Min. & Max. velocity flow for sewer pipes 1

[Will007]

I have been told by the city that I need to design sewer pipes flowing half full to have a minimum of 2 fps velocity.

Is this standard practice, or does anyone know what the min. & max flow velocities are for sewer pipes.

Sizes 8-15" inside diameter.

Thanks in advance for any help.

 

[RWF7437]

Yes, this is standard practice in many areas. 2 ft/sec is considered to be a self cleaning velocity and has been adopted by many, if not all, sewer agencies.

Maximum velocity is not so standardized, in my experience. Somewhere around 10 ft/sec is common practice. Higher velocities lead to problems of water hammer, erosion, gas release (H2S), etc.

You might look at ASCE's Manual of Practice. No. 9 or the Ten State Standards as widely cited authorities.

good luck

 

[redbridge]

I have always been told 2 fps full flow.

 

[Will007]

So the minimum velocity is 2 fps, rather the pipe is flowing full or half-full.

Thanks

 

[Maury]

I have always designed for a minimum velocity of 2 fps for sanitary sewers and 3 fps for storm sewers. The velocity will be same whether the pipe is full or half full.

 

[LCruiser]

Maury -
That is if you are ignoring inlet and outlet conditions - a dangerous practice...

 

[bimr]

The ususal practice is to design the slopes for sanitary sewers to ensure a minimum velocity of 2 ft/sec with flow at one-half full or full depth. However, a mean velocity of 1 ft/sec is usally suficient to prevent solids deposition. The minimum and maximum velocities are typically specified in state and local standards.

In general, maximum mean velocities of 8 to 10 ft/sec at the design depth of flow will not damage the sewer.

However, high velocities in small pipe sewers and corresponding low depths of flow may allo larger objects, which at times enter all sanitary sewer systems to remain on the inverts, where they may become lodged so firmly that the next rush of wastewater will not detach the objects.

 

[Hoagie]

2 fps is the minimum, 3fps is recommended (Merritt); 10 fps is your upper limit (Ibid). Be sure to check low-flow conditions (with Mannings).

 

[PipelineTZM]

The mostly used minimum self-cleaning velocity for sewer lines is 0.76m/s (metric)

 

[hokie66]

.76, that's a nice round number. So somebody couldn't decide whether it was to be 2fps or 3fps, used 2.5, then converted to metric. Very scientific.

 

[rconner]

I believe I have seen that some authors over the years have indicated that it may take at least a little more than the oft quoted "minimum" velocity to move particularly large "grit" etc. (if this could be an issue) in particularly large sized sewers. If you did a search e.g. under "critical shear stress sewers" you would proably find more information (e.g. see page 74 at ftp://phoenix.gov/pub/payf/wtrstndrds.pdf ).