. That's about the max flow rate you can do. Probably higher than flowing full.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[bimr]

Is the piping originating at the tank bottoms, tank tops, or some midpoint? A sketch would be helpful.