Force Main/pump Staion

[jerryone]

I am trying to understand a sewage pump and force main design.
The total pipe required for the design is about 5600 ft .
The high point of the design 676 ft is at 1200ft from the lift station.(the elevation at the pump station is 658ft)
Then the elevation constantly decreases and at the discharge manhole the elevation becomes 645 ft. (exacavation is an issue and forcemain is a must)
My question here is for my head loss due to friction calculation, do I have to use 5600ft or 1200ft. my assumption after the high point the flow will run by gravity even though it will be just the same force main .
Could you please give me a good website that has a good example sewage pump station design .
Thanks for your help

 

[francesca]

You need to establish the controlling point. It is not necessarily the high point. Flow will transition to gravity flow where the HGL crosses the soffit of the pipe. Depending on the slope after the high point, it may be downstream. Your pump will pump to the controlling point, so that is the length you will use.

Remember to model various flow conditions and to plan for the entire drainage area, not just this particular development.

 

[bimr]

You would have a 5,600 ft forcemain. Your option is to install a shorter forcemain and use a gravity sewer for the balance of the pipe length.

The cost difference would vary depending on the size of the force main, size of the sewer, number of road crossings, depth of excavation, and subsurface conditions.

Go buy the book:

Collection and Pumping of Wastewater
by Metcalf & Eddy.