How to find velocities in sanitary sewers given size, q, slope 1

[lilyanne22]

I know this is probably a silly question but maybe I am over analyzing it? I am checking velocities and have the pipe size, discharge and slopes but how do I determine the velocity?

 

[dicksewerrat]

Plug your numbers into Manning's equation.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[stanier]

Suggest you search for Flowpro2 on the net. This freeware is useful in modelling free surface flows.

 

[bimr]

Here is the Mannings formula:

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Manning_s_Equation.htm

 

[lilyanne22]

I not able to download at this time. As for plugging in to mannings, Q = A x (1.49/n) x (R^2/3) x (s^.5) where I have Q,n, and my question would be, is A what I am trying to find so I can find V from the equation Q=VA? if so how do I find R?

 

[RWF7437]

You cannot simply "plug" numbers into Manning's Equation.

You need first to determine whether or not the pipes in your system are flowing full, surcharged or partly full. Suggest you do the following:

1 Calculate the full flow capacity of each pipe in the system
2. Compare the actual flow to that capacity
3. If the flow is less than capacity then the pipe is probably flowing partly full. If the flow is greater than capacity then the pipe is surcharged ( i.e. flowing under some pressure ). If the flow exactly equals capacity then the pipe is flowing full but this is very unlikely in a real system.

Remember that the slope in the Manning Eq. is the slope of the hydraulic grade line; NOT the slope of the pipe.

There are a number of programs around that will model this for you. I use HydraFlow Storm Drains but there are others. Some are free and some low cost ( <$ 1000>.

good luck

 

[bimr]

Check wikipedia, it has the definition of R - hydraulic radius.

http://en.wikipedia.org/wiki/Open_channel_flow

There is a free calculator on there as well:

http://www.wq.uiuc.edu/dg/Equations/Mannings.exe

Here is a chart that shows the minimum slope that is typically acceptable:

c) Slope
1) All sewers shall be designed and constructed to give mean velocities, when flowing full, of not less than 2.0 feet per second, based on Manning's formula using an "n" value of 0.013. The following minimum slopes shall be provided; however, slopes greater than these are desirable:

Minimum Slope in Feet

Sewer Size....Per 100 Feet....... Flow (mgd)
8 inch....... 0.40............... 0.49
10 inch.......0.28................ 0.75
12 inch.......0.22................ 1.07
14 inch.......0.17.................1.43
15 inch...... 0.15................ 1.61
16 inch.......0.14................ 1.85
18 inch...... 0.12................ 2.35
21 inch...... 0.10................ 3.23
24 inch...... 0.08................ 4.13
27 inch...... 0.067............... 5.17
30 inch...... 0.058............... 6.37
33 inch...... 0.050............... 7.66
36 inch...... 0.046............... 9.23
42 inch...... 0.036.............. 12.41

http://www.ipcb.state.il.us/documents/dsweb/Get/Document-12042/

 

[PEDARRIN2]

There are charts that are based on manning.

They give flows of different diameter pipes at 1/2 - 3/4 and total full for various slopes.

This will give you a ballpark to know what flow or what slope you will have or need.

 

[lilyanne22]

Thank you to all

 

[chanel86]

hi ,

to calculate velocity u may use this eqation:

V(m/s)=Q/Pix(d^2)/4

Q=flow rate (l/s)
d=diameter or pipe in (m)

u may also see my attached file to calculate head losses and velocity using Darcy-weisbach.

hope that will be usefull .

looking for your forward.

best regards

Chanel

 

[chanel86]

hi again ,

in selecting pipe size we put into consideration that velocity in pipe should be 0.75 m/s < v <3 m/s

so we try in the formulas above to find the appropriate size where the velocity is arround 1.5 m/s.

best regards,

chanel

 

[lilyanne22]

Thank you Chanel.

 

[chanel86]

Welcome anytime

lilyanne

 

[kimhi]

lilyanne22 (Civil/Environme)
5 Mar 08 17:31
I not able to download at this time. As for plugging in to mannings, Q = A x (1.49/n) x (R^2/3) x (s^.5) where I have Q,n, and my question would be, is A what I am trying to find so I can find V from the equation Q=VA? if so how do I find R?

====
From what I understand, "A" is the cross-sectional area of your pipe (is this correct?). Then all your parameters are known and you can solve directly for Q.

Follow that Wikipedia link that BimR posted. Thats what I have been using.

Someone else said:
"Remember that the slope in the Manning Eq. is the slope of the hydraulic grade line; NOT the slope of the pipe."

WHAT IS THE HYDRAULIC GRADE LINE??? I thought slope was slope of pipe.

 

[lilyanne22]

Here is the situation. I have a calculations I am to check. I already know Q, Slope, and n but it is unknown how full the pipe is so the hydraulic radius and wetted perimeter are unknown. How do you acturally calculate the velocity if you don't know the area of water? Do I assume it is half full?

 

[lilyanne22]

Also, has anyone else having problem with the Manning online calcualtor from the Wikipedia link that BimR posted? It has locked up on me several times and I think the slope button is supposed to be ft/ft. Please correct me if I am wrong

 

[jartgo]

lilyanne22,
I am assuming you are asking about pipes flowing partially full (i.e., not under pressure).

In the simplest response to your latest question, you need to determine how full the pipe is based on the Q. Easier said than done, but quite easy with one of the many available "circular pipe ratio" nomographs.

First, use known slope, n, pipe diameter, and Manning's to determine the "Vfull" and "Qfull" which is the V and Q, respectfully when the pipe is at full capacity. You apparently know what flow (Q) you want to analyze. With this, you can enter one side of the abovementioned nomograph with the ratio of "Q" vs. "Qfull" then, depending on which type of nomograph you use, the graph will tell you what the velocity is, as a ratio of full velocity.

Two sources for these nomographs are the Concrete Pipe Design Manual, by ACPA and the Civil Engineering Reference Manual by Lindeburg. They are both common references and should be easy to locate.

kimhi,
you appear to be mistaken on several points. First, the "A" in the equation is not the area of the pipe, or channel, but the flow area.
Similarly, as RWF pointed out, the slope in the equation is the slope of the hydraulic grade line, not necessarily the slope of the pipe. The hydraulic grade line is essentially the head difference divided by the length of the conveyance you are analyzing. For free surface flows (gravity systems), this most nearly coincides with the surface of the water and is approximated by the slope of the pipe. However, when the pipe starts to flow under pressure (it's more than full), the hydraulic grade line begins to deviate from the pipe slope, at which point the slope of the pipe has no influence on the velocity. Water flows from higher head to lower head, regardless of the slope of the pipe.

This is why water can flow uphill, as long as the hydraulic grade line is down, in that direction. Clear as mud?

 

[lilyanne22]

Thank you jthompson. Great explanation and very helpful.

 

[elev8848]

lilyanne22,
Try my simple

http://www.hydrocalc.com