Design of sewer pipes 1

[hayderg2000]

Greeting

I want to design sewer network and when i reading about design criteria i found two , one for minimum slope and other for minimum velocity and both of them theoretically in one relation actually what's the control ( slope or velocity),mean
if get the minimum velocity but less than the minimum slope is that acceptable or not

thanks

 

[Ron]

They are obviously interrelated; however, the function of the pipe is more closely related to the velocity, since you want enough velocity to keep sediment from building up, but not so much that you scour and degrade the pipe.

 

[dicksewerrat]

You need 2-3 fps to keep the pipes fairly clean. Look for the slope that gives you that velocity for each size pipe. Less slope means less depth and therefore lower installation cost.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

The local agency having jurisdiction will set the minimum requirements for sewer design. The requirement for a minimum velocity in the sewer to prevent solids deposition calculates into a minimum design slope requirement.

Whether you are expressing the sewer design criteria as the minimum velocity in terms of ft/sec or in slope of the sewer, you are essentially setting the same minimum design criteria.

Here are a few examples:

Gravity sewers shall be designed for a minimum velocity of three fps that exists at the time the pipe is placed into service. City Engineer approval must be obtained to use lower design velocities, except in the extreme upper reaches of the system with few connections. In these cases, 8-inch diameter minimum pipe size and 0.0044 ft/ft minimum slope will govern except for the last upstream reach to a terminal maintenance hole where 0.0060 ft/ft minimum slope will govern.

Sewer Pipe Design • Sewer pipes shall be designed for minimum velocity of 2 feet per second.
• The minimum pipe size for sanitary sewer is 8” inside diameter.
• If a flatter slope is proposed, the pipe shall be designed to prevent settlement of solids in the pipe.


3. Slope -
a. All sewers shall be so 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. Based on an "n" value of 0.013, the following are the minimum slopes which should be provided; however, slopes greater than these are desirable:

Sewer Minimum Slope
Ft/100 feet
4 inch 1.05
6 inch 0.60
8 inch 0.40
10 inch 0.28
12 inch 0.22
14 inch 0.17
15 inch 0.15
16 inch 0.14
18 inch 0.12
21 inch 0.10
24 inch 0.08
27 inch 0.067
30 inch 0.058
36 inch 0.046

http://denr.sd.gov/documents/designnumber.pdf

 

[hayderg2000]

Thanks to Ron, dicksewerrat, bimr

another question the velocity will be checked for actual loads or load multiply by peaking factor ( i use multiplied by P.F)

 

[bimr]

The design for various components of the wastewater collection system (collector sewers,interceptor sewers, pumping stations) is usually based on peak hourly flow.

Peak Hourly Flow is the ratio of peak diurnal flow to daily average flow. The ratio is determined from Fair & Geyer's chart "Ratio of Peak Hourly Flow to Design Average Flow".