Acceptable Belly/Sag in Wastewater Force Main

[drewnje]

I am looking for a design reference for the acceptable belly/sag in a wastewater force main. The wastewater is little more than water, but due to existing infrastructure may have to temporarily cross below and obstruction. Flow is less than 100 gpm and the pipe size is 3 inches.

Any guidance would be much appreciated.

 

[bimr]

You are describing this as a force main which means that the piping is always flowing full. A typical velocity of 3.5 ft/sec will flush air out of a pipeline with a reasonable horizontal slope (not vertical). As long as you have the velocity you have nothing to worry about.

If you are concerned with the pipeline breaking from the sag, indicate what the piping is fabricated from and the length that you want to bridge. In general if the sag is less than 1/360 of the length you don't need to be concerned with the sag from a structural aspect. Generally, 3-Inch PVC requires supports on 4' centers as the link shows. 3" ductile iron can go 11' between supports. Note that just because the pipe sags doesn't mean there is an issue.

Ductile

PVC