Sewer Line Capacity.

[dhot]

I have looked through the forum trying to find something to help me. I am trying to find the capacity of an old sewer line. I am Upgrading a Treatment plant and need to know the capacity of the trunk sewer leading to the plant. The trunk line is bass ackwards. It goes from a 12" VCP to an 8" DI/PVC into the plant. The 8" line was fixed with PVC thats why its half DI PVC. Do I use Mannings or HW equation? Also I have the Slope of the lines in % how do I convert it for the equations?

 

[dicksewerrat]

Use Manning's for the 12 inch. then compare that to the 8 inch. My guess will be that the 8 inch will be the controling Q. Slope is in feet per foot. 1% = .01 ft./foot or 1 foot per hundred.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[dhot]

So if I had a 0.57 foot drop in 27.81 feet i would have 0.57/27.81 = 0.20496 or 2% slope.

 

[cvg]

you probably need to calculate pressure flow since most influent lines I have seen are surcharged near the plant. The surcharge / pressure flow probably explains why ductile iron or pvc pipe was used instead of the bell and spigot vcp gravity pipe. In that case, use bernoulli equation and slope does not influence the flow rate. But do account for the minor losses at the transitions between the different size and types of pipe as well as any bends, manholes or other fittings.

 

[jartgo]

I'm not sure I follow what you're doing, but if you are truly upgrading the plant, the way to do it would be size it for the trib. flow to the plant based on the demand, not on the capacity of the influent line. This would likely be set by the state (based on population and uses) in which you are working, assuming you're working in the U.S.

If you're just needing the identify the capacity of the line (independent of the plant upgrades), I agree with cvg's recommendations above. You'll have to make some assumptions about what a "reasonable" headwater would be. Again, I'd also check the regulations to see whether they'll allow you to consider pressure flow for a gravity line. Most regulations I'm familiar with require gravity pipes to pass design flow at half full.

 

[BRIS]

You appear to have a chicken and egg situation and to be solving for neither:

The 8" pipe no doubt flows full>

The capacity of the trunk sewer is its surcharged capacity not its free flow capacity. So the equation you should be using is a full pipe flow equation and the gradient you should be using is the hydraulic gradient not the pipe gradient. The hydraulic gradient is the maximum upstream level in the manhole - discharge level in the plant.