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

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

Source：Internet Author：Unknow Pubdate：2008-04-01

lilyanne22 (Civil/Environme) 5 Mar 08 17:00
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 (Civil/Environme) 5 Mar 08 17:03
Plug your numbers into Manning's equation. Richard A. Cornelius, P.E.
WWW.amlinereast.com

stanier (Mechanical) 5 Mar 08 17:14
Suggest you search for Flowpro2 on the net. This freeware is useful in modelling free surface flows.

bimr (Civil/Environme) 5 Mar 08 17:31
Here is the Mannings formula:

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Manning_s_Equation.htm 字串9

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?

RWF7437 (Civil/Environme) 5 Mar 08 18:01
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. 字串7

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 (Civil/Environme) 5 Mar 08 21:59
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: 字串8

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 (Mechanical) 6 Mar 08 6:24
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.

字串7

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

lilyanne22 (Civil/Environme) 6 Mar 08 9:49
Thank you to all

chanel86 (Electrical) 19 Mar 08 16:36
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

http://files.engineering.com/getfile.aspx?folder=7ee45cbe-c072-45c7-ad68-55

chanel86 (Electrical) 19 Mar 08 16:45
hi again ,

字串3

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 (Civil/Environme) 19 Mar 08 16:47
Thank you Chanel.

chanel86 (Electrical) 20 Mar 08 4:15
Welcome anytime lilyanne

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